Flexible Electronics

**67**

**Chapter 5**

**Abstract**

*and Hyun-Joong Kim*

display, flexible, foldable

**1. Introduction**

features possible [2].

Pressure-Sensitive Adhesives for

Pressure-sensitive adhesives (PSA) have been used in electronics for not only attachment of two materials but also shock absorption, thermal and electrical conductivity, electromagnetic shielding, and optical property. Optically clear adhesives (OCA) have been used as a core material for optical performance of display. In addition to basic properties of OCA such as adhesion strength, transmittance, haze, and reliability, it has required dielectric constant, gap filling, and anticorrosion according to a substrate. However, the structural and functional changes of flexible display bring challenges to OCA that protects vulnerable components such as thin-film transistor, OLED, and thin-film encapsulation by stress dispersion and adjustment of a neutral plane. At the same time, flexibility and existing properties are essential. In this chapter, the development of components and performance of

**Keywords:** pressure-sensitive adhesives, PSA, optically clear adhesives, OCA,

PSA is a material attached by slight pressure and detached easily from the substrate [1]. PSA's semisolid property caused by low glass transition temperature (Tg, generally 25–45°C below usage temperature) at usage temperature makes these

PSA can be classified by type and proportion of components into natural and synthetic rubber, acrylic, silicone, and urethane PSA [3]. The acrylic PSA is widely used in display, mobile phones, and automotive applications due to its high transparency, weather resistance, heat resistance, and high adhesion strength [4–6]. Furthermore, the acrylic PSA can have a wide range of properties because of a variety of acrylic monomers. Although the silicone PSA is not as universal as the acrylic PSA, special applications have been used that require high reliability because

PSA has better processability than liquid-type adhesive because it can adhere without the hardening process and to the three-dimensional substrate. Also in the automobile industry, PSA has been considered as a substitutive process of the traditional mechanical joining method using bolts and screws because of the requirement of weight reduction. In particular, as the application of electronic and automobile industries is expanded, it is required to have not only adhesion strength

Flexible Display Applications

*Tae-Hyung Lee, Ji-Soo Kim, Jung-Hun Lee* 

OCA, and evaluation methods will be discussed.

of its high resistance to high and low temperatures.

#### **Chapter 5**

## Pressure-Sensitive Adhesives for Flexible Display Applications

*Tae-Hyung Lee, Ji-Soo Kim, Jung-Hun Lee and Hyun-Joong Kim*

#### **Abstract**

Pressure-sensitive adhesives (PSA) have been used in electronics for not only attachment of two materials but also shock absorption, thermal and electrical conductivity, electromagnetic shielding, and optical property. Optically clear adhesives (OCA) have been used as a core material for optical performance of display. In addition to basic properties of OCA such as adhesion strength, transmittance, haze, and reliability, it has required dielectric constant, gap filling, and anticorrosion according to a substrate. However, the structural and functional changes of flexible display bring challenges to OCA that protects vulnerable components such as thin-film transistor, OLED, and thin-film encapsulation by stress dispersion and adjustment of a neutral plane. At the same time, flexibility and existing properties are essential. In this chapter, the development of components and performance of OCA, and evaluation methods will be discussed.

**Keywords:** pressure-sensitive adhesives, PSA, optically clear adhesives, OCA, display, flexible, foldable

#### **1. Introduction**

PSA is a material attached by slight pressure and detached easily from the substrate [1]. PSA's semisolid property caused by low glass transition temperature (Tg, generally 25–45°C below usage temperature) at usage temperature makes these features possible [2].

PSA can be classified by type and proportion of components into natural and synthetic rubber, acrylic, silicone, and urethane PSA [3]. The acrylic PSA is widely used in display, mobile phones, and automotive applications due to its high transparency, weather resistance, heat resistance, and high adhesion strength [4–6]. Furthermore, the acrylic PSA can have a wide range of properties because of a variety of acrylic monomers. Although the silicone PSA is not as universal as the acrylic PSA, special applications have been used that require high reliability because of its high resistance to high and low temperatures.

PSA has better processability than liquid-type adhesive because it can adhere without the hardening process and to the three-dimensional substrate. Also in the automobile industry, PSA has been considered as a substitutive process of the traditional mechanical joining method using bolts and screws because of the requirement of weight reduction. In particular, as the application of electronic and automobile industries is expanded, it is required to have not only adhesion strength and processability but also complex functions such as electrical and thermal conductivity, high thermal resistance, and reliability of humidity or chemicals.

OCA is used for attachment of layers in the display including cover window, touch panel, polarizers, and the light-emitting layer, which commonly require high transmittance, low haze, and corrosion resistance to ITO (indium tin oxide) film. When OCA is directly bonded to ITO film, the acid component must be excluded for the durability of the ITO film. Prolonged contact with acid can cause touch problems by reacting with metal, which makes surface resistance to increase [7]. It is also important for OCA to minimize air bubbles to reduce defects.

The demand of thin and flexible displays is increasing as interest in small and diverse designs grows [8]. Flexible displays can be distinguished by their intended use and function [9]. Recently, significant progress has been made in achieving active-matrix organic light-emitting diodes with bendable and rollable displays [10–12]. Several PSA properties are additionally required to create these flexible displays. Generally, OCA for flexible displays requires low shear modulus and Tg [13]. The recovery and stress relaxation properties of OCA are important for flexible displays because a high recovery of PSA prevents it from deformation under repeated folding-unfolding conditions [14].

#### **2. Component of PSA**

As mentioned above, acrylic PSA has been widely used for not only household or general industrial product but also electronics and automobile industries due to various advantages such as adhesion properties, optical properties, high reliability, and easy modification. Especially in OCA manufacturing, the synthesis and curing by ultraviolet (UV) radiation are used for short process time, high molecular weight, and nonsolvent process. The acrylate is one of the most suitable materials for the UV process. In this chapter, we would like to describe the materials that compose acrylic PSA including OCA.

acrylic PSA is synthesized by selecting several acrylic monomers, and the monomers can be classified into the alkyl (meth)acrylate and functional (meth) acrylate. Also, the alkyl (meth)acrylate is divided into monomers with a low Tg and a high Tg. The acrylic monomer of low Tg has a linear carbon chain that consists of 4–17 atoms, and the monomer of high Tg has a short linear chain (1–3 carbon atoms) or bulky chemical structure such as a cyclic hydrocarbon or aromatic ring. The functional monomer has a hydrophilic functional group such as a carboxyl group and hydroxyl group, and this gives an acrylic polymer reaction site for crosslinking and hydrophilicity to enhance adhesion and cohesion strength. The low Tg monomers are the main component of acrylic PSA, and the high Tg monomers and the functional monomers are added to adjust the characteristics of acrylic PSA. A typical acrylic PSA consists of 50–90% of the low Tg monomer, 10–40% of the high Tg monomer, and 2–20% of the functional monomer [3, 15].

The adhesion properties of PSA are determined by both the viscoelasticity and chemical characteristics. The relationship between the viscoelasticity and the adhesion has been studied, and the range of storage modulus (G′) and loss modulus (G″) is suggested as a viscoelastic window [3, 16–19]. The surface free energy can represent the chemical characteristic of PSA. Wettability between PSA and substrate is determined by the relation of surface free energy of PSA and substrate. The immediate adhesion by quick wetting should be demonstrated right after contact with the substrate for effective use of PSA [20]. Kowalski et al. conducted a study about the tack value of acrylic PSA that increased from 300 to 700% depending on

**69**

**Figure 1.**

*Pressure-Sensitive Adhesives for Flexible Display Applications*

[21].

monomers can be evaporated in the final product [27].

**3. Development in display and OCA**

*can be placed on the encapsulation by OCA or direct deposition.*

monomers with longer carbon chains are considered for flexibility.

the cross-linking degree when surface free energy of a polymer substrate increased

Silicone PSA has extreme resistance at a wide temperature range from −40 to 300°C. In addition, because it can be applied to substrates having various surface energies, it is used for masking tape for printed circuit board plating, electrical insulation tape, and OCA. However, due to its drawbacks such as high price, high process temperature, and high release strength to release film, it has been applied only to specific fields that can withstand harsh environments [22]. This PSA consists of silanol-terminated silicone polymer and silanol-functional siloxane resin. The silicone polymer for PSA is a semisolid gum having a high viscosity and

Many manufacturers of OCA adopt the UV radiation process for the mentioned reasons, and the process is divided into two steps. The first step is a synthesis of an acrylic prepolymer by UV radiation to a mixture of the monomers and photoinitiator. The monomer mixture, except for the photoinitiator, is similar to the composition described above. But, because the acrylic acid, the major functional monomer, cannot be used, other functional monomers are evaluated to enhance adhesion strength. N-vinyl caprolactam (NVC) and N, N-dimethyl acrylamide (DMAA) are the representative monomers [24, 25]. Since nitrogen atom has a high electronegativity, it can improve the cohesion and at the same time prevent the risk of corrosion by acid [26]. In the second step, the synthesized acrylic prepolymer is mixed with multifunctional acrylate as a cross-linking agent and cross-linked by UV radiation. However, the unreacted monomers can be remained after the curing process. Because a drying process is not included in the UV radiation process, the unreacted

Currently, the development issue facing OCA is flexibility, and this not only prohibits the use of acidic monomers but also limits the selection of other common monomers. The high Tg monomers and functional monomers cause high modulus, and this property restricts the deformation of OCA. 2-Ethylhexyl acrylate (2-EHA) and butyl acrylate (BA) are used as a conventional low Tg monomer, but other

All kinds of displays have a multilayer structure, and most of these layers are combined with OCA or PSA (**Figure 1**). Without OCA, the light from the backlight

*Multilayer structures of the display. OCA or PSA can be applied to red layers in the structures. A touch sensor* 

*DOI: http://dx.doi.org/10.5772/intechopen.90619*

600–1000 kDa of molecular mass [23].

from 20.5 to 42.9 mJ/m2

*Hybrid Nanomaterials - Flexible Electronics Materials*

repeated folding-unfolding conditions [14].

compose acrylic PSA including OCA.

**2. Component of PSA**

and processability but also complex functions such as electrical and thermal conductivity, high thermal resistance, and reliability of humidity or chemicals.

OCA is used for attachment of layers in the display including cover window, touch panel, polarizers, and the light-emitting layer, which commonly require high transmittance, low haze, and corrosion resistance to ITO (indium tin oxide) film. When OCA is directly bonded to ITO film, the acid component must be excluded for the durability of the ITO film. Prolonged contact with acid can cause touch problems by reacting with metal, which makes surface resistance to increase [7]. It

The demand of thin and flexible displays is increasing as interest in small and diverse designs grows [8]. Flexible displays can be distinguished by their intended use and function [9]. Recently, significant progress has been made in achieving active-matrix organic light-emitting diodes with bendable and rollable displays [10–12]. Several PSA properties are additionally required to create these flexible displays. Generally, OCA for flexible displays requires low shear modulus and Tg [13]. The recovery and stress relaxation properties of OCA are important for flexible displays because a high recovery of PSA prevents it from deformation under

As mentioned above, acrylic PSA has been widely used for not only household or general industrial product but also electronics and automobile industries due to various advantages such as adhesion properties, optical properties, high reliability, and easy modification. Especially in OCA manufacturing, the synthesis and curing by ultraviolet (UV) radiation are used for short process time, high molecular weight, and nonsolvent process. The acrylate is one of the most suitable materials for the UV process. In this chapter, we would like to describe the materials that

acrylic PSA is synthesized by selecting several acrylic monomers, and the monomers can be classified into the alkyl (meth)acrylate and functional (meth) acrylate. Also, the alkyl (meth)acrylate is divided into monomers with a low Tg and a high Tg. The acrylic monomer of low Tg has a linear carbon chain that consists of 4–17 atoms, and the monomer of high Tg has a short linear chain (1–3 carbon atoms) or bulky chemical structure such as a cyclic hydrocarbon or aromatic ring. The functional monomer has a hydrophilic functional group such as a carboxyl group and hydroxyl group, and this gives an acrylic polymer reaction site for crosslinking and hydrophilicity to enhance adhesion and cohesion strength. The low Tg monomers are the main component of acrylic PSA, and the high Tg monomers and the functional monomers are added to adjust the characteristics of acrylic PSA. A typical acrylic PSA consists of 50–90% of the low Tg monomer, 10–40% of the high

The adhesion properties of PSA are determined by both the viscoelasticity and chemical characteristics. The relationship between the viscoelasticity and the adhesion has been studied, and the range of storage modulus (G′) and loss modulus (G″) is suggested as a viscoelastic window [3, 16–19]. The surface free energy can represent the chemical characteristic of PSA. Wettability between PSA and substrate is determined by the relation of surface free energy of PSA and substrate. The immediate adhesion by quick wetting should be demonstrated right after contact with the substrate for effective use of PSA [20]. Kowalski et al. conducted a study about the tack value of acrylic PSA that increased from 300 to 700% depending on

Tg monomer, and 2–20% of the functional monomer [3, 15].

is also important for OCA to minimize air bubbles to reduce defects.

**68**

the cross-linking degree when surface free energy of a polymer substrate increased from 20.5 to 42.9 mJ/m2 [21].

Silicone PSA has extreme resistance at a wide temperature range from −40 to 300°C. In addition, because it can be applied to substrates having various surface energies, it is used for masking tape for printed circuit board plating, electrical insulation tape, and OCA. However, due to its drawbacks such as high price, high process temperature, and high release strength to release film, it has been applied only to specific fields that can withstand harsh environments [22]. This PSA consists of silanol-terminated silicone polymer and silanol-functional siloxane resin. The silicone polymer for PSA is a semisolid gum having a high viscosity and 600–1000 kDa of molecular mass [23].

Many manufacturers of OCA adopt the UV radiation process for the mentioned reasons, and the process is divided into two steps. The first step is a synthesis of an acrylic prepolymer by UV radiation to a mixture of the monomers and photoinitiator. The monomer mixture, except for the photoinitiator, is similar to the composition described above. But, because the acrylic acid, the major functional monomer, cannot be used, other functional monomers are evaluated to enhance adhesion strength. N-vinyl caprolactam (NVC) and N, N-dimethyl acrylamide (DMAA) are the representative monomers [24, 25]. Since nitrogen atom has a high electronegativity, it can improve the cohesion and at the same time prevent the risk of corrosion by acid [26]. In the second step, the synthesized acrylic prepolymer is mixed with multifunctional acrylate as a cross-linking agent and cross-linked by UV radiation. However, the unreacted monomers can be remained after the curing process. Because a drying process is not included in the UV radiation process, the unreacted monomers can be evaporated in the final product [27].

Currently, the development issue facing OCA is flexibility, and this not only prohibits the use of acidic monomers but also limits the selection of other common monomers. The high Tg monomers and functional monomers cause high modulus, and this property restricts the deformation of OCA. 2-Ethylhexyl acrylate (2-EHA) and butyl acrylate (BA) are used as a conventional low Tg monomer, but other monomers with longer carbon chains are considered for flexibility.

#### **3. Development in display and OCA**

All kinds of displays have a multilayer structure, and most of these layers are combined with OCA or PSA (**Figure 1**). Without OCA, the light from the backlight

#### **Figure 1.**

*Multilayer structures of the display. OCA or PSA can be applied to red layers in the structures. A touch sensor can be placed on the encapsulation by OCA or direct deposition.*

unit is reflected at the interface between each film and the air due to the differences of the refractive index. Finally, less than 10% of light reach to user's eyes [28]. The presence of OCA affects the clarity of the display screen because OCA between films not only holds the film together but also prevents loss of light. Also, the difference in the refractive index between layers decreases, and a similar refractive index allows the straight progress of light without loss [29]. So, OCA should basically be transparent and have a low haze. It should also be optically isotropic, with less coloring and discoloration in environmental conditions [30].

LCD is the most widely used display to date. As shown in **Figure 1**, because it has to have a more complex structure than OLED display, more PSA layers are used. While the polarizer is used for blocking reflection of external light in OLED, it plays an important role in displaying images in LCD. The most important thing for the polarizer in LCD is to minimize the light leaking [31–34]. According to the study of Ma et al., OCA for polarizer can improve light leaking by reducing the shrinkage stress between a glass substrate and polarizer [35].

OLED is based on pixels that emit light on their own, and it can display pure black by turning off the pixels. The structures between LCD and OLED show stark differences because of their self light-emitting layer and vulnerability to oxygen and moisture [36]. Although there are many technical differences between OLED and LCD, the requirements for OCA are not much different.

A key to next-generation display technology is flexibility. The flexible displays are divided into quasi-flexible and real-flexible. The quasi-flexible display contains curved and bended displays, and Samsung's Galaxy Round is the first smartphone with a curved display. The manufacturing process of the curved display includes the bending process of the OLED panel. When the display panel is bent, spring back force is generated in the plastic film of the module to reverse the elastic deformation [37]. So, OCA of the curved module should have high adhesion property to resist this force. Otherwise, there is a risk of creating air bubbles when OCA is separated away from the glass. In order to achieve high adhesion and filling thick ink step at

**71**

*Pressure-Sensitive Adhesives for Flexible Display Applications*

the same time, 3 M has developed OCA products that have low modulus and good wetting performance with semicured structure (CEF3806, 3 M). Semicured OCA shows optimal adhesion performance through additional UV curing after lamina-

The foldable display is a united panel containing a variety of layers and substrates. For a flexible OLED display, it is desirable to have either an OCA that perfectly mechanically decouples the layers from each other in bending (i.e., a material with no modulus) to minimize strain on critical layers or to have OCA that can tune the position of the neutral plane during the folding process [38]. Also, although the thickness of OCA is thinner than that of OCA for a rigid display for reducing strain caused by folding, the strong adhesion, flexibility, and durability to withstand

OLED type of display has been chosen to implement flexible display. Major components of OLED display are thin-film transistors (TFTs), organic light-emitting diodes (OLEDs), and thin-film encapsulation (TFE). However, these layers are easily damaged by minor deformations and lose their functions because they are fragile in common [39]. To avoid this problem, the materials that have low modulus are used to absorb stress, and structural design to place these parts in the neutral plane is applied [40]. When the display composed with multilayers is bent, tensile or compressive deformation occurs depending on the position, but for each design, the layer at a particular position is simply bent without such deformation. This position is called the neutral plane. Masumi Nishimura *et al*. give the following equation

> <sup>2</sup> − *hi*−1 2 )/2∑ *i*=1 *n*

The λ means the distance from the surface of the innermost layer of the multilayer to the neutral plane, while Ei, hi, and ti mean the modulus of i-th layer, the distance from the innermost surface to the layer, and the thickness of the layer, respectively. As expressed in the equation, the position of neutral plane is determined by the modulus and thickness of each layer. Because OCA layers are relatively easy to control modulus and thickness without losing function, OCA layers are able

The studies were conducted to protect several layers by splitting neutral planes within a multilayer. Su et al. utilized PSA layers in flexible piezoelectric mechanical energy harvesters to split the neutral planes, thereby implementing a flexible structure without destroying the components vulnerable to deformation [42]. Based on this research, Nishimura et al. published a study about splitting neutral plane using OCA in a foldable AMOLED structure. The trend of neutral plane formation according to the modulus of adhesive was identified, and the splitting was observed

Recently, foldable mobile phone, which is a type of flexible electronics, has been commercialized. So, manufacturers of mobile devices, display, and materials for electronic devices and research institutes have been set evaluation methods for foldable characteristics. The most representative and essential method is the folding

*Ei ti* (1)

*DOI: http://dx.doi.org/10.5772/intechopen.90619*

hundreds of thousands of bends are required.

**4. Stress dispersion and neutral plane**

for calculating the position of the neutral plane [41]:

*i*=1 *n Ei*(*hi*

to be used for adjustment of the location of the neutral plane.

by the use of low modulus adhesive in the system [41].

**5. Test methods for foldable OCA**

λ = ∑

tion as shown in **Figure 2**.

**Figure 2.**

*Application process of the 3 M CEF3806.*

*Pressure-Sensitive Adhesives for Flexible Display Applications DOI: http://dx.doi.org/10.5772/intechopen.90619*

*Hybrid Nanomaterials - Flexible Electronics Materials*

coloring and discoloration in environmental conditions [30].

and LCD, the requirements for OCA are not much different.

stress between a glass substrate and polarizer [35].

unit is reflected at the interface between each film and the air due to the differences of the refractive index. Finally, less than 10% of light reach to user's eyes [28]. The presence of OCA affects the clarity of the display screen because OCA between films not only holds the film together but also prevents loss of light. Also, the difference in the refractive index between layers decreases, and a similar refractive index allows the straight progress of light without loss [29]. So, OCA should basically be transparent and have a low haze. It should also be optically isotropic, with less

LCD is the most widely used display to date. As shown in **Figure 1**, because it has to have a more complex structure than OLED display, more PSA layers are used. While the polarizer is used for blocking reflection of external light in OLED, it plays an important role in displaying images in LCD. The most important thing for the polarizer in LCD is to minimize the light leaking [31–34]. According to the study of Ma et al., OCA for polarizer can improve light leaking by reducing the shrinkage

OLED is based on pixels that emit light on their own, and it can display pure black by turning off the pixels. The structures between LCD and OLED show stark differences because of their self light-emitting layer and vulnerability to oxygen and moisture [36]. Although there are many technical differences between OLED

A key to next-generation display technology is flexibility. The flexible displays are divided into quasi-flexible and real-flexible. The quasi-flexible display contains curved and bended displays, and Samsung's Galaxy Round is the first smartphone with a curved display. The manufacturing process of the curved display includes the bending process of the OLED panel. When the display panel is bent, spring back force is generated in the plastic film of the module to reverse the elastic deformation [37]. So, OCA of the curved module should have high adhesion property to resist this force. Otherwise, there is a risk of creating air bubbles when OCA is separated away from the glass. In order to achieve high adhesion and filling thick ink step at

**70**

**Figure 2.**

*Application process of the 3 M CEF3806.*

the same time, 3 M has developed OCA products that have low modulus and good wetting performance with semicured structure (CEF3806, 3 M). Semicured OCA shows optimal adhesion performance through additional UV curing after lamination as shown in **Figure 2**.

The foldable display is a united panel containing a variety of layers and substrates. For a flexible OLED display, it is desirable to have either an OCA that perfectly mechanically decouples the layers from each other in bending (i.e., a material with no modulus) to minimize strain on critical layers or to have OCA that can tune the position of the neutral plane during the folding process [38]. Also, although the thickness of OCA is thinner than that of OCA for a rigid display for reducing strain caused by folding, the strong adhesion, flexibility, and durability to withstand hundreds of thousands of bends are required.

#### **4. Stress dispersion and neutral plane**

OLED type of display has been chosen to implement flexible display. Major components of OLED display are thin-film transistors (TFTs), organic light-emitting diodes (OLEDs), and thin-film encapsulation (TFE). However, these layers are easily damaged by minor deformations and lose their functions because they are fragile in common [39]. To avoid this problem, the materials that have low modulus are used to absorb stress, and structural design to place these parts in the neutral plane is applied [40]. When the display composed with multilayers is bent, tensile or compressive deformation occurs depending on the position, but for each design, the layer at a particular position is simply bent without such deformation. This position is called the neutral plane. Masumi Nishimura *et al*. give the following equation for calculating the position of the neutral plane [41]:

$$\lambda = \sum\_{t=1}^{n} E\_i \left( h\_i^{\ 2} - h\_{i-1} \right) / 2 \sum\_{t=1}^{n} E\_i t\_i \tag{1}$$

The λ means the distance from the surface of the innermost layer of the multilayer to the neutral plane, while Ei, hi, and ti mean the modulus of i-th layer, the distance from the innermost surface to the layer, and the thickness of the layer, respectively. As expressed in the equation, the position of neutral plane is determined by the modulus and thickness of each layer. Because OCA layers are relatively easy to control modulus and thickness without losing function, OCA layers are able to be used for adjustment of the location of the neutral plane.

The studies were conducted to protect several layers by splitting neutral planes within a multilayer. Su et al. utilized PSA layers in flexible piezoelectric mechanical energy harvesters to split the neutral planes, thereby implementing a flexible structure without destroying the components vulnerable to deformation [42]. Based on this research, Nishimura et al. published a study about splitting neutral plane using OCA in a foldable AMOLED structure. The trend of neutral plane formation according to the modulus of adhesive was identified, and the splitting was observed by the use of low modulus adhesive in the system [41].

#### **5. Test methods for foldable OCA**

Recently, foldable mobile phone, which is a type of flexible electronics, has been commercialized. So, manufacturers of mobile devices, display, and materials for electronic devices and research institutes have been set evaluation methods for foldable characteristics. The most representative and essential method is the folding test. Samsung Electronics, a major manufacturer of electronics including mobile phones, released folding test images with the launch of Galaxy Fold in September 2019 [43]. Folding durability not only of the assembled mobile phones but also of single or laminated film used as part of electronics has been evaluated. This test is performed with various environments and test modes such as low or high temperature, high humidity, the radius of curvature, static fold, and dynamic fold for simulation of various usage environments [44].

The folding test can obtain results from a complex interaction between multilayered components. So, the following various methods have been conducted to evaluate OCA independently.

#### **5.1 Shear strain on folded OCA**

When a book is bent, it can be seen that the unbound edge is deformed into an inclined form. Because unlike the book, however, each layer of the foldable display is attached to each other by PSA, each layer of multilayered display is deformed by different type and extent of stress. In this case, as shown in **Figure 3**, the outer layers and inner layers of the neutral plane are subjected to tensile stress and compressive stress, respectively. As such, the stress applied to each layer during folding can be calculated by the following Equation [41]:

$$
\sigma = E\_i \mathbf{e} = E \ i \left( \mathbf{y} - \lambda \right) / \rho \tag{2}
$$

**73**

preferred.

*Pressure-Sensitive Adhesives for Flexible Display Applications*

for increasing adhesion strength as described above.

Although the adhesion test is not a specific method for OCA applied to the foldable display, it is an important and fundamental method to evaluate all kinds of PSA. The test mode is selected according to the external force applied between PSA and substrate, and the general methods are peel, tack, lap shear, and pulloff test. The 180° peel test and lap shear test are conducted for foldable OCA

Although the evaluation of adhesion is not a specific test for foldable OCA, this is considered an important factor because of several limits for satisfying adhesion strength. One of these limits is the substitution of the glass cover window to the plastic cover window for foldable properties [47]. The surface energies of several substrates such as plastic, glass, and SUS are listed in **Table 1** [48, 49]. The polyimide (PI) film has been used for the plastic cover window, and the surface energy of plastic substrate including PI is significantly lower than the surface energy of glass and SUS substrates. According to the rule of thumb, the lower surface energy of a substrate is able to decrease adhesion strength between PSA and substrate [50]. It is necessary to increase adhesion strength to the plastic substrate with minimization of rising Tg and modulus of OCA. If sufficient adhesion strength is not developed, it will not be possible to withstand the stress applied between OCA and substrate in continuous folding. The other is the limitation of the usable monomers

The representative methods to measure shear modulus of PSA are the shear sandwich method by Dynamic Mechanical Analyzer (DMA) and the torsion method by Advanced Rheometric Expansion System (ARES) (**Figure 6**). Shear sandwich and torsion methods are subject to uniaxial and rotational shear stresses, respectively. In both of these methods, the material responds to repetitive deformation, which has a constant frequency, and its viscoelastic property is evaluated by this reaction. The elastic property reacted immediately is represented by the storage modulus, and the viscous property reacted belatedly is represented by the loss modulus [3]. Generally, the shear modulus is the storage modulus and is presented G′ separated with the tensile modulus (E′). Since the shear sandwich method can have inaccuracy of pressure in the loading sample, the torsion method has been

When measuring the shear modulus of foldable OCA, Campbell et al. suggest that the storage modulus should be maintained and has a low Tg in the operating range (−20 to 80°C) [38]. Because, as presented in **Figure 7**, Tg is shown at the primary reduction of storage modulus, lowering Tg can result in modulus reduction below the operating range. Since the temperature affects the movement of molecules, polymers recognize the time or frequency as a different degree

*DOI: http://dx.doi.org/10.5772/intechopen.90619*

*Schematic diagram of shear deformation of a single layer.*

**5.2 Adhesion strength**

(**Figure 5**).

**Figure 4.**

**5.3 Shear modulus**

Using a simple equation to calculate the stress (σ) by modulus (Ei) and strain (ε), the stress can be obtained by the positional difference between the target layer and the neutral plane (y − λ) and the folding radius (ρ) when the modulus of the material is fixed. Since these deviations of strain occur not only between layers but also within a single layer as shown in **Figure 4**, each layer including OCA is subjected to shear stress. So, most tests for evaluating foldable OCA are conducted under shear stress.

Although the shear strain depends on the thickness and placement of other layers, the shear strain of >300% is generally applied to the adhesive layer at a 5 mm radius of curvature. If the radius was reduced to 3 mm, the shear strain is increased up to 500–700% [45]. For the reduction of increase of shear strain by change of radius, the minimization of thickness and optimization of the structural design of multilayers have been studied [46].

**Figure 3.** *Schematic diagram of applied stresses of folded multilayered display including neutral plane [41].*

*Pressure-Sensitive Adhesives for Flexible Display Applications DOI: http://dx.doi.org/10.5772/intechopen.90619*

**Figure 4.** *Schematic diagram of shear deformation of a single layer.*

#### **5.2 Adhesion strength**

*Hybrid Nanomaterials - Flexible Electronics Materials*

simulation of various usage environments [44].

be calculated by the following Equation [41]:

evaluate OCA independently.

under shear stress.

multilayers have been studied [46].

**5.1 Shear strain on folded OCA**

test. Samsung Electronics, a major manufacturer of electronics including mobile phones, released folding test images with the launch of Galaxy Fold in September 2019 [43]. Folding durability not only of the assembled mobile phones but also of single or laminated film used as part of electronics has been evaluated. This test is performed with various environments and test modes such as low or high temperature, high humidity, the radius of curvature, static fold, and dynamic fold for

The folding test can obtain results from a complex interaction between multilayered components. So, the following various methods have been conducted to

When a book is bent, it can be seen that the unbound edge is deformed into an inclined form. Because unlike the book, however, each layer of the foldable display is attached to each other by PSA, each layer of multilayered display is deformed by different type and extent of stress. In this case, as shown in **Figure 3**, the outer layers and inner layers of the neutral plane are subjected to tensile stress and compressive stress, respectively. As such, the stress applied to each layer during folding can

σ = *Ei* ε = *E i*(y − λ)/ρ (2)

Using a simple equation to calculate the stress (σ) by modulus (Ei) and strain (ε), the stress can be obtained by the positional difference between the target layer and the neutral plane (y − λ) and the folding radius (ρ) when the modulus of the material is fixed. Since these deviations of strain occur not only between layers but also within a single layer as shown in **Figure 4**, each layer including OCA is subjected to shear stress. So, most tests for evaluating foldable OCA are conducted

Although the shear strain depends on the thickness and placement of other layers, the shear strain of >300% is generally applied to the adhesive layer at a 5 mm radius of curvature. If the radius was reduced to 3 mm, the shear strain is increased up to 500–700% [45]. For the reduction of increase of shear strain by change of radius, the minimization of thickness and optimization of the structural design of

*Schematic diagram of applied stresses of folded multilayered display including neutral plane [41].*

**72**

**Figure 3.**

Although the adhesion test is not a specific method for OCA applied to the foldable display, it is an important and fundamental method to evaluate all kinds of PSA. The test mode is selected according to the external force applied between PSA and substrate, and the general methods are peel, tack, lap shear, and pulloff test. The 180° peel test and lap shear test are conducted for foldable OCA (**Figure 5**).

Although the evaluation of adhesion is not a specific test for foldable OCA, this is considered an important factor because of several limits for satisfying adhesion strength. One of these limits is the substitution of the glass cover window to the plastic cover window for foldable properties [47]. The surface energies of several substrates such as plastic, glass, and SUS are listed in **Table 1** [48, 49]. The polyimide (PI) film has been used for the plastic cover window, and the surface energy of plastic substrate including PI is significantly lower than the surface energy of glass and SUS substrates. According to the rule of thumb, the lower surface energy of a substrate is able to decrease adhesion strength between PSA and substrate [50]. It is necessary to increase adhesion strength to the plastic substrate with minimization of rising Tg and modulus of OCA. If sufficient adhesion strength is not developed, it will not be possible to withstand the stress applied between OCA and substrate in continuous folding. The other is the limitation of the usable monomers for increasing adhesion strength as described above.

#### **5.3 Shear modulus**

The representative methods to measure shear modulus of PSA are the shear sandwich method by Dynamic Mechanical Analyzer (DMA) and the torsion method by Advanced Rheometric Expansion System (ARES) (**Figure 6**). Shear sandwich and torsion methods are subject to uniaxial and rotational shear stresses, respectively. In both of these methods, the material responds to repetitive deformation, which has a constant frequency, and its viscoelastic property is evaluated by this reaction. The elastic property reacted immediately is represented by the storage modulus, and the viscous property reacted belatedly is represented by the loss modulus [3]. Generally, the shear modulus is the storage modulus and is presented G′ separated with the tensile modulus (E′). Since the shear sandwich method can have inaccuracy of pressure in the loading sample, the torsion method has been preferred.

When measuring the shear modulus of foldable OCA, Campbell et al. suggest that the storage modulus should be maintained and has a low Tg in the operating range (−20 to 80°C) [38]. Because, as presented in **Figure 7**, Tg is shown at the primary reduction of storage modulus, lowering Tg can result in modulus reduction below the operating range. Since the temperature affects the movement of molecules, polymers recognize the time or frequency as a different degree

**Figure 5.** *180° peel and lap shear test for adhesion evaluation.*


**Table 1.**

*Surface energy of substrates.*

according to temperature [51]. Therefore, reducing modulus at low temperatures maintains the flexibility not only in low usage temperature but also in rapid deformation.

The maintenance of the shear modulus ensures adhesion strength [38], strain recovery, and processability at high temperature. If the modulus decreased as the temperature rise, the adhesion strength and strain recovery are able to decrease by cohesion reduction.

#### **5.4 Shear creep and stress relaxation**

The creep is the strain change with time under constant stress, and the stress relaxation is the stress change with time under a constant strain. Furthermore, the strain recovery after removing stress is an important factor to assess the restoration of PSA after folding. By the folding test, only the functional and macroscopic properties occurred after repetitive or static deformation can be observed, so the flexibility and resilience of a single material can be evaluated through creep and stress relaxation.

**75**

recovered [53].

**Figure 8.**

*The creep behavior of OCA.*

*Pressure-Sensitive Adhesives for Flexible Display Applications*

In the creep test, after immediate deformation of OCA by constant stress, the strain increases continuously because of its viscoelasticity. The elastic property makes the strain to be maintained after the initial deformation, but the viscous property results in an increase of continuous deformation. Lee et al. evaluated creep recovery by dividing it into elastic recovery and residual strain after a specific time (**Figure 8**) [52]. While the elastic deformation is recovered after the removal of stress, the viscous deformation causes permanent deformation that cannot be

*DOI: http://dx.doi.org/10.5772/intechopen.90619*

*Shear sandwich and torsion methods for testing shear modulus.*

**Figure 6.**

**Figure 7.**

*The graph of shear modulus and tan delta.*

*Pressure-Sensitive Adhesives for Flexible Display Applications DOI: http://dx.doi.org/10.5772/intechopen.90619*

*Hybrid Nanomaterials - Flexible Electronics Materials*

according to temperature [51]. Therefore, reducing modulus at low temperatures maintains the flexibility not only in low usage temperature but also in rapid

**Substrate Polypropylene Polyester Polyimide Glass SUS** Surface energy (mN/m) 29 43 50 200–300 700–1100

The maintenance of the shear modulus ensures adhesion strength [38], strain recovery, and processability at high temperature. If the modulus decreased as the temperature rise, the adhesion strength and strain recovery are able to decrease by

The creep is the strain change with time under constant stress, and the stress relaxation is the stress change with time under a constant strain. Furthermore, the strain recovery after removing stress is an important factor to assess the restoration of PSA after folding. By the folding test, only the functional and macroscopic properties occurred after repetitive or static deformation can be observed, so the flexibility and resilience of a single material can be evaluated through creep and

**74**

deformation.

**Figure 5.**

**Table 1.**

cohesion reduction.

*Surface energy of substrates.*

stress relaxation.

**5.4 Shear creep and stress relaxation**

*180° peel and lap shear test for adhesion evaluation.*

**Figure 6.** *Shear sandwich and torsion methods for testing shear modulus.*

**Figure 7.** *The graph of shear modulus and tan delta.*

In the creep test, after immediate deformation of OCA by constant stress, the strain increases continuously because of its viscoelasticity. The elastic property makes the strain to be maintained after the initial deformation, but the viscous property results in an increase of continuous deformation. Lee et al. evaluated creep recovery by dividing it into elastic recovery and residual strain after a specific time (**Figure 8**) [52]. While the elastic deformation is recovered after the removal of stress, the viscous deformation causes permanent deformation that cannot be recovered [53].

**Figure 9.** *The stress relaxation behavior of OCA.*

In the stress relaxation test, the continued decrease of stress is observed after the highest stress appears in response to the initial rapid deformation (**Figure 9**). This reduction is due to molecular movement, bond break, and bond interchange [51]. Energy reduction due to the stretching of molecules and intermolecular motion within a range fixed by cross-linking or entanglement is mostly recoverable after stress removal, but the breakage or interchange of the intermolecular bond and intermolecular slip is irreversible.

Permanent deformation that cannot be recovered after the removal of stress in creep and stress relaxation measurement can cause wrinkles or optical defects during the folding test of the display module or final product. Considering the viscoelasticity of PSA and interaction with adjacent films that have higher modulus, the perfect recovery is not required for OCA. But, high resilience as possible may be advantageous in long-term reliability.

#### **6. Researches of OCA for flexible display**

Lee et al. researched several factors that affect the foldable properties of OCA. First is the molecular weight of the prepolymer. The acrylic resins with a molecular weight in the rage of 360,000–690,000 are synthesized, and stress relaxation and recovery properties are evaluated under a constant shear strain of 400%. Although the initial stress of the stress relaxation test increases as the molecular weight increases, the ratio of stress relaxation decreases. The recovery after stress relaxation and creep is also improved [52]. Further, as the content of the cross-linking agent increased, the recovery increases in stress relaxation evaluation. But, it is confirmed that it decreases at over specific content. As the content of the cross-linking agent increases, the cross-link point increases. And the viscous deformation decreases. But, since too many cross-link points restrict the deformation, it results in bond breakage in the strain of 400% and decrease recovery [54]. As such, the entanglement and chemical connection between molecules play a major role in deformation and recovery.

They reported adhesion properties and recovery behavior of cross-linked silicone OCA using a platinum catalyst. The effect of the degree of cross-link is similar to that of acrylic OCA. But, the speed and degree of elastic recovery after stress relaxation is much faster and better than acrylic OCA [55]. Lee et al. also attempted to improve adhesion properties and recovery by connecting a styrene-isoprenestyrene (SIS) elastomer to acrylic OCA. The entanglement of the elastomer shows a positive effect on adhesion and recovery. However, the styrene groups of SIS elastomer cause the low transparency of OCA [56].

**77**

*Pressure-Sensitive Adhesives for Flexible Display Applications*

Campbell et al. present the shear modulus of standard OCA, foldable OCA, and improved foldable OCA, and results of repetitive tension tests. Compared to standard OCA, the foldable OCA's low modulus is suitable for foldable equipment, and the improved foldable OCA has the modulus of 10–100 kPa even at high temperatures by preventing modulus decrease at high temperature. The decrease of modulus causes poor adhesion and mechanical durability in high temperature. So, they are researching that the foldable OCA has a stable modulus while maintaining

Some studies conduct a simulation method to figure out the roles of

foldability, because it reduces its own strain and risk of delamination [58].

and control of component have been conducted.

In the flexible display, OCA is used for protecting TFT, TFE, and OLED in addition to the existing roles in the flat display. The low Tg and fixed modulus at operating temperature range are required to foldable OCA, while it secures high adhesion strength to the plastic substrate. Some high Tg and functional monomers used for the increase of adhesion strength have a limit to be used because of its high Tg and modulus. Also, the low Tg monomers that have long side chains are considered for OCA's high elongation and recovery. The foldable durability can be evaluated by the creep and stress relaxation test, and the strain and recovery of OCA are a combination of the elastic and viscous part. The viscous part can cause an optical defect in the display by permanent deformation. Because the perfect recovery of OCA, viscoelastic material with log Tg, and modulus is difficult, studies to compensate permanent deformation by the interaction with adjacent layers, structural design,

OCA. Salmon et al. demonstrate the effect of the modulus and elasticity of OCA by modeling of foldable OLED panels, and two types of OCA are used. The first one is 3 M foldable OCA and has lower modulus and higher elasticity, and the other one is 3 M OCA 8180. The softer OCA, 3 M foldable OCA, contributes less bending stress of display panel and less tensile strain of OLED layer than the other OCA. Also, they show buckling due to residual strain after unfolding. The elasticity of 3 M foldable OCA prevents continuous shear deformation and shear creep, and results in lower buckling than 3 M OCA 8180 [57]. Jia et al. presented nonlinear viscoelastic behavior of OCA in a folded display by constitutive model. They show that each layer in a multilayered display has its own neutral plane when OCA exists between the layers and this phenomenon due to the decoupling effect of the soft OCA. Also, they conduct the folding simulation of panel structures with various thicknesses of OCA. The major factor that affects the strain of the first OCA layer laid under a cover window is its own thickness. Consequentially, using thicker first OCA layer improves thermal

*DOI: http://dx.doi.org/10.5772/intechopen.90619*

a low Tg [38].

**7. Summary**

*Pressure-Sensitive Adhesives for Flexible Display Applications DOI: http://dx.doi.org/10.5772/intechopen.90619*

Campbell et al. present the shear modulus of standard OCA, foldable OCA, and improved foldable OCA, and results of repetitive tension tests. Compared to standard OCA, the foldable OCA's low modulus is suitable for foldable equipment, and the improved foldable OCA has the modulus of 10–100 kPa even at high temperatures by preventing modulus decrease at high temperature. The decrease of modulus causes poor adhesion and mechanical durability in high temperature. So, they are researching that the foldable OCA has a stable modulus while maintaining a low Tg [38].

Some studies conduct a simulation method to figure out the roles of OCA. Salmon et al. demonstrate the effect of the modulus and elasticity of OCA by modeling of foldable OLED panels, and two types of OCA are used. The first one is 3 M foldable OCA and has lower modulus and higher elasticity, and the other one is 3 M OCA 8180. The softer OCA, 3 M foldable OCA, contributes less bending stress of display panel and less tensile strain of OLED layer than the other OCA. Also, they show buckling due to residual strain after unfolding. The elasticity of 3 M foldable OCA prevents continuous shear deformation and shear creep, and results in lower buckling than 3 M OCA 8180 [57]. Jia et al. presented nonlinear viscoelastic behavior of OCA in a folded display by constitutive model. They show that each layer in a multilayered display has its own neutral plane when OCA exists between the layers and this phenomenon due to the decoupling effect of the soft OCA. Also, they conduct the folding simulation of panel structures with various thicknesses of OCA. The major factor that affects the strain of the first OCA layer laid under a cover window is its own thickness. Consequentially, using thicker first OCA layer improves thermal foldability, because it reduces its own strain and risk of delamination [58].

#### **7. Summary**

*Hybrid Nanomaterials - Flexible Electronics Materials*

intermolecular slip is irreversible.

*The stress relaxation behavior of OCA.*

**Figure 9.**

advantageous in long-term reliability.

deformation and recovery.

**6. Researches of OCA for flexible display**

elastomer cause the low transparency of OCA [56].

In the stress relaxation test, the continued decrease of stress is observed after the highest stress appears in response to the initial rapid deformation (**Figure 9**). This reduction is due to molecular movement, bond break, and bond interchange [51]. Energy reduction due to the stretching of molecules and intermolecular motion within a range fixed by cross-linking or entanglement is mostly recoverable after stress removal, but the breakage or interchange of the intermolecular bond and

Permanent deformation that cannot be recovered after the removal of stress in creep and stress relaxation measurement can cause wrinkles or optical defects during the folding test of the display module or final product. Considering the viscoelasticity of PSA and interaction with adjacent films that have higher modulus, the perfect recovery is not required for OCA. But, high resilience as possible may be

Lee et al. researched several factors that affect the foldable properties of OCA. First is the molecular weight of the prepolymer. The acrylic resins with a molecular weight in the rage of 360,000–690,000 are synthesized, and stress relaxation and recovery properties are evaluated under a constant shear strain of 400%. Although the initial stress of the stress relaxation test increases as the molecular weight increases, the ratio of stress relaxation decreases. The recovery after stress relaxation and creep is also improved [52]. Further, as the content of the cross-linking agent increased, the recovery increases in stress relaxation evaluation. But, it is confirmed that it decreases at over specific content. As the content of the cross-linking agent increases, the cross-link point increases. And the viscous deformation decreases. But, since too many cross-link points restrict the deformation, it results in bond breakage in the strain of 400% and decrease recovery [54]. As such, the entanglement and chemical connection between molecules play a major role in

They reported adhesion properties and recovery behavior of cross-linked silicone OCA using a platinum catalyst. The effect of the degree of cross-link is similar to that of acrylic OCA. But, the speed and degree of elastic recovery after stress relaxation is much faster and better than acrylic OCA [55]. Lee et al. also attempted to improve adhesion properties and recovery by connecting a styrene-isoprenestyrene (SIS) elastomer to acrylic OCA. The entanglement of the elastomer shows a positive effect on adhesion and recovery. However, the styrene groups of SIS

**76**

In the flexible display, OCA is used for protecting TFT, TFE, and OLED in addition to the existing roles in the flat display. The low Tg and fixed modulus at operating temperature range are required to foldable OCA, while it secures high adhesion strength to the plastic substrate. Some high Tg and functional monomers used for the increase of adhesion strength have a limit to be used because of its high Tg and modulus. Also, the low Tg monomers that have long side chains are considered for OCA's high elongation and recovery. The foldable durability can be evaluated by the creep and stress relaxation test, and the strain and recovery of OCA are a combination of the elastic and viscous part. The viscous part can cause an optical defect in the display by permanent deformation. Because the perfect recovery of OCA, viscoelastic material with log Tg, and modulus is difficult, studies to compensate permanent deformation by the interaction with adjacent layers, structural design, and control of component have been conducted.

*Hybrid Nanomaterials - Flexible Electronics Materials*

### **Author details**

Tae-Hyung Lee, Ji-Soo Kim, Jung-Hun Lee and Hyun-Joong Kim\* Laboratory of Adhesion and Bio-Composites, Program in Environmental Materials Science, College of Agriculture and Life Sciences, Seoul National University, Seoul, Republic of Korea

\*Address all correspondence to: hjokim@snu.ac.kr

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**79**

*Pressure-Sensitive Adhesives for Flexible Display Applications*

displays. IEEE Transactions on Electron Devices. 2011;**58**(10):3609-3615. DOI:

[9] Crawford GP. Flexible flat panel display technology. West Sussex: Wiley; 2005:1-4. DOI: 10.1002/0470870508

[10] Jin D-U, Kim T-W, Koo H-W, Stryakhilev D, Kim H-S, Seo S-J, et al. 47.1: Invited paper: Highly robust flexible AMOLED display on plastic substrate with new structure. SID Symposium Digest of Technical Papers. 2010;**41**(1):703-705.

[11] An S, Lee J, Kim Y, Kim T, Jin D, Min H, et al. 47.2: 2.8-inch WQVGA flexible AMOLED using high performance low temperature Polysilicon TFT on plastic substrates. SID Symposium Digest of Technical Papers. 2010;**41**(1):706-709. DOI:

[12] Kim S, Kwon H-J, Lee S, Shim H, Chun Y, Choi W, et al. Flexible displays: Low-power flexible organic light-

[13] Brotzman, R, Paskiewicz D-M. U.S. Patent Application No.10/033,015;

[14] Chen C-J, Lin K-L. Internal stress and adhesion of amorphous Ni–Cu–P alloy on aluminum. Thin Solid Films. 2000;**370**(1-2):106-113. DOI: 10.1016/

Pocius AV. Surface energy and adhesion studies on acrylic pressure sensitive

[16] Fang C, Jing Y, Zong Y, Lin Z. Effect of trifunctional cross-linker triallyl

emitting diode display device. Advanced Materials. 2011;**23**(31):3475-3475. DOI:

DOI: 10.1889/1.3500565

10.1889/1.3500566

10.1002/adma.201190120

S0040-6090(00)00859-2

adhesive. The Journal of

[15] Li L, Tirrell M, Korba GA,

Adhesion. 2001;**76**:307-334. DOI: 10.1080/00218460108030724

2018

10.1109/TED.2011.2162844

*DOI: http://dx.doi.org/10.5772/intechopen.90619*

[1] Webster I. Recent developments in pressure-sensitive adhesives for medical applications. International Journal of Adhesion and Adhesives. 1997;**17**(1):69-73. DOI: 10.1016/ S0143-7496(96)00024-3

[2] Creton C. Pressure-sensitive

[3] Satas D. Handbook of Pressure Sensitive Adhesive Technology. 3rd ed. Vol. 171-202. Warwick, Rhode Island (USA): Satas & Associates; 1999.

[4] Gower M-D, Shanks R-A. The effect of varied monomer composition on adhesive performance and peeling master curves for acrylic pressuresensitive adhesives. Journal of Applied Polymer Science. 2004;**93**(6):2909-2917.

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DOI: 10.1002/app.20873

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[5] Benedek I. Development and Manufacture of Pressure-Sensitive Products. United States: CRC Press; 1998

[6] Chun H, Kim H-A, Kim G, Kim J, Lim K-Y. Effect of the stress relaxation property of acrylic pressure-sensitive adhesive on light-leakage phenomenon of polarizer in liquid crystal display. Journal of Applied Polymer Science. 2007;**106**(4):2746-2752.

[7] Park J-W, Yoo T-M, Chung H-W, Jang W-B, Kim H-J. Evaluation of UV-curability of photo-curable

[8] Kim S, Choi W, Rim W, Chun Y, Shim H, Kwon H, et al. A highly sensitive capacitive touch sensor integrated on a thin-film-encapsulated active-matrix OLED for ultrathin

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*Pressure-Sensitive Adhesives for Flexible Display Applications DOI: http://dx.doi.org/10.5772/intechopen.90619*

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*Hybrid Nanomaterials - Flexible Electronics Materials*

**78**

**Author details**

Republic of Korea

Tae-Hyung Lee, Ji-Soo Kim, Jung-Hun Lee and Hyun-Joong Kim\*

\*Address all correspondence to: hjokim@snu.ac.kr

provided the original work is properly cited.

Laboratory of Adhesion and Bio-Composites, Program in Environmental Materials Science, College of Agriculture and Life Sciences, Seoul National University, Seoul,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

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[44] Komatsu R, Nakazato R, Sasaki T, Suzuki A, Senda N, Kawata T, et al. Repeatedly foldable AMOLED display. Journal of the Society for Information Display. 2018;**23**(2):41-49. DOI:

[45] Abrahamson J, Beagi H, Salmon F, Campbell C. Optically clear adhesives for OLED. In: OLED Technology and

10.1002/sdtp.12071

sdtp.11812

jsid.796

10.1063/1.4927677

10.1002/jsid.276

*DOI: http://dx.doi.org/10.5772/intechopen.90619*

2016;**100**:130-141. DOI: 10.1016/j. reactfunctpolym.2016.01.012

eurpolymj.2017.04.039

DOI: 10.1143/JJAP.41.4553

s10043-009-0035-2

[33] Cristaldi DJ, Pennisi S,

New York: Springer; 2009

[30] Baek SS, Hwang SH. Preparation of biomass-based transparent pressure sensitive adhesives for optically clear adhesive and their adhesion performance. European Polymer Journal. 2017;**92**:97-104. DOI: 10.1016/j.

[31] Ishinabe T, Miyashita T, Uchida T. Wide-viewing-angle polarizer with a large wavelength range. Japanese Journal of Applied Physics. 2002;**41**:4553-4558.

[32] Yeh P. Leakage of light in liquid crystal displays and birefringent thin film compensators. Optical Review. 2009;**16**(2):192-198. DOI: 10.1007/

Pulvirenti F. Liquid Crystal Display Drivers: Techniques and Circuits.

[34] Koden M. Wide viewing angle technologies of TFT-LCDs. Sharp Technical Journal. 1999;**74**(8):55-60

[35] Ma J, Ye X, Jin B. Structure and application of polarizer film for thinfilm-transistor liquid crystal displays. Displays. 2011;**32**(2):49-57. DOI: 10.1016/j.displa.2010.12.006

[36] Kumar RS, Auch M, Ou E, Ewald G, Jin CS. Low moisture permeation measurement through polymer substrates for organic light emitting devices. Thin Solid Films. 2002;**417**(1-2):120-126. DOI: 10.1016/

S0040-6090(02)00584-9

2001;**43**(1):5-37

[37] Asnafi N. On springback of doublecurved autobody panels. International Journal of Mechanical Sciences.

[38] Campbell CJ, Clapper J, Behling RE, Erdogan B, Beagi HZ, Abrahamson JT, et al. Optically clear adhesives enabling

*Pressure-Sensitive Adhesives for Flexible Display Applications DOI: http://dx.doi.org/10.5772/intechopen.90619*

2016;**100**:130-141. DOI: 10.1016/j. reactfunctpolym.2016.01.012

*Hybrid Nanomaterials - Flexible Electronics Materials*

in silicone pressure-sensitive adhesives. Journal of Adhesion Science and Technology. 2007;**21**(7):605-623. DOI:

[24] Park C-H, Lee S-J, Lee T-H, Kim H-J. Characterization of an acrylic polymer under hygrothermal aging as an optically clear adhesive for touch screen panels. International Journal of Adhesion and Adhesives. 2015;**63**:137-144. DOI: 10.1016/j.

[25] Fang C, Jing Y, Zong Y, Lin Z. Effect of N,N-dimethylacrylamide (DMA) on the comprehensive properties of acrylic latex pressure sensitive adhesives. International Journal of Adhesion and Adhesives. 2016;**71**:105-111. DOI: 10.1016/j.ijadhadh.2016.09.003

[26] Kuo C-FJ, Chen J-B, Chang S-H. Low corrosion optically clear adhesives for conducting glass: I. effects of N,N-diethylacrylamide and acrylic acid mixtures on optically clear adhesives. Journal of Applied Polymer Science. 2018;**135**(21):46277. DOI:

[27] Czech Z, Milker R. Solventfree radiation-curable polyacrylate pressure-sensitive adhesive systems. Journal of Applied Polymer Science. 2003;**87**(2):182-191. DOI: 10.1002/

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[29] Park CH, Lee SJ, Lee TH, Kim HJ. Characterization of an acrylic pressure-

sensitive adhesive blended with hydrophilic monomer exposed to hygrothermal aging: Assigning cloud point resistance as an optically clear adhesive for a touch screen panel. Reactive and Functional Polymers.

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app.11303

rd.423.0527

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ijadhadh.2015.08.012

isocyanurate (TAIC) on the surface morphology and viscoelasticity of the acrylic emulsion pressure-sensitive adhesives. Journal of Adhesion Science and Technology. 2017;**31**(8):858-873. DOI: 10.1080/01694243.2016.1235005

[17] Tse MF. Studies of triblock

by viscoelasticity and adhesive performance. Journal of Adhesion Science and Technology. 1989;**3**(7):551- 570. DOI: 10.1163/156856189X00407

[19] Gdalin BE, Bermesheva EV, Shandryuk GA, Feldstein MM. Effect

adhesion: Extension of the Dahlquist criterion of tack. The Journal of Adhesion. 2011;**87**(2):111-138. DOI: 10.1080/00218464.2011.545325

[21] Kowalski A, Czech Z, Byczynski L. How does the surface free energy influence the tack of acrylic pressuresensitive adhesives (PSAs)? Journal of Coatings Technology and Research. 2013;**10**(6):879-885. DOI: 10.1007/

[22] Czech Z, Goracy K. Characterization of the crosslinking process of silicone pressure-sensitive adhesives. Polymer. 2005;**50**(10):762-764. DOI: 10.14314/

[23] Lin SB, Durfee LD, Ekeland RA, McVie J, Schalau GK. Recent advances

s11998-013-9522-2

polimery.2005.762

of temperature on probe tack

[20] Krenceski MA, Johnson JF, Temin SC. Chemical and physical factors affecting performance of pressure-sensitive adhesives. Journal of Macromolecular Science - Reviews in Macromolecular Chemistry and Physics. 1986;**C26**(1):143-182. DOI: 10.1080/07366578608081971

copolymer-tackifying resin interactions

[18] Tse MF, Jacob L. Pressure sensitive adhesives based on vector sis polymers I. rheological model and adhesive design pathways. The Journal of Adhesion. 1996;**56**(1-4):79-95. DOI: 10.1080/00218469608010500

**80**

[30] Baek SS, Hwang SH. Preparation of biomass-based transparent pressure sensitive adhesives for optically clear adhesive and their adhesion performance. European Polymer Journal. 2017;**92**:97-104. DOI: 10.1016/j. eurpolymj.2017.04.039

[31] Ishinabe T, Miyashita T, Uchida T. Wide-viewing-angle polarizer with a large wavelength range. Japanese Journal of Applied Physics. 2002;**41**:4553-4558. DOI: 10.1143/JJAP.41.4553

[32] Yeh P. Leakage of light in liquid crystal displays and birefringent thin film compensators. Optical Review. 2009;**16**(2):192-198. DOI: 10.1007/ s10043-009-0035-2

[33] Cristaldi DJ, Pennisi S, Pulvirenti F. Liquid Crystal Display Drivers: Techniques and Circuits. New York: Springer; 2009

[34] Koden M. Wide viewing angle technologies of TFT-LCDs. Sharp Technical Journal. 1999;**74**(8):55-60

[35] Ma J, Ye X, Jin B. Structure and application of polarizer film for thinfilm-transistor liquid crystal displays. Displays. 2011;**32**(2):49-57. DOI: 10.1016/j.displa.2010.12.006

[36] Kumar RS, Auch M, Ou E, Ewald G, Jin CS. Low moisture permeation measurement through polymer substrates for organic light emitting devices. Thin Solid Films. 2002;**417**(1-2):120-126. DOI: 10.1016/ S0040-6090(02)00584-9

[37] Asnafi N. On springback of doublecurved autobody panels. International Journal of Mechanical Sciences. 2001;**43**(1):5-37

[38] Campbell CJ, Clapper J, Behling RE, Erdogan B, Beagi HZ, Abrahamson JT, et al. Optically clear adhesives enabling

foldable and flexible OLED displays. SID Symposium Digest of Technical Papers. 2017;**48**(1):2009-2011. DOI: 10.1002/sdtp.12071

[39] Kao SC, Li LJ, Hsieh MC, Zhang S, Tsai PM, Sun ZY, et al. 71-1: Invited paper: The challenges of flexible OLED display development. SID Symposium Digest of Technical Papers. 2017;**48**(1):1034-1037. DOI: 10.1002/ sdtp.11812

[40] Yan J-Y, Ho J-C, Chen J. Foldable AMOLED display development: Progress and challenges. Information Display. 2018;**31**(1):12-16. DOI: 10.1002/j.2637-496X.2015.tb00780.x

[41] Nishimura M, Takebayashi K, Hishinuma M, Yamaguchi H, Murayama A. A 5.5-inch full HD foldable AMOLED display based on neutral-plane splitting concept. Journal of the Society for Information Display. 2019;**27**(8):480-486. DOI: 10.1002/ jsid.796

[42] Su Y, Li S, Li R, Dagdeviren C. Splitting of neutral mechanical plane of conformal, multilayer piezoelectric mechanical energy harvester. Applied Physics Letters. 2015;**107**:041905. DOI: 10.1063/1.4927677

[43] SAMSUNG Newsroom. [Video] New Form Factor, New Rules: Watch the Galaxy Fold's Folding Test [Internet]. Available from: https://news.samsung. com/global/video-new-form-factornew-rules-watch-the-galaxy-foldsfolding-test [Accessed: 11 August 2019]

[44] Komatsu R, Nakazato R, Sasaki T, Suzuki A, Senda N, Kawata T, et al. Repeatedly foldable AMOLED display. Journal of the Society for Information Display. 2018;**23**(2):41-49. DOI: 10.1002/jsid.276

[45] Abrahamson J, Beagi H, Salmon F, Campbell C. Optically clear adhesives for OLED. In: OLED Technology and

Applications. London: IntechOpen; 2019. DOI: 10.5772/intechopen.88659

[46] Cheng A, Chen Y, Jin J. Su T. study on mechanical behavior and effect of adhesive layers in foldable AMOLED display by finite element analysis. SID Symposium Digest of Technical Papers. 2019;**50**(1):1060-1063. DOI: 10.1002/ sdtp.13110

[47] Lin L, Dang P, Hu K, Gao X, Huang X. Challenges and Progress of small bending radius foldable AMOLED display module technology. SID Symposium Digest of Technical Papers. 2017;**48**(1):445-446. DOI: 10.1002/sdtp.11673

[48] 3M. Substrates and Adhesion [Internet]. Available from: https:// www.3m.com/3M/en\_US/bondingand-assembly-us/resources/ full-story/?storyid=1d2481ca-5c8c-455d-952d-5ed90e04e8a7 [Accessed: 11 August 2019]

[49] Kim HJ, Park YJ, Choi JH, Han HS, Hong YT. Surface modification of polyimide film by coupling reaction for copper metallization. Journal of Industrial and Engineering Chemistry. 2009;**15**(1):23-30. DOI: 10.1016/j. jiec.2008.08.016

[50] Bichler CH, Langowski HC, Moosheimer U, Seifert B. Adhesion mechanism of aluminum, aluminum oxide, and silicon oxide on biaxially oriented polypropylene (BOPP), poly (ethyleneterephthalate)(PET), and poly (vinyl chloride)(PVC). Journal of Adhesion Science and Technology. 1997;**11**(2):233-246. DOI: 10.1163/156856197X00336

[51] Sperling LH. Introduction to Physical Polymer Science. 4th ed. Vol. 508-509. New Jersey: John Wiley & Sons; 2006. p. 530

[52] Lee JH, Lee TH, Shim KS, Park JW, Kim HJ, Kim Y, et al. Molecular weight and crosslinking on the adhesion

performance and flexibility of acrylic PSAs. Journal of Adhesion Science and Technology. 2016;**30**(21):2316-2328. DOI: 10.1080/01694243.2016.1182382

**Chapter 6**

**Abstract**

Electronics

SmartSpice and Gateway.

**1. Introduction**

**83**

Numerical Simulation and

*Arun Dev Dhar Dwivedi, Sushil Kumar Jain, Rajeev Dhar Dwivedi and Shubham Dadhich*

Compact Modeling of Thin Film

In this chapter, we present a finite element method (FEM)-based numerical device simulation of low-voltage DNTT-based organic thin film transistor (OTFT) by considering field-dependent mobility model and double-peak Gaussian density of states model. Device simulation model is able to reproduce output characteristics

**Keywords:** OTFTs, numerical simulation, compact modeling, flexible electronics

The interest in organic thin film transistors (OTFTs) has increased significantly in the past few years and has been proven for various applications such as flexible low-cost displays [1], organic memory [2], key components of RFID [3] tags, lowend electronic products, and polymer circuits and sensors [4]. Flexible electronics is a new technology that builds electronic circuits by depositing electronic products on flexible substrates like plastics, paper, and even cloth. Compared with inorganic electronics, organic or flexible electronics have the various following advantages. First, it can be manufactured at a very low cost at low temperatures. Second, it is thin, lightweight, foldable, and bendable and has a strong light absorption, no crushing, mechanical flexibility, low energy consumption, and high emission efficiency. Third, the cost is lower due to cheaper materials and lower-cost deposition processes [5], and it is also used for large area applications. Actually a stack of organic semiconductors (OSC) and low-temperature polymer gate dielectrics and

in linear and saturation region and transfer characteristics below and above threshold region. We also demonstrate an approach for compact modeling and compact model parameter extraction of organic thin film transistors (OTFTs) using universal organic TFT (UOTFT) model by comparing the compact modeling results with the experimental results. Results obtained from technology computer-aided design (TCAD) simulation and compact modeling are compared and contrasted with experimental results. Further we present simulations of voltage transfer characteristic (VTC) plot of polymer P-channel thin film transistor (PTFT)-based inverter to assess the compact model against simple logic circuit simulation using

Transistors for Future Flexible

[53] Rubinstein M, Colby RH. Polymer Physics. New York: Oxford University Press Inc.; 2003. p. 290

[54] Lee J-H, Lee T-H, Shim K-S, Park J-W, Kim H-J, Kim Y, et al. Effect of crosslinking density on adhesion performance and flexibility properties of acrylic pressure sensitive adhesives for flexible display applications. International Journal of Adhesion and Adhesives. 2017;**74**:137-143. DOI: 10.1016/j.ijadhadh.2017.01.005

[55] Lee J-H, Lee T-H, Shim K-S, Park J-W, Kim H-J, Kim Y. Adhesion performance and recovery of platinum catalyzed silicone PSAs under various temperature conditions for flexible display applications. Materials Letters. 2017;**208**:86-88. DOI: 10.1016/j. matlet.2017.05.042

[56] Lee J-H, Shim G-S, Park J-W, Kim H-J, Kim Y. Adhesion performance and recovery of acrylic pressure-sensitive adhesives thermally crosslinked with styrene–isoprene–styrene elastomer blends for flexible display applications. Journal of Industrial and Engineering Chemistry. 2019;**78**:461-467. DOI: 10.1016/j.jiec.2019.05.019

[57] Salmon F, Everaerts A, Campbell C, Pennington B, Erdogan-Haug B, Gregg C. Modeling the mechanical performance of a foldable display panel bonded by 3M optically clear adhesives. SID Symposium Digest of Technical Papers. 2017;**48**(1):938- 941. DOI: 10.1002/sdtp.11796

[58] Jia Y, Liu Z, Wu D, Chen J, Meng H. Mechanical simulation of foldable AMOLED panel with a module structure. Organic Electronics. 2019;**65**:185-192. DOI: 10.1016/j. orgel.2018.11.026

#### **Chapter 6**

*Hybrid Nanomaterials - Flexible Electronics Materials*

performance and flexibility of acrylic PSAs. Journal of Adhesion Science and Technology. 2016;**30**(21):2316-2328. DOI: 10.1080/01694243.2016.1182382

[53] Rubinstein M, Colby RH. Polymer Physics. New York: Oxford University

[54] Lee J-H, Lee T-H, Shim K-S, Park J-W, Kim H-J, Kim Y, et al. Effect of crosslinking density on adhesion performance and flexibility properties of acrylic pressure sensitive adhesives for flexible display applications. International Journal of Adhesion and Adhesives. 2017;**74**:137-143. DOI: 10.1016/j.ijadhadh.2017.01.005

[55] Lee J-H, Lee T-H, Shim K-S, Park J-W, Kim H-J, Kim Y. Adhesion performance and recovery of platinum catalyzed silicone PSAs under various temperature conditions for flexible display applications. Materials Letters.

2017;**208**:86-88. DOI: 10.1016/j.

[56] Lee J-H, Shim G-S, Park J-W, Kim H-J, Kim Y. Adhesion performance and recovery of acrylic pressure-sensitive adhesives thermally crosslinked with styrene–isoprene–styrene elastomer blends for flexible display applications. Journal of Industrial and Engineering Chemistry. 2019;**78**:461-467. DOI:

matlet.2017.05.042

10.1016/j.jiec.2019.05.019

[57] Salmon F, Everaerts A,

[58] Jia Y, Liu Z, Wu D, Chen J, Meng H. Mechanical simulation of foldable AMOLED panel with a module structure. Organic Electronics. 2019;**65**:185-192. DOI: 10.1016/j.

orgel.2018.11.026

Campbell C, Pennington B, Erdogan-Haug B, Gregg C. Modeling the mechanical performance of a foldable display panel bonded by 3M optically clear adhesives. SID Symposium Digest of Technical Papers. 2017;**48**(1):938- 941. DOI: 10.1002/sdtp.11796

Press Inc.; 2003. p. 290

Applications. London: IntechOpen; 2019. DOI: 10.5772/intechopen.88659

[46] Cheng A, Chen Y, Jin J. Su T. study on mechanical behavior and effect of adhesive layers in foldable AMOLED display by finite element analysis. SID Symposium Digest of Technical Papers. 2019;**50**(1):1060-1063. DOI: 10.1002/

[47] Lin L, Dang P, Hu K, Gao X, Huang X. Challenges and Progress of small bending radius foldable AMOLED display module technology. SID Symposium Digest of Technical Papers. 2017;**48**(1):445-446.

sdtp.13110

DOI: 10.1002/sdtp.11673

11 August 2019]

jiec.2008.08.016

[48] 3M. Substrates and Adhesion [Internet]. Available from: https:// www.3m.com/3M/en\_US/bondingand-assembly-us/resources/ full-story/?storyid=1d2481ca-5c8c-455d-952d-5ed90e04e8a7 [Accessed:

[49] Kim HJ, Park YJ, Choi JH, Han HS, Hong YT. Surface modification of polyimide film by coupling reaction for copper metallization. Journal of Industrial and Engineering Chemistry. 2009;**15**(1):23-30. DOI: 10.1016/j.

[50] Bichler CH, Langowski HC, Moosheimer U, Seifert B. Adhesion mechanism of aluminum, aluminum oxide, and silicon oxide on biaxially oriented polypropylene (BOPP), poly (ethyleneterephthalate)(PET), and poly (vinyl chloride)(PVC). Journal of Adhesion Science and Technology. 1997;**11**(2):233-246. DOI:

10.1163/156856197X00336

Sons; 2006. p. 530

[51] Sperling LH. Introduction to Physical Polymer Science. 4th ed. Vol. 508-509. New Jersey: John Wiley &

[52] Lee JH, Lee TH, Shim KS, Park JW, Kim HJ, Kim Y, et al. Molecular weight and crosslinking on the adhesion

**82**

## Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible Electronics

*Arun Dev Dhar Dwivedi, Sushil Kumar Jain, Rajeev Dhar Dwivedi and Shubham Dadhich*

#### **Abstract**

In this chapter, we present a finite element method (FEM)-based numerical device simulation of low-voltage DNTT-based organic thin film transistor (OTFT) by considering field-dependent mobility model and double-peak Gaussian density of states model. Device simulation model is able to reproduce output characteristics in linear and saturation region and transfer characteristics below and above threshold region. We also demonstrate an approach for compact modeling and compact model parameter extraction of organic thin film transistors (OTFTs) using universal organic TFT (UOTFT) model by comparing the compact modeling results with the experimental results. Results obtained from technology computer-aided design (TCAD) simulation and compact modeling are compared and contrasted with experimental results. Further we present simulations of voltage transfer characteristic (VTC) plot of polymer P-channel thin film transistor (PTFT)-based inverter to assess the compact model against simple logic circuit simulation using SmartSpice and Gateway.

**Keywords:** OTFTs, numerical simulation, compact modeling, flexible electronics

#### **1. Introduction**

The interest in organic thin film transistors (OTFTs) has increased significantly in the past few years and has been proven for various applications such as flexible low-cost displays [1], organic memory [2], key components of RFID [3] tags, lowend electronic products, and polymer circuits and sensors [4]. Flexible electronics is a new technology that builds electronic circuits by depositing electronic products on flexible substrates like plastics, paper, and even cloth. Compared with inorganic electronics, organic or flexible electronics have the various following advantages. First, it can be manufactured at a very low cost at low temperatures. Second, it is thin, lightweight, foldable, and bendable and has a strong light absorption, no crushing, mechanical flexibility, low energy consumption, and high emission efficiency. Third, the cost is lower due to cheaper materials and lower-cost deposition processes [5], and it is also used for large area applications. Actually a stack of organic semiconductors (OSC) and low-temperature polymer gate dielectrics and

the rapid annealing process are suitable with high-throughput, low-cost printing manufacturing [6]. Researchers replaced semiconductors with organic materials such as DNTT [7], poly(3-octylthiophene) (P3OT), poly(3-hexylthiophene) (P3HT), and poly(3-alkylthiophene) layers, and dielectric layers are used to create complete flexibility. A bigger challenge is to enhance the real performance of organic devices so as to expand their usage in real-time commercial applications [8]. To enhance the speed of the device, a very great deal of the research efforts has been dedicated to increasing the mobility of organic materials by improving the deposition conditions [9]. In addition to mobility, other methods of improving OTFT performance include scaling the length of channel and changing the active layer thickness. The OTFT is usually fabricated in an inverted structure with gate at the bottom, and source and drain will be at the top. Gundlach et al. [10] show that the bottom contact structure has a strong dependence on the contact barrier and due to the different nature of the interface between the channel and the insulator, the device exhibits different electrical properties [11]. Recently, for p-type OSCs, very high mobility values of several tens of cm2 V<sup>1</sup> s <sup>1</sup> have been reported for polymers and small molecules indicating that OSC has great potential for improved performance through chemical structures and process optimization [12]. In addition to performance, deep understanding of instability issues of OTFTs and finding stable and reliable solutions for OTFT is therefore very important [13]. Since the systematic cost of experimental investigation is very high and it requires a lot of time also, technology computeraided design (TCAD) simulation of semiconductor devices is becoming very important for investigating the design and electrical characteristics of the device prior to fabrication of the device. Organic semiconductor technology has emerged in the past 20 years. Depending on the model, these devices have been developed and studied over the past decade. Compared to the silicon industry, for which public model is clearly defined and commonly used to provide designers with a relatively good description of the process, organic transistors still lack to have complete device models that can fully describe their electrical characteristics. Therefore TCAD simulation and compact modeling of organic transistors become very important.

1.7-nm-thick n-tetradecylphosphonic acid self-assembled monolayer (SAM) was used [24]. Next, an organic semiconductor layer having a thickness of 11 nm was placed on the AlOx/SAM gate dielectric. The AlOx/SAM gate dielectric (5.3 nm) is very small in thickness and has a large capacitance per unit area, so transistors and circuits can operate at a low voltage of about 3 V. The OTFT has a channel length of

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible…*

The energy band diagram of a metal insulator semiconductor (MIS) structure is given in **Figure 1**. Maximum valence band energy (EV) and minima of conduction band energy (EC) of the inorganic semiconductor are substantially similar between the HOMO and the LUMO of the organic semiconductor. Especially for DNTT,

200 μm and a channel width of 400 μm, Lov = 10 μm.

*DOI: http://dx.doi.org/10.5772/intechopen.90301*

*Energy band diagram of a metal insulator semiconductor (MIS) structure.*

*Schematic cross-sectional diagram of organic TFTs along with the chemical structure of SAM and organic*

**Figure 1.**

**Figure 2.**

**85**

*semiconductor.*

In this chapter we present 2D device simulation of low-voltage DNTT-based OTFT using Silvaco's ATLAS 2D simulator which uses Poisson semiconductor device equation [14–22] continuity equation for charge carriers, drift diffusion transport model, and density of defect states model for simulation electrical characteristics of the device. Silvaco's UTMOST IV model parameter extraction software is used to get compact model parameters using UOTFT model [23]. Also TCAD simulation results and compact modeling results were compared and contrasted with the experimentally measured results of the device. Compact model has been applied for logic circuit simulation, and voltage transfer characteristics of PTFTbased inverter circuit have been simulated using the compact model parameters extracted from UOTFT model. This chapter contains five sections. This section introduces the content of the paper. The device structure and simulation are described in Section 2. The compact modeling, model verification, and parameter extraction are explained in Section 3. The results and discussion are explained in Section 4. Finally, conclusions drawn are given in Section 5.

#### **2. Simulation**

#### **2.1 Device structure and finite element-based numerical simulation**

The OTFT is designed on the bottom-gate top-contact of a flexible PEN substrate. A gate dielectric composed of a 3.6-nm-thick aluminum oxide layer and a *Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible… DOI: http://dx.doi.org/10.5772/intechopen.90301*

1.7-nm-thick n-tetradecylphosphonic acid self-assembled monolayer (SAM) was used [24]. Next, an organic semiconductor layer having a thickness of 11 nm was placed on the AlOx/SAM gate dielectric. The AlOx/SAM gate dielectric (5.3 nm) is very small in thickness and has a large capacitance per unit area, so transistors and circuits can operate at a low voltage of about 3 V. The OTFT has a channel length of 200 μm and a channel width of 400 μm, Lov = 10 μm.

The energy band diagram of a metal insulator semiconductor (MIS) structure is given in **Figure 1**. Maximum valence band energy (EV) and minima of conduction band energy (EC) of the inorganic semiconductor are substantially similar between the HOMO and the LUMO of the organic semiconductor. Especially for DNTT,

**Figure 1.**

the rapid annealing process are suitable with high-throughput, low-cost printing manufacturing [6]. Researchers replaced semiconductors with organic materials such as DNTT [7], poly(3-octylthiophene) (P3OT), poly(3-hexylthiophene) (P3HT), and poly(3-alkylthiophene) layers, and dielectric layers are used to create complete flexibility. A bigger challenge is to enhance the real performance of organic devices so as to expand their usage in real-time commercial applications [8]. To enhance the speed of the device, a very great deal of the research efforts has been dedicated to increasing the mobility of organic materials by improving the deposition conditions [9]. In addition to mobility, other methods of improving OTFT performance include scaling the length of channel and changing the active layer thickness. The OTFT is usually fabricated in an inverted structure with gate at the bottom, and source and drain will be at the top. Gundlach et al. [10] show that the bottom contact structure has a strong dependence on the contact barrier and due to the different nature of the interface between the channel and the insulator, the device exhibits different electrical properties [11]. Recently, for p-type OSCs, very high mobility

molecules indicating that OSC has great potential for improved performance through chemical structures and process optimization [12]. In addition to performance, deep understanding of instability issues of OTFTs and finding stable and reliable solutions for OTFT is therefore very important [13]. Since the systematic cost of experimental investigation is very high and it requires a lot of time also, technology computeraided design (TCAD) simulation of semiconductor devices is becoming very important for investigating the design and electrical characteristics of the device prior to fabrication of the device. Organic semiconductor technology has emerged in the past 20 years. Depending on the model, these devices have been developed and studied over the past decade. Compared to the silicon industry, for which public model is clearly defined and commonly used to provide designers with a relatively good description of the process, organic transistors still lack to have complete device models that can fully describe their electrical characteristics. Therefore TCAD simulation and compact modeling of organic transistors become very important. In this chapter we present 2D device simulation of low-voltage DNTT-based OTFT using Silvaco's ATLAS 2D simulator which uses Poisson semiconductor device equation [14–22] continuity equation for charge carriers, drift diffusion transport model, and density of defect states model for simulation electrical characteristics of the device. Silvaco's UTMOST IV model parameter extraction software is used to get compact model parameters using UOTFT model [23]. Also TCAD simulation results and compact modeling results were compared and contrasted with the experimentally measured results of the device. Compact model has been applied for logic circuit simulation, and voltage transfer characteristics of PTFTbased inverter circuit have been simulated using the compact model parameters extracted from UOTFT model. This chapter contains five sections. This section introduces the content of the paper. The device structure and simulation are described in Section 2. The compact modeling, model verification, and parameter extraction are explained in Section 3. The results and discussion are explained in

Section 4. Finally, conclusions drawn are given in Section 5.

**2.1 Device structure and finite element-based numerical simulation**

The OTFT is designed on the bottom-gate top-contact of a flexible PEN substrate. A gate dielectric composed of a 3.6-nm-thick aluminum oxide layer and a

**2. Simulation**

**84**

<sup>1</sup> have been reported for polymers and small

values of several tens of cm2 V<sup>1</sup> s

*Hybrid Nanomaterials - Flexible Electronics Materials*

*Energy band diagram of a metal insulator semiconductor (MIS) structure.*

#### **Figure 2.**

*Schematic cross-sectional diagram of organic TFTs along with the chemical structure of SAM and organic semiconductor.*

HOMO is approximately �5.19 eV, and LUMO is about �1.81 eV [7, 24]. This introduces a large enough 3.38 eV HOMO-LUMO energy gap, which is sufficient for transistor operation.

To start the ATLAS simulation, we defined the physical structure and device dimensions, including the location of the electrical contacts. **Figure 2** shows a crosssectional view of the bottom-gate, top-contact DNTT-based OTFT.

#### **2.2 Device physical equation**

We can calculate these charge carrier densities by solving basic device equations simultaneously including Poisson equation [14–22], electron and hole continuity equation, charge transfer equation, and defect density of states equation. The first three equations are the default equations that ATLAS uses to find the electrical behavior of the device.

The Poisson equation determines the electric field intensity in the given device based on the internal movement of the carriers and the distribution of the fixed charges given by Eq. (1):

$$\nabla \mathbf{E} = -\text{div}(\nabla \Psi) = \frac{\rho(\mathbf{x}, \mathbf{y})}{\mathbf{E}} \tag{1}$$

In these equations, q is the magnitude of the electronic charge, n is the electron

density, G is the corresponding charge generation rate, and R is the corresponding charge recombination rate. For organic/metal oxide semiconductor field-effect transistors (MOSFETs), there is no optical absorption, so the term is simplified and the properties of the material are described by the minority carrier recombination lifetime. Since MOSFETs are majority carrier devices, the characteristics of carrier generation and recombination are relatively unimportant. The physical properties of organic semiconductors depend on the generation and movement

A third important set of equations for describing the device physics for the

It contains drift and diffusion parts. These equations determine the current density based on the carrier mobility (μ), the electric field (E), the carrier density (n, p), and the diffusion coefficient of the carrier (D). Diffusion coefficient opera-

> *Dn* <sup>¼</sup> *kT q*

> *Dp* <sup>¼</sup> *kT q*

In summary, the ATLAS software solves Poisson equations, continuity equations, and current density equations [26, 27] at each node in a two-dimensional grid for a given device structure simultaneously with itself and is subject to boundary conditions (including those applied at the contacts). With the help of ATLAS, the electric field distribution and electron and hole current density are calculated at

The assumed total density (DOS), g(E), consists of four bands: two tail bands (analogous to acceptor-like conduction band and donor-like valence band) and two deep energy bands (one donor-like and the other acceptor-like); they are modeled

Here, E is the trap energy, *EC* is the conduction band energy, *EV* is the valence band energy, and the subscripts (*T*, *G*, *A*, *D*) stand for tail, Gaussian (deep level),

*gTA*ð Þ¼ *E NTA* exp

*gTD*ð Þ¼ *E NTD* exp

*g E*ð Þ¼ *gTA*ð Þþ *E gTD*ð Þþ *E gGA*ð Þþ *E gGD*ð Þ *E* (10)

*E* � *Ec*

*Ev* � *E*

*WTA* (11)

*WTD* (12)

*Jp* ¼ qnμ*pE* � *qDp*∇*p* (6) *Jn* ¼ qnμ*nE* þ *qDn*∇*n* (7)

μ*<sup>n</sup>* (8)

μ*<sup>p</sup>* (9)

carrier density, p is the hole carrier density, J is the corresponding current

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible…*

of polarons [25].

charge carrier is given by

*DOI: http://dx.doi.org/10.5772/intechopen.90301*

tors are related to Einstein's mobility relationship:

each node and terminal current at electrode.

**2.3 Density of defect states model**

using a Gaussian distribution [15–22, 28]:

acceptor, and donor states, respectively:

**87**

where ∈ is the permittivity of the region and *ρ*(x,y) is the charge density given by.

$$\rho(\mathbf{x}, \mathbf{y}) = \mathbf{q}[\mathbf{p}(\mathbf{x}, \mathbf{y}) - \mathbf{n}(\mathbf{x}, \mathbf{y}) + \mathbf{N}\_{\mathrm{D}}{}^{+}\left(\mathbf{x}, \mathbf{y}\right) - \mathbf{N}\_{\mathrm{A}}{}^{-}\left(\mathbf{x}, \mathbf{y}\right)] \tag{2}$$

where p(x,y) is the hole density, n(x,y) is the electron density, ND + (x,y) is the ionization donor density, and NA � (x,y) is the ionization acceptor density.

To account for the trapped charge, Poisson equations are modified by adding an additional term QT, representing trapped charge given in (A):

$$\text{div}(\varepsilon \nabla \Psi) = -\rho(\mathbf{x}, \mathbf{y}) = \mathbf{q}[\mathbf{n}(\mathbf{x}, \mathbf{y}) - \mathbf{p}(\mathbf{x}, \mathbf{y}) - \mathbf{N}\_{\mathcal{D}}^{+}\left(\mathbf{x}, \mathbf{y}\right) + \mathbf{N}\_{\mathcal{A}}^{-}\left(\mathbf{x}, \mathbf{y}\right)] - \mathbf{Q}\_{\mathcal{T}} \tag{3}$$

where QT = q(N<sup>+</sup> tD + N� tA), N<sup>+</sup> tD = density � FtD, and N� tA = density � FtA. Here, N+ tD and N� tA are ionized density of donor-like trap and ionized density of acceptor-like traps, respectively, and FtD and FtA are probability of ionization of donor-like traps and acceptor-like traps, respectively.

Due to charge accumulation, a potential is generated, which affects the intensity of electric field distribution and current. The voltage applied to the gate electrode generates an electric field that attracts a few or majority carriers. In addition, for OTFTs, the voltage potential between the source and the drain establishes another electric field along the channel that drives the charge carriers and produces a current.

The continuity equation describes the dynamics of charge carrier distribution over time as given in Eqs. (4) and (5):

$$\frac{\partial \mathbf{n}}{\partial \mathbf{t}} = \frac{1}{\mathbf{q}} \nabla J\_n + G\_n \mathbf{-} R\_n \tag{4}$$

$$\frac{\partial \mathbf{p}}{\partial \mathbf{t}} = -\frac{1}{\mathbf{q}} \nabla J\_p + G\_p \mathbf{-} R\_p \tag{5}$$

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible… DOI: http://dx.doi.org/10.5772/intechopen.90301*

In these equations, q is the magnitude of the electronic charge, n is the electron carrier density, p is the hole carrier density, J is the corresponding current density, G is the corresponding charge generation rate, and R is the corresponding charge recombination rate. For organic/metal oxide semiconductor field-effect transistors (MOSFETs), there is no optical absorption, so the term is simplified and the properties of the material are described by the minority carrier recombination lifetime. Since MOSFETs are majority carrier devices, the characteristics of carrier generation and recombination are relatively unimportant. The physical properties of organic semiconductors depend on the generation and movement of polarons [25].

A third important set of equations for describing the device physics for the charge carrier is given by

$$J\_p = \text{eqn}\mu\_p E - qD\_p \nabla p \tag{6}$$

$$J\_n = \text{qn}\mu\_n E + qD\_n \nabla n \tag{7}$$

It contains drift and diffusion parts. These equations determine the current density based on the carrier mobility (μ), the electric field (E), the carrier density (n, p), and the diffusion coefficient of the carrier (D). Diffusion coefficient operators are related to Einstein's mobility relationship:

$$D\_n = \frac{kT}{q} \mu\_n \tag{8}$$

$$D\_p = \frac{kT}{q}\mu\_p\tag{9}$$

In summary, the ATLAS software solves Poisson equations, continuity equations, and current density equations [26, 27] at each node in a two-dimensional grid for a given device structure simultaneously with itself and is subject to boundary conditions (including those applied at the contacts). With the help of ATLAS, the electric field distribution and electron and hole current density are calculated at each node and terminal current at electrode.

#### **2.3 Density of defect states model**

The assumed total density (DOS), g(E), consists of four bands: two tail bands (analogous to acceptor-like conduction band and donor-like valence band) and two deep energy bands (one donor-like and the other acceptor-like); they are modeled using a Gaussian distribution [15–22, 28]:

$$\mathbf{g}(E) = \mathbf{g}\_{TA}(E) + \mathbf{g}\_{TD}(E) + \mathbf{g}\_{GA}(E) + \mathbf{g}\_{GD}(E) \tag{10}$$

Here, E is the trap energy, *EC* is the conduction band energy, *EV* is the valence band energy, and the subscripts (*T*, *G*, *A*, *D*) stand for tail, Gaussian (deep level), acceptor, and donor states, respectively:

$$\log\_{TA}(E) = NTA \exp\left[\frac{E - E\_c}{WTA}\right] \tag{11}$$

$$\mathbf{g}\_{\rm TD}(E) = N \text{TD} \exp\left[\frac{E\_v - E}{\mathcal{W} \text{TD}}\right] \tag{12}$$

HOMO is approximately �5.19 eV, and LUMO is about �1.81 eV [7, 24]. This introduces a large enough 3.38 eV HOMO-LUMO energy gap, which is sufficient for

sectional view of the bottom-gate, top-contact DNTT-based OTFT.

To start the ATLAS simulation, we defined the physical structure and device dimensions, including the location of the electrical contacts. **Figure 2** shows a cross-

We can calculate these charge carrier densities by solving basic device equations simultaneously including Poisson equation [14–22], electron and hole continuity equation, charge transfer equation, and defect density of states equation. The first three equations are the default equations that ATLAS uses to find the electrical

The Poisson equation determines the electric field intensity in the given device based on the internal movement of the carriers and the distribution of the fixed

<sup>∇</sup>*:*<sup>E</sup> ¼ �divð Þ¼ <sup>∇</sup><sup>Ψ</sup> <sup>ρ</sup> x, y

where ∈ is the permittivity of the region and *ρ*(x,y) is the charge density

where p(x,y) is the hole density, n(x,y) is the electron density, ND

<sup>þ</sup> ND

To account for the trapped charge, Poisson equations are modified by adding an

acceptor-like traps, respectively, and FtD and FtA are probability of ionization of

Due to charge accumulation, a potential is generated, which affects the intensity of electric field distribution and current. The voltage applied to the gate electrode generates an electric field that attracts a few or majority carriers. In addition, for OTFTs, the voltage potential between the source and the drain establishes another electric field along the channel that drives the charge carriers and produces a

The continuity equation describes the dynamics of charge carrier distribution

� ND

tD = density � FtD, and N�

tA are ionized density of donor-like trap and ionized density of

<sup>þ</sup> x, y � NA � x, y (2)

� (x,y) is the ionization acceptor density.

<sup>þ</sup> x, y <sup>þ</sup> NA

∇*:Jn* þ *Gn*–*Rn* (4)

∇*:Jp* þ *Gp*–*Rp* (5)

� x, y � QT

<sup>∈</sup> (1)

+

tA = density � FtA.

(x,y) is the

(3)

transistor operation.

**2.2 Device physical equation**

*Hybrid Nanomaterials - Flexible Electronics Materials*

behavior of the device.

charges given by Eq. (1):

ρ x, y

ionization donor density, and NA

divð Þ¼� *ε*∇*Ψ* ρ x, y

tD and N�

over time as given in Eqs. (4) and (5):

where QT = q(N<sup>+</sup>

<sup>¼</sup> q p x, y � n x, y

tD + N�

donor-like traps and acceptor-like traps, respectively.

additional term QT, representing trapped charge given in (A):

<sup>¼</sup> q n x, y � p x, y

tA), N<sup>+</sup>

∂n <sup>∂</sup><sup>t</sup> <sup>¼</sup> <sup>1</sup> q

∂p <sup>∂</sup><sup>t</sup> ¼ � <sup>1</sup> q

given by.

Here, N+

current.

**86**

$$\mathbf{g}\_{GA}(E) = \text{NGA} \exp\left[-\left[\frac{EGA - E}{\text{WGA}}\right]^2\right] \tag{13}$$

*ftTD*ð Þ¼ *E*, *n*, *p*

*DOI: http://dx.doi.org/10.5772/intechopen.90301*

*ftGD*ð Þ¼ *E*, *n*, *p*

for donor states [8].

**2.5 Poole-Frenkel mobility model**

<sup>μ</sup>*nPF*ð Þ¼ *<sup>E</sup>* <sup>μ</sup>*<sup>n</sup>*<sup>0</sup> exp � *DELTAEN:PFMOB*

<sup>μ</sup>*pPF*ð Þ¼ *<sup>E</sup>* <sup>μ</sup>*<sup>p</sup>*<sup>0</sup> exp � *DELTAEP:PFMOB*

the effective temperature for holes.

**verification**

**89**

*<sup>v</sup>*<sup>p</sup> *SIGTDH:<sup>p</sup>* <sup>þ</sup> *vn SIGTDE:ni* exp *<sup>E</sup>*�*Ei*

*vp SIGGDH:<sup>p</sup>* <sup>þ</sup> *vn SIGGDE:ni* exp *<sup>E</sup>*�*Ei*

where *vn* is the thermal velocity of electron, *vp* is the thermal velocity of hole, and ni is intrinsic carrier concentration. SIGGAE and SIGTAE are the electron capture cross sections subject to the Gaussian states and main tail, respectively. SIGGAH and SIGTAH are hole trap cross sections of the Gaussian states and acceptor tail, respectively. SIGTDE, SIGGDE, SIGTDH, and SIGGDH are the equivalent

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible…*

Firstly, Miller et al. [29] described the rate of monophonic jumps for simulating

BETAN � PFMOB *kTneff*

BETAP � PFMOB *kTpeff*

where μ*pPF*ð Þ *<sup>E</sup>* and μ*nPF*ð Þ *<sup>E</sup>* are the Poole-Frenkel mobilities for holes and electrons, respectively; μ*<sup>n</sup>*<sup>0</sup> and μ*<sup>p</sup>*<sup>0</sup> are defined as the zero-field mobilities for electrons and holes, respectively; and *E* is the electric field. DELTAEN.PFMOB and DELTAEP.PFMOB are the activation energy at zero electric field for electrons and holes, respectively. BETAN.PFMOB is the electron Poole-Frenkel factor, and BETAP.PFMOB is the hole Poole-Frenkel factor. *Tneff* is the effective temperature for electrons, and *Tpeff* is

j j *<sup>E</sup>* p !

j j *<sup>E</sup>* p !

� GAMMAN*:*PFMOB ! ffiffiffiffiffiffi

� GAMMAP*:*PFMOB ! ffiffiffiffiffiffi

hopping in inorganic semiconductors. Later, Vissenberg et al. [30] studied the dependency related to carrier transport on the energy distribution and the jump distance in amorphous transistors, which further helps to find the carrier mobility.

þ

þ

**3. Compact modeling, model parameter extraction, and model**

density of states, interface traps and space charge-limited carrier transport,

The technology and operation of organic thin film transistors (OTFTs) have various unique features that require a dedicated compact TFT model. The important features of OTFT include operation in carrier accumulation mode, exponential

nonlinear parasitic resistance, source and drain contacts without junction isolation, dependence of mobility on career concentration, electric field, and temperature. The universal organic TFT (UOTFT) model [23] is a modeling expression that

The very popular Poole-Frenkel mobility model [31] is given by

*kTneff*

*kTpeff*

*kT* � � � � <sup>þ</sup> *vpSIGTDH p* <sup>þ</sup> *ni* exp *Ei*�*<sup>E</sup>*

*kT* � � � � <sup>þ</sup> *vpSIGGDH p* <sup>þ</sup> *ni* exp *Ei*�*<sup>E</sup>*

*vnSIGTDE n* <sup>þ</sup> *ni* exp *<sup>E</sup>*�*Ei*

*vnSIGGDE n* <sup>þ</sup> *ni* exp *<sup>E</sup>*�*Ei*

*kT* � �

*kT* � �

*kT* � � � �

*kT* � � � �

(23)

(24)

(25)

(26)

$$\lg\_{GD}(E) = NGD \exp\left[-\left[\frac{E - EGD}{WGD}\right]^2\right] \tag{14}$$

For exponential tails, DOS is defined by its conduction and valence band edge intercept densities (NTA and NTD) and by its characteristic attenuation energy (WTD and WTA).

For Gaussian distribution, DOS is given by the total state density (NGD and NGA), its characteristic attenuation energy (WGD and WGA), and its peak energy distribution (EGD and EGA).

#### **2.4 Trapped carrier density**

The ionized densities of donor and acceptor states are given by Eqs. (14) and (15):

$$
\mathfrak{n}\_T = \mathfrak{n}\_{\rm TD} + \mathfrak{n}\_{\rm GD} \tag{15}
$$

$$p\_T = p\_{TA} + p\_{GA} \tag{16}$$

where *pTA*, *pGA*, *nTD*, and *nGD* are given below

$$p\_{TA} = \int\_{Ev}^{E} \mathbf{g}\_{TA}(E) f\_{t\_{TA}}(E, n, p) dE \tag{17}$$

$$p\_{GA} = \int\_{Ev}^{E} \mathbf{g}\_{GA}(E) f\_{\mathbf{t}\_{GA}}(E, n, p) dE \tag{18}$$

$$n\_{\rm TD} = \int\_{E^p}^{E} \mathbf{g}\_{\rm TD}(E) f\_{\rm tTD}(E, n, p) dE \tag{19}$$

$$n\_{GD} = \int\_{Ev}^{E} g\_{GD}(E) f\_{tGD}(E, n, p) dE \tag{20}$$

ftGA(E,n,p) and ftTA(E,n,p) are the ionization probabilities for the Gaussian acceptor and tail DOS, while ftTD(E,n,p) and ftGD(E,n,p) are defined as the probability of occupation of a trap level at energy E for the Gaussian and tail acceptor, and donor states in steady state are given by following equations [24–27]:

$$f\_{IGA}(E, n, p) = \frac{v\_n \text{SIGTA}.n + v\_p \text{SIGTA}.n\_i \exp\left[\frac{E\_i - E}{kT}\right]}{v\_n \text{SIGTA}\left(n + n\_i \exp\left[\frac{E\_i - E}{kT}\right]\right) + v\_p \text{SIGTA}(p + n\_i \exp\left[\frac{E\_i - E}{kT}\right])} \tag{21}$$

$$f\_{IGA}(E, n, p) = \frac{v\_n \text{SIGGA.n} + v\_p \text{SIGGA.n}\_i \exp\left[\frac{E\_i - E}{kT}\right]}{v\_n \text{SIGGA}\left(n + n\_i \exp\left[\frac{E\_i - E}{kT}\right]\right) + v\_p \text{SIGGA}(p + n\_i \exp\left[\frac{E\_i - E}{kT}\right])} \tag{22}$$

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible… DOI: http://dx.doi.org/10.5772/intechopen.90301*

$$f\_{t\text{TD}}(E, n, p) = \frac{v\_p \text{SIGT}DH.p + v\_n \text{SIGT}DE.n\_i \exp\left[\frac{E - E\_i}{kT}\right]}{v\_n \text{SIGT}DE \left(n + n\_i \exp\left[\frac{E - E\_i}{kT}\right]\right) + v\_p \text{SIGT}DH \left(p + n\_i \exp\left[\frac{E - E}{kT}\right]\right)} \tag{23}$$

$$f\_{t\text{GD}}(E, n, p) = \frac{v\_p \text{SIGG}DH.p + v\_n \text{SIGG}DE.n\_i \exp\left[\frac{E - E\_i}{kT}\right]}{v\_n \text{SIGG}DE \left(n + n\_i \exp\left[\frac{E - E\_i}{kT}\right]\right) + v\_p \text{SIGG}DH \left(p + n\_i \exp\left[\frac{E - E}{kT}\right]\right)} \tag{24}$$

where *vn* is the thermal velocity of electron, *vp* is the thermal velocity of hole, and ni is intrinsic carrier concentration. SIGGAE and SIGTAE are the electron capture cross sections subject to the Gaussian states and main tail, respectively. SIGGAH and SIGTAH are hole trap cross sections of the Gaussian states and acceptor tail, respectively. SIGTDE, SIGGDE, SIGTDH, and SIGGDH are the equivalent for donor states [8].

#### **2.5 Poole-Frenkel mobility model**

*gGA*ð Þ¼ *<sup>E</sup> NGA* exp � *EGA* � *<sup>E</sup>*

*gGD*ð Þ¼ *<sup>E</sup> NGD* exp � *<sup>E</sup>* � *EGD*

(WTD and WTA).

and (15):

*ftTA*ð Þ¼ *E*, *n*, *p*

**88**

*ftGA*ð Þ¼ *E*, *n*, *p*

distribution (EGD and EGA).

*Hybrid Nanomaterials - Flexible Electronics Materials*

**2.4 Trapped carrier density**

where *pTA*, *pGA*, *nTD*, and *nGD* are given below

*pTA* ¼

*pGA* ¼

*nTD* ¼

*nGD* ¼

states in steady state are given by following equations [24–27]:

*vnSIGGAE n* <sup>þ</sup> *ni* exp *Ei*�*<sup>E</sup>*

*vnSIGTAE n* <sup>þ</sup> *ni* exp *Ei*�*<sup>E</sup>*

*Ec* ð

*Ev*

*Ec* ð

*Ev*

*Ec* ð

*Ev*

*Ec* ð

*Ev*

ftGA(E,n,p) and ftTA(E,n,p) are the ionization probabilities for the Gaussian acceptor

*vnSIGTAE:<sup>n</sup>* <sup>þ</sup> *vpSIGTAH:ni* exp *Ei*�*<sup>E</sup>*

*kT*

� � � � <sup>þ</sup> *vpSIGTAH p* <sup>þ</sup> *ni* exp *Ei*�*<sup>E</sup>*

� � � � <sup>þ</sup> *vpSIGGAH p* <sup>þ</sup> *ni* exp *Ei*�*<sup>E</sup>*

*vnSIGGAE:<sup>n</sup>* <sup>þ</sup> *vpSIGGAH:ni* exp *Ei*�*<sup>E</sup>*

and tail DOS, while ftTD(E,n,p) and ftGD(E,n,p) are defined as the probability of occupation of a trap level at energy E for the Gaussian and tail acceptor, and donor

*kT*

For exponential tails, DOS is defined by its conduction and valence band edge intercept densities (NTA and NTD) and by its characteristic attenuation energy

For Gaussian distribution, DOS is given by the total state density (NGD and NGA), its characteristic attenuation energy (WGD and WGA), and its peak energy

The ionized densities of donor and acceptor states are given by Eqs. (14)

*WGA* � �<sup>2</sup> " #

*WGD* � �<sup>2</sup> " #

*nT* ¼ *nTD* þ *nGD* (15) *pT* ¼ *pTA* þ *pGA* (16)

*gTA*ð Þ *E :ftTA* ð Þ *E*, *n*, *p dE* (17)

*gGA*ð Þ *E :ftGA* ð Þ *E*, *n*, *p dE* (18)

*gTD*ð Þ *E :ftTD*ð Þ *E*, *n*, *p dE* (19)

*gGD*ð Þ *E :ftGD*ð Þ *E*, *n*, *p dE* (20)

*kT* � �

> *kT* � �

*kT* � � � � (21)

� � � �

*kT*

(22)

(13)

(14)

Firstly, Miller et al. [29] described the rate of monophonic jumps for simulating hopping in inorganic semiconductors. Later, Vissenberg et al. [30] studied the dependency related to carrier transport on the energy distribution and the jump distance in amorphous transistors, which further helps to find the carrier mobility. The very popular Poole-Frenkel mobility model [31] is given by

$$
\mu\_{\text{nyr}(E)\to\mu\_{\text{n}0}} \exp\left(-\frac{\text{DETAEN.PFMOB}}{kT\_{\text{ngr}}} + \left(\frac{\text{BETAN}\cdot\text{PFMOB}}{kT\_{\text{ngr}}} - \text{GAMMAN.PFMOB}\right)\sqrt{|E|}\right) \tag{25}
$$

$$
\mu\_{\text{pry}(E)\to\mu\_{\text{p0}}} \exp\left(-\frac{\text{DETAEP.PFMOB}}{kT\_{\text{pgf}}} + \left(\frac{\text{BETAP}\cdot\text{PFMOB}}{kT\_{\text{pgf}}} - \text{GAMMAP.PFMOB}\right)\sqrt{|E|}\right) \tag{26}
$$

where μ*pPF*ð Þ *<sup>E</sup>* and μ*nPF*ð Þ *<sup>E</sup>* are the Poole-Frenkel mobilities for holes and electrons, respectively; μ*<sup>n</sup>*<sup>0</sup> and μ*<sup>p</sup>*<sup>0</sup> are defined as the zero-field mobilities for electrons and holes, respectively; and *E* is the electric field. DELTAEN.PFMOB and DELTAEP.PFMOB are the activation energy at zero electric field for electrons and holes, respectively. BETAN.PFMOB is the electron Poole-Frenkel factor, and BETAP.PFMOB is the hole Poole-Frenkel factor. *Tneff* is the effective temperature for electrons, and *Tpeff* is the effective temperature for holes.

#### **3. Compact modeling, model parameter extraction, and model verification**

The technology and operation of organic thin film transistors (OTFTs) have various unique features that require a dedicated compact TFT model. The important features of OTFT include operation in carrier accumulation mode, exponential density of states, interface traps and space charge-limited carrier transport, nonlinear parasitic resistance, source and drain contacts without junction isolation, dependence of mobility on career concentration, electric field, and temperature. The universal organic TFT (UOTFT) model [23] is a modeling expression that

extends the uniform charge control model (UCCM) previously used for a-Si and poly-Si TFTs [23, 32] to OTFTs and introduces a general expression of modeling for conductivity of channel of OTFTs [30, 33]. In this way, the UOTFT model is applicable to various OTFT device architectures, specifications of material, and manufacturing technologies.

#### **3.1 Model features**

UOTFT model depends on a general-purpose compact modeling approach [23, 32], which provides smooth interpolation of drain currents between linear and saturated operating regions including channel length modulation effects and also provides the unified expression of the gate-induced charge in the conductive channel which is valid in all operating states. This model also gives a unified chargebased mobility description, drain-source current, and gate-to-source and gate-todrain capacitances.

#### **3.2 Model description**

The control equation for the UOTFT model for the n-channel OTFT case is described here. The p-channel condition can be obtained by direct change in voltage, charge polarity, and current.

The charge accumulation in channel per unit area at zero-channel potential (�Qacc)o is calculated by the help of the solution of the UCCM equation given by [34].

$$(-Q\_{\text{acc}})\_{\mathfrak{o}} = \mathbf{C}\_{i}.V\_{\text{gsc}}\tag{27}$$

MUACC, VACC, and GAMMA are model parameters. MUACC is a temperaturerelated parameter which defines effective channel mobility at the onset of strong accumulation of channel. This onset point is controlled by model parameter VACC. The power-law dependence of the mobility on carrier concentration is defined by

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible…*

Drain-source current of intrinsic transistor due to charge carriers accumulated in

*Isat*ð Þ <sup>1</sup>þ*LAMBDA:Vds MSAT* <sup>1</sup> *MSAT*

where Gch is the effective channel conductance in the linear region, Vdse is the effective intrinsic drain-source voltage, Vds is the intrinsic drain-source voltage, parameter LAMBDA defines the finite output conductance in the saturation region, and MSAT is the model parameter that provides a smooth transition between linear and saturated transistor operation. Isat is the ideal intrinsic drain-source saturation current, and the effective channel conductance in the linear region Gch is obtained

*GCh* <sup>¼</sup> *Gch*<sup>0</sup>

*Rds* <sup>¼</sup> *RDS T*ð Þ <sup>1</sup> <sup>þ</sup> *Vgse VRDS*

where Gch0 is the intrinsic effective conductance of channel in the linear region and Rds is the nonlinear bias-dependent series resistance for intrinsic channel region defined by temperature-dependent model parameter RDS and the model parameter VRDS; on the other hand, Weff and Leff are effective channel widths and length,

The drain saturation current Isat is determined by the following formula:

�*Q acc* ð Þ<sup>0</sup> *GAMMA T*ð Þþ <sup>2</sup> <sup>þ</sup>

where *ASAT* is the temperature-dependent model parameter.

*Vds Vth* � <sup>1</sup>

*:* exp �NSGL

where *Vsat* is the saturation voltage obtained as

The drain-source leakage current is obtained as

*Vsat* <sup>¼</sup> *ASAT T*ð Þ *Ci*

fIOL Tð Þ*:* exp NSDL*:*

*Gch*<sup>0</sup> <sup>¼</sup> *Weff Leff*

1 þ *Gcho:Rds*

*ds* ¼ *Gch:Vdse* (33)

*:*μ*<sup>c</sup>:* �*Q acc* ð Þ<sup>0</sup> (36)

*Isat* ¼ *Gch:Vsat* (38)

*CiVO T*ð Þ *GAMMA T*ð Þþ 1

(39)

*Vgs*

*Vth* <sup>þ</sup> SIGMAO*:Vds* (40)

(34)

(35)

(37)

the temperature-dependent model parameter GAMMA.

the channel is defined by general interpolation expressions [23]:

*I acc*

*Vdse* <sup>¼</sup> *Vds*

<sup>1</sup> <sup>þ</sup> *<sup>G</sup>:Vds*

**3.4 Intrinsic drain-source current**

*DOI: http://dx.doi.org/10.5772/intechopen.90301*

in the following way:

respectively.

*I leak ds* <sup>¼</sup> *weff Leff*

**91**

$$V\_{g\epsilon\epsilon} = \text{VO(T)} \cdot \ln\left[\mathbf{1} + \frac{e^{\mu+1}}{\mathbf{1} + k(\mu+2)\ln\left(\mathbf{1} + e^{\mu+1}\right)}\right] \tag{28}$$

$$
\mu = \frac{V\_{\text{ges}} - VT(T)}{VO(T)} \tag{29}
$$

$$k(\varkappa) = 1 - \frac{84.4839}{\varkappa^2 + 150.864} \tag{30}$$

$$\mathbf{C}\_{i} = \mathbf{C}\_{0} \frac{EPSI}{TINS} \tag{31}$$

where *Ci* is the gate insulator capacitance per unit area, Vgse is the effective intrinsic gate-source voltage, Vgs is the gate-source voltage (intrinsic), VT is the temperature-dependent threshold voltage parameter, and VO is characteristic voltage (temperature-dependent); for carrier density of states including the influence of interface traps, ∈<sup>0</sup> is the vacuum permittivity, and EPSI and TINS are model parameters representing the relative permittivity and thickness of the gate insulator, respectively.

#### **3.3 Effective channel mobility**

For accurate modeling of OTFTs, we should consider the characteristic power-law dependence of mobility on carrier concentration. According to the results of percolation theory [30], effective channel mobility is expressed in the UOTFT model as

$$\mu\_C = \text{MUACC}(T) \cdot \left(\frac{(-Q\_{acc})\_0}{\mathbf{C}\_i \cdot \mathbf{VACC}}\right)^{\text{GAMMA}(T)} \tag{32}$$

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible… DOI: http://dx.doi.org/10.5772/intechopen.90301*

MUACC, VACC, and GAMMA are model parameters. MUACC is a temperaturerelated parameter which defines effective channel mobility at the onset of strong accumulation of channel. This onset point is controlled by model parameter VACC. The power-law dependence of the mobility on carrier concentration is defined by the temperature-dependent model parameter GAMMA.

#### **3.4 Intrinsic drain-source current**

extends the uniform charge control model (UCCM) previously used for a-Si and poly-Si TFTs [23, 32] to OTFTs and introduces a general expression of modeling for conductivity of channel of OTFTs [30, 33]. In this way, the UOTFT model is applicable to various OTFT device architectures, specifications of material, and

UOTFT model depends on a general-purpose compact modeling approach [23, 32], which provides smooth interpolation of drain currents between linear and saturated operating regions including channel length modulation effects and also provides the unified expression of the gate-induced charge in the conductive channel which is valid in all operating states. This model also gives a unified chargebased mobility description, drain-source current, and gate-to-source and gate-to-

The control equation for the UOTFT model for the n-channel OTFT case is described here. The p-channel condition can be obtained by direct change in volt-

The charge accumulation in channel per unit area at zero-channel potential (�Qacc)o is calculated by the help of the solution of the UCCM equation given by [34].

*<sup>u</sup>* <sup>¼</sup> *Vgs* � *VT T*ð Þ

*k x*ð Þ¼ <sup>1</sup> � <sup>84</sup>*:*<sup>4839</sup>

*EPSI*

*Ci* ¼ ∈<sup>0</sup>

where *Ci* is the gate insulator capacitance per unit area, Vgse is the effective intrinsic gate-source voltage, Vgs is the gate-source voltage (intrinsic), VT is the temperature-dependent threshold voltage parameter, and VO is characteristic voltage (temperature-dependent); for carrier density of states including the influence of interface traps, ∈<sup>0</sup> is the vacuum permittivity, and EPSI and TINS are model parameters representing the relative permittivity and thickness of the gate insula-

For accurate modeling of OTFTs, we should consider the characteristic power-law dependence of mobility on carrier concentration. According to the results of percolation theory [30], effective channel mobility is expressed in the UOTFT model as

*Ci:VACC*

*GAMMA T*ð Þ

<sup>μ</sup>*<sup>C</sup>* <sup>¼</sup> *MUACC T*ð Þ*:* �*<sup>Q</sup> acc* ð Þ<sup>0</sup>

*Vgse* ¼ VO Tð Þ*:* ln 1 þ

�*Q acc* ð Þ*<sup>o</sup>* ¼ *Ci:Vgse* (27)

*VO T*ð Þ (29)

*<sup>x</sup>*<sup>2</sup> <sup>þ</sup> <sup>150</sup>*:*<sup>864</sup> (30)

*TINS* (31)

(28)

(32)

*e<sup>u</sup>*þ<sup>1</sup> 1 þ *k u*ð Þ þ 2 ln 1 þ *eu*þ<sup>1</sup> ð Þ 

manufacturing technologies.

*Hybrid Nanomaterials - Flexible Electronics Materials*

**3.1 Model features**

drain capacitances.

tor, respectively.

**90**

**3.3 Effective channel mobility**

**3.2 Model description**

age, charge polarity, and current.

Drain-source current of intrinsic transistor due to charge carriers accumulated in the channel is defined by general interpolation expressions [23]:

$$I\_{ds}^{\text{act}} = \mathcal{G}\_{ch} \mathcal{V}\_{de} \tag{33}$$

$$Vdee = \frac{V\_{ds}}{\left[\mathbf{1} + \left(\frac{\_{G\cdot V\_{ds}}}{I\_{\rm int}(1 + LAMBDA\cdot Vds)}\right)^{\rm MSAT}\right]^{\frac{1}{K\rm SAT}}}\tag{34}$$

where Gch is the effective channel conductance in the linear region, Vdse is the effective intrinsic drain-source voltage, Vds is the intrinsic drain-source voltage, parameter LAMBDA defines the finite output conductance in the saturation region, and MSAT is the model parameter that provides a smooth transition between linear and saturated transistor operation. Isat is the ideal intrinsic drain-source saturation current, and the effective channel conductance in the linear region Gch is obtained in the following way:

$$G\_{Ch} = \frac{G\_{ch0}}{1 + G\_{cho}R\_{ds}} \tag{35}$$

$$\mathbf{G}\_{ch0} = \frac{\mathbf{W}\_{\rm eff}}{L\_{\rm eff}} \boldsymbol{\mu}\_{c.} (-\mathbf{Q}\_{acc})\_0 \tag{36}$$

$$R\_{ds} = \frac{RDS(T)}{1 + \frac{V\_{F^\*}}{VRDS}}\tag{37}$$

where Gch0 is the intrinsic effective conductance of channel in the linear region and Rds is the nonlinear bias-dependent series resistance for intrinsic channel region defined by temperature-dependent model parameter RDS and the model parameter VRDS; on the other hand, Weff and Leff are effective channel widths and length, respectively.

The drain saturation current Isat is determined by the following formula:

$$I\_{\rm sat} = \mathcal{G}\_{\rm ch} \cdot V\_{\rm sat} \tag{38}$$

where *Vsat* is the saturation voltage obtained as

$$V\_{sat} = \frac{ASAT(T)}{C\_i} \left[ \frac{(-Q\_{acc})\_0}{GAMMA(T) + 2} + \frac{C\_iVO(T)}{GAMMA(T) + 1} \right] \tag{39}$$

where *ASAT* is the temperature-dependent model parameter. The drain-source leakage current is obtained as

$$I\_{ds}^{\text{lank}} = \frac{w\_{\text{eff}}}{L\_{\text{eff}}} \{ \text{IOL(T)}.\left[ \exp\left( \text{NSDL}.\frac{V\_{ds}}{V\_{th}} \right) - 1 \right].\exp\left( -\text{NSGL}\frac{V\_{\text{g}}}{V\_{th}} \right) + \text{SIGMAO}.V\_{ds} \tag{40}$$

The IOL is a temperature-dependent leakage saturation current model parameter; NGSL and NDSL are non-ideal factors for gate and drain bias, respectively, and SIGMAO is a model parameter representing zero-bias drain-source conductivity:

$$V\_{th} = \frac{kT}{q} \tag{41}$$

where *Vth* is the thermal voltage at device operating temperature. The total intrinsic drain-source current is

$$I\_{ds} = I\_{ds}^{acc} + I\_{ds}^{leak} \tag{42}$$

#### **4. Results and discussion**

#### **4.1 Material parameters used for DNTT**

The DNTT-based OTFT is designed in a bottom-gate, top-contact configuration. The designed structure has a channel length of 200 μm and a channel width of 400 μm with Lov = 10 μm as shown in **Figure 2**. For the simulation of DNTT-based OTFT structure, the following parameters [24] used are listed in **Table 1**.

#### **4.2 Comparison of TCAD-based numerical simulation characteristics and compact model-based simulation characteristics with experimental characteristics**

**Figure 3** shows the transfer characteristics obtained for the TCAD-based numerical simulation, compact model-based simulation of DNTT-based organic thin film transistor, and the measured characteristic of DNTT-based OTFT [24]. The transfer characteristics are obtained by varying the gate-to-source voltage (VGS) from 0 to �3 V keeping drain voltage constant at �2 V. There is very good agreement between TCAD-based numerical simulation, compact model-based simulation of the transfer characteristics of OTFT, and experimental transfer characteristics of the fabricated device. **Figure 4** shows the output characteristics obtained from the TCAD-based numerical simulation, compact model-based simulation of DNTT-based organic thin film transistor, and the measured output characteristics of DNTT-based OTFT [24]. Output characteristics are obtained by varying the


drain-to-source voltage (VDS) from 0 to 3 V and keeping the gate-to-source voltage (VGS) constant at 1.5, 1.8, 2.1, 2.4, 2.7, and 3.0 V. The simulated output characteristic matched with the experimental output characteristic of the

*Comparisons of output characteristics of the measured data, the TCAD-simulated data, and the modeled data.*

*Comparisons of transfer characteristics of the measured data, the TCAD-simulated data, and the modeled data.*

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible…*

*DOI: http://dx.doi.org/10.5772/intechopen.90301*

Extracted OTFT model parameters for low-voltage DNTT-based OTFT using UOTFT model is given in **Table 2**. The extraction process starts with the collection

fabricated device.

**Figure 4.**

**93**

**Figure 3.**

**4.3 Parameter extraction**

#### **Table 1.**

*Simulation parameters of material of the OTFT [17].*

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible… DOI: http://dx.doi.org/10.5772/intechopen.90301*

**Figure 3.** *Comparisons of transfer characteristics of the measured data, the TCAD-simulated data, and the modeled data.*

**Figure 4.** *Comparisons of output characteristics of the measured data, the TCAD-simulated data, and the modeled data.*

drain-to-source voltage (VDS) from 0 to 3 V and keeping the gate-to-source voltage (VGS) constant at 1.5, 1.8, 2.1, 2.4, 2.7, and 3.0 V. The simulated output characteristic matched with the experimental output characteristic of the fabricated device.

#### **4.3 Parameter extraction**

Extracted OTFT model parameters for low-voltage DNTT-based OTFT using UOTFT model is given in **Table 2**. The extraction process starts with the collection

The IOL is a temperature-dependent leakage saturation current model parameter; NGSL and NDSL are non-ideal factors for gate and drain bias, respectively, and SIGMAO is a model parameter representing zero-bias drain-source conductivity:

(41)

*ds* (42)

*Vth*<sup>¼</sup> *kT q*

where *Vth* is the thermal voltage at device operating temperature.

*Ids* ¼ *I acc ds* þ *I leak*

The DNTT-based OTFT is designed in a bottom-gate, top-contact configuration.

The designed structure has a channel length of 200 μm and a channel width of 400 μm with Lov = 10 μm as shown in **Figure 2**. For the simulation of DNTT-based

**4.2 Comparison of TCAD-based numerical simulation characteristics and compact model-based simulation characteristics with experimental**

**Figure 3** shows the transfer characteristics obtained for the TCAD-based numerical simulation, compact model-based simulation of DNTT-based organic thin film transistor, and the measured characteristic of DNTT-based OTFT [24]. The transfer characteristics are obtained by varying the gate-to-source voltage (VGS) from 0 to �3 V keeping drain voltage constant at �2 V. There is very good agreement between TCAD-based numerical simulation, compact model-based simulation of the transfer characteristics of OTFT, and experimental transfer characteristics of the fabricated device. **Figure 4** shows the output characteristics obtained from the TCAD-based numerical simulation, compact model-based simulation of DNTT-based organic thin film transistor, and the measured output characteristics of DNTT-based OTFT [24]. Output characteristics are obtained by varying the

**Material simulation parameters Value** DNTT energy band gap (eV) 3.38 eV Occupied molecular orbital Of DNTT (highest) �5.19 eV Occupied molecular orbital Of DNTT (lowest) �1.81 eV Intrinsic p-type doping in DNTT 10<sup>16</sup> cm�<sup>3</sup> Fixed interface charge concentration <sup>5</sup> � 1016 cm�<sup>3</sup> Work function of aluminum gate 4.1 eV Work function of Au contact 5.0 eV Semiconductor thickness of DNTT 11 nm Dielectric thickness 5.3 nm

OTFT structure, the following parameters [24] used are listed in **Table 1**.

The total intrinsic drain-source current is

*Hybrid Nanomaterials - Flexible Electronics Materials*

**4.1 Material parameters used for DNTT**

**4. Results and discussion**

**characteristics**

**Table 1.**

**92**

*Simulation parameters of material of the OTFT [17].*


#### **Table 2.**

*Model parameters extracted for UOTFT model.*

of data for ID-VG and ID-VD characteristic and providing it in UTMOST IV database in .uds format. Further we performed simulation of ID-VD and ID-VG characteristic using UOTFT model and optimization of this characteristic using Levenberg– Marquardt optimization technique with respect to experimental data for extraction of model parameters.

(V). The given inverter circuit works like a potential divider between the driver and the load OTFT. When the input voltage is lower than the threshold voltage (more positive than VT), the driver OTFT turns off. On the other side, when it is more than the threshold voltage (more negative than VT), the driver OTFT turns on. The operation of the inverter also depends on load TFT size relatively with the driver TFT. To assess whether the simulation correctly reproduces this dependence, the size of load OTFT and its gate voltage (V) remain at the same value, while the size and gate voltage of driver OTFT change. **Figure 6** shows the voltage transfer characteristic (VTC) plot of the inverter circuit under consideration for W/L ratio of 10, 120 1140 of driver TFT. As W/L ratio of the driver OTFT increases, its impedance decreases, and the transition between high and low states becomes

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible…*

*DOI: http://dx.doi.org/10.5772/intechopen.90301*

*Voltage transfer characteristics of inverter circuit shown for different W/L ratios of driver OTFT.*

We presented a finite element method (FEM)-based device simulation of lowvoltage DNTT-based OTFT by considering field-dependent mobility model and double-peak Gaussian density of states model using device simulator ATLAS. We also presented the application of UOTFT model and parameter extraction method to organic TFTs. We can also conclude that numerical simulations, experiments, and compact modeling-based simulation characteristics demonstrate the same behavior as matched in **Figure 3** and **Figure 4**. We simulated an OTFT based on DNTT and demonstrated the application of the UOTFT model to organic TFTs and also use experimental data from DNTT-based OTFTs to extract parameters for Silvaco's general-purpose organic TFT compact model. The model has been verified against logic circuit simulation. It has been concluded that UOTFTs provide more accurate modeling of the simpler parameter extraction methods for various organic TFTs. The results show that the UOTFT model correctly simulates the behavior of the devices reported in this study and is expected to be used for more complex

clearer.

**95**

**Figure 6.**

**5. Conclusion**

circuits based on organic thin film transistors.

#### **4.4 Simulation of logic circuit**

For UOTFT model validity, simple logic circuit has been implemented and simulated based on p-type OTFTs only. The schematic in **Figure 5** shows the simple inverter circuit used in the simulation of a load transistor with auxiliary gate voltage

**Figure 5.** *A circuit diagram of the inverter circuit used for assessing the simulation results.*

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible… DOI: http://dx.doi.org/10.5772/intechopen.90301*

**Figure 6.** *Voltage transfer characteristics of inverter circuit shown for different W/L ratios of driver OTFT.*

(V). The given inverter circuit works like a potential divider between the driver and the load OTFT. When the input voltage is lower than the threshold voltage (more positive than VT), the driver OTFT turns off. On the other side, when it is more than the threshold voltage (more negative than VT), the driver OTFT turns on. The operation of the inverter also depends on load TFT size relatively with the driver TFT. To assess whether the simulation correctly reproduces this dependence, the size of load OTFT and its gate voltage (V) remain at the same value, while the size and gate voltage of driver OTFT change. **Figure 6** shows the voltage transfer characteristic (VTC) plot of the inverter circuit under consideration for W/L ratio of 10, 120 1140 of driver TFT. As W/L ratio of the driver OTFT increases, its impedance decreases, and the transition between high and low states becomes clearer.

#### **5. Conclusion**

of data for ID-VG and ID-VD characteristic and providing it in UTMOST IV database in .uds format. Further we performed simulation of ID-VD and ID-VG characteristic using UOTFT model and optimization of this characteristic using Levenberg– Marquardt optimization technique with respect to experimental data for extraction

**Parameter name Symbol UNIT Typical values** The thickness of gate insulator TINS m 5.3 <sup>10</sup><sup>9</sup> Relative dielectric permittivity of the insulator at gate EPSI — 3.37 Relative dielectric permittivity of the semiconductor EPS — 3.0 Zero-bias threshold voltage VT V 0.884542 Trap density states characteristic voltage VO V 0.0314021

Characteristic voltage of the effective mobility VACC V 1.0 Output conductance parameter LAMBDA 1/V 0.0 Knee-shape parameter MSAT — 5.0 Saturation modulation parameter ASAT — 1.52 Leakage saturation current IOL A 1 <sup>10</sup><sup>10</sup> Contact resistance RS + RD Kilo Ohm 116.892

/Vs 1.85

Characteristic effective accumulation channel mobility MUACC cm2

*Hybrid Nanomaterials - Flexible Electronics Materials*

For UOTFT model validity, simple logic circuit has been implemented and simulated based on p-type OTFTs only. The schematic in **Figure 5** shows the simple inverter circuit used in the simulation of a load transistor with auxiliary gate voltage

*A circuit diagram of the inverter circuit used for assessing the simulation results.*

of model parameters.

**Table 2.**

**Figure 5.**

**94**

**4.4 Simulation of logic circuit**

*Model parameters extracted for UOTFT model.*

We presented a finite element method (FEM)-based device simulation of lowvoltage DNTT-based OTFT by considering field-dependent mobility model and double-peak Gaussian density of states model using device simulator ATLAS. We also presented the application of UOTFT model and parameter extraction method to organic TFTs. We can also conclude that numerical simulations, experiments, and compact modeling-based simulation characteristics demonstrate the same behavior as matched in **Figure 3** and **Figure 4**. We simulated an OTFT based on DNTT and demonstrated the application of the UOTFT model to organic TFTs and also use experimental data from DNTT-based OTFTs to extract parameters for Silvaco's general-purpose organic TFT compact model. The model has been verified against logic circuit simulation. It has been concluded that UOTFTs provide more accurate modeling of the simpler parameter extraction methods for various organic TFTs. The results show that the UOTFT model correctly simulates the behavior of the devices reported in this study and is expected to be used for more complex circuits based on organic thin film transistors.

#### **Acknowledgements**

The authors are thankful to SERB, DST, Government of India, for the financial support under Early Career Research Award (ECRA) for Project No. ECR/2017/ 000179.

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[1] Mizukami M, Hirohata N, Iseki T, Ohtawara K, Tada T, Yagyu S, et al. Flexible AMOLED panel driven by bottom-contact OTFTs. IEEE Electron

*DOI: http://dx.doi.org/10.5772/intechopen.90301*

[9] Han CY, Ma YX, Tang WM,

organic thin-film transistor with different gate materials on various substrates. IEEE Electron Device

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*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible…*

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nmat4785

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[15] Dwivedi ADD. Numerical simulation and spice modeling of organic thin film transistors (OTFTs). International Journal of Advanced Applied Physics Research. 2014;**1**:14-21

Effect of gate dielectric on the performance of ZnO based thin film

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transistor. Superlattices and Microstructures. 2018;**120**:223-234

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[10] Gundlach DJ, Zhou L, Nichols JA, Jackson TN. An experimental study of contact effects in organic thin film transistors. Journal of Applied Physics.

[11] Street RA, Salleo A. Contact effects in polymer transistors. Applied Physics

[12] Lee BH, Bazan GC, Heeger AJ. Doping-induced carrier density modulation in polymer field-effect transistors. Advanced Materials. 2016;

[13] Nikolka M et al. High operational and environmental stability of highmobility conjugated polymer fieldeffect transistors through the use of molecular additives. Nature Materials. Dec. 2017;**16**:356-362. DOI: 10.1038/

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[5] Lodha A, Singh R. Prospects of manufacturing organic semiconductor-

based integrated circuits. IEEE Transactions on Semiconductor Manufacturing. Aug 2001;**14**(3)

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semiconductor thin-film transistors for flexible electronics. Applied Physics

[7] Yamamoto T, Takimiya K. Facile synthesis of highly *π*-extended heteroarenes, dinaphtho[2,3-b,2<sup>0</sup>

chalcogenophenes, and their application to field-effect transistors. Journal of the American Chemical Society. 2007;

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,3'f]

### **Author details**

Arun Dev Dhar Dwivedi\*, Sushil Kumar Jain, Rajeev Dhar Dwivedi and Shubham Dadhich Department of Electrical and Electronics Engineering, Poornima University, Jaipur, India

\*Address all correspondence to: adddwivedi@gmail.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Numerical Simulation and Compact Modeling of Thin Film Transistors for Future Flexible… DOI: http://dx.doi.org/10.5772/intechopen.90301*

#### **References**

**Acknowledgements**

*Hybrid Nanomaterials - Flexible Electronics Materials*

000179.

**Author details**

Shubham Dadhich

Jaipur, India

**96**

The authors are thankful to SERB, DST, Government of India, for the financial support under Early Career Research Award (ECRA) for Project No. ECR/2017/

Arun Dev Dhar Dwivedi\*, Sushil Kumar Jain, Rajeev Dhar Dwivedi and

\*Address all correspondence to: adddwivedi@gmail.com

provided the original work is properly cited.

Department of Electrical and Electronics Engineering, Poornima University,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

[1] Mizukami M, Hirohata N, Iseki T, Ohtawara K, Tada T, Yagyu S, et al. Flexible AMOLED panel driven by bottom-contact OTFTs. IEEE Electron Device Letters. 2006;**27**:249

[2] Takamiya M, Sekitani T, Kato Y, Kawaguchi H, Someya T, Sakurai T. An organic FET SRAM with back gate to increase static noise margin and its application to braille sheet display. IEEE Journal of Solid-State Circuits. 2007;**42**:93

[3] Cantatore E, Geuns TCT, Gelinck GH, Veenendaal EV, Gruijthuijsen AFA, Schrijnemakers L, et al. A 13.56 MHz RFID system based on organic transponers. IEEE Journal of Solid-State Circuits. 2007;**42**:84

[4] Brianda D, Opreab A, Courbata J, Barsanb N. Making environmental sensors on plastic foils. Materials Today. 2011;**14**:416

[5] Lodha A, Singh R. Prospects of manufacturing organic semiconductorbased integrated circuits. IEEE Transactions on Semiconductor Manufacturing. Aug 2001;**14**(3)

[6] Petti L et al. Metal oxide semiconductor thin-film transistors for flexible electronics. Applied Physics Reviews. 2016;**3**(2):021303

[7] Yamamoto T, Takimiya K. Facile synthesis of highly *π*-extended heteroarenes, dinaphtho[2,3-b,2<sup>0</sup> ,3'f] chalcogenopheno[3,2-b] chalcogenophenes, and their application to field-effect transistors. Journal of the American Chemical Society. 2007; **129**(8):2224-2225

[8] Jeon J, Murmann B, Bao Z. Fully inkjet-printed short-channel organic thin-film transistors and inverter arrays on flexible substrates. IEEE Electron Device Letters. 2010;**31**:1488

[9] Han CY, Ma YX, Tang WM, Wang XL, Lai PT. A study on pentacene organic thin-film transistor with different gate materials on various substrates. IEEE Electron Device Letters. 2017;**38**(6)

[10] Gundlach DJ, Zhou L, Nichols JA, Jackson TN. An experimental study of contact effects in organic thin film transistors. Journal of Applied Physics. 2006;**100**:024509

[11] Street RA, Salleo A. Contact effects in polymer transistors. Applied Physics Letters. 2002;**81**:2887

[12] Lee BH, Bazan GC, Heeger AJ. Doping-induced carrier density modulation in polymer field-effect transistors. Advanced Materials. 2016; **28**(1):57-62

[13] Nikolka M et al. High operational and environmental stability of highmobility conjugated polymer fieldeffect transistors through the use of molecular additives. Nature Materials. Dec. 2017;**16**:356-362. DOI: 10.1038/ nmat4785

[14] Buonomo A, di Bello C. On solving Poisson's equation in two-dimensional semiconductor devices. Electronics Letters. 1984;**20**:4

[15] Dwivedi ADD. Numerical simulation and spice modeling of organic thin film transistors (OTFTs). International Journal of Advanced Applied Physics Research. 2014;**1**:14-21

[16] Vyas S, Dwivedi ADD, Dwivedi RD. Effect of gate dielectric on the performance of ZnO based thin film transistor. Superlattices and Microstructures. 2018;**120**:223-234

[17] Dwivedi ADD, Dwivedi RD, Dwivedi RD, Vyas S, Chakrabarti P. Numerical simulation of P3HT based organic thin film transistors (OTFTs). International Journal of Microelectronics and Digital Integrated Circuits. 2015;**1**:13-20

[18] Kumari P, Dwivedi ADD. Modeling and simulation of pentacene based organic thin film transistors with organic gate dielectrics. Journal of Microelectronics and Solid State Devices. 2017;**4**:13-18

[19] Kushwah NS, Dwivedi ADD. Computer modeling of organic thin film transistors (OTFTs) using Verilog-A. Journal of Microelectronics and Solid State Devices. 2018;**5**:1-7

[20] Dwivedi ADD, Kumari P. Numerical simulation and characterization of pentacene based organic thin film transistors with top and bottom gate configurations. Global Journal of Research in Engineering-F. 2019;**19**:7-12

[21] Dwivedi ADD, Kumari P. TCAD simulation and performance analysis of single and dual gate OTFTs. Surface Review Letters. 2019;**1950145**:1-7. DOI: 10.1142/S0218625X19501452

[22] Dwivedi ADD, Dwivedi RD, Dwivedi RD, Zhao Q. Technology computer aided design (TCAD) based simulation and compact modeling of organic thin film transistors (OTFTs) for circuit simulation. International Journal of Advanced Applied Physics Research. 2019;**6**:1-5

[23] Fjeldly A, Ytterdal T, Shur M. Introduction to Device Modeling and Circuit Simulation. New York, NY, USA: Wiley; 1998

[24] Zaki T, Scheinert S, Hörselmann I, Rödel R, Letzkus F, Richter H, et al. Accurate capacitance modeling and characterization of organic thin-film transistors. IEEE Transactions on Electron Devices. 2014;**61**:98

[25] Hertel D, Bässler H. Photoconduction in amorphous organic solids. ChemPhysChem. 2008;**9**(5):666

[26] van Roosbroeck W. Theory of the flow of electrons and holes in germanium and other semiconductors. The Bell System Technical Journal. 1950; **29**:560

[27] Juengel A. Drift-Diffusion Equations. Springer; 2009. pp. 99-127. Chap. 5

[28] SILVACO. ATLAS User's Manual— Device Simulation Software. USA; 2010

[29] Miller A, Abrahams E. Impurity conduction at low concentrations. Physics Review. 1960;**120**:745

[30] Vissenberg MCJM, Matters M. Theory of the field-effect mobility in amorphous organic transistors. Physical Review B. 1998;**57**:964

[31] Shim CH, Maruoka F, Hattori R. Structural analysis on organic thin-film transistor with device simulation. IEEE Transactions on Electron Devices. 2010; **57**:195

[32] Iniguez B, Picos R, Veksler D, Koudymov A, Shur MS, Ytterdal T, et al. Universal compact model for long- and short-channel thin-film transistors. Solid-State Electronics. 2008;**52**:400

[33] Estrada M, Cerdeira A, Puigdollers J, Resendiz L, Pallares J, Marsal LF, et al. Accurate modeling and parameter extraction for organic TFTs. Solid-State Electronics. 2005;**49**:1009

[34] UTMOST IV Spice Models Manual. Santa Clara, CA, USA: Silvaco International; 2018

**99**

**Chapter 7**

**Abstract**

are also explored.

**1. Introduction**

large area electronics, roll to roll

Electronics

Smart Manufacturing

Technologies for Printed

*Saleem Khan, Shawkat Ali and Amine Bermak*

Fabrication of electronic devices on different flexible substrates is an area of significant interest due to low cost, ease of fabrication, and manufacturing at ambient conditions over large areas. Over the time, a number of printing technologies have been developed to fabricate a wide range of electronic devices on nonconventional substrates according to the targeted applications. As an increasing interest of electronic industry in printed electronics, further expansion of printed technologies is expected in near future to meet the challenges of the field in terms of scalability, yield, and diversity and biocompatibility. This chapter presents a comprehensive review of various printing electronic technologies commonly used in the fabrication of electronic devices, circuits, and systems. The different printing techniques based on contact/noncontact approach of the printing tools with the target substrates have been explored. These techniques are assessed on the basis of ease of operation, printing resolutions, processability of materials, and ease of optimization of printed structures. The various technical challenges in printing techniques, their solutions with possible alternatives, and the potential research directions are highlighted. The latest developments in assembling various printing tools for enabling high speed and batch manufacturing through roll-to-roll systems

**Keywords:** printed electronic technology, printed electronics, flexible electronics,

Printing electronics is a special type of manufacturing, where electronic components, circuits, and systems are developed on a wide variety of substrates in a similar fashion as drawing text and figures on a paper, textile, and handicrafts [1, 2]. The difference between normal printing and printed electronics is that in printed electronics, functional material is used as ink that exhibits functionalities of insulator, conductor, and semiconductor materials, which are essential for the electronic devices [3]. In most of the fabrication of electronic devices, these materials are sprayed over the substrate with the help of printing technology as low as few nanometers thick (thin film) and few micrometers width (pattern) [4, 5]. The combination of thin films and patterns can make any electronic device to be used in the electronic circuits [6]. Although the process seems simple, but the limitations posed by the various parameters such as uniformly dispersed and stable colloidal

#### **Chapter 7**

organic thin film transistors (OTFTs).

*Hybrid Nanomaterials - Flexible Electronics Materials*

[25] Hertel D, Bässler H.

**29**:560

Chap. 5

**57**:195

Photoconduction in amorphous organic solids. ChemPhysChem. 2008;**9**(5):666

[26] van Roosbroeck W. Theory of the

germanium and other semiconductors. The Bell System Technical Journal. 1950;

Equations. Springer; 2009. pp. 99-127.

[28] SILVACO. ATLAS User's Manual— Device Simulation Software. USA; 2010

[29] Miller A, Abrahams E. Impurity conduction at low concentrations. Physics Review. 1960;**120**:745

[30] Vissenberg MCJM, Matters M. Theory of the field-effect mobility in amorphous organic transistors. Physical

[31] Shim CH, Maruoka F, Hattori R. Structural analysis on organic thin-film transistor with device simulation. IEEE Transactions on Electron Devices. 2010;

[32] Iniguez B, Picos R, Veksler D, Koudymov A, Shur MS, Ytterdal T, et al. Universal compact model for long- and short-channel thin-film transistors. Solid-State Electronics. 2008;**52**:400

[33] Estrada M, Cerdeira A, Puigdollers J, Resendiz L, Pallares J, Marsal LF, et al. Accurate modeling and parameter extraction for organic TFTs. Solid-State

[34] UTMOST IV Spice Models Manual.

Electronics. 2005;**49**:1009

International; 2018

Santa Clara, CA, USA: Silvaco

Review B. 1998;**57**:964

flow of electrons and holes in

[27] Juengel A. Drift-Diffusion

Microelectronics and Digital Integrated

[18] Kumari P, Dwivedi ADD. Modeling and simulation of pentacene based organic thin film transistors with organic gate dielectrics. Journal of Microelectronics and Solid State

International Journal of

Circuits. 2015;**1**:13-20

Devices. 2017;**4**:13-18

State Devices. 2018;**5**:1-7

2019;**19**:7-12

[20] Dwivedi ADD, Kumari P. Numerical simulation and

characterization of pentacene based organic thin film transistors with top and bottom gate configurations. Global Journal of Research in Engineering-F.

[21] Dwivedi ADD, Kumari P. TCAD simulation and performance analysis of single and dual gate OTFTs. Surface Review Letters. 2019;**1950145**:1-7. DOI:

10.1142/S0218625X19501452

Research. 2019;**6**:1-5

Wiley; 1998

**98**

[22] Dwivedi ADD, Dwivedi RD, Dwivedi RD, Zhao Q. Technology computer aided design (TCAD) based simulation and compact modeling of organic thin film transistors (OTFTs) for circuit simulation. International Journal of Advanced Applied Physics

[23] Fjeldly A, Ytterdal T, Shur M. Introduction to Device Modeling and Circuit Simulation. New York, NY, USA:

[24] Zaki T, Scheinert S, Hörselmann I, Rödel R, Letzkus F, Richter H, et al. Accurate capacitance modeling and characterization of organic thin-film transistors. IEEE Transactions on Electron Devices. 2014;**61**:98

[19] Kushwah NS, Dwivedi ADD. Computer modeling of organic thin film transistors (OTFTs) using Verilog-A. Journal of Microelectronics and Solid

## Smart Manufacturing Technologies for Printed Electronics

*Saleem Khan, Shawkat Ali and Amine Bermak*

#### **Abstract**

Fabrication of electronic devices on different flexible substrates is an area of significant interest due to low cost, ease of fabrication, and manufacturing at ambient conditions over large areas. Over the time, a number of printing technologies have been developed to fabricate a wide range of electronic devices on nonconventional substrates according to the targeted applications. As an increasing interest of electronic industry in printed electronics, further expansion of printed technologies is expected in near future to meet the challenges of the field in terms of scalability, yield, and diversity and biocompatibility. This chapter presents a comprehensive review of various printing electronic technologies commonly used in the fabrication of electronic devices, circuits, and systems. The different printing techniques based on contact/noncontact approach of the printing tools with the target substrates have been explored. These techniques are assessed on the basis of ease of operation, printing resolutions, processability of materials, and ease of optimization of printed structures. The various technical challenges in printing techniques, their solutions with possible alternatives, and the potential research directions are highlighted. The latest developments in assembling various printing tools for enabling high speed and batch manufacturing through roll-to-roll systems are also explored.

**Keywords:** printed electronic technology, printed electronics, flexible electronics, large area electronics, roll to roll

#### **1. Introduction**

Printing electronics is a special type of manufacturing, where electronic components, circuits, and systems are developed on a wide variety of substrates in a similar fashion as drawing text and figures on a paper, textile, and handicrafts [1, 2]. The difference between normal printing and printed electronics is that in printed electronics, functional material is used as ink that exhibits functionalities of insulator, conductor, and semiconductor materials, which are essential for the electronic devices [3]. In most of the fabrication of electronic devices, these materials are sprayed over the substrate with the help of printing technology as low as few nanometers thick (thin film) and few micrometers width (pattern) [4, 5]. The combination of thin films and patterns can make any electronic device to be used in the electronic circuits [6]. Although the process seems simple, but the limitations posed by the various parameters such as uniformly dispersed and stable colloidal

solutions, substrate treatments, and above all, optimized printing recipes make the printing process much more challenging [7, 8]. The prominent challenges in printed electronics are material compatibility, substrate surface energy, viscosity of the materials' solution, and compatibility of dissimilar materials in multilayer structure, technology limitations in terms of film thickness, width, and height. As compared to conventional electronic manufacturing, printing technologies are revolutionizing the incredible field of flexible/bendable electronics by providing cost-effective routes for processing diverse electronic materials on nonplanar substrates at compatible temperatures [9–12]. Simplified processing steps, reduced materials' wastage, low-fabrication costs, and simple patterning techniques make printing technologies very attractive when compared to standard microfabrication in clean room processes. Attractive features of the printed electronics have allowed researchers to explore new avenues for material processing to develop electronic devices, circuits, and systems on such surfaces, which are difficult to realize with the conventional wafer-based fabrication techniques [13, 14]. In accordance with the electronic industry roadmap, the research in this field is slowly inching toward a merge of well-established microelectronics and the age-old printing technologies. Traditionally, prepatterned parts of a printing module are brought in conformal contact to the target flexible (nonflexible) substrates, resulting in the transfer of functional inks/solutions on target surfaces [15–21]. The mechanisms to transfer inks/solutions on target substrates divide printing technologies into two main streams, where the surfaces come in physical contact in one, and in the second approach, ink is deposited through noncontacting the surfaces. The different printing technologies based on contact/noncontact approach are summarized in **Figure 1**. In contact printing process, the patterned structures with inked surfaces are interfaced physically at controlled pressures with the target substrate.

In noncontact process, the solution is dispensed through the openings or nozzles, and structures are defined by moving the substrate holder in a preprogrammed pattern. The contact-based printing technologies consist of gravure printing, gravure-offset printing, screen printing, flexographic printing, microcontact printing, nanoimprint, and dry transfer printing. The prominent noncontact printing techniques include slot-die coating, electrohydrodynamics, and inkjet printing. The noncontact printing techniques have received greater attractions due to their distinct capabilities such as simplicity, affordability, speed, adaptability to the fabrication process, reduced material wastage, high resolution of patterns, and easy control by adjusting few process parameters [6, 21–27].

**101**

**Figure 2.**

*Droplet's actuation mechanism of inkjet material printer.*

*Smart Manufacturing Technologies for Printed Electronics*

The fascinating field of printed electronics is enabled by the rapid developments in printing technologies for precise deposition of functional inks in easy and cost-efficient way. The ultimate goal of developing the printed electronic technology is to revolutionize the device manufacturing and maximize the throughput by covering areas larger than wafer scales as well as increase the production through roll-to-roll processes. The attraction of printed electronic technology is that it can be executed at ambient conditions, thus enabling the fabrication of biocompatible electronics. With the help of printed electronic technologies and biocompatible materials, it is possible to fabricate electronic device on plastic substrates and even on human skin. The most commonly used and reliable printed electronic technologies are explained in the below sections.

Inkjet printing has gained a significant interest in recent years for processing solution-based nanomaterials and patterning on diverse substrates in a single step. Nanoparticles of the functional materials are mixed in compatible solvents to prepare printable ink. Besides, chemical-based solutions are also prepared and adjusted to the jetting parameters of the inkjet printing systems. Materials are ejected in micrometersized droplets through miniaturized nozzle printheads. Mainly two mechanisms for the actuation of inkjet nozzle head have been developed, i.e., thermal and piezoelectric. Droplets in very small diameters are ejected at each corresponding pulse and generated by either thermal or piezoelectric actuators used in the inkjet nozzle head. Schematic in **Figure 2** shows mechanism of droplet ejection through piezoelectric/ thermal inkjet material printer. Typical nozzle diameters used for inkjet heads range

The actuating element is either thermal or piezoelectric, in case of thermal actuator, a filament is embedded in the nozzle head, and upon the voltage application, a bubble is generated in the nozzle that pushes ink to the nozzle tip and generates droplets. In case of piezoelectric element, piezoelectric crystal is placed inside the nozzle, and upon the voltage application, piezoelectric crystal vibrates, and as a result, pressure exerts on the ink inside the nozzle and droplet generates. **Figure 2** shows the droplet actuation mechanism of inkjet material printer; ink is filled in the reservoir,

from 10 to 150 μm and reservoir ink with a volume capacity of 3 ml.

*DOI: http://dx.doi.org/10.5772/intechopen.89377*

**2. Printed electronic technologies**

**2.1 Noncontact printing technologies**

*2.1.1 Inkjet material printer*

**Figure 1.**

*The classification of common printing technologies.*

#### **2. Printed electronic technologies**

*Hybrid Nanomaterials - Flexible Electronics Materials*

solutions, substrate treatments, and above all, optimized printing recipes make the printing process much more challenging [7, 8]. The prominent challenges in printed electronics are material compatibility, substrate surface energy, viscosity of the materials' solution, and compatibility of dissimilar materials in multilayer structure, technology limitations in terms of film thickness, width, and height. As compared to conventional electronic manufacturing, printing technologies are revolutionizing the incredible field of flexible/bendable electronics by providing cost-effective routes for processing diverse electronic materials on nonplanar substrates at compatible temperatures [9–12]. Simplified processing steps, reduced materials' wastage, low-fabrication costs, and simple patterning techniques make printing technologies very attractive when compared to standard microfabrication in clean room processes. Attractive features of the printed electronics have allowed researchers to explore new avenues for material processing to develop electronic devices, circuits, and systems on such surfaces, which are difficult to realize with the conventional wafer-based fabrication techniques [13, 14]. In accordance with the electronic industry roadmap, the research in this field is slowly inching toward a merge of well-established microelectronics and the age-old printing technologies. Traditionally, prepatterned parts of a printing module are brought in conformal contact to the target flexible (nonflexible) substrates, resulting in the transfer of functional inks/solutions on target surfaces [15–21]. The mechanisms to transfer inks/solutions on target substrates divide printing technologies into two main streams, where the surfaces come in physical contact in one, and in the second approach, ink is deposited through noncontacting the surfaces. The different printing technologies based on contact/noncontact approach are summarized in **Figure 1**. In contact printing process, the patterned structures with inked surfaces

are interfaced physically at controlled pressures with the target substrate.

control by adjusting few process parameters [6, 21–27].

In noncontact process, the solution is dispensed through the openings or nozzles, and structures are defined by moving the substrate holder in a preprogrammed pattern. The contact-based printing technologies consist of gravure printing, gravure-offset printing, screen printing, flexographic printing, microcontact printing, nanoimprint, and dry transfer printing. The prominent noncontact printing techniques include slot-die coating, electrohydrodynamics, and inkjet printing. The noncontact printing techniques have received greater attractions due to their distinct capabilities such as simplicity, affordability, speed, adaptability to the fabrication process, reduced material wastage, high resolution of patterns, and easy

**100**

**Figure 1.**

*The classification of common printing technologies.*

The fascinating field of printed electronics is enabled by the rapid developments in printing technologies for precise deposition of functional inks in easy and cost-efficient way. The ultimate goal of developing the printed electronic technology is to revolutionize the device manufacturing and maximize the throughput by covering areas larger than wafer scales as well as increase the production through roll-to-roll processes. The attraction of printed electronic technology is that it can be executed at ambient conditions, thus enabling the fabrication of biocompatible electronics. With the help of printed electronic technologies and biocompatible materials, it is possible to fabricate electronic device on plastic substrates and even on human skin. The most commonly used and reliable printed electronic technologies are explained in the below sections.

#### **2.1 Noncontact printing technologies**

#### *2.1.1 Inkjet material printer*

Inkjet printing has gained a significant interest in recent years for processing solution-based nanomaterials and patterning on diverse substrates in a single step. Nanoparticles of the functional materials are mixed in compatible solvents to prepare printable ink. Besides, chemical-based solutions are also prepared and adjusted to the jetting parameters of the inkjet printing systems. Materials are ejected in micrometersized droplets through miniaturized nozzle printheads. Mainly two mechanisms for the actuation of inkjet nozzle head have been developed, i.e., thermal and piezoelectric. Droplets in very small diameters are ejected at each corresponding pulse and generated by either thermal or piezoelectric actuators used in the inkjet nozzle head. Schematic in **Figure 2** shows mechanism of droplet ejection through piezoelectric/ thermal inkjet material printer. Typical nozzle diameters used for inkjet heads range from 10 to 150 μm and reservoir ink with a volume capacity of 3 ml.

The actuating element is either thermal or piezoelectric, in case of thermal actuator, a filament is embedded in the nozzle head, and upon the voltage application, a bubble is generated in the nozzle that pushes ink to the nozzle tip and generates droplets. In case of piezoelectric element, piezoelectric crystal is placed inside the nozzle, and upon the voltage application, piezoelectric crystal vibrates, and as a result, pressure exerts on the ink inside the nozzle and droplet generates. **Figure 2** shows the droplet actuation mechanism of inkjet material printer; ink is filled in the reservoir,

**Figure 2.** *Droplet's actuation mechanism of inkjet material printer.*

#### **Figure 3.** *Continuous and drop-on-demand inkjet systems.*

and driving signal is applied to the actuating element that generates droplets according to the designed geometry. Inkjet printers are further divided into two types as continues inkjet (CIJ) and drop on demand (DoD) systems. In continuous inkjet printer, the ink is stored in a reservoir supplied to the nozzles. A charging electrode is used to pull ink out of the nozzle and form a droplet. After droplet formation, deflection plates are used to direct the droplets on the targeted area on the substrate. This kind of printing system is used in the industry for printing on the packages such as expiry date commonly seen on boxes and drinks of liquor bottles. As the droplets are not controlled by any input signal, the CIJ system continuously produces droplets with certain frequency; hence, the unused droplets are directed to the recycling system, where the ink is stored and reused. Schematic diagram of the CIJ system is shown in **Figure 3**. On the other hand, DoD system is controlled by a digital input signal that comes from the design of the pattern. In DoD system, each droplet is generated on demand; hence, there is no need of deflection and charge electrodes. The substrate is kept perpendicular to the nozzle, and droplets sit on the substrate one by one. The substrate is moved separately through a control system synchronized with the droplet generation in order to make a pattern or thin film on the substrate. The number of nozzles in commercial inkjet printhead ranges from 1 to 128 with ink droplets as small as 1.0 pL.

#### *2.1.2 Electrohydrodynamic (EHD) printing*

Electrohydrodynamics (EHD) is another interesting type of inkjet printing systems used to deposit functional ink in the form of thin films as well as high-resolution patterns. It is consisted of high-speed camera, light source, nozzle and head, ink storage and supply mechanism, stage movement, and activity display unit as shown in **Figure 4**. Working principle of the EHD system is that the ink to be deposited on substrate is pumped from ink storage tank to nozzle with appropriate ink flow rate to make stable cone jet [28]. Positive voltage is supplied to the nozzle, and ground is connected with substrate holder. The induction of the surface charges on the pendent meniscus emerging at the nozzle outlet results in an electric stress over the ink surface. If the electric field and flow rate are in some operating range, then this will overcome the surface tension stress over the ink surface and results in deformation of the droplet at the orifice of the nozzle into a conical shape. While sweeping the high voltage from zero to a required value, different modes of the spray occur including dripping, unstable, stable, and multi jet mode as shown in **Figure 4**. For stable cone

**103**

**Figure 5.**

*Smart Manufacturing Technologies for Printed Electronics*

*Electrohydrodynamic system and different modes of jetting ink.*

jet, appropriate voltage and flow rate are required, which result in uniform spray. The tangential electric field acting on the surface of the ink cone a thin jet emanates at the cone apex which further breaks up into a number of small droplets under the effect of coulomb forces [29–31]. In the stable cone jet mode, EHD is ideal for printing and can be used for thin films or pattern deposition on various substrates. The cone jet expands at the substrate sides and makes spray as the distance between nozzle and substrate, i.e., standoff is increased [32]. This distance needs to be optimized to achieve stable cone jet desired for thin film deposition [5]. Whereas, during small

Aerosol jet printing is an interesting technique in noncontact printing, which has attracted a significant interest in the manufacturing of high-resolution pattering. A wide variety of materials including insulators, semiconductors, and metallic conductors are processed in viscosity ranges of 1–1000 cps [1, 33]. The aerosol process is driven by the gas flows where a mist of microdroplets is generated as a result of pneumatic atomization or through ultrasonication. The capability to process a wide variety of materials and to pattern higher resolutions, as high as 10 μm on diverse substrates, makes aerosol jet printing most attractive among other contact-less printing techniques. Aerosol jet printing is divided into two main categories, i.e., pneumatic and ultrasonic, as shown in **Figure 5**. Both the techniques are based on different operational procedures and are used targeting specific set of requirements

standoff distance, cone jet is narrow and used for patterning [11, 21].

*Schematics of (a) pneumatic and (b) ultrasonic aerosol jet printing systems.*

*2.1.3 Aerosol jet printing*

**Figure 4.**

*DOI: http://dx.doi.org/10.5772/intechopen.89377*

*Smart Manufacturing Technologies for Printed Electronics DOI: http://dx.doi.org/10.5772/intechopen.89377*

#### **Figure 4.**

*Hybrid Nanomaterials - Flexible Electronics Materials*

and driving signal is applied to the actuating element that generates droplets according to the designed geometry. Inkjet printers are further divided into two types as continues inkjet (CIJ) and drop on demand (DoD) systems. In continuous inkjet printer, the ink is stored in a reservoir supplied to the nozzles. A charging electrode is used to pull ink out of the nozzle and form a droplet. After droplet formation, deflection plates are used to direct the droplets on the targeted area on the substrate. This kind of printing system is used in the industry for printing on the packages such as expiry date commonly seen on boxes and drinks of liquor bottles. As the droplets are not controlled by any input signal, the CIJ system continuously produces droplets with certain frequency; hence, the unused droplets are directed to the recycling system, where the ink is stored and reused. Schematic diagram of the CIJ system is shown in **Figure 3**. On the other hand, DoD system is controlled by a digital input signal that comes from the design of the pattern. In DoD system, each droplet is generated on demand; hence, there is no need of deflection and charge electrodes. The substrate is kept perpendicular to the nozzle, and droplets sit on the substrate one by one. The substrate is moved separately through a control system synchronized with the droplet generation in order to make a pattern or thin film on the substrate. The number of nozzles in commercial

inkjet printhead ranges from 1 to 128 with ink droplets as small as 1.0 pL.

Electrohydrodynamics (EHD) is another interesting type of inkjet printing systems used to deposit functional ink in the form of thin films as well as high-resolution patterns. It is consisted of high-speed camera, light source, nozzle and head, ink storage and supply mechanism, stage movement, and activity display unit as shown in **Figure 4**. Working principle of the EHD system is that the ink to be deposited on substrate is pumped from ink storage tank to nozzle with appropriate ink flow rate to make stable cone jet [28]. Positive voltage is supplied to the nozzle, and ground is connected with substrate holder. The induction of the surface charges on the pendent meniscus emerging at the nozzle outlet results in an electric stress over the ink surface. If the electric field and flow rate are in some operating range, then this will overcome the surface tension stress over the ink surface and results in deformation of the droplet at the orifice of the nozzle into a conical shape. While sweeping the high voltage from zero to a required value, different modes of the spray occur including dripping, unstable, stable, and multi jet mode as shown in **Figure 4**. For stable cone

*2.1.2 Electrohydrodynamic (EHD) printing*

*Continuous and drop-on-demand inkjet systems.*

**102**

**Figure 3.**

*Electrohydrodynamic system and different modes of jetting ink.*

jet, appropriate voltage and flow rate are required, which result in uniform spray. The tangential electric field acting on the surface of the ink cone a thin jet emanates at the cone apex which further breaks up into a number of small droplets under the effect of coulomb forces [29–31]. In the stable cone jet mode, EHD is ideal for printing and can be used for thin films or pattern deposition on various substrates. The cone jet expands at the substrate sides and makes spray as the distance between nozzle and substrate, i.e., standoff is increased [32]. This distance needs to be optimized to achieve stable cone jet desired for thin film deposition [5]. Whereas, during small standoff distance, cone jet is narrow and used for patterning [11, 21].

#### *2.1.3 Aerosol jet printing*

Aerosol jet printing is an interesting technique in noncontact printing, which has attracted a significant interest in the manufacturing of high-resolution pattering. A wide variety of materials including insulators, semiconductors, and metallic conductors are processed in viscosity ranges of 1–1000 cps [1, 33]. The aerosol process is driven by the gas flows where a mist of microdroplets is generated as a result of pneumatic atomization or through ultrasonication. The capability to process a wide variety of materials and to pattern higher resolutions, as high as 10 μm on diverse substrates, makes aerosol jet printing most attractive among other contact-less printing techniques. Aerosol jet printing is divided into two main categories, i.e., pneumatic and ultrasonic, as shown in **Figure 5**. Both the techniques are based on different operational procedures and are used targeting specific set of requirements

*Schematics of (a) pneumatic and (b) ultrasonic aerosol jet printing systems.*

and goals [1, 34]. For instance, in pneumatic atomizer, aerosol mist is generated by supplying pressurized air/gas inside a closed chamber containing the ink, which results in the generation of microdroplets close at the ink and air interface. The microdroplets ranging in ≤5 μm diameter are entrained in the supplied gas and are driven toward the nozzle printhead. Whereas in the second case, the ink contained in a delicate vial is subjected to ultrasonication, and microdroplets are generated as a result of ultrasonic pressure waves. The microdroplets in the size ranges of ≤5 μm are entrained in the atomizer gas, which is driven toward the nozzle printhead. Another accompanying gas, i.e., sheath gas flow along with the nozzle orifice size, leads to define the pattern size to be printed on the target substrate. Pneumatic atomizer requires more materials, i.e., more than 10 mL, and is usually used for wide area printing. On the other hand, ultrasonic aerosol system requires about 0.5 mL of solution and can be used for printing at very high-resolution patterns, i.e., down to 10 μm, for instance. The annular gas flow reduces the size of the mist inside the carrier tube, which is accompanied by the sheath gas at the nozzle printhead. This sheath gas flow further converges the aerosol stream and reaches the surface at higher impact. The aerosol mist is ejected in the form of an intact jet, and the stage speed adjusted in such a way that the desired patterns are deposited in a continuous fashion.

#### *2.1.4 Slot-die coater*

Slot die is a special deposition technique, where the material is printed on a moving substrate and installed directly on a roll-to-roll system, as shown in **Figure 6**. The solution is directly dispensed off the slot-die printhead in a controlled manner. The slot-die coating is executed in two steps, where a uniform and stable flow of the coated material is achieved in the first phase. In the second phase, other processing parameters such as standoff distance between the slot-die opening and the target substrate speed of the rolling substrate and sintering conditions inline to promote a multilayer structure coating capability [35, 36]. System is connected to a continuous flow of materials into a temporary reservoir at the printhead and applied at appropriate amount on the rolling substrate as shown in **Figure 6**. This type of coating technique is ideal for large area deposition, such as solar cells and light emitting diodes, where high-resolution patterning is not required. Besides optimizing the rheological properties of the ink/solution, system specific parameters such as speed of the roll play a significant role in establishing the process in safe and stable operating mode. The coating process can also be affected by various defects such as uncontrolled

**105**

*Smart Manufacturing Technologies for Printed Electronics*

result in deviating from device real dimensions [17, 37, 38].

techniques are used by the industry for the mass productions.

meniscus, resulting in dripping out of the solution, air bubbles entraps, and ribbing. Proper control at the start-up and shut-down cycles is highly demanding, as minor deviations could lead to deposition of the materials at unwanted positions on the substrate. Proper tuning of the material and process parameters is needed; otherwise, it would result in the wastage of the coating solution and also compromising on the shape of the patterns. This could also significantly affect the thin film quality and

Materials' properties such as viscosity and surface tension along with the system parameters, i.e., slot size and gap, standoff distance, and decreased dip lip length, reduce the dimension of the printing jet. Proper adjustment of these parameters leads to shortening the trial time required to reach the steady-state conditions [38]. Despite the high-speed coating capabilities, the challenges involved in reaching stable operating conditions make the process less attractive than other printing systems and are adopted seldomly in the manufacturing of printed electronic devices.

In contact-based printing techniques, prepatterned structures of the printing tools are physically brought in conformal contact to the target substrate. Similarly, micron-scale dispensing nozzles are also used for high-resolution patterning by contacting the target surface in a similar fashion as drawing. Almost all the techniques used in contact-based printing are precise and rapid; therefore, these

Screen printing remains the top priority when it comes to rapid, fast, and large area manufacturing. The technology has been using from the early developmental stages of microelectronic industry, especially for printing electrodes and interconnections. Screen printing is advantageous when compared to other printing systems as it is more versatile, and processing is simple, capable of reproducing similar structures in large batches with minimum dimensional variations. Results are duplicated by repeating the similar printing parameters and optimized solution pastes [12, 39]. Screen printing process can be established following the two different assemblies, i.e., in flatbed and installing a rotating surface [23, 40]. Flatbed systems are usually employed for low-throughput production and lab-level research activities. Whereas rotary systems are installed on a fast production platforms such as R2R, where all the solution and printing parameters are optimized first and applied afterward for high-speed production of devices. **Figure 7** shows schematics of flatbed and rotary screen printing systems. The system setup is simple, which contains screen, squeegee, press bed, and substrates placed on the movable stage. In flatbed systems, the printable solution is applied on the screen, and a squeegee is used to coat the ink all the way over the structured mesh on the stencil. A controlled pressure ensures the dispensing of ink through the holes in the mesh, and the squeegee recollects the extra ink for the consequent layers. Flatbed screen printers are ideal for the optimization of the process and structures on a small-scale lab level research. For high-speed production, a folded screen with squeegee inside the rotating and ink filled in a tube are applied on a continuously rotating system. However, the screens for rotary screen systems are expansive and challenging to clean in case of material clogging [23, 41]. The print quality in both the approaches is affected greatly by the similar set of parameters such as solution viscosity, print speed, angle and geometry of the squeegee, standoff between screen and substrate, and mesh size [42–44]. Material properties are tuned separately to have the right viscosities and surface tension for complete dispensing through the screens.

*DOI: http://dx.doi.org/10.5772/intechopen.89377*

**2.2 Contact-based printing technologies**

*2.2.1 Screen printing*

**Figure 6.** *Slot-die coater schematic and slot-die coater in operation diagram.*

#### *Smart Manufacturing Technologies for Printed Electronics DOI: http://dx.doi.org/10.5772/intechopen.89377*

*Hybrid Nanomaterials - Flexible Electronics Materials*

*2.1.4 Slot-die coater*

and goals [1, 34]. For instance, in pneumatic atomizer, aerosol mist is generated by supplying pressurized air/gas inside a closed chamber containing the ink, which results in the generation of microdroplets close at the ink and air interface. The microdroplets ranging in ≤5 μm diameter are entrained in the supplied gas and are driven toward the nozzle printhead. Whereas in the second case, the ink contained in a delicate vial is subjected to ultrasonication, and microdroplets are generated as a result of ultrasonic pressure waves. The microdroplets in the size ranges of ≤5 μm are entrained in the atomizer gas, which is driven toward the nozzle printhead. Another accompanying gas, i.e., sheath gas flow along with the nozzle orifice size, leads to define the pattern size to be printed on the target substrate. Pneumatic atomizer requires more materials, i.e., more than 10 mL, and is usually used for wide area printing. On the other hand, ultrasonic aerosol system requires about 0.5 mL of solution and can be used for printing at very high-resolution patterns, i.e., down to 10 μm, for instance. The annular gas flow reduces the size of the mist inside the carrier tube, which is accompanied by the sheath gas at the nozzle printhead. This sheath gas flow further converges the aerosol stream and reaches the surface at higher impact. The aerosol mist is ejected in the form of an intact jet, and the stage speed adjusted in such a way that the desired patterns are deposited in a continuous fashion.

Slot die is a special deposition technique, where the material is printed on a moving substrate and installed directly on a roll-to-roll system, as shown in **Figure 6**. The solution is directly dispensed off the slot-die printhead in a controlled manner. The slot-die coating is executed in two steps, where a uniform and stable flow of the coated material is achieved in the first phase. In the second phase, other processing parameters such as standoff distance between the slot-die opening and the target substrate speed of the rolling substrate and sintering conditions inline to promote a multilayer structure coating capability [35, 36]. System is connected to a continuous flow of materials into a temporary reservoir at the printhead and applied at appropriate amount on the rolling substrate as shown in **Figure 6**. This type of coating technique is ideal for large area deposition, such as solar cells and light emitting diodes, where high-resolution patterning is not required. Besides optimizing the rheological properties of the ink/solution, system specific parameters such as speed of the roll play a significant role in establishing the process in safe and stable operating mode. The coating process can also be affected by various defects such as uncontrolled

**104**

**Figure 6.**

*Slot-die coater schematic and slot-die coater in operation diagram.*

meniscus, resulting in dripping out of the solution, air bubbles entraps, and ribbing. Proper control at the start-up and shut-down cycles is highly demanding, as minor deviations could lead to deposition of the materials at unwanted positions on the substrate. Proper tuning of the material and process parameters is needed; otherwise, it would result in the wastage of the coating solution and also compromising on the shape of the patterns. This could also significantly affect the thin film quality and result in deviating from device real dimensions [17, 37, 38].

Materials' properties such as viscosity and surface tension along with the system parameters, i.e., slot size and gap, standoff distance, and decreased dip lip length, reduce the dimension of the printing jet. Proper adjustment of these parameters leads to shortening the trial time required to reach the steady-state conditions [38]. Despite the high-speed coating capabilities, the challenges involved in reaching stable operating conditions make the process less attractive than other printing systems and are adopted seldomly in the manufacturing of printed electronic devices.

#### **2.2 Contact-based printing technologies**

In contact-based printing techniques, prepatterned structures of the printing tools are physically brought in conformal contact to the target substrate. Similarly, micron-scale dispensing nozzles are also used for high-resolution patterning by contacting the target surface in a similar fashion as drawing. Almost all the techniques used in contact-based printing are precise and rapid; therefore, these techniques are used by the industry for the mass productions.

#### *2.2.1 Screen printing*

Screen printing remains the top priority when it comes to rapid, fast, and large area manufacturing. The technology has been using from the early developmental stages of microelectronic industry, especially for printing electrodes and interconnections. Screen printing is advantageous when compared to other printing systems as it is more versatile, and processing is simple, capable of reproducing similar structures in large batches with minimum dimensional variations. Results are duplicated by repeating the similar printing parameters and optimized solution pastes [12, 39].

Screen printing process can be established following the two different assemblies, i.e., in flatbed and installing a rotating surface [23, 40]. Flatbed systems are usually employed for low-throughput production and lab-level research activities. Whereas rotary systems are installed on a fast production platforms such as R2R, where all the solution and printing parameters are optimized first and applied afterward for high-speed production of devices. **Figure 7** shows schematics of flatbed and rotary screen printing systems. The system setup is simple, which contains screen, squeegee, press bed, and substrates placed on the movable stage. In flatbed systems, the printable solution is applied on the screen, and a squeegee is used to coat the ink all the way over the structured mesh on the stencil. A controlled pressure ensures the dispensing of ink through the holes in the mesh, and the squeegee recollects the extra ink for the consequent layers. Flatbed screen printers are ideal for the optimization of the process and structures on a small-scale lab level research. For high-speed production, a folded screen with squeegee inside the rotating and ink filled in a tube are applied on a continuously rotating system. However, the screens for rotary screen systems are expansive and challenging to clean in case of material clogging [23, 41]. The print quality in both the approaches is affected greatly by the similar set of parameters such as solution viscosity, print speed, angle and geometry of the squeegee, standoff between screen and substrate, and mesh size [42–44]. Material properties are tuned separately to have the right viscosities and surface tension for complete dispensing through the screens.

**Figure 7.** *Flatbed and rotary screen printing systems.*

Viscosity of the pastes used with screen printing is kept higher than other conventional printing technologies in order to avoid undesired flow through the screen masks [27, 45]. Screen printing is used for both printing patterned structures and coating larger areas [46]. Pattern resolution in the ranges of 80–100 μm can be achieved after proper tuning the solution properties and optimizing the screen printing parameters. A good compromise between the surface energies of the substrate and the surface tension of the ink is desired to achieve higher resolutions [45, 47, 48]. The type, material, and strength of mesh used in the screen also contribute to the high-resolution patterning through screen printing. Different materials such as nylon, polyester, and stainless steel are used in the screen mesh. For printing stability during mass production, a screen made of stainless steel mesh with three times more in strength than conventional stainless steel mesh has also been developed [43, 49]. The possibility of printing relatively thick layers could enable printing of low-resistance structures, also with conducting polymers, by compensating the high-volume resistivity with a thicker layer [6].

Screen printing has been successfully used for demonstrating various fabricated devices. For instance, an all-screen printing has been adopted for developing thin film transistors (TFTs) [50–52]. OLED devices are also presented by exploring the different process parameters, such as viscosity and mesh count, and their effect on the printed structures [43]. An advanced screen-printing approach is adopted to develop multilayer high-density flexible electronic circuits connected through holes with embedded passive and optical devices [42]. Printing interconnect lines between discrete devices on a same substrate or tape out for data reading are usually printed with screen printing. Screen printed electrical interconnects for temperature sensor on PET substrate are reported by Shi et al. [48]. Screen printing does not require high-capital investment like many other manufacturing techniques, and setup can be established at lower installation costs. A high-speed production line can be established by assembling rotary printing setup along with supplemental methods such as inkjet, vapor deposition, and laser/flash photonic sintering tools [49, 53]. Although the advantages and attractions using screen printing are higher, there are also some challenges that need to be addressed. Printing multiple layered structures, higher wet thicknesses, and exposure of the ink to ambient environment while remaining on the screen bed present more risks for developing repeatable and reliable electronic devices. The rapid evaporation of solvents and surfactants from the printing paste, when the system is idle and ink applied on the screen lead to blockage of the screen masks [54]. Therefore, proper tuning of the material properties and developing reliable procedures are highly desired for developing a repeatable printing recipe.

**107**

**Figure 8.**

*Schematic of a typical standard and microgravure-offset printing process.*

*Smart Manufacturing Technologies for Printed Electronics*

Gravure printing is the most prominent and representative technique in the contact-based printing category. Structures are transferred through a prepatterned surface, where the solvent is deposited on the target surface upon contacting. The engraved structures are designed in cylindrical shaped objects, and substrate controlled through moving rolls of the system represents a typical R2R process. The gravure printing tools consist of a large cylinder electroplated with copper and engraved with microcells, as shown in **Figure 8**. The microcells are engraved by either using electromechanical means or using laser [23, 26, 41, 55]. The physical contact between the printing surface and the substrate leads to wear and tear of the engraved structure; therefore, chrome electroplating is performed for protecting it from deterioration. The size of engraved microcells is responsible for ink pickup from the reservoir lying beneath the cylinder or filled with dispensing nozzles from the top. The extra ink is removed from the surface using a doctor blade to avoid

cross contamination or unwanted deposition of ink on the substrate.

Surface properties of the substrate and rheological properties of ink are tuned to promote the efficient deposition. Capillary action of the ink plays a significant role in complete transfer of the ink from microcells of the engraved cylinder to the substrate. Pressure on the impression cylinder is also properly controlled to expedite the ink transfer and to reduce the deterioration of contacting surfaces. Width and depth ratio of the microcells in the engraved cylinder also plays a significant role in manipulating the ink transfer [41]. Viscosity and surface tension of the desired material solution have to be in acceptable ranges to prevent the bleeding out of the solution from the microcells. The optimal viscosities help in rapid prototyping by increasing the printing speed and allow full emptying of the ink from engraved

An advanced version of gravure printing, i.e., gravure offset uses an extra elastic blanket to avoid potential risks occurring from the deteriorations of the contacting surfaces, as shown in **Figure 8**. The elastic blanket serves as an intermediate step between the contacting surfaces, where the ink from engraved microcells is picked up by the blanket and transferred finally on the target substrate. This avoids the direct interface between the patterned cylinder and the substrate. Few of the printing parameters affecting significantly the print quality are speed, pressure, and blanket's dimensions plus thickness. Surface properties of the blanket and its thickness combined with the speed are more dominant factors that control and enhance the efficient transfer of the ink [16, 41, 57]. The limited time of contact and

*DOI: http://dx.doi.org/10.5772/intechopen.89377*

*2.2.2 Gravure printing*

microcells [56].

#### *2.2.2 Gravure printing*

*Hybrid Nanomaterials - Flexible Electronics Materials*

Viscosity of the pastes used with screen printing is kept higher than other conventional printing technologies in order to avoid undesired flow through the screen masks [27, 45]. Screen printing is used for both printing patterned structures and coating larger areas [46]. Pattern resolution in the ranges of 80–100 μm can be achieved after proper tuning the solution properties and optimizing the screen printing parameters. A good compromise between the surface energies of the substrate and the surface tension of the ink is desired to achieve higher resolutions [45, 47, 48]. The type, material, and strength of mesh used in the screen also contribute to the high-resolution patterning through screen printing. Different materials such as nylon, polyester, and stainless steel are used in the screen mesh. For printing stability during mass production, a screen made of stainless steel mesh with three times more in strength than conventional stainless steel mesh has also been developed [43, 49]. The possibility of printing relatively thick layers could enable printing of low-resistance structures, also with conducting polymers, by compensating the high-volume resistivity with a thicker layer [6].

Screen printing has been successfully used for demonstrating various fabricated devices. For instance, an all-screen printing has been adopted for developing thin film transistors (TFTs) [50–52]. OLED devices are also presented by exploring the different process parameters, such as viscosity and mesh count, and their effect on the printed structures [43]. An advanced screen-printing approach is adopted to develop multilayer high-density flexible electronic circuits connected through holes with embedded passive and optical devices [42]. Printing interconnect lines between discrete devices on a same substrate or tape out for data reading are usually printed with screen printing. Screen printed electrical interconnects for temperature sensor on PET substrate are reported by Shi et al. [48]. Screen printing does not require high-capital investment like many other manufacturing techniques, and setup can be established at lower installation costs. A high-speed production line can be established by assembling rotary printing setup along with supplemental methods such as inkjet, vapor deposition, and laser/flash photonic sintering tools [49, 53]. Although the advantages and attractions using screen printing are higher, there are also some challenges that need to be addressed. Printing multiple layered structures, higher wet thicknesses, and exposure of the ink to ambient environment while remaining on the screen bed present more risks for developing repeatable and reliable electronic devices. The rapid evaporation of solvents and surfactants from the printing paste, when the system is idle and ink applied on the screen lead to blockage of the screen masks [54]. Therefore, proper tuning of the material properties and developing reliable procedures are highly desired for developing a repeatable printing recipe.

**106**

**Figure 7.**

*Flatbed and rotary screen printing systems.*

Gravure printing is the most prominent and representative technique in the contact-based printing category. Structures are transferred through a prepatterned surface, where the solvent is deposited on the target surface upon contacting. The engraved structures are designed in cylindrical shaped objects, and substrate controlled through moving rolls of the system represents a typical R2R process. The gravure printing tools consist of a large cylinder electroplated with copper and engraved with microcells, as shown in **Figure 8**. The microcells are engraved by either using electromechanical means or using laser [23, 26, 41, 55]. The physical contact between the printing surface and the substrate leads to wear and tear of the engraved structure; therefore, chrome electroplating is performed for protecting it from deterioration. The size of engraved microcells is responsible for ink pickup from the reservoir lying beneath the cylinder or filled with dispensing nozzles from the top. The extra ink is removed from the surface using a doctor blade to avoid cross contamination or unwanted deposition of ink on the substrate.

Surface properties of the substrate and rheological properties of ink are tuned to promote the efficient deposition. Capillary action of the ink plays a significant role in complete transfer of the ink from microcells of the engraved cylinder to the substrate. Pressure on the impression cylinder is also properly controlled to expedite the ink transfer and to reduce the deterioration of contacting surfaces. Width and depth ratio of the microcells in the engraved cylinder also plays a significant role in manipulating the ink transfer [41]. Viscosity and surface tension of the desired material solution have to be in acceptable ranges to prevent the bleeding out of the solution from the microcells. The optimal viscosities help in rapid prototyping by increasing the printing speed and allow full emptying of the ink from engraved microcells [56].

An advanced version of gravure printing, i.e., gravure offset uses an extra elastic blanket to avoid potential risks occurring from the deteriorations of the contacting surfaces, as shown in **Figure 8**. The elastic blanket serves as an intermediate step between the contacting surfaces, where the ink from engraved microcells is picked up by the blanket and transferred finally on the target substrate. This avoids the direct interface between the patterned cylinder and the substrate. Few of the printing parameters affecting significantly the print quality are speed, pressure, and blanket's dimensions plus thickness. Surface properties of the blanket and its thickness combined with the speed are more dominant factors that control and enhance the efficient transfer of the ink [16, 41, 57]. The limited time of contact and

**Figure 8.**

*Schematic of a typical standard and microgravure-offset printing process.*

higher speed also enable high-resolution patterning on the substrate and increase reliability of the system. Reliability of gravure offset is more critical for assembling in a high-speed production line of printed electronics on rollable substrates [58]. Combinations of various manipulating forces such as adhesive force between the blanket and the ink, cohesive force within the ink when on the blanket, adhesive force between the ink and the gravure, and adhesive force between the ink and the target substrate are of particular importance to control and tune for efficient transfer of ink. A complete dispensation of the ink is desired as a minor mismatch or open hole within the pickup ink can lead to incomplete printing of the structures [16, 59]. Speed of the roll is also a main contributing factor to optimize for complete transfer of the ink, and a uniform impression pressure in suitable ranges would increase the uniformity of the print edges and product yield. Despite the advantages offered by using an intermediate medium of transfer, gravure offset also poses some serious challenges. For instance, the lifespan of the blanket is of serious concern, as continuous absorbance of the ink leads to saturation of the surface and needs to be changed quite often. Prolonged use of the blanket greatly affects the resolution of the printed patterns on target substrates as it reduces temporarily the ink viscosity, which results in spread out of the ink during the setting process [60]. Therefore, a proper and timely maintenance of the printing tools, especially the blanket, is needed to avoid undesired printing structures.

#### *2.2.3 Flexographic printing*

Flexographic printing ensures high-speed printing and produces high-resolution patterned structures as compared to gravure and gravure-offset printing approaches [23]. A rubber- or polymer-based plate with elevated patterns on the surface and developed through photolithography is used in flexographic printing. The plate is attached to the printing cylinder as shown in **Figure 9**. A wide variety of ink including but not limited to solvent-based, wafer-based, UV-curable inks and two parts chemically curing inks, etc. can be processed to pattern high-resolution structures on target surfaces [18, 61, 62]. The Anilox cylinder picks up the ink from the reservoir, and upon contacting the inked areas with the plate cylinder, it transfers the ink and prints on the running substrate between plate and impression cylinders as shown in **Figure 9**. Transfer of the ink is more efficient in flexographic printing and results in very thin patterns with sharper edges. Amount of ink pick up is controlled predominantly by the Anilox roll, where the size and frequency of the engraved cells

**109**

materials [66].

*2.2.4 Microcontact printing (μCP)*

*Smart Manufacturing Technologies for Printed Electronics*

are responsible indirectly. A proper balance between the solution properties such as nanoparticles mixing ratios plus the carrier fluids is essentially required for filling the Anilox engraved cells. Relatively higher-solution concentrations are required within the specific range of viscosities to achieve good resolution patterns [18, 61, 62]. Typical resolution obtained with flexographic printing is in the range of 50–100 μm; however, further higher resolutions down to ~20 μm could be made possible by proper tuning the solution properties and process the parameters [18, 61, 62]. Film quality printed with flexography is uniform as compared to other competitive printing technologies [60]. Film instability and dewetting of the printing plates cause many defects such as open lines, overlapped lines, and edge waviness. Controlling the load pressure and cell aspect ratio is very critically important to avoid these issues, especially in the case of targeting high-resolution printing and high-end devices. Maintenance plus observation of the engraved cells is very important, as the blockade or erosion of any of the cells could lead to discontinuous printed patterns. An acceptable margin in terms of resolution needs to be allowed, as the pattern dimensions could vary as a result of pressure from impression cylinder on the flexible or polymer plate [18, 27, 61–63]. An optimum range of width and thickness is needed for the printed patterns to decrease the ohmic losses and also increase the efficiency of the printed devices [47]. For thick film deposition through flexographic, several printing passes with similar parameters are required, which also minimize sheet resistance. Repeating the same procedure needs proper alignment of the equipment for subsequent layers, which adds to the complexities of the system [64]. The current technology limits the highly desirable features such as high-switching speed and reduced supply voltage that are needed for many applications. These limitations result in degraded device parameters such as charge-carrier mobility, parasitic capacitances, and overlay precision registration accuracy [65]. Challenges to overcome for fine patterning are surface irregularities and pores, nonuniform films, ragged lines, and nonavailability of suitable functional

Microcontact printing is a special type of contact-based printing approach, where an inked surface is brought in conformal contact and transfers the patterns on target surface. The contact is controlled through micromanipulation, and surface conditions are set to release and receive the ink consequently. A conformal contact of prepatterned elastomeric stamp with precise control and alignment to the target surface on micron scale is key requirement for successful transfer of the structures. A master mold is developed using conventional microfabrication or photolithography techniques, and multiple copies of the stamp with desired structures are reproduces. A moldable and elastomeric material is usually used to develop the stamp, which can easy be casted into the master mold and delaminated without causing any degradation of the microscale structures on stamp surface [3, 19, 67, 68]. Poly(dimthylsiloxane) (PDMS) is the frequently used elastomer due to its extraordinary properties as compared to other elastomers such as polyurethanes, polyimides, and cross-linked Novolac resin. PDMS has few distinguishing properties such as conformability to larger areas, deformable to mount on nonplanar surfaces, elasticity for easy release, isotropic, and optically transparent. Low-surface free energy, chemically inertness, and durability for multiple uses make PDMS an attractive candidate for the purpose of using it as a stamp [19, 68]. Microcontact is an effective rapid prototyping technique for preparing the substrates and patterning a wide range of materials that are sensitive to light and chemical etchants. The lower surface energies offered by PDMS stamp due to elasticity of siloxane chain and the lower intermolecular forces between the methyl groups promote the peeling

*DOI: http://dx.doi.org/10.5772/intechopen.89377*

**Figure 9.** *Flexographic printing schematic diagram.*

#### *Smart Manufacturing Technologies for Printed Electronics DOI: http://dx.doi.org/10.5772/intechopen.89377*

*Hybrid Nanomaterials - Flexible Electronics Materials*

needed to avoid undesired printing structures.

*2.2.3 Flexographic printing*

higher speed also enable high-resolution patterning on the substrate and increase reliability of the system. Reliability of gravure offset is more critical for assembling in a high-speed production line of printed electronics on rollable substrates [58]. Combinations of various manipulating forces such as adhesive force between the blanket and the ink, cohesive force within the ink when on the blanket, adhesive force between the ink and the gravure, and adhesive force between the ink and the target substrate are of particular importance to control and tune for efficient transfer of ink. A complete dispensation of the ink is desired as a minor mismatch or open hole within the pickup ink can lead to incomplete printing of the structures [16, 59]. Speed of the roll is also a main contributing factor to optimize for complete transfer of the ink, and a uniform impression pressure in suitable ranges would increase the uniformity of the print edges and product yield. Despite the advantages offered by using an intermediate medium of transfer, gravure offset also poses some serious challenges. For instance, the lifespan of the blanket is of serious concern, as continuous absorbance of the ink leads to saturation of the surface and needs to be changed quite often. Prolonged use of the blanket greatly affects the resolution of the printed patterns on target substrates as it reduces temporarily the ink viscosity, which results in spread out of the ink during the setting process [60]. Therefore, a proper and timely maintenance of the printing tools, especially the blanket, is

Flexographic printing ensures high-speed printing and produces high-resolution patterned structures as compared to gravure and gravure-offset printing approaches [23]. A rubber- or polymer-based plate with elevated patterns on the surface and developed through photolithography is used in flexographic printing. The plate is attached to the printing cylinder as shown in **Figure 9**. A wide variety of ink including but not limited to solvent-based, wafer-based, UV-curable inks and two parts chemically curing inks, etc. can be processed to pattern high-resolution structures on target surfaces [18, 61, 62]. The Anilox cylinder picks up the ink from the reservoir, and upon contacting the inked areas with the plate cylinder, it transfers the ink and prints on the running substrate between plate and impression cylinders as shown in **Figure 9**. Transfer of the ink is more efficient in flexographic printing and results in very thin patterns with sharper edges. Amount of ink pick up is controlled predominantly by the Anilox roll, where the size and frequency of the engraved cells

**108**

**Figure 9.**

*Flexographic printing schematic diagram.*

are responsible indirectly. A proper balance between the solution properties such as nanoparticles mixing ratios plus the carrier fluids is essentially required for filling the Anilox engraved cells. Relatively higher-solution concentrations are required within the specific range of viscosities to achieve good resolution patterns [18, 61, 62]. Typical resolution obtained with flexographic printing is in the range of 50–100 μm; however, further higher resolutions down to ~20 μm could be made possible by proper tuning the solution properties and process the parameters [18, 61, 62]. Film quality printed with flexography is uniform as compared to other competitive printing technologies [60]. Film instability and dewetting of the printing plates cause many defects such as open lines, overlapped lines, and edge waviness. Controlling the load pressure and cell aspect ratio is very critically important to avoid these issues, especially in the case of targeting high-resolution printing and high-end devices. Maintenance plus observation of the engraved cells is very important, as the blockade or erosion of any of the cells could lead to discontinuous printed patterns. An acceptable margin in terms of resolution needs to be allowed, as the pattern dimensions could vary as a result of pressure from impression cylinder on the flexible or polymer plate [18, 27, 61–63]. An optimum range of width and thickness is needed for the printed patterns to decrease the ohmic losses and also increase the efficiency of the printed devices [47]. For thick film deposition through flexographic, several printing passes with similar parameters are required, which also minimize sheet resistance. Repeating the same procedure needs proper alignment of the equipment for subsequent layers, which adds to the complexities of the system [64]. The current technology limits the highly desirable features such as high-switching speed and reduced supply voltage that are needed for many applications. These limitations result in degraded device parameters such as charge-carrier mobility, parasitic capacitances, and overlay precision registration accuracy [65]. Challenges to overcome for fine patterning are surface irregularities and pores, nonuniform films, ragged lines, and nonavailability of suitable functional materials [66].

#### *2.2.4 Microcontact printing (μCP)*

Microcontact printing is a special type of contact-based printing approach, where an inked surface is brought in conformal contact and transfers the patterns on target surface. The contact is controlled through micromanipulation, and surface conditions are set to release and receive the ink consequently. A conformal contact of prepatterned elastomeric stamp with precise control and alignment to the target surface on micron scale is key requirement for successful transfer of the structures. A master mold is developed using conventional microfabrication or photolithography techniques, and multiple copies of the stamp with desired structures are reproduces. A moldable and elastomeric material is usually used to develop the stamp, which can easy be casted into the master mold and delaminated without causing any degradation of the microscale structures on stamp surface [3, 19, 67, 68]. Poly(dimthylsiloxane) (PDMS) is the frequently used elastomer due to its extraordinary properties as compared to other elastomers such as polyurethanes, polyimides, and cross-linked Novolac resin. PDMS has few distinguishing properties such as conformability to larger areas, deformable to mount on nonplanar surfaces, elasticity for easy release, isotropic, and optically transparent. Low-surface free energy, chemically inertness, and durability for multiple uses make PDMS an attractive candidate for the purpose of using it as a stamp [19, 68]. Microcontact is an effective rapid prototyping technique for preparing the substrates and patterning a wide range of materials that are sensitive to light and chemical etchants. The lower surface energies offered by PDMS stamp due to elasticity of siloxane chain and the lower intermolecular forces between the methyl groups promote the peeling

**Figure 10.** *Microcontact printing (μCP) steps during processing.*

and stamping capabilities, which are ideal for microcontact printing. Surface energies of the stamp and target surface play a significant role in efficient transfer of the ink. For instance, a higher energy of PDMS stamp is desired for the pickup of ink from the donating surface, whereas higher energy than the PDMS stamp is desired for the receiving surface to complete detachment of the ink. To avoid collapsing of the stamp during peeling or capillary action during inking, a specific ratio of the height to width of the features on the stamp is required [69]. **Figure 10** shows schematic of the process flow of typical microcontact printing approach.

Robustness of the stamp by having sufficient flexibility and mechanical strength for maintaining the dimensional integrity of the printable structures is of great importance. Elastomeric properties of the stamp material enhance the efficient ink delivery by providing a good interface between stamp, ink, and target substrate. Submicron scale is challenging to reach as due to the elastomeric nature of stamp, it tends to collapse, and noncontact areas of the stamp also interface with the substrate leading to uneven printing. Besides the printing tools and processing condition, chemical composition of the ink also contributes to the microcontact process. For instance, polar molecules are challenging to print with stamp because of the hydrophobicity of the PDMS. Therefore, treatment at right conditions of the PDMS stamp is central for stamping such type of materials [19]. Similar challenge faced by the flexographic technique is microcontact printing, as diffusion of molecules in the unpatterned areas of the stamp broadens the feature size and thus results in unwanted printing or stamping of the materials besides the intended structures [27, 70, 71]. Repeated use of the same stamp results in swelling due to the absorption of the ink and results in deterioration of the micron-scale structures on the stamp. A proper check and observation of the stamp are needed because the continuous use can cause pairing, buckling, or roof collapse of the microstructured patterns upon physical contact. Therefore, for reproducing the similar structures on target substrate, full operating conditions need to be explored considering especially the applied pressure, peeling the stamp from master mold, polarity of the molecules, and so on to guarantee uniform printing [69–71].

#### *2.2.5 Nanoimprinting (NI)*

Nanoimprinting, as the name suggests, is used to produce structures at nanoscale by using an imprint approach. NI uses mechanical and physical deformation of wet

**111**

*Smart Manufacturing Technologies for Printed Electronics*

layers through molding accompanied by different thermal procedures. The NI operating principles are quite straight forward, as shown in **Figure 11**. As against microcontact printing, NI uses a mold having nanoscale structures developed through standard clean room processes and is pressed against a uniformly coated wet surface at controlled pressure and temperature. A thin-residual layer of polymeric material is intentionally left underneath the mold protrusions and acts as a soft cushioning layer that prevents direct impact of the hard mold on the substrate and effectively protects the delicate nano-scale features on the mold surface [72]. Resist filling and demold characteristics are the two primary and critical processing steps affecting the print quality and throughput. Controlling the pressure precisely is central during the demolding process, which helps in maintaining the imprinted patterns at the desired dimensions [15]. Various approaches are adopted for executing NI process such as thermal, ultraviolet (UV), step and flash, and roller imprinting. They are all selected

on the basis of type of material and processing conditions. In UV-NI process, transparent medium is required for allowing the UV light to penetrate through and in-situ sintering of the imprinted structures. UV-IN is thus considered the most reliable and robust approach among the practiced NI procedures. Common materials for the molding in UV-IN are quartz and silica, which are molded by using very high

Compared to the quartz and mechanically rigid, polymer-based molds are advantageous as they can replicate nanostructures over larger areas. Besides, the material cost is lower, and it can be used to develop manufacturing platform at depreciated costs. The mechanical molding makes the process a bit challenging embarking into new challenges compared to the traditional manufacturing processes [76]. The spatial confinement of solution-based materials in nanoscale ranges gives rise to drastic changes in the physical, electrical, and chemical properties as compared to the patterned structures in micro or macrostructures. NI offers several advantages such as very high resolution patterning, high pattern transfer fidelity, 3D patterning, covering large areas, reduced fabrication steps, high throughput, and lower processing cost. However, the challenges faced by NI process overshadow the attractions and keep it restricted for a special case of uses. The overlay alignment, template fabrication, defect control, yield, and seeking a suitable application with a good payback are few of the critical challenges faced by NI process [15, 22, 72, 75, 77]. Probability of defect density is higher, and the mechanical and physical inabilities to withstand the uneven pressures on the molding masks lead to the lateral and vertical collapse of the nanostructures. This adds to the reproducing cast, and attaining repeatable structures with similar dimensions are challenging due to the less margin in dimensional variability. The thermal-based NI involves in-situ heating while applying the mask on the wet film, and slight variations in the temperature or thermal mismatch between the mask and the printed materials could lead to deterioration of the pattern structures on the mask. Time required, i.e., 10–15 min per replication for heating and cooling cycles, is longer than other soft printing techniques [74]. The thermal budget of about 125°C is challenging for some of the plastic substrates with lower glass transition temperature, which can create dimensional instabilities. Several electronic devices

resolution electron beam lithography techniques [72–75].

*DOI: http://dx.doi.org/10.5772/intechopen.89377*

*Nanoimprint steps for patterning the substrate.*

**Figure 11.**

*Smart Manufacturing Technologies for Printed Electronics DOI: http://dx.doi.org/10.5772/intechopen.89377*

#### **Figure 11.**

*Hybrid Nanomaterials - Flexible Electronics Materials*

*Microcontact printing (μCP) steps during processing.*

and stamping capabilities, which are ideal for microcontact printing. Surface energies of the stamp and target surface play a significant role in efficient transfer of the ink. For instance, a higher energy of PDMS stamp is desired for the pickup of ink from the donating surface, whereas higher energy than the PDMS stamp is desired for the receiving surface to complete detachment of the ink. To avoid collapsing of the stamp during peeling or capillary action during inking, a specific ratio of the height to width of the features on the stamp is required [69]. **Figure 10** shows

Robustness of the stamp by having sufficient flexibility and mechanical strength

Nanoimprinting, as the name suggests, is used to produce structures at nanoscale by using an imprint approach. NI uses mechanical and physical deformation of wet

schematic of the process flow of typical microcontact printing approach.

for maintaining the dimensional integrity of the printable structures is of great importance. Elastomeric properties of the stamp material enhance the efficient ink delivery by providing a good interface between stamp, ink, and target substrate. Submicron scale is challenging to reach as due to the elastomeric nature of stamp, it tends to collapse, and noncontact areas of the stamp also interface with the substrate leading to uneven printing. Besides the printing tools and processing condition, chemical composition of the ink also contributes to the microcontact process. For instance, polar molecules are challenging to print with stamp because of the hydrophobicity of the PDMS. Therefore, treatment at right conditions of the PDMS stamp is central for stamping such type of materials [19]. Similar challenge faced by the flexographic technique is microcontact printing, as diffusion of molecules in the unpatterned areas of the stamp broadens the feature size and thus results in unwanted printing or stamping of the materials besides the intended structures [27, 70, 71]. Repeated use of the same stamp results in swelling due to the absorption of the ink and results in deterioration of the micron-scale structures on the stamp. A proper check and observation of the stamp are needed because the continuous use can cause pairing, buckling, or roof collapse of the microstructured patterns upon physical contact. Therefore, for reproducing the similar structures on target substrate, full operating conditions need to be explored considering especially the applied pressure, peeling the stamp from master mold, polarity of the molecules, and so on to guarantee uniform printing [69–71].

**110**

**Figure 10.**

*2.2.5 Nanoimprinting (NI)*

#### *Nanoimprint steps for patterning the substrate.*

layers through molding accompanied by different thermal procedures. The NI operating principles are quite straight forward, as shown in **Figure 11**. As against microcontact printing, NI uses a mold having nanoscale structures developed through standard clean room processes and is pressed against a uniformly coated wet surface at controlled pressure and temperature. A thin-residual layer of polymeric material is intentionally left underneath the mold protrusions and acts as a soft cushioning layer that prevents direct impact of the hard mold on the substrate and effectively protects the delicate nano-scale features on the mold surface [72]. Resist filling and demold characteristics are the two primary and critical processing steps affecting the print quality and throughput. Controlling the pressure precisely is central during the demolding process, which helps in maintaining the imprinted patterns at the desired dimensions [15]. Various approaches are adopted for executing NI process such as thermal, ultraviolet (UV), step and flash, and roller imprinting. They are all selected on the basis of type of material and processing conditions. In UV-NI process, transparent medium is required for allowing the UV light to penetrate through and in-situ sintering of the imprinted structures. UV-IN is thus considered the most reliable and robust approach among the practiced NI procedures. Common materials for the molding in UV-IN are quartz and silica, which are molded by using very high resolution electron beam lithography techniques [72–75].

Compared to the quartz and mechanically rigid, polymer-based molds are advantageous as they can replicate nanostructures over larger areas. Besides, the material cost is lower, and it can be used to develop manufacturing platform at depreciated costs. The mechanical molding makes the process a bit challenging embarking into new challenges compared to the traditional manufacturing processes [76]. The spatial confinement of solution-based materials in nanoscale ranges gives rise to drastic changes in the physical, electrical, and chemical properties as compared to the patterned structures in micro or macrostructures. NI offers several advantages such as very high resolution patterning, high pattern transfer fidelity, 3D patterning, covering large areas, reduced fabrication steps, high throughput, and lower processing cost. However, the challenges faced by NI process overshadow the attractions and keep it restricted for a special case of uses. The overlay alignment, template fabrication, defect control, yield, and seeking a suitable application with a good payback are few of the critical challenges faced by NI process [15, 22, 72, 75, 77]. Probability of defect density is higher, and the mechanical and physical inabilities to withstand the uneven pressures on the molding masks lead to the lateral and vertical collapse of the nanostructures. This adds to the reproducing cast, and attaining repeatable structures with similar dimensions are challenging due to the less margin in dimensional variability. The thermal-based NI involves in-situ heating while applying the mask on the wet film, and slight variations in the temperature or thermal mismatch between the mask and the printed materials could lead to deterioration of the pattern structures on the mask. Time required, i.e., 10–15 min per replication for heating and cooling cycles, is longer than other soft printing techniques [74]. The thermal budget of about 125°C is challenging for some of the plastic substrates with lower glass transition temperature, which can create dimensional instabilities. Several electronic devices

have been presented by different research groups following the NI manufacturing. The NI process is ideal for a single-layer structure, as perfect alignment at nanoscale on multilayer devices is a challenging task. As a final goal to transfer the NI process on a R2R manufacturing, introducing multilayer steps and misalignment of the stamp during the imprint needs to be taken care of [69, 70, 78–80].

#### *2.2.6 Transfer printing*

Transfer printing is relatively a new technique to fabricate flexible electronics through physical transfer of prefabricated structures using a stamp [81]. Microstructures in the shape of wires or membranes are developed using standard photolithography processes in clean room, etched underway, and then used a stamp to pick and stamp on a target surface. Transfer printing can be executed in two ways such as direct transfer and stamp-assisted transfer. In direct transfer, an adhesive coating is performed on the receiving surface, and the donor wafer is directly contacted with it. After releasing, the structures are transferred to flipped surfaces on the target surface. Whereas, stamp-assisted transfer is completed in two steps: in first, the microstructures from donor substrate are picked up the stamp, and the stamp is contacted with the target substrate, thus transferring the structures with the top processed surface facing upward. The elastomeric stamp usually developed by using a PDMS is used, where the viscoelastic properties of the stamp are exploited to detach the fabricated structures from donor substrates and place them deterministically on a secondary substrate [82–86]. This technique is ideal for developing high-speed electronic devices on unconventional substrates, by merging both organic- and inorganicbased materials. The heterogeneous integration of these dissimilar materials helps in reducing the cost and maintaining the reliability of developed electronic circuits and systems [87]. The lower cost comes from the large area processing units such as inkjet printing for the electrodes and interconnections as well as insulator layers, where the high-mobility semiconductor layers are integrated into discrete circuit or into full circuit through stamp-assisted transfer technology. This is an effective approach toward manufacturing of high-end devices on larger areas as the state-of-the-art processing of electronic grade silicon, and other compound semiconductors in clean rooms constitute high level of purity, surface smoothness, control over crystallinity, doping levels, and types, resulting in higher carrier mobilities.

The structures are developed and finished in clean room processes and underetched afterward to release and be ready for the PDMS transfer. **Figure 12** shows schematics of the processing steps involved in transfer printing approach. Here, the structures are shown in the shape of wires, which are released from the underlying layers using the relevant etchants. Tethered points are designed as per the dimensional requirements and are sufficient enough to keep intact the released structures with the donor wafer before contacting the PDMS stamp. The dimension of tethered points is essential and useful for efficient transfer, especially for structures with dimensions in the micron scale. The conformal contact of soft elastomeric stamp having surface activated temporarily through oxygen plasma with the microwires or microribbons guarantees the efficient pickup. The wires attach to the stamp surface and, when peeled back, retrieve the microstructures with fast speed, enhancing the kinetic control of adhesion [88]. The rate-dependent adhesion and printing of the solid structures with high-peel velocity (typically 10 cm/s) and low-stamping velocity (~1 mm/s), respectively, have been investigated [89]. The mechanics of kinetic dependence of switching of adhesion between the microstructures and the stamp has its origin in the viscoelastic response of the elastomeric materials, i.e., PDMS. Adhesiveless stamping like this is very valuable for wafer-based microstructure printing to operate it from moderate to high temperatures [82].

**113**

**Figure 13.**

*modules are installed.*

*Smart Manufacturing Technologies for Printed Electronics*

*DOI: http://dx.doi.org/10.5772/intechopen.89377*

**3. Mass production perspective**

*patterns transferred to final substrate.*

**Figure 12.**

Developing printing technologies is motivated by the development of fast and efficient production line by assembling different manufacturing units, substrate treatments, and sintering after fabrication, as shown in **Figure 13**. The highly optimized techniques on lab level are merged together to develop a single manufacturing platform. The scope of such developments is rendered from the high-speed text printing-based technology. A similar approach is foreseen to establish for batch manufacturing of electronic components and systems on larger areas as much higher speeds. Printing processes matured at lab level need to be transferred to large-scale fast production lines with the same level of performance. However, assembling different printing techniques into a single production platform inline is a challenging task, as all the processing parameters and conditions need to be properly tuned and need to be in close ranges of the process conditions, especially when the substrate is moving at higher speeds as 5–50 m/min. Therefore, investigation of the optimal and matching processing conditions is desired for the new arrangement, which is not at all obvious, especially when considering that there are boundary conditions

*Schematic illustration of typical roll-to-roll system where different deposition, patterning, and sintering* 

*Conformal contact of polymeric stamp with the Si patterns, pickup of patterns by peeling off the stamp, and* 

*Smart Manufacturing Technologies for Printed Electronics DOI: http://dx.doi.org/10.5772/intechopen.89377*

**Figure 12.**

*Hybrid Nanomaterials - Flexible Electronics Materials*

*2.2.6 Transfer printing*

during the imprint needs to be taken care of [69, 70, 78–80].

doping levels, and types, resulting in higher carrier mobilities.

ture printing to operate it from moderate to high temperatures [82].

The structures are developed and finished in clean room processes and underetched afterward to release and be ready for the PDMS transfer. **Figure 12** shows schematics of the processing steps involved in transfer printing approach. Here, the structures are shown in the shape of wires, which are released from the underlying layers using the relevant etchants. Tethered points are designed as per the dimensional requirements and are sufficient enough to keep intact the released structures with the donor wafer before contacting the PDMS stamp. The dimension of tethered points is essential and useful for efficient transfer, especially for structures with dimensions in the micron scale. The conformal contact of soft elastomeric stamp having surface activated temporarily through oxygen plasma with the microwires or microribbons guarantees the efficient pickup. The wires attach to the stamp surface and, when peeled back, retrieve the microstructures with fast speed, enhancing the kinetic control of adhesion [88]. The rate-dependent adhesion and printing of the solid structures with high-peel velocity (typically 10 cm/s) and low-stamping velocity (~1 mm/s), respectively, have been investigated [89]. The mechanics of kinetic dependence of switching of adhesion between the microstructures and the stamp has its origin in the viscoelastic response of the elastomeric materials, i.e., PDMS. Adhesiveless stamping like this is very valuable for wafer-based microstruc-

have been presented by different research groups following the NI manufacturing. The NI process is ideal for a single-layer structure, as perfect alignment at nanoscale on multilayer devices is a challenging task. As a final goal to transfer the NI process on a R2R manufacturing, introducing multilayer steps and misalignment of the stamp

Transfer printing is relatively a new technique to fabricate flexible electronics through physical transfer of prefabricated structures using a stamp [81]. Microstructures in the shape of wires or membranes are developed using standard photolithography processes in clean room, etched underway, and then used a stamp to pick and stamp on a target surface. Transfer printing can be executed in two ways such as direct transfer and stamp-assisted transfer. In direct transfer, an adhesive coating is performed on the receiving surface, and the donor wafer is directly contacted with it. After releasing, the structures are transferred to flipped surfaces on the target surface. Whereas, stamp-assisted transfer is completed in two steps: in first, the microstructures from donor substrate are picked up the stamp, and the stamp is contacted with the target substrate, thus transferring the structures with the top processed surface facing upward. The elastomeric stamp usually developed by using a PDMS is used, where the viscoelastic properties of the stamp are exploited to detach the fabricated structures from donor substrates and place them deterministically on a secondary substrate [82–86]. This technique is ideal for developing high-speed electronic devices on unconventional substrates, by merging both organic- and inorganicbased materials. The heterogeneous integration of these dissimilar materials helps in reducing the cost and maintaining the reliability of developed electronic circuits and systems [87]. The lower cost comes from the large area processing units such as inkjet printing for the electrodes and interconnections as well as insulator layers, where the high-mobility semiconductor layers are integrated into discrete circuit or into full circuit through stamp-assisted transfer technology. This is an effective approach toward manufacturing of high-end devices on larger areas as the state-of-the-art processing of electronic grade silicon, and other compound semiconductors in clean rooms constitute high level of purity, surface smoothness, control over crystallinity,

**112**

*Conformal contact of polymeric stamp with the Si patterns, pickup of patterns by peeling off the stamp, and patterns transferred to final substrate.*

#### **3. Mass production perspective**

Developing printing technologies is motivated by the development of fast and efficient production line by assembling different manufacturing units, substrate treatments, and sintering after fabrication, as shown in **Figure 13**. The highly optimized techniques on lab level are merged together to develop a single manufacturing platform. The scope of such developments is rendered from the high-speed text printing-based technology. A similar approach is foreseen to establish for batch manufacturing of electronic components and systems on larger areas as much higher speeds. Printing processes matured at lab level need to be transferred to large-scale fast production lines with the same level of performance. However, assembling different printing techniques into a single production platform inline is a challenging task, as all the processing parameters and conditions need to be properly tuned and need to be in close ranges of the process conditions, especially when the substrate is moving at higher speeds as 5–50 m/min. Therefore, investigation of the optimal and matching processing conditions is desired for the new arrangement, which is not at all obvious, especially when considering that there are boundary conditions

#### **Figure 13.**

*Schematic illustration of typical roll-to-roll system where different deposition, patterning, and sintering modules are installed.*

(i.e., materials, solvents, multilayer processing, overlay registration accuracy, drying temperature, speed, etc.) involved in fast R2R processing [20, 23, 53, 90].

R2R as a commonly shared platform has the potential for a continuous and high-throughput process for deposition of diverse materials on large substrate rolls (often called "web") [53, 90]. Besides the instrumentation and hardware for control system, R2R line is equipped with several rollers over which the web (flexible substrates) passes with controlled tension. As described in Section 3, these webs are the backbone of a R2R system and should be accurately controlled during passage through different rollers and processing sections.

#### **4. 3D packaging**

Packaging is an important step during electronic fabrication, which enables user to interface with electronic devices, circuits, and systems. In printed electronics, devices are relatively large size as compared to conventional technologies and easy to handle. However, they need packaging to protect from the ambient environment such as humidity, light, and temperature.

On the other hand, the device itself needs protection from the user touches as it can damage the thin films and patterns. 3D printing (also called additive manufacturing) is a manufacturing technology, which is based on imposing the material layers to create the 3D objects. A 3D object is fabricated through melting filament material with controlled temperature and flow rate in combination with X, Y, and Z axis control, as shown in **Figure 14**. The object is design in CAD tool, i.e., AutoCAD or any other tool that can create 3D structures and converted into printer supported file format. The file is then loaded into the printer to create the object in 3D form. There are several ways to create a 3D object, which defines the types of 3D printers.

**115**

**5. Conclusion**

*Smart Manufacturing Technologies for Printed Electronics*

The development of 3D object is made by either microdrops or melt near-field

Stereolithography (SLA) uses a UV laser instead of melting the filament through heater and making microdrops. An SLA printer uses two mirrors in combination with UV laser, known as galvanometers, positioned on the X-axis and on the Y-axis. Both galvanometers rapidly aim a laser beam across a vat of resin, the area under light beam selectively curing and solidifying a cross section of the object inside this

This 3D printing technology is almost the same as stereolithography, the only difference that instead of laser and two mirrors, DLP uses image of the 3D object to make one layer and repeat until the job is finished. DLP is much faster than SLA as it uses digital image array to produce a structure in the vertical sequential order frame by frame. The digital image of the target object is consisted of small rectangles called voxels. This 3D printing technology is based on selective laser sintering (SLS), and commonly used materials are thermoplastic powders (Nylon 6, Nylon 11, and Nylon 12). Potential applications of this technology are functional parts, complex ducting (hollow designs), and low run part production. This technology can be used for strong and elastic mechanical property parts and also for the

SLS 3D technology creates an object with powder bed fusion technology and polymer powder. Working of this technology, i.e., polymer powder, is preheated to a temperature slightly below the melting point. Then, a very thin layer of the powdered material is deposited with the help of a blade normally 0.1 mm thick on the object platform. The surface is then scanned with a CO2 laser beam, as it selectively sinters the powder and solidifies a cross section of the object according to the designed geometry. Same as SLA 3D technology, the laser is precisely focused on to the correct location with the help of two galvos. Once the entire cross-sectional area of the object is scanned that creates one layer of the object, the build platform will move down one layer thickness in height to make the next step happen. The powder recoating blade deposits a new layer of the powder on the top of prescanned layer, and the laser will sinter the next cross section of the object area the same as previ-

Commonly used and advanced printed electronic technologies were discussed briefly in this chapter that covers almost all the technologies to fabricate electronic devices, circuits, and systems. Although the printed electronic technologies come

ous layer. This process is continuous until the object is created.

electrospinning of melted thermoplastics of consecutive layers, which solidifies after a certain time. Commonly used filament materials are PVA, PLA, ABS, nylon, and some composites. This 3D printing technology is often used in the rapid

build area, building it up layer by layer and forming a 3D structure.

*DOI: http://dx.doi.org/10.5772/intechopen.89377*

**4.1 Fused deposition modeling**

**4.3 Digital light processing (DLP)**

complex geometry printing.

**4.4 Selective laser sintering (SLS)**

prototyping objects.

**4.2 Stereolithography**

**Figure 14.** *3D printing system schematic diagram.*

#### **4.1 Fused deposition modeling**

The development of 3D object is made by either microdrops or melt near-field electrospinning of melted thermoplastics of consecutive layers, which solidifies after a certain time. Commonly used filament materials are PVA, PLA, ABS, nylon, and some composites. This 3D printing technology is often used in the rapid prototyping objects.

#### **4.2 Stereolithography**

*Hybrid Nanomaterials - Flexible Electronics Materials*

through different rollers and processing sections.

such as humidity, light, and temperature.

**4. 3D packaging**

(i.e., materials, solvents, multilayer processing, overlay registration accuracy, drying

Packaging is an important step during electronic fabrication, which enables user to interface with electronic devices, circuits, and systems. In printed electronics, devices are relatively large size as compared to conventional technologies and easy to handle. However, they need packaging to protect from the ambient environment

On the other hand, the device itself needs protection from the user touches as it can damage the thin films and patterns. 3D printing (also called additive manufacturing) is a manufacturing technology, which is based on imposing the material layers to create the 3D objects. A 3D object is fabricated through melting filament material with controlled temperature and flow rate in combination with X, Y, and Z axis control, as shown in **Figure 14**. The object is design in CAD tool, i.e., AutoCAD or any other tool that can create 3D structures and converted into printer supported file format. The file is then loaded into the printer to create the object in 3D form. There are several ways to create a 3D object, which defines the types of 3D printers.

R2R as a commonly shared platform has the potential for a continuous and high-throughput process for deposition of diverse materials on large substrate rolls (often called "web") [53, 90]. Besides the instrumentation and hardware for control system, R2R line is equipped with several rollers over which the web (flexible substrates) passes with controlled tension. As described in Section 3, these webs are the backbone of a R2R system and should be accurately controlled during passage

temperature, speed, etc.) involved in fast R2R processing [20, 23, 53, 90].

**114**

**Figure 14.**

*3D printing system schematic diagram.*

Stereolithography (SLA) uses a UV laser instead of melting the filament through heater and making microdrops. An SLA printer uses two mirrors in combination with UV laser, known as galvanometers, positioned on the X-axis and on the Y-axis. Both galvanometers rapidly aim a laser beam across a vat of resin, the area under light beam selectively curing and solidifying a cross section of the object inside this build area, building it up layer by layer and forming a 3D structure.

#### **4.3 Digital light processing (DLP)**

This 3D printing technology is almost the same as stereolithography, the only difference that instead of laser and two mirrors, DLP uses image of the 3D object to make one layer and repeat until the job is finished. DLP is much faster than SLA as it uses digital image array to produce a structure in the vertical sequential order frame by frame. The digital image of the target object is consisted of small rectangles called voxels. This 3D printing technology is based on selective laser sintering (SLS), and commonly used materials are thermoplastic powders (Nylon 6, Nylon 11, and Nylon 12). Potential applications of this technology are functional parts, complex ducting (hollow designs), and low run part production. This technology can be used for strong and elastic mechanical property parts and also for the complex geometry printing.

#### **4.4 Selective laser sintering (SLS)**

SLS 3D technology creates an object with powder bed fusion technology and polymer powder. Working of this technology, i.e., polymer powder, is preheated to a temperature slightly below the melting point. Then, a very thin layer of the powdered material is deposited with the help of a blade normally 0.1 mm thick on the object platform. The surface is then scanned with a CO2 laser beam, as it selectively sinters the powder and solidifies a cross section of the object according to the designed geometry. Same as SLA 3D technology, the laser is precisely focused on to the correct location with the help of two galvos. Once the entire cross-sectional area of the object is scanned that creates one layer of the object, the build platform will move down one layer thickness in height to make the next step happen. The powder recoating blade deposits a new layer of the powder on the top of prescanned layer, and the laser will sinter the next cross section of the object area the same as previous layer. This process is continuous until the object is created.

#### **5. Conclusion**

Commonly used and advanced printed electronic technologies were discussed briefly in this chapter that covers almost all the technologies to fabricate electronic devices, circuits, and systems. Although the printed electronic technologies come

with their limitations of scalability, mass production, and life time, it is not perfect replacement of the conventional electronic technology; however, it allows free design, rapid prototyping, and unlimited application areas, especially lowtemperature fabrication. Printed electronic technology benefits from new printing techniques, solution-based materials, and the combination of other manufacturing processes. Printing technology has a prominent impact on the electronic industry, such as flexibility and biocompatibility; otherwise, it was impossible to achieve with the conventional techniques and materials. Different printing technologies, processing requirements, operation, materials, and fabrication limitations were highlighted. The possibility of combining printing technologies to enable mass production of the devices, circuits, and systems, i.e., R2R system, was also presented. Moreover, it was discussed that 3D printing plays an important role in the packaging and test platform fabrication for the printed electronics. At present stage, printed electronics suffers from limitations such as operating frequency, scalability, mass production, shelf life, and robustness. However, as the research is continuous in the field, breakthroughs are expected in near future to meet all these challenges.

### **Acknowledgements**

This work was supported by NPRP from the Qatar National Research Fund (a member of Qatar 846 Foundation) under Grant NPRP10-0201-170315 and NPRP11S-0110-180246. The publication charges were supported by Qatar National Library (QNL). The findings herein reflect the work and are solely the responsibility of the authors.

### **Conflict of interest**

Authors declare no conflict of interests.

### **Author details**

Saleem Khan1 \*, Shawkat Ali1,2 and Amine Bermak1

1 College of Science and Engineering, Hamad Bin Khalifa University, Doha, Qatar

2 Electrical Engineering, National University of Computer and Emerging Sciences (FAST-NU), Islamabad, Pakistan

\*Address all correspondence to: sakhan3@hbku.edu.qa

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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**References**

*Smart Manufacturing Technologies for Printed Electronics DOI: http://dx.doi.org/10.5772/intechopen.89377*

#### **References**

*Hybrid Nanomaterials - Flexible Electronics Materials*

with their limitations of scalability, mass production, and life time, it is not perfect replacement of the conventional electronic technology; however, it allows free design, rapid prototyping, and unlimited application areas, especially lowtemperature fabrication. Printed electronic technology benefits from new printing techniques, solution-based materials, and the combination of other manufacturing processes. Printing technology has a prominent impact on the electronic industry, such as flexibility and biocompatibility; otherwise, it was impossible to achieve with the conventional techniques and materials. Different printing technologies, processing requirements, operation, materials, and fabrication limitations were highlighted. The possibility of combining printing technologies to enable mass production of the devices, circuits, and systems, i.e., R2R system, was also presented. Moreover, it was discussed that 3D printing plays an important role in the packaging and test platform fabrication for the printed electronics. At present stage, printed electronics suffers from limitations such as operating frequency, scalability, mass production, shelf life, and robustness. However, as the research is continuous in the

field, breakthroughs are expected in near future to meet all these challenges.

This work was supported by NPRP from the Qatar National Research Fund (a member of Qatar 846 Foundation) under Grant NPRP10-0201-170315 and NPRP11S-0110-180246. The publication charges were supported by Qatar National Library (QNL). The findings herein reflect the work and are solely the responsibil-

**116**

**Author details**

**Acknowledgements**

ity of the authors.

**Conflict of interest**

Authors declare no conflict of interests.

(FAST-NU), Islamabad, Pakistan

provided the original work is properly cited.

\*, Shawkat Ali1,2 and Amine Bermak1

\*Address all correspondence to: sakhan3@hbku.edu.qa

1 College of Science and Engineering, Hamad Bin Khalifa University, Doha, Qatar

2 Electrical Engineering, National University of Computer and Emerging Sciences

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Saleem Khan1

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Someya T. Applied Physics Letters.

Micro and Nanosystems. 2009;

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[51] Noguchi Y, Sekitani T,

2007;**91**(13):133502

Cells. 2009;**93**(4):422

**1**(1):46

[50] Kwack YJ, Choi WS. Journal of the Korean Physical Society. 2011;**59**:3410

[47] Siden J, Nilsson HE. Presented at the IEEE Intl. Symp. Antennas Propag. Soc.

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2007 (unpublished)

2001;**7**(5):769

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Nakayama M, Kawasaki H. Presented at the IPC Printed Circuit Expo/APEX

Engineering and Science.

(Basel). 2019;**19**(5):1197

printed electronics. 2010

2008 (unpublished)

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2009;**49**(6):1158

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[62] Maksud MI, Yusof MS, Jamil A, Mahadi M. International Journal of Integrated Engineering. 2014;**5**(3):679

[63] Lo C-Y, Keinänen JH, Huttunen O-H, Petäjä J, Hast J, Maaninen A, et al. Microelectronic Engineering. 2009;**86**(4):979

[64] Kwak MK, Shin KH, Yoon EY, Suh KY. Journal of Colloid and Interface Science. 2010;**343**(1):301

[65] Kempa H, Hambsch M, Reuter K, Stanel M, Schmidt GC, Meier B, et al. IEEE Transactions on Electron Devices. 2011;**58**(8):2765

[66] Lee HK, Joyce MK, Fleming PD. Journal of Imaging Science and Technology. 2005;**49**(1):54 [67] Kim P, Kwon KW, Park MC, Lee SH, Kim SM, Suh KY. BioChip Journal. 2008;**2**(1):1

[68] Rogers JA, Nuzzo RG. Materials Today. 2005;**8**(2):50

[69] Ruiz SA, Chen CS. Soft Matter. 2007;**3**(2):168

[70] Perl A, Reinhoudt DN, Huskens J. Advanced. Materials. 2009;**21**(22):2257

[71] Quist AP, Pavlovic E, Oscarsson S. Analytical and Bioanalytical Chemistry. 2005;**381**(3):591

[72] Guo LJ. Advanced Materials. 2007;**19**(4):495

[73] Nagato K. Polymers. 2014;**6**(3):604

[74] Wolfe DB, Love JC, Whitesides GM. Encyclopedia of Nanoscience and Nanotechnology. New York: Marcel Dekker, Inc; 2004. pp. 2657-2666

[75] Gilles S, Meier M, Prömpers M, van der Hart A, Kügeler C, Offenhäusser A, et al. Microelectronic Engineering. 2009;**86**(4):661

[76] Schift H. Journal of Vacuum Science and Technology B. 2008;**26**(2):458

[77] Jiang L, Chi L. Strategies for High Resolution Patterning of Conducting Polymers. Rijeka: IntechOpen; 2010. p. 656

[78] Auner C, Palfinger U, Gold H, Kraxner J, Haase A, Haber T, et al. Organic Electronics. 2010;**11**(4):552

[79] Li B, Zhang J, Ge H. Applied Physics A. 2013;**110**(1):123

[80] Moonen PF, Vratzov B, Smaal WTT, Kjellander BKC, Gelinck GH, Meinders ER, et al. Organic Electronics. 2012;**13**(12):3004

[81] Khan S, Lorenzelli L, Dahiya R. Semiconductor Science and Technology. 2017;**32**(8):085013

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**121**

**Chapter 8**

**Abstract**

**1. Introduction**

ing, and integration [6–8].

Plastic Inorganic Semiconductors

Featured with bendability and deformability, smartness and lightness, flexible materials and devices have wide applications in electronics, optoelectronics, and energy utilization. The key for flexible electronics is the integration of flexibility and decent electrical performance of semiconductors. It has long been realized that high-performance inorganic semiconductors are brittle, and the thinning-downinduced flexibility does not change the intrinsic brittleness. This inconvenient fact severely restricts the fabrication and service of inorganic semiconductors in flexible and deformable electronics. By contrast, flexible and soft polymers can be readily deformed but behave poorly in terms of electrical properties. Recently, Ag2S was discovered as the room-temperature ductile inorganic semiconductor. The intrinsic flexibility and plasticity of Ag2S are attributed to multicentered chemical bonding and solid linkage among easy slip planes. Furthermore, the electrical and thermoelectric properties of Ag2S can be readily optimized by Se/Te alloying while the ductility is maintained, giving birth to a high-efficiency full inorganic flexible thermoelectric device. This chapter briefly reviews this big discovery, relevant backgrounds, and research advances and tries to demonstrate a clear structure-performance correlation between crystal structure/chemical bonding and mechanical/electrical properties.

**Keywords:** flexible electronics, plastic inorganic semiconductors, chemical bonds

Flexible electronics endows electronic circuits with novel flexibility, foldability, and scalability, breaking the restriction of wafers and greatly expanding the application in various fields, e.g., flexible displays, electronic textiles, and sensory skins [1–3]. Consequently, flexible electronics has attracted widespread attention from both academic and industrial communities and has witnessed marvelous breakthroughs in terms of material development [4–5], device fabrication, packag-

In flexible electronics, semiconductors and devices are mounted onto flexible substrates, mostly polymers [1, 9]. The key to flexible electronics is realizing both high flexibility and desired physical/chemical properties (mostly electrical performance) of the semiconductors. Due to the intrinsic softness and flexibility, polymer semiconductors have long been a popular candidate for flexible electronics [10–13]. However, carriers in organic materials are rather localized, leading to poor electrical conduction. Another way to realize "flexibility" is to process inorganic materials into ultrathin format to reduce the stiffness [5, 14], and this is why 2D materials or thin films are widely used [15–16]. However, the thinning-induced flexibility does

*Tian-Ran Wei, Heyang Chen, Xun Shi and Lidong Chen*

for Flexible Electronics

[90] Huang YY, Chen J, Yin Z, Xiong Y. IEEE Transactions on Components, Packaging, and Manufacturing Technology. 2011;**1**(9):1368

#### **Chapter 8**

*Hybrid Nanomaterials - Flexible Electronics Materials*

[81] Khan S, Lorenzelli L, Dahiya R. Semiconductor Science and Technology.

[82] Meitl MA, Zhu ZT, Kumar V, Lee KJ, Feng X, Huang YY, et al. Nature

[83] Zhang K, Seo JH, Zhou W, Ma Z. Journal of Physics D: Applied

Collini C, Lorenzelli L. Microelectronic

[85] Oh TY, Jeong SW, Chang S, Choi K, Ha HJ, Ju BK. Applied Physics Letters.

[86] Khan S, Dahiya R, Lorenzelli L. Presented at the ESSDERC 2014, the 44th European Solid-State Device Research Conference; Venice, Italy. 2014

[87] Khan S, Yogeswaran N, Taube W, Lorenzelli L, Dahiya R. Journal of Micromechanics and Microengineering.

[88] Mack S, Meitl MA, Baca AJ, Zhu ZT, Rogers JA. Applied Physics Letters.

Huang Y, Nuzzo RG, Rogers JA. Advanced

Physics. 2012;**45**(14):143001

[84] Dahiya RS, Adami A,

Engineering. 2012;**98**:502

2013;**102**(2):021106

(unpublished)

2015;**25**(12):125019

2006;**88**(21):213101

2011;**1**(9):1368

[89] Carlson A, Bowen AM,

Materials. 2012;**24**(39):5284

[90] Huang YY, Chen J, Yin Z, Xiong Y. IEEE Transactions on Components, Packaging, and Manufacturing Technology.

2017;**32**(8):085013

Materials. 2006;**5**(1):33

**120**

## Plastic Inorganic Semiconductors for Flexible Electronics

*Tian-Ran Wei, Heyang Chen, Xun Shi and Lidong Chen*

#### **Abstract**

Featured with bendability and deformability, smartness and lightness, flexible materials and devices have wide applications in electronics, optoelectronics, and energy utilization. The key for flexible electronics is the integration of flexibility and decent electrical performance of semiconductors. It has long been realized that high-performance inorganic semiconductors are brittle, and the thinning-downinduced flexibility does not change the intrinsic brittleness. This inconvenient fact severely restricts the fabrication and service of inorganic semiconductors in flexible and deformable electronics. By contrast, flexible and soft polymers can be readily deformed but behave poorly in terms of electrical properties. Recently, Ag2S was discovered as the room-temperature ductile inorganic semiconductor. The intrinsic flexibility and plasticity of Ag2S are attributed to multicentered chemical bonding and solid linkage among easy slip planes. Furthermore, the electrical and thermoelectric properties of Ag2S can be readily optimized by Se/Te alloying while the ductility is maintained, giving birth to a high-efficiency full inorganic flexible thermoelectric device. This chapter briefly reviews this big discovery, relevant backgrounds, and research advances and tries to demonstrate a clear structure-performance correlation between crystal structure/chemical bonding and mechanical/electrical properties.

**Keywords:** flexible electronics, plastic inorganic semiconductors, chemical bonds

#### **1. Introduction**

Flexible electronics endows electronic circuits with novel flexibility, foldability, and scalability, breaking the restriction of wafers and greatly expanding the application in various fields, e.g., flexible displays, electronic textiles, and sensory skins [1–3]. Consequently, flexible electronics has attracted widespread attention from both academic and industrial communities and has witnessed marvelous breakthroughs in terms of material development [4–5], device fabrication, packaging, and integration [6–8].

In flexible electronics, semiconductors and devices are mounted onto flexible substrates, mostly polymers [1, 9]. The key to flexible electronics is realizing both high flexibility and desired physical/chemical properties (mostly electrical performance) of the semiconductors. Due to the intrinsic softness and flexibility, polymer semiconductors have long been a popular candidate for flexible electronics [10–13]. However, carriers in organic materials are rather localized, leading to poor electrical conduction. Another way to realize "flexibility" is to process inorganic materials into ultrathin format to reduce the stiffness [5, 14], and this is why 2D materials or thin films are widely used [15–16]. However, the thinning-induced flexibility does

not change the intrinsic brittleness and rigidity [17–18] of the inorganic semiconductors, which are constituted mainly by covalent or ion-covalent bonding [19].

Regarding this issue, another important mechanical property, plasticity, should be considered. As a matter of fact, however, plasticity is a long-sought target for inorganic materials, e.g., ceramics [20]. On the one hand, plasticity means machinability, that is, plastic ceramics can be mechanically deformed and processed just like metals do. On the other hand, plastic deformation can prevent the sudden, catastrophic, brittle fracture, which is essential to not only structural materials but also functional materials. Hence, the discovery of the room-temperature plastic inorganic semiconductor Ag2S [21] and the fabrication of full-inorganic Ag2S-based thermoelectric (TE) power generation modules [22] are ground breaking, opening a new avenue toward next-generation flexible electronics.

This chapter will provide an in-time overview for the newly discovered plastic/ flexible inorganic semiconductors. We shall first clarify the concept of flexibility and then illustrate the intrinsic plasticity for metals and brittleness for inorganic materials. Then, we will mention the special plasticity and the chemical bonding origins in a few ionic crystals such as AgCl. After that, we will systematically review the extraordinary mechanical properties of Ag2S and fully flexible thermoelectric devices. Finally, the prospect and challenge for plastic inorganic semiconductors as flexible electronic materials will be discussed.

#### **2. Fundamental concepts: flexibility**

As a matter of fact, "flexibility," unlike "ductility" or "rigidity," is not a scientifically clear concept. For flexible electronics, it is widely conceived that only elastic deformation is needed or allowed. In this sense, "flexibility" refers to the ability of the material/device to bend easily in an elastic way. According to Peng and Snyder [23], flexibility *f* is quantified by the largest curvature of bending, 1/*r*b, where *r*<sup>b</sup> is the minimum bending radius. A material with a thickness *h* bent about a radius *r*b experiences the greatest tensile and compressive stresses on the outer and inner faces, respectively. The maximum strain is readily calculated from the geometry considering that the middle (neutral layer) of the material is unstrained:

$$\text{Substituting that the middle (neutral layer) of the material is unstrained:}$$

$$\varepsilon = \frac{\left[2\pi \left(r\_b + \frac{h}{2}\right) - 2\pi r\_b\right]}{2\pi r\_b} = \frac{h}{2r\_b} \tag{1}$$

The maximum elastic strain is reached when plastic deformation occurs: ε = *σy*/*E*, where *E* is the elastic (Young's) modulus. Thus, the flexibility becomes:

$$f = \frac{1}{r\_b} = \frac{2}{h} \varepsilon\_p = \frac{2}{h} \frac{\sigma\_\mathcal{V}}{E} \tag{2}$$

**123**

**Figure 2.**

*distances.*

**Figure 1.**

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

a plastic material gains intrinsic flexibility without the restrict of size, that is, it can be readily deformed without breaking even in the bulk format, which is essentially important for applications requiring energy densities. Nonetheless, plasticity is rarely seen in inorganic semiconductors, which will be discussed in Section 3.

*The ratio of yield strength to Young's modulus for various materials. Raw data are taken from Ref. [23].*

Ductile and brittle behaviors are schematically shown in **Figure 2(a)**. Inorganic semiconductors are mostly constituted by covalent or ion-covalent bonds [25], which assure an appreciable electron orbital overlap, dispersive electronic band, and decent carrier mobility [26]. Covalent bonds are directional, saturated, and localized. Therefore, even a trivial bonding distortion will cause a large instability, which is vividly demonstrated by the deep and steep curve in the interatomic potential versus

*(a) Stress-strain curves for brittle and ductile materials and (b) atomic potential varying with atomic* 

**3. Prevalent brittleness for inorganic semiconductors**

Removing the shape factor (2/*h*), the material flexibility is *f*fom = *σy*/*E*.

The value of *ffom* is plotted in **Figure 1** for various materials. It is seen that this definition with the plot well distinguishes commonly sensed flexible materials such as rubber between rigid ones like ceramics. In fact, this definition is essentially consistent with the commonly seen bending stiffness or flexural rigidity, *fr* ~ *h*<sup>3</sup> *E* [14, 24]. That is, intrinsically stiff materials with a large thickness tend to be rigid or not flexible. Accordingly, (elastic) flexibility can be realized in thin films or flakes. Therefore, two-dimensional (2D) materials are popular candidates for flexible electronics.

The above definition treats flexibility as elastic deformability, which is reasonable considering the real application. However, as discussed in Section 1, plasticity is important, particularly for efficient material processing. A plastic material can be easily processed into target geometry with little fracture or waste. In addition,

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

*Hybrid Nanomaterials - Flexible Electronics Materials*

new avenue toward next-generation flexible electronics.

flexible electronic materials will be discussed.

**2. Fundamental concepts: flexibility**

*ε* = [2π(*rb* <sup>+</sup>

*<sup>f</sup>* = \_1

not change the intrinsic brittleness and rigidity [17–18] of the inorganic semiconductors, which are constituted mainly by covalent or ion-covalent bonding [19]. Regarding this issue, another important mechanical property, plasticity, should be considered. As a matter of fact, however, plasticity is a long-sought target for inorganic materials, e.g., ceramics [20]. On the one hand, plasticity means machinability, that is, plastic ceramics can be mechanically deformed and processed just like metals do. On the other hand, plastic deformation can prevent the sudden, catastrophic, brittle fracture, which is essential to not only structural materials but also functional materials. Hence, the discovery of the room-temperature plastic inorganic semiconductor Ag2S [21] and the fabrication of full-inorganic Ag2S-based thermoelectric (TE) power generation modules [22] are ground breaking, opening a

This chapter will provide an in-time overview for the newly discovered plastic/ flexible inorganic semiconductors. We shall first clarify the concept of flexibility and then illustrate the intrinsic plasticity for metals and brittleness for inorganic materials. Then, we will mention the special plasticity and the chemical bonding origins in a few ionic crystals such as AgCl. After that, we will systematically review the extraordinary mechanical properties of Ag2S and fully flexible thermoelectric devices. Finally, the prospect and challenge for plastic inorganic semiconductors as

As a matter of fact, "flexibility," unlike "ductility" or "rigidity," is not a scientifically clear concept. For flexible electronics, it is widely conceived that only elastic deformation is needed or allowed. In this sense, "flexibility" refers to the ability of the material/device to bend easily in an elastic way. According to Peng and Snyder [23], flexibility *f* is quantified by the largest curvature of bending, 1/*r*b, where *r*<sup>b</sup> is the minimum bending radius. A material with a thickness *h* bent about a radius *r*b experiences the greatest tensile and compressive stresses on the outer and inner faces, respectively. The maximum strain is readily calculated from the geometry

considering that the middle (neutral layer) of the material is unstrained:

\_ *h*2 ) <sup>−</sup> 2π *rb*] \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ 2π *rb*

The maximum elastic strain is reached when plastic deformation occurs: ε = *σy*/*E*, where *E* is the elastic (Young's) modulus. Thus, the flexibility becomes:

> *rb* = \_2 *h <sup>ε</sup>y* = \_2 *h* \_ *σy*

Removing the shape factor (2/*h*), the material flexibility is *f*fom = *σy*/*E*. The value of *ffom* is plotted in **Figure 1** for various materials. It is seen that this definition with the plot well distinguishes commonly sensed flexible materials such as rubber between rigid ones like ceramics. In fact, this definition is essentially consis-

That is, intrinsically stiff materials with a large thickness tend to be rigid or not flexible. Accordingly, (elastic) flexibility can be realized in thin films or flakes. Therefore, two-dimensional (2D) materials are popular candidates for flexible electronics. The above definition treats flexibility as elastic deformability, which is reasonable considering the real application. However, as discussed in Section 1, plasticity is important, particularly for efficient material processing. A plastic material can be easily processed into target geometry with little fracture or waste. In addition,

tent with the commonly seen bending stiffness or flexural rigidity, *fr* ~ *h*<sup>3</sup>

= \_*<sup>h</sup>*

2 *rb*

(1)

*E* [14, 24].

*<sup>E</sup>* (2)

**122**

**Figure 1.** *The ratio of yield strength to Young's modulus for various materials. Raw data are taken from Ref. [23].*

a plastic material gains intrinsic flexibility without the restrict of size, that is, it can be readily deformed without breaking even in the bulk format, which is essentially important for applications requiring energy densities. Nonetheless, plasticity is rarely seen in inorganic semiconductors, which will be discussed in Section 3.

#### **3. Prevalent brittleness for inorganic semiconductors**

Ductile and brittle behaviors are schematically shown in **Figure 2(a)**. Inorganic semiconductors are mostly constituted by covalent or ion-covalent bonds [25], which assure an appreciable electron orbital overlap, dispersive electronic band, and decent carrier mobility [26]. Covalent bonds are directional, saturated, and localized. Therefore, even a trivial bonding distortion will cause a large instability, which is vividly demonstrated by the deep and steep curve in the interatomic potential versus

#### **Figure 2.**

*(a) Stress-strain curves for brittle and ductile materials and (b) atomic potential varying with atomic distances.*

atomic distance diagram (**Figure 2(b)**) [20]. As a contrast, ionic bonding is unidirectional and nonsaturated. The columbic force is somewhat diffuse and extended within a large space. Consequently, the bonding distortion will induce a smaller variation in energy, as reflected by the shallow and flat potential-distance curve [20]. Nonetheless, charges in ionic materials are localized around anions, leading to a poor electrical conductivity. Metallic bonding strength is mediate between covalent and ionic bonds. Metal atoms use their outer-shell electrons for two- and multicenter polar interactions, which open the possibility to unite charge transfer with a nonnegligible density of states at the Fermi level. This makes up good electrical conductivity and ductility.

Beyond the covalent bonding characteristics, the absence of plasticity and flexibility in inorganic materials can be interpreted in terms of dislocations. Various defects exist in inorganic materials (e.g., ceramics) such as vacancies, interstitials, and voids, strongly inhibiting the movement of dislocations. In addition, grain boundaries can cause dislocation pile-up, which may be a kind of crack source. For metals, the pile-up of dislocations near grain boundaries will cause hardening in mechanical properties. For ceramics, ductility is limited by crack nucleation and glide. In fact, the main deformation mechanism for most ceramics is creep. This process is mainly controlled by dislocation climb and diffusion, both of which are strongly dependent on temperature.

#### **4. Plastic ionic inorganic materials**

Although most inorganic materials are generally brittle, some ionic crystals have been found to exhibit some degree of plasticity upon certain deformation even at room temperature, such as AgCl [27–28], KCl [29], and LiF [30]. Under a slow strain rate, these crystals exhibit tensile properties like metals and show the phenomenon of neck down [29]. This ductile behavior is attributed to the wavy slip [31–32]. During the deformation, each grain has its own change and should conform to the distortion of neighbors [33]. The wavy slipping allows to change planes in the vicinity of grain boundaries to permit distortions. The ability of crystals to relax the stress is essential for such a ductile deformation. At higher temperature, dislocations near the grain boundaries are easier to change the slipping planes due to the thermal active diffusion mentioned above, spreading out from boundaries to another grain. Thus, plastic deformation above transition temperature is mainly caused by the increase in slip systems and the dislocate-diffusion-induced creep and cross-slip.

First-principle calculations have been performed to compare the plasticity between AgCl and NaCl [34]. The generalized stacking fault energy (GSFE) and the double of the surface energy 2*γ*s were used as slipping barrier energy and cleavage energy, respectively. If GSFE > 2*γ*s, rupture occurs. It was found that in NaCl, GSFE >2*γ*s happens before reaching the middle of slipping process for most slipping systems. While for AgCl, interestingly, GSFE remains smaller than 2*γ*s over the whole slipping process for <110>{110}/{111}/{001} systems. It means that the three slipping systems are stable for plastic deformation in AgCl.

The reason for the different plastic behaviors between NaCl and AgCl was further attributed to their fundamental differences in electronic structures (**Figure 3**). The calculations [34] show that the smaller band gap and larger atomic-orbital overlap (Ag-5s and Cl-3p) contribute to a weak ionic bonding in AgCl. Crystal Orbital Hamilton population (COHP) analysis shows that the energies of initial Ag-Cl bonds gradually decrease as slipping proceeds. Particularly, in the (110), (111) planes, new Ag-Cl bonds are formed; a strong Ag-Ag bond is also formed when slipping on (100) planes. These newly formed bonds lower the unstable GSF energies in these slipping systems. While in NaCl, no newly formed bonds are observed.

**125**

**Figure 3.**

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

The plasticity in AgCl was also understood from the perspective of dislocations [35]. Electronic structures of dislocations in AgCl exhibit larger bonding interactions between atomic orbitals than that of NaCl. Also, the nearest neighbor distances around the dislocation core are tend to be shorter than that in NaCl. The two properties lead to a lower core energy (*E*core) for AgCl. Far from the core, atoms are displaced elastically, and the energy stored elastically (*E*elastic) for AgCl is also estimated to be lower than that in NaCl. The dislocation excess energies (*E*elastic + *E*core) in NaCl are at least two times higher than that in AgCl, regardless of the type of dislocation. Therefore, it is expected that dislocations in AgCl are much

*The ratio of GSF energy to the double surface energy 2γs of the {001} plane for different slipping systems in NaCl and AgCl. u denotes displacement, and b is the Burgers vector. The data are taken from Ref. [34].*

Very recently, ZnS, a well-known brittle material, was also reported to exhibit extraordinary "plasticity" in complete darkness [36]. ZnS crystals fractured immediately when they deformed under light irradiation. However, the crystals could be plastically deformed to a compression strain of 45% in complete darkness as shown in **Figure 4**. It is also found that the optical band gap decreased by 0.6 eV after deformation as also clearly reflected by the apparent colors, which is probably due to the formation of extra energy levels at the bandgap edge in the presence of dislocations. Based on optical and electronic microscopies, the plastic deformation in complete darkness is caused by glide and multiplication of dislocations belonging to the primary slip system. By contrast, plastic deformation under light irradiation involves deformation twinning. Obviously, the latter corresponds to a much poorer plasticity. The origins for different deformation types are explained below. The dislocations in ZnS decompose into two partial dislocations. In darkness, the synergetic

easier to nucleate and multiply from various sources.

**5. Plasticity of ZnS in darkness**

**Figure 3.**

*Hybrid Nanomaterials - Flexible Electronics Materials*

strongly dependent on temperature.

**4. Plastic ionic inorganic materials**

slipping systems are stable for plastic deformation in AgCl.

systems. While in NaCl, no newly formed bonds are observed.

atomic distance diagram (**Figure 2(b)**) [20]. As a contrast, ionic bonding is unidirectional and nonsaturated. The columbic force is somewhat diffuse and extended within a large space. Consequently, the bonding distortion will induce a smaller variation in energy, as reflected by the shallow and flat potential-distance curve [20]. Nonetheless, charges in ionic materials are localized around anions, leading to a poor electrical conductivity. Metallic bonding strength is mediate between covalent and ionic bonds. Metal atoms use their outer-shell electrons for two- and multicenter polar interactions, which open the possibility to unite charge transfer with a nonnegligible density of states at the Fermi level. This makes up good electrical conductivity and ductility. Beyond the covalent bonding characteristics, the absence of plasticity and flexibility in inorganic materials can be interpreted in terms of dislocations. Various defects exist in inorganic materials (e.g., ceramics) such as vacancies, interstitials, and voids, strongly inhibiting the movement of dislocations. In addition, grain boundaries can cause dislocation pile-up, which may be a kind of crack source. For metals, the pile-up of dislocations near grain boundaries will cause hardening in mechanical properties. For ceramics, ductility is limited by crack nucleation and glide. In fact, the main deformation mechanism for most ceramics is creep. This process is mainly controlled by dislocation climb and diffusion, both of which are

Although most inorganic materials are generally brittle, some ionic crystals have been found to exhibit some degree of plasticity upon certain deformation even at room temperature, such as AgCl [27–28], KCl [29], and LiF [30]. Under a slow strain rate, these crystals exhibit tensile properties like metals and show the phenomenon of neck down [29]. This ductile behavior is attributed to the wavy slip [31–32]. During the deformation, each grain has its own change and should conform to the distortion of neighbors [33]. The wavy slipping allows to change planes in the vicinity of grain boundaries to permit distortions. The ability of crystals to relax the stress is essential for such a ductile deformation. At higher temperature, dislocations near the grain boundaries are easier to change the slipping planes due to the thermal active diffusion mentioned above, spreading out from boundaries to another grain. Thus, plastic deformation above transition temperature is mainly caused by the increase in slip systems and the dislocate-diffusion-induced creep and cross-slip. First-principle calculations have been performed to compare the plasticity between AgCl and NaCl [34]. The generalized stacking fault energy (GSFE) and the double of the surface energy 2*γ*s were used as slipping barrier energy and cleavage energy, respectively. If GSFE > 2*γ*s, rupture occurs. It was found that in NaCl, GSFE >2*γ*s happens before reaching the middle of slipping process for most slipping systems. While for AgCl, interestingly, GSFE remains smaller than 2*γ*s over the whole slipping process for <110>{110}/{111}/{001} systems. It means that the three

The reason for the different plastic behaviors between NaCl and AgCl was further attributed to their fundamental differences in electronic structures (**Figure 3**). The calculations [34] show that the smaller band gap and larger atomic-orbital overlap (Ag-5s and Cl-3p) contribute to a weak ionic bonding in AgCl. Crystal Orbital Hamilton population (COHP) analysis shows that the energies of initial Ag-Cl bonds gradually decrease as slipping proceeds. Particularly, in the (110), (111) planes, new Ag-Cl bonds are formed; a strong Ag-Ag bond is also formed when slipping on (100) planes. These newly formed bonds lower the unstable GSF energies in these slipping

**124**

*The ratio of GSF energy to the double surface energy 2γs of the {001} plane for different slipping systems in NaCl and AgCl. u denotes displacement, and b is the Burgers vector. The data are taken from Ref. [34].*

The plasticity in AgCl was also understood from the perspective of dislocations [35]. Electronic structures of dislocations in AgCl exhibit larger bonding interactions between atomic orbitals than that of NaCl. Also, the nearest neighbor distances around the dislocation core are tend to be shorter than that in NaCl. The two properties lead to a lower core energy (*E*core) for AgCl. Far from the core, atoms are displaced elastically, and the energy stored elastically (*E*elastic) for AgCl is also estimated to be lower than that in NaCl. The dislocation excess energies (*E*elastic + *E*core) in NaCl are at least two times higher than that in AgCl, regardless of the type of dislocation. Therefore, it is expected that dislocations in AgCl are much easier to nucleate and multiply from various sources.

#### **5. Plasticity of ZnS in darkness**

Very recently, ZnS, a well-known brittle material, was also reported to exhibit extraordinary "plasticity" in complete darkness [36]. ZnS crystals fractured immediately when they deformed under light irradiation. However, the crystals could be plastically deformed to a compression strain of 45% in complete darkness as shown in **Figure 4**. It is also found that the optical band gap decreased by 0.6 eV after deformation as also clearly reflected by the apparent colors, which is probably due to the formation of extra energy levels at the bandgap edge in the presence of dislocations.

Based on optical and electronic microscopies, the plastic deformation in complete darkness is caused by glide and multiplication of dislocations belonging to the primary slip system. By contrast, plastic deformation under light irradiation involves deformation twinning. Obviously, the latter corresponds to a much poorer plasticity.

The origins for different deformation types are explained below. The dislocations in ZnS decompose into two partial dislocations. In darkness, the synergetic

#### **Figure 4.**

*Characterizations of plastic deformation. (a) Stress-strain curves of ZnS single crystals under white or UV light (365 nm) or in complete darkness. (b) An undeformed specimen. (c and d) The specimens deformed under (c) white light-emitting diode (LED) light and (d) UV LED light (365 nm). (e–g) The specimens deformed up to (e)* ε *= 11%, (f)* ε *= 25%, and (g)* ε *= 35% in complete darkness. (h) A stress-strain curve obtained by a deformation in complete darkness up to* ε *= 10% and the subsequent deformation under UV light. Adapted from Ref. [36] with the permission from AAAS, Copyright 2019.*

glide motion of the two partials will cause large slip deformations. On the contrary, under light irradiation, photo-excited electrons or holes can be trapped, thus charging some dislocations. The mobility of the dislocation can be limited by dragging the surrounding charge cloud compensating the dislocation charge. Therefore, the different charge states of the two dislocations in ZnS will lead to the great difference of their mobility, which will lead to the observed deformation twinning.

The work suggests that the mechanical properties are strongly intercorrelated to optical and electronic properties. It also implies that inorganic semiconductors are not necessarily "intrinsically" brittle.

#### **6. Room-temperature plastic semiconductor Ag2S**

The plastic ionic materials like AgCl are nearly insulating and cannot be used as semiconductors. Recently, α-Ag2S was discovered as the room-temperature ductile inorganic semiconductor as shown in **Figure 5** [21]. The plasticity of Ag2S is extraordinary: the engineering strains are ~4.5% in tension, 50% in compression, and above 20% in three-point bending, typical characteristics of metals as shown in **Figure 6**. α-Ag2S is a typical nondegenerate *n*-type semiconductor with a low electron carrier concentration about (1014–1015) cm−<sup>3</sup> and a large, negative Seebeck coefficient (around −1000 μV/K) at room temperature. The band gap is around 1 eV, and RT electrical conductivity ranges from 0.09 to 0.16 Sm<sup>−</sup><sup>1</sup> . The carrier mobility *μ*H is around 100 cm2 /Vs [21].

The marvelous plasticity of α-Ag2S comes from its special crystal structure. α-Ag2S adopts a monoclinic symmetry with the space group *P21/c*, consisting of

**127**

**Figure 6.**

*Copyright 2018.*

**Figure 5.**

*permissions from Springer Nature, Copyright 2018.*

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

*Elongation versus electrical conductivity for α-Ag2S and various materials. Adapted from Ref. [21] with* 

*Room-temperature mechanical properties of the semiconductor α-Ag2S. (a) A machined cylinder for the compression test (top) and its deformations under hammering (bottom). (b–d) Strain-stress curves for compression (b), bending, (c) and tension (d) tests at room temperature. Typical materials such as the ceramics yttria-stabilized zirconia (YSZ) and Ti, SiC; the metals Al, Nb, Ni, Cu, Ag, Cu–8.5%Zr alloy, and Fe3C; and the intermetallic compound TiAl are shown for comparison. The inset in c shows the as-cast ingot samples before and after the bending test. Adapted from Ref. [21] with permissions from Springer Nature,* 

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

#### **Figure 5.**

*Hybrid Nanomaterials - Flexible Electronics Materials*

glide motion of the two partials will cause large slip deformations. On the contrary, under light irradiation, photo-excited electrons or holes can be trapped, thus charging some dislocations. The mobility of the dislocation can be limited by dragging the surrounding charge cloud compensating the dislocation charge. Therefore, the different charge states of the two dislocations in ZnS will lead to the great difference

*Characterizations of plastic deformation. (a) Stress-strain curves of ZnS single crystals under white or UV light (365 nm) or in complete darkness. (b) An undeformed specimen. (c and d) The specimens deformed under (c) white light-emitting diode (LED) light and (d) UV LED light (365 nm). (e–g) The specimens deformed up to (e)* ε *= 11%, (f)* ε *= 25%, and (g)* ε *= 35% in complete darkness. (h) A stress-strain curve obtained by a deformation in complete darkness up to* ε *= 10% and the subsequent deformation under UV light.* 

The work suggests that the mechanical properties are strongly intercorrelated to optical and electronic properties. It also implies that inorganic semiconductors are

The plastic ionic materials like AgCl are nearly insulating and cannot be used as semiconductors. Recently, α-Ag2S was discovered as the room-temperature ductile inorganic semiconductor as shown in **Figure 5** [21]. The plasticity of Ag2S is extraordinary: the engineering strains are ~4.5% in tension, 50% in compression, and above 20% in three-point bending, typical characteristics of metals as shown in **Figure 6**. α-Ag2S is a typical nondegenerate *n*-type semiconductor with a low electron carrier concentration

at room temperature. The band gap is around 1 eV, and RT electrical conductivity

The marvelous plasticity of α-Ag2S comes from its special crystal structure. α-Ag2S adopts a monoclinic symmetry with the space group *P21/c*, consisting of

and a large, negative Seebeck coefficient (around −1000 μV/K)

/Vs [21].

. The carrier mobility *μ*H is around 100 cm2

of their mobility, which will lead to the observed deformation twinning.

**6. Room-temperature plastic semiconductor Ag2S**

*Adapted from Ref. [36] with the permission from AAAS, Copyright 2019.*

not necessarily "intrinsically" brittle.

**126**

**Figure 4.**

about (1014–1015) cm−<sup>3</sup>

ranges from 0.09 to 0.16 Sm<sup>−</sup><sup>1</sup>

*Elongation versus electrical conductivity for α-Ag2S and various materials. Adapted from Ref. [21] with permissions from Springer Nature, Copyright 2018.*

#### **Figure 6.**

*Room-temperature mechanical properties of the semiconductor α-Ag2S. (a) A machined cylinder for the compression test (top) and its deformations under hammering (bottom). (b–d) Strain-stress curves for compression (b), bending, (c) and tension (d) tests at room temperature. Typical materials such as the ceramics yttria-stabilized zirconia (YSZ) and Ti, SiC; the metals Al, Nb, Ni, Cu, Ag, Cu–8.5%Zr alloy, and Fe3C; and the intermetallic compound TiAl are shown for comparison. The inset in c shows the as-cast ingot samples before and after the bending test. Adapted from Ref. [21] with permissions from Springer Nature, Copyright 2018.*

tetramolecular units. Four S and four Ag atoms constitute eight-atomic ring fragments interlinked by the sulfur atoms. Precisely among (100) plane, a wrinkled structure formed by two S and six Ag atoms stacking along [100] direction, and this structure provides channels for slipping. In addition, it was found that Ag sites were occupied only for 70%, while S sites occupied completely. These unfixed Ag atoms may induce additional Ag-S and Ag-Ag bonds.

The multicentered, diffuse, and relatively weak bonding gives rise to the small slipping energy and large cleavage energy, i.e., plastic material can slip easily without cleavage as shown in **Figure 7**. As for α-Ag2S, it is assumed that slip plane is (100) and slip direction [001]. According to the calculation, the slipping energy (*E*B) is 150 meV per atom for Ag2S, which is comparably small with conventionally ductile metals (Ti, Mg); also, the cleavage energy is relatively large for Ag2S, which is 148 meV per atom, indicating certain relatively strong forces interlinking those slip planes instead of cleaving. In comparison, the values are, respectively, less than 60 meV for NaCl, graphite, and diamond; 570 meV for Mg; and 2150 meV per atom for Ti.

The distribution of the electron localizability indicator (ELI-D) shows a local maximum on the outer side of each S atom, and the basin of this maximum is caused by the formation of a lone pair or a strong Ag-S interaction shown in quantum theory of atoms in molecules (QTAIM). Besides, these lone pairs form double layers in the (100) plane. Thus, the *E*B is supposed to be small due to a relative weak interaction between the lone pairs. The COHP calculations reveal chemical bonds changing during the glide. During the slipping process, some bonds vanish, while new bonds form continuously during the whole process, and these new bonds are comparable with the Ag-S bonds between layers in strength. Therefore, S atoms are always bonded with Ag atoms, resulting in a large *ΔE*C to prevent materials from cleaving. In short, S atoms move along Ag-formed tracks easily due to small energy difference in steps, while these are difficult to cleavage for tight bonding with surrounded Ag atoms.

Li et al. [37] applied *ab initio-*based DFT to investigate the structural response of Ag2S under pure shear, uniaxial tension, and biaxial shear deformations. To simulate quasi-static mechanical loading process, they imposed shear or tensile strain on a particular system gradually. It is found that Ag2S has a theoretical shear strength of 1.02 GPa in the (001) [010] and (100) [010] slip systems (**Figure 8(a)**) lower than that of common metal and ceramics. The ideal tensile strength along [100] direction is 2.2 GPa, significantly higher than its shear strength (**Figure 8(b)**). The very low shear strength under these slip systems is expected to create pathways for easy slip.

The Ag-S octagon structure would be highly distorted under shear deformation. However, the Ag-S bond lengths change only slightly during shearing. Only when the strain is larger than ~110%, bond breakage would happen. This feature implies that Ag2S would retain its structural integrity even under very large shear deformation. Under (100) [010] shear load, Ag-Ag metallic bonds would form at 0.671 strain. These newly formed bonds strengthen the Ag-S frameworks and contribute to its structural integrity. Under [100] tensile load, the processes of breakage and the formation of Ag-S bond happen at the same time. The structural integrity could thus be preserved to a large strain. Based on these discoveries, they proposed that the easy slip pathways with good structural integrity under shear load are the origin of ductility in Ag2S.

Recent research also found that the single-layer Ag2S is a kind of auxetic materials with unusual negative Poisson's ratio [38]. According to their calculations, single-layer Ag2S has relatively low Young's modulus with 61.61 N·m<sup>−</sup><sup>1</sup> along [010] direction and 2.78 N·m<sup>−</sup><sup>1</sup> along [100] direction. This is the lowest value found in various 2D materials such as graphite and MoS2. This means that along [100] direction, single-layer Ag2S will show extraordinary flexibility. Moreover, single-layer Ag2S shows negative Poisson's ratio in both in-plane and out-of-plane directions, which is unique among quasi-2D materials.

**129**

mobility (~100 cm<sup>2</sup>

K<sup>−</sup><sup>1</sup>

(~0.5 Wm<sup>−</sup><sup>1</sup>

**Figure 8.**

**Figure 7.**

**7. Full inorganic flexible thermoelectric materials**

*Adapted from Ref. [37] under the Creative Commons CC BY license.*

*from Ref. [21] with permissions from Springer Nature, Copyright 2018.*

The discovery of plastic Ag2S makes it possible to fabricate full-inorganic flexible devices. Pristine Ag2S exhibits a medium band gap (~1.0 eV), high

*Stress response of α-Ag2S against pure shear strain and biaxial tensile strain, respectively. (a) Shear-stress-shearstrain relations along various slip systems. (b) Tensile-stress-tensile-strain relations along various tensile systems.* 

*(a) Crystal structure of α-Ag2S along [001] direction. (b) Schematic map for energy variation as a function of interlayer distance* d *during slipping. (c) (EL − E0) and (d) (EInf − EL) behavior during slipping. Adapted* 

/Vs), and extremely low lattice thermal conductivity

near room temperature) [21], which makes it a potential candidate

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

#### **Figure 7.**

*Hybrid Nanomaterials - Flexible Electronics Materials*

may induce additional Ag-S and Ag-Ag bonds.

tetramolecular units. Four S and four Ag atoms constitute eight-atomic ring fragments interlinked by the sulfur atoms. Precisely among (100) plane, a wrinkled structure formed by two S and six Ag atoms stacking along [100] direction, and this structure provides channels for slipping. In addition, it was found that Ag sites were occupied only for 70%, while S sites occupied completely. These unfixed Ag atoms

The multicentered, diffuse, and relatively weak bonding gives rise to the small slipping energy and large cleavage energy, i.e., plastic material can slip easily without cleavage as shown in **Figure 7**. As for α-Ag2S, it is assumed that slip plane is (100) and slip direction [001]. According to the calculation, the slipping energy (*E*B) is 150 meV per atom for Ag2S, which is comparably small with conventionally ductile metals (Ti, Mg); also, the cleavage energy is relatively large for Ag2S, which is 148 meV per atom, indicating certain relatively strong forces interlinking those slip planes instead of cleaving. In comparison, the values are, respectively, less than 60 meV for NaCl,

The distribution of the electron localizability indicator (ELI-D) shows a local maximum on the outer side of each S atom, and the basin of this maximum is caused by the formation of a lone pair or a strong Ag-S interaction shown in quantum theory of atoms in molecules (QTAIM). Besides, these lone pairs form double layers in the (100) plane. Thus, the *E*B is supposed to be small due to a relative weak interaction between the lone pairs. The COHP calculations reveal chemical bonds changing during the glide. During the slipping process, some bonds vanish, while new bonds form continuously during the whole process, and these new bonds are comparable with the Ag-S bonds between layers in strength. Therefore, S atoms are always bonded with Ag atoms, resulting in a large *ΔE*C to prevent materials from cleaving. In short, S atoms move along Ag-formed tracks easily due to small energy difference in steps, while these are difficult to cleavage for tight bonding with surrounded Ag atoms.

Li et al. [37] applied *ab initio-*based DFT to investigate the structural response of Ag2S under pure shear, uniaxial tension, and biaxial shear deformations. To simulate quasi-static mechanical loading process, they imposed shear or tensile strain on a particular system gradually. It is found that Ag2S has a theoretical shear strength of 1.02 GPa in the (001) [010] and (100) [010] slip systems (**Figure 8(a)**) lower than that of common metal and ceramics. The ideal tensile strength along [100] direction is 2.2 GPa, significantly higher than its shear strength (**Figure 8(b)**). The very low shear strength under these slip systems is expected to create pathways for easy slip. The Ag-S octagon structure would be highly distorted under shear deformation. However, the Ag-S bond lengths change only slightly during shearing. Only when the strain is larger than ~110%, bond breakage would happen. This feature implies that Ag2S would retain its structural integrity even under very large shear deformation. Under (100) [010] shear load, Ag-Ag metallic bonds would form at 0.671 strain. These newly formed bonds strengthen the Ag-S frameworks and contribute to its structural integrity. Under [100] tensile load, the processes of breakage and the formation of Ag-S bond happen at the same time. The structural integrity could thus be preserved to a large strain. Based on these discoveries, they proposed that the easy slip pathways with good structural integrity under shear load are the origin of ductility in Ag2S. Recent research also found that the single-layer Ag2S is a kind of auxetic materials with unusual negative Poisson's ratio [38]. According to their calculations,

graphite, and diamond; 570 meV for Mg; and 2150 meV per atom for Ti.

single-layer Ag2S has relatively low Young's modulus with 61.61 N·m<sup>−</sup><sup>1</sup>

various 2D materials such as graphite and MoS2. This means that along [100] direction, single-layer Ag2S will show extraordinary flexibility. Moreover, single-layer Ag2S shows negative Poisson's ratio in both in-plane and out-of-plane directions,

along [100] direction. This is the lowest value found in

along [010]

**128**

direction and 2.78 N·m<sup>−</sup><sup>1</sup>

which is unique among quasi-2D materials.

*(a) Crystal structure of α-Ag2S along [001] direction. (b) Schematic map for energy variation as a function of interlayer distance* d *during slipping. (c) (EL − E0) and (d) (EInf − EL) behavior during slipping. Adapted from Ref. [21] with permissions from Springer Nature, Copyright 2018.*

**Figure 8.**

*Stress response of α-Ag2S against pure shear strain and biaxial tensile strain, respectively. (a) Shear-stress-shearstrain relations along various slip systems. (b) Tensile-stress-tensile-strain relations along various tensile systems. Adapted from Ref. [37] under the Creative Commons CC BY license.*

#### **7. Full inorganic flexible thermoelectric materials**

The discovery of plastic Ag2S makes it possible to fabricate full-inorganic flexible devices. Pristine Ag2S exhibits a medium band gap (~1.0 eV), high mobility (~100 cm<sup>2</sup> /Vs), and extremely low lattice thermal conductivity (~0.5 Wm<sup>−</sup><sup>1</sup> K<sup>−</sup><sup>1</sup> near room temperature) [21], which makes it a potential candidate for thermoelectric application. However, the extremely low carrier concentration (about 1.6 × 1014 cm−<sup>3</sup> at room temperature) leads to its poor electrical conductivity. Therefore, the thermoelectric performance of Ag2S should be further optimized, and its plasticity should be maintained at the same time.

By alloying with Se/Te, Liang et al. successfully tuned the carrier concentration of Ag2S by virtue of the lowered defect formation energy of Ag interstitial atoms. Consequently, the electrical conductivity and power factor were largely optimized: the electrical conductivities of Ag2S0.5Se0.5, Ag2S0.8Te0.2, and Ag2S0.5Se0.45Te0.05 reach around 104 S m<sup>−</sup><sup>1</sup> at room temperature, which are comparable to state-of-the-art brittle inorganic TE materials. The power factors of Ag2S-based materials can reach 5 μW·cm<sup>−</sup><sup>1</sup> ·K<sup>−</sup><sup>2</sup> at room temperature. Meanwhile, Ag2(S, Se), Ag2(S, Te), and Ag2(S, Se, Te) have record low thermal conductivities of 0.3~0.6 Wm<sup>−</sup><sup>1</sup> K<sup>−</sup><sup>1</sup> at 300–450 K, among the lowest values observed in fully densified inorganic solids. The band gap of Se or Te alloyed Ag2S-based materials is also reduced, shifting the peak value of *zT* toward a lower temperature. Thus, a highest *zT* value about 0.44 could be realized for Ag2S0.5Se0.45Te0.05 at room temperature.

More interestingly, alloying does not severely impair the plasticity. According to the mechanical property test, the ductility and flexibility of Ag2S-based materials would be maintained if the Se content is less than 60% or the Te content is less than 70%. Hence, the materials would possess both good ductility and TE performance when the Se/Te content is in the range of 20~60%, as shown in **Figure 9(c)**. To further verify the robustness of Ag2S-based materials under different usage scenarios, bending cycle test was conducted on Ag2S0.5Se0.5 strip with a thickness of about 10 μm. As shown in **Figure 9(f )**, the variation in Seebeck coefficient and electrical conductivity is less than 10% after 1000 bending cycles with a bending radius less than 3 mm. In addition, the relative resistance variation in the strip under different bending radius is also estimated.

#### **Figure 9.**

*(a) TE figure of merit* zT *for Ag2(S, Se), Ag2(S, Te), and Ag2(S, Se, Te) at 300 K. Several representative organic TE materials are included for comparison. (b) Calculated band structure for Ag2S and Ag2S0.5Se0.5. (c) Flexibility-*zT *phase diagram of Ag2S-Ag2Se-Ag2Te system. (d) The Ag2S0.5Se0.5 and Ag2S0.8Te0.2 samples twisted to various shapes. (e) Bending tests of Ag2S1–xSe*x *(*x *= 0, 0.1, 0.3, 0.5). (f) Relative electrical conductivity variation σ*/*σ0 of the Ag2S0.5Se0.5 foil after various number of times of bending cycles. Reproduced from Ref. [22] with permissions from The Royal Society of Chemistry. Copyright 2019.*

**131**

*Chemistry. Copyright 2019.*

**Figure 10.**

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

flexible thermoelectrics.

ture difference of 30 K.

Using Ag2S0.5Se0.5 strips as *n*-type legs and Pt-Rh wires as *p*-type legs, fullinorganic thermoelectric devices were fabricated as shown in **Figure 10** [22]. Under temperature difference 20 K, the maximum power of the device is 10 μW, and the

higher than those for current organic TE devices. This research has solved the most fundamental and challenging problems for the fabrication of full-inorganic and high-performance flexible thermoelectrics, opening a new direction for inorganic

The conventional strategy toward flexible TE devices is mounting TE thinfilm materials onto the intrinsically flexible substrates. Ding et al. [39] fabricated flexible TE devices based on Ag2Se nanowires and plastic nylon substrate. The hybrid film was hot pressed at 200°C and 1 MPa for 30 min, which endows the film high TE performance and excellent flexibility at the same time. The highest power

almost the highest value among reported *n*-type flexible TE materials. Although the internal resistance of the four legs of Ag2Se/nylon devices is up to 250 Ω, the

*(a) Upper panel: A schematic of the Ag2S0.5Se0.5/Pt-Rh in-plane device with Ag2S0.5Se0.5 as* n*-type legs and Pt-Ru wire as* p*-type legs. Bottom panel: Optical image of a six-couple flexible Ag2S0.5Se0.5/Pt-Rh TE device. The as-shown in-plane device is merely for the purpose of demonstration. (b) Output voltage* V *and power output* P*out as a function of current (*I*) for a six-couple Ag2S0.5Se0.5/Pt-Rh device under different operating temperature differences. The cold side temperature is fixed at 293 K. (c) Comparison of normalized maximum power density (*P*max*L*/*A*) among the Ag2(S, Se)-based inorganic TE device, inorganic-organic hybrid flexible TE devices, and organic flexible TE devices. (d) Relative electrical resistance variation* R*/*R*0 of the Ag2S0.5Se0.5/ Pt-Rh TE device after bending various number of times. The inset shows the optical image for the bended device. The bending radius is 10 mm. Reproduced from Ref. [22] with permissions from The Royal Society of* 

, which is much

at 300 K,

under a tempera-

K<sup>−</sup><sup>2</sup>

normalized maximum power density *P*max*L*/*A* reaches 0.08 W·m<sup>−</sup><sup>1</sup>

factor value of as-prepared Ag2Se/nylon film reaches 9.87 μWm<sup>−</sup><sup>1</sup>

in-plane TE device exhibits a high-power density of 2.3 W m<sup>−</sup><sup>2</sup>

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

*Hybrid Nanomaterials - Flexible Electronics Materials*

and its plasticity should be maintained at the same time.

Se, Te) have record low thermal conductivities of 0.3~0.6 Wm<sup>−</sup><sup>1</sup>

ized for Ag2S0.5Se0.45Te0.05 at room temperature.

(about 1.6 × 1014 cm−<sup>3</sup>

S m<sup>−</sup><sup>1</sup>

bending radius is also estimated.

·K<sup>−</sup><sup>2</sup>

around 104

5 μW·cm<sup>−</sup><sup>1</sup>

for thermoelectric application. However, the extremely low carrier concentration

Therefore, the thermoelectric performance of Ag2S should be further optimized,

By alloying with Se/Te, Liang et al. successfully tuned the carrier concentration of Ag2S by virtue of the lowered defect formation energy of Ag interstitial atoms. Consequently, the electrical conductivity and power factor were largely optimized: the electrical conductivities of Ag2S0.5Se0.5, Ag2S0.8Te0.2, and Ag2S0.5Se0.45Te0.05 reach

brittle inorganic TE materials. The power factors of Ag2S-based materials can reach

among the lowest values observed in fully densified inorganic solids. The band gap of Se or Te alloyed Ag2S-based materials is also reduced, shifting the peak value of *zT* toward a lower temperature. Thus, a highest *zT* value about 0.44 could be real-

More interestingly, alloying does not severely impair the plasticity. According to the mechanical property test, the ductility and flexibility of Ag2S-based materials would be maintained if the Se content is less than 60% or the Te content is less than 70%. Hence, the materials would possess both good ductility and TE performance when the Se/Te content is in the range of 20~60%, as shown in **Figure 9(c)**. To further verify the robustness of Ag2S-based materials under different usage scenarios, bending cycle test was conducted on Ag2S0.5Se0.5 strip with a thickness of about 10 μm. As shown in **Figure 9(f )**, the variation in Seebeck coefficient and electrical conductivity is less than 10% after 1000 bending cycles with a bending radius less than 3 mm. In addition, the relative resistance variation in the strip under different

*(a) TE figure of merit* zT *for Ag2(S, Se), Ag2(S, Te), and Ag2(S, Se, Te) at 300 K. Several representative organic TE materials are included for comparison. (b) Calculated band structure for Ag2S and Ag2S0.5Se0.5. (c) Flexibility-*zT *phase diagram of Ag2S-Ag2Se-Ag2Te system. (d) The Ag2S0.5Se0.5 and Ag2S0.8Te0.2 samples twisted to various shapes. (e) Bending tests of Ag2S1–xSe*x *(*x *= 0, 0.1, 0.3, 0.5). (f) Relative electrical conductivity variation σ*/*σ0 of the Ag2S0.5Se0.5 foil after various number of times of bending cycles. Reproduced from Ref. [22]* 

*with permissions from The Royal Society of Chemistry. Copyright 2019.*

at room temperature, which are comparable to state-of-the-art

at room temperature. Meanwhile, Ag2(S, Se), Ag2(S, Te), and Ag2(S,

K<sup>−</sup><sup>1</sup>

at 300–450 K,

at room temperature) leads to its poor electrical conductivity.

**130**

**Figure 9.**

Using Ag2S0.5Se0.5 strips as *n*-type legs and Pt-Rh wires as *p*-type legs, fullinorganic thermoelectric devices were fabricated as shown in **Figure 10** [22]. Under temperature difference 20 K, the maximum power of the device is 10 μW, and the normalized maximum power density *P*max*L*/*A* reaches 0.08 W·m<sup>−</sup><sup>1</sup> , which is much higher than those for current organic TE devices. This research has solved the most fundamental and challenging problems for the fabrication of full-inorganic and high-performance flexible thermoelectrics, opening a new direction for inorganic flexible thermoelectrics.

The conventional strategy toward flexible TE devices is mounting TE thinfilm materials onto the intrinsically flexible substrates. Ding et al. [39] fabricated flexible TE devices based on Ag2Se nanowires and plastic nylon substrate. The hybrid film was hot pressed at 200°C and 1 MPa for 30 min, which endows the film high TE performance and excellent flexibility at the same time. The highest power factor value of as-prepared Ag2Se/nylon film reaches 9.87 μWm<sup>−</sup><sup>1</sup> K<sup>−</sup><sup>2</sup> at 300 K, almost the highest value among reported *n*-type flexible TE materials. Although the internal resistance of the four legs of Ag2Se/nylon devices is up to 250 Ω, the in-plane TE device exhibits a high-power density of 2.3 W m<sup>−</sup><sup>2</sup> under a temperature difference of 30 K.

#### **Figure 10.**

*(a) Upper panel: A schematic of the Ag2S0.5Se0.5/Pt-Rh in-plane device with Ag2S0.5Se0.5 as* n*-type legs and Pt-Ru wire as* p*-type legs. Bottom panel: Optical image of a six-couple flexible Ag2S0.5Se0.5/Pt-Rh TE device. The as-shown in-plane device is merely for the purpose of demonstration. (b) Output voltage* V *and power output* P*out as a function of current (*I*) for a six-couple Ag2S0.5Se0.5/Pt-Rh device under different operating temperature differences. The cold side temperature is fixed at 293 K. (c) Comparison of normalized maximum power density (*P*max*L*/*A*) among the Ag2(S, Se)-based inorganic TE device, inorganic-organic hybrid flexible TE devices, and organic flexible TE devices. (d) Relative electrical resistance variation* R*/*R*0 of the Ag2S0.5Se0.5/ Pt-Rh TE device after bending various number of times. The inset shows the optical image for the bended device. The bending radius is 10 mm. Reproduced from Ref. [22] with permissions from The Royal Society of Chemistry. Copyright 2019.*

#### **8. Summaries and outlook**

This chapter reviews the newly emerging plastic inorganic semiconductors (e.g., Ag2S) for next-generation flexible electronics. The term "flexibility" is clarified at the very beginning. It should be recognized that plasticity is important for flexible electronics due to the availability of feasible processing and deformability free of size restricts. The intrinsic brittleness for inorganic semiconductors and ceramics is explained from unidirectional and saturated characteristics of covalent bonds. Historically, ionic crystals like AgCl have been found to exhibit certain plasticity but lack decent electrical conductivity. Groundbreakingly, Ag2S was discovered as the room-temperature ductile semiconductor. From the chemical bonding perspective, the multicentered, diffuse, weak interactions induce easy slipping while maintaining the integrity, which holds well not only for Ag2S but also for other plastic materials. The generalized bonding features are useful guidance for developing new flexible/plastic semiconductors. The electrical properties and thermoelectric performance of Ag2S are readily optimized upon Se/Te alloying while maintaining the plasticity and flexibility. Successively, full-inorganic thermoelectric devices are fabricated based on plastic and flexible Ag2S-based semiconductors, yielding much higher output power density than organic counterparts.

The discovery and application demonstration of plastic Ag2S inorganic semiconductor pave a new way toward next-generation flexible electronics. Facing large-scale applications in electronics and energy conversions, several key challenges lie ahead. First and basically, the mechanisms for plastic deformation in Ag2S needs further investigation, especially on the individual and synergetic effects of both chemical bonding and dislocations, which calls for tremendous efforts of both experimentalists and theorists from a variety of disciplines. Second, practical criteria are required to rapidly screen potentially new, plastic/flexible semiconductors. These performance indicators should be easily available yet insightful, and it is better that they can be implemented into the high-throughput calculations. Third, all the techniques are to be renewed including material processing, electrode/substrate selection, device fabrication, and circuit integration.

Facing all these exciting challenges and fascinating opportunities, there is no doubt that the flexible/plastic inorganic semiconductors will bring a revolution to academic communities, electronic/energy industries, and worldwide market. The next-generation flexible electronics is meant to deeply change our life and reshape the world. The future has come.

**133**

**Author details**

Shanghai, China

Tian-Ran Wei1,2, Heyang Chen2

, Xun Shi1,2 and Lidong Chen1

1 State Key Laboratory of High Performance Ceramics and Superfine

and Engineering, Shanghai Jiao Tong University, Shanghai, China

\*Address all correspondence to: cld@mail.sic.ac.cn

provided the original work is properly cited.

Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences,

2 State Key Laboratory of Metal Matrix Composites, School of Materials Science

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

*Plastic Inorganic Semiconductors for Flexible Electronics DOI: http://dx.doi.org/10.5772/intechopen.91195*

*Hybrid Nanomaterials - Flexible Electronics Materials*

higher output power density than organic counterparts.

selection, device fabrication, and circuit integration.

the world. The future has come.

This chapter reviews the newly emerging plastic inorganic semiconductors (e.g., Ag2S) for next-generation flexible electronics. The term "flexibility" is clarified at the very beginning. It should be recognized that plasticity is important for flexible electronics due to the availability of feasible processing and deformability free of size restricts. The intrinsic brittleness for inorganic semiconductors and ceramics is explained from unidirectional and saturated characteristics of covalent bonds. Historically, ionic crystals like AgCl have been found to exhibit certain plasticity but lack decent electrical conductivity. Groundbreakingly, Ag2S was discovered as the room-temperature ductile semiconductor. From the chemical bonding perspective, the multicentered, diffuse, weak interactions induce easy slipping while maintaining the integrity, which holds well not only for Ag2S but also for other plastic materials. The generalized bonding features are useful guidance for developing new flexible/plastic semiconductors. The electrical properties and thermoelectric performance of Ag2S are readily optimized upon Se/Te alloying while maintaining the plasticity and flexibility. Successively, full-inorganic thermoelectric devices are fabricated based on plastic and flexible Ag2S-based semiconductors, yielding much

The discovery and application demonstration of plastic Ag2S inorganic semiconductor pave a new way toward next-generation flexible electronics. Facing large-scale applications in electronics and energy conversions, several key challenges lie ahead. First and basically, the mechanisms for plastic deformation in Ag2S needs further investigation, especially on the individual and synergetic effects of both chemical bonding and dislocations, which calls for tremendous efforts of both experimentalists and theorists from a variety of disciplines. Second, practical criteria are required to rapidly screen potentially new, plastic/flexible semiconductors. These performance indicators should be easily available yet insightful, and it is better that they can be implemented into the high-throughput calculations. Third, all the techniques are to be renewed including material processing, electrode/substrate

Facing all these exciting challenges and fascinating opportunities, there is no doubt that the flexible/plastic inorganic semiconductors will bring a revolution to academic communities, electronic/energy industries, and worldwide market. The next-generation flexible electronics is meant to deeply change our life and reshape

**8. Summaries and outlook**

**132**

### **Author details**

Tian-Ran Wei1,2, Heyang Chen2 , Xun Shi1,2 and Lidong Chen1 \*

1 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China

2 State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China

\*Address all correspondence to: cld@mail.sic.ac.cn

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### *Edited by Rafael Vargas-Bernal, Peng He and Shuye Zhang*

Two of the hottest research topics today are hybrid nanomaterials and flexible electronics. As such, this book covers both topics with chapters written by experts from across the globe. Chapters address hybrid nanomaterials, electronic transport in black phosphorus, three-dimensional nanocarbon hybrids, hybrid ion exchangers, pressure-sensitive adhesives for flexible electronics, simulation and modeling of transistors, smart manufacturing technologies, and inorganic semiconductors.

Published in London, UK © 2020 IntechOpen © v\_alex / iStock

Hybrid Nanomaterials - Flexible Electronics Materials

Hybrid Nanomaterials

Flexible Electronics Materials

*Edited by Rafael Vargas-Bernal,* 

*Peng He and Shuye Zhang*