**1. Overview of printing techniques**

#### **1.1. Introduction to printing**

The beginning of printing techniques can be followed back to ancient history [1]. Here the use of stencils was used to define patterns as part of cave paintings. As time progressed, techniques that are more complex were developed, with the most common being seals used to pattern clay and wax [2]. The idea behind was to replicate patterns that were commonly

© 2016 The Author(s). Licensee InTech. 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. © 2018 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.

used. The seal is an inverted image that is pressed into a soft material to create the desired pattern. Such a technique requires the material to be malleable as such with a similar idea, block printing was developed where a pre-defined carved block is inked and then brought into contact with the samples, in early times this being cloth and later following, paper. While this method allowed for the fast multiplication of the single block, it required a long time to alter the master block. Following this, it gave rise to the movable type where a big block would be composed of smaller blocks that could be permutated to create various designs. This along with the invention of the printing press gave rise to the printing revolution in Europe. With the tremendous technological advances of the twentieth century, fully automated printing technologies were developed leading to the digital household printers becoming the norm. Making use of these advances, throughput for printed media surged.

From an electronic point of view, inorganic electronics are predominantly used due to the inherent material properties of naturally found elements. Transition metals such as copper, gold or silver, display very good electrical conductance while silicon, germanium and III–V elements (among others) are employed as semiconductors in elements such as transistors and diodes. Although these materials offer ideal electrical characteristics, they have certain drawbacks when it comes to mechanical and or optical features. In terms of processing, these materials also tend to have limiting factors such as high processing temperatures, require high vacuum and/or involve chemical processes that are environmentally unfriendly. To overcome these shortcomings organic and nanomaterials have been developed, in the past decades, with the promise to enhance and cover the gaps in terms of possible devices. While extending the portfolio of material properties, the aforementioned materials can in most cases be brought into solution usually in form of a dispersion lending themselves to processing techniques widely used in printed technology. Such techniques have inherent advantages in comparison to commonly used CMOS technologies, both in terms of processing condition as well as throughput potential. In the following sub sections commonly used printed techniques used in the printed electronics community will be explored accompanied by state of the art devices fabricated herewith. fully cover

#### **1.2. Gravure and flexographic printing**

Gravure printing is one of the most common printing techniques and shares similarities to flexographic printing. The main difference is in the application of the ink to the substrate. Flexographic printing is a relief printing technique that employs a soft plate similar to stamping. In gravure printing the image is etched or laser written into a metal plate essentially creating pockets into which the ink can be filled. **Figure 1** displays the two processing techniques with their core components as well as the inking of the printing roll and transfer onto the substrate. Both these processes excel when it comes to throughput in comparison to other printing techniques. In terms of printed electronics gravure and flexographic printed have been employed to fabricate a plethora of devices spanning from organic photovoltaic (OPV) [3], transparent conductive films (TCFs) [4], thin-film transistors (TFTs) [5] and organic light emitting diodes (OLEDs) [6]. In addition Kraus et al. [7] have used gravure printing to deposit nanoparticles in the sub 100 nm range. As such proving that gravure printing not only profits from high throughput but can also provide very good resolution. A general drawback of this printing technique are the required costs in setting up the production in comparison to, for example, ink jetting where the pattern can be altered digitally. a

**1.3. Screen printing**

depicted.

ily scaled to an industrial scale.

Screen printing is a contact printing method that makes use of a patterned mesh to define structures on a variety of substrates. Highly viscous ink is pushed through the mesh that supports an ink-blocking stencil, with the aid of a squeegee. A typical printing process consists of inking the screen followed by a pass over with the squeegee. Due to the force exerted on the screen, by the squeegee, the screen is brought into contact with the substrate and the ink passes through creating a pattern. **Figure 2** displays the main components of the screen printer while in operation. Resolution is limited by ink formulation, thread thickness and density. Although commonly used in the printing of labels, signs as well clothing, screen printing has been widely used in flexible and printed electronics for a variety of devices including conductors [8], TFTs [9], RFID antennas [10]. An impressive work was done by Krebs et al. [11], who presented a fully screen printed, flexible solar cell based on a blend of organic acceptor and donor materials. The cells were integrated into household items to show the feasibility of such devices. Due to the possibility of integration into R2R framework, screen printing can be eas-

**Figure 2.** Schematic drawing of screen printer during a printing process. Key components as well resulting pattern are

**Figure 1.** Schematic displaying the core principle of (left) gravure printing and (right) flexographic printing as well as the

Technological Integration in Printed Electronics http://dx.doi.org/10.5772/intechopen.76520 95

inking of the printing cylinders and transfer to the substrate (adapted from [3] with authorization).

**Figure 1.** Schematic displaying the core principle of (left) gravure printing and (right) flexographic printing as well as the inking of the printing cylinders and transfer to the substrate (adapted from [3] with authorization).

**Figure 2.** Schematic drawing of screen printer during a printing process. Key components as well resulting pattern are depicted.

#### **1.3. Screen printing**

used. The seal is an inverted image that is pressed into a soft material to create the desired pattern. Such a technique requires the material to be malleable as such with a similar idea, block printing was developed where a pre-defined carved block is inked and then brought into contact with the samples, in early times this being cloth and later following, paper. While this method allowed for the fast multiplication of the single block, it required a long time to alter the master block. Following this, it gave rise to the movable type where a big block would be composed of smaller blocks that could be permutated to create various designs. This along with the invention of the printing press gave rise to the printing revolution in Europe. With the tremendous technological advances of the twentieth century, fully automated printing technologies were developed leading to the digital household printers becoming the norm.

From an electronic point of view, inorganic electronics are predominantly used due to the inherent material properties of naturally found elements. Transition metals such as copper, gold or silver, display very good electrical conductance while silicon, germanium and III–V elements (among others) are employed as semiconductors in elements such as transistors and diodes. Although these materials offer ideal electrical characteristics, they have certain drawbacks when it comes to mechanical and or optical features. In terms of processing, these materials also tend to have limiting factors such as high processing temperatures, require high vacuum and/or involve chemical processes that are environmentally unfriendly. To overcome these shortcomings organic and nanomaterials have been developed, in the past decades, with the promise to enhance and cover the gaps in terms of possible devices. While extending the portfolio of material properties, the aforementioned materials can in most cases be brought into solution usually in form of a dispersion lending themselves to processing techniques widely used in printed technology. Such techniques have inherent advantages in comparison to commonly used CMOS technologies, both in terms of processing condition as well as throughput potential. In the following sub sections commonly used printed techniques used in the printed electronics community will be explored accompanied by state of

Gravure printing is one of the most common printing techniques and shares similarities to flexographic printing. The main difference is in the application of the ink to the substrate. Flexographic printing is a relief printing technique that employs a soft plate similar to stamping. In gravure printing the image is etched or laser written into a metal plate essentially creating pockets into which the ink can be filled. **Figure 1** displays the two processing techniques with their core components as well as the inking of the printing roll and transfer onto the substrate. Both these processes excel when it comes to throughput in comparison to other printing techniques. In terms of printed electronics gravure and flexographic printed have been employed to fabricate a plethora of devices spanning from organic photovoltaic (OPV) [3], transparent conductive films (TCFs) [4], thin-film transistors (TFTs) [5] and organic light emitting diodes (OLEDs) [6]. In addition Kraus et al. [7] have used gravure printing to deposit nanoparticles in the sub 100 nm range. As such proving that gravure printing not only profits from high throughput but can also provide very good resolution. A general drawback of this printing technique are the required costs in setting up the production in comparison to, for

Making use of these advances, throughput for printed media surged.

the art devices fabricated herewith.

94 Flexible Electronics

**1.2. Gravure and flexographic printing**

example, ink jetting where the pattern can be altered digitally.

Screen printing is a contact printing method that makes use of a patterned mesh to define structures on a variety of substrates. Highly viscous ink is pushed through the mesh that supports an ink-blocking stencil, with the aid of a squeegee. A typical printing process consists of inking the screen followed by a pass over with the squeegee. Due to the force exerted on the screen, by the squeegee, the screen is brought into contact with the substrate and the ink passes through creating a pattern. **Figure 2** displays the main components of the screen printer while in operation. Resolution is limited by ink formulation, thread thickness and density. Although commonly used in the printing of labels, signs as well clothing, screen printing has been widely used in flexible and printed electronics for a variety of devices including conductors [8], TFTs [9], RFID antennas [10]. An impressive work was done by Krebs et al. [11], who presented a fully screen printed, flexible solar cell based on a blend of organic acceptor and donor materials. The cells were integrated into household items to show the feasibility of such devices. Due to the possibility of integration into R2R framework, screen printing can be easily scaled to an industrial scale.

#### **1.4. Inkjet**

Inkjet is a widespread technology in the printing world, commonly employed in personal small printers. Although limited in terms of throughput, in comparison to technologies such as gravure printing, inkjet printing provides an affordable technology that offers itself especially to quick prototyping and alterations in print design. The functioning principle of this technology is the 'jetting' of individual droplets from a print head. Although there are a multitude of techniques to generate the individual droplets, the core principle is shared. Ink is brought into a chamber and through the addition of either thermal or mechanical energy, the fluid is perturbed. This creates a droplet that is released from the printing head. **Figure 3** displays the core components of an inkjet printer and its operation mode. In order to create closed layers droplets are overlapped and tessellated in order to form desired shapes. The processing speed can be dramatically increased with the use of a multitude of print heads simultaneously working. A difficulty arising from this is the sensitivity to clogging of individual nozzles leading to missing droplets and openings in the film. A further differentiation can be made in processing between drop on demand and continuous-mode. The name is indicative of the processing where drop are released as is required by the source image while the other mode jets continuously.

**1.5. Spray deposition**

tioned will be briefly discussed below.

setup. The insert displays the point of atomization.

gas to fluid, fluid properties as well as nozzle dimensions.

spray technology the reader is referred to the work of Lefebrve [17].

Although spray technology has a variety of applications ranging from humidification to combustion, the more relevant for printed electronics is its use as surface coating. Although spray deposition inherently is a coating technique, it can easily be enhanced to fit the criteria of printing with the aid of a shadow mask. Spray deposition is the deposition of, usually a material in liquid state, with the aid of a gas stream. The bulk liquid is separated into droplets; this process is known as atomization and subsequently carried to the substrate in a mix of gas and a stream of droplets. The atomization can be achieved by either mixing with an air stream (air-assisted atomization) or with kinetic energy (ultrasonic atomization). The two aforemen-

Technological Integration in Printed Electronics http://dx.doi.org/10.5772/intechopen.76520 97

Air-assisted nozzles make use of a high velocity gas stream to produce atomization. The liquid is fed into the nozzle either under pressure or as a gravity fed alternative. At the orifice, the liquid is mixed with the gas stream, which disrupts the liquid causing atomization. **Figure 4** displays a standard air assisted nozzle with the inset displaying the point of atomization. The droplet size is dependent on many factors such as the velocity of the gas stream, the ratio of

Ultrasonic-assisted nozzles utilize a vibrating body based on a piezoelectric transducer that vibrates at ultrasonic frequencies. When this body is brought into contact with the body of fluid the liquid becomes unstable generating a mist of droplets. In most cases, this mist is mixed with a carrier gas that transports the droplets to the surface. Based on the applied frequency the droplet size can be varied. As with air-assisted nozzles the droplet formation also depends on a multitude of factors, both nozzle as well as fluid related. For further detail on

In terms of printed electronics, spray deposition has been used for a multitude of device. Both conducting as well as semiconducting devices being produced. Some of these devices include TFTs [18], TCFs [19], chemical/bio sensors [20, 21], OPVs [22] as well as OPDs [23] to name a few. Falco et al. [24] presented a fully spray deposited, TCO free, flexible OPD based

**Figure 4.** Schematic displaying a typical air-assisted atomization nozzle including key elements in a spray deposition

Due to the flexibility in terms of processable ink, inkjet printing has been successfully employed in the printed electronics world with great success. Devices presented include interconnects [12], chemical sensors [13], capacitors [14], OPV [15] and TFTs [16].

**Figure 3.** Schematic of an inkjet printer displaying key components and displaying operational mode.

#### **1.5. Spray deposition**

**1.4. Inkjet**

96 Flexible Electronics

the other mode jets continuously.

Inkjet is a widespread technology in the printing world, commonly employed in personal small printers. Although limited in terms of throughput, in comparison to technologies such as gravure printing, inkjet printing provides an affordable technology that offers itself especially to quick prototyping and alterations in print design. The functioning principle of this technology is the 'jetting' of individual droplets from a print head. Although there are a multitude of techniques to generate the individual droplets, the core principle is shared. Ink is brought into a chamber and through the addition of either thermal or mechanical energy, the fluid is perturbed. This creates a droplet that is released from the printing head. **Figure 3** displays the core components of an inkjet printer and its operation mode. In order to create closed layers droplets are overlapped and tessellated in order to form desired shapes. The processing speed can be dramatically increased with the use of a multitude of print heads simultaneously working. A difficulty arising from this is the sensitivity to clogging of individual nozzles leading to missing droplets and openings in the film. A further differentiation can be made in processing between drop on demand and continuous-mode. The name is indicative of the processing where drop are released as is required by the source image while

Due to the flexibility in terms of processable ink, inkjet printing has been successfully employed in the printed electronics world with great success. Devices presented include

interconnects [12], chemical sensors [13], capacitors [14], OPV [15] and TFTs [16].

**Figure 3.** Schematic of an inkjet printer displaying key components and displaying operational mode.

Although spray technology has a variety of applications ranging from humidification to combustion, the more relevant for printed electronics is its use as surface coating. Although spray deposition inherently is a coating technique, it can easily be enhanced to fit the criteria of printing with the aid of a shadow mask. Spray deposition is the deposition of, usually a material in liquid state, with the aid of a gas stream. The bulk liquid is separated into droplets; this process is known as atomization and subsequently carried to the substrate in a mix of gas and a stream of droplets. The atomization can be achieved by either mixing with an air stream (air-assisted atomization) or with kinetic energy (ultrasonic atomization). The two aforementioned will be briefly discussed below. fit

Air-assisted nozzles make use of a high velocity gas stream to produce atomization. The liquid is fed into the nozzle either under pressure or as a gravity fed alternative. At the orifice, the liquid is mixed with the gas stream, which disrupts the liquid causing atomization. **Figure 4** displays a standard air assisted nozzle with the inset displaying the point of atomization. The droplet size is dependent on many factors such as the velocity of the gas stream, the ratio of gas to fluid, fluid properties as well as nozzle dimensions.

Ultrasonic-assisted nozzles utilize a vibrating body based on a piezoelectric transducer that vibrates at ultrasonic frequencies. When this body is brought into contact with the body of fluid the liquid becomes unstable generating a mist of droplets. In most cases, this mist is mixed with a carrier gas that transports the droplets to the surface. Based on the applied frequency the droplet size can be varied. As with air-assisted nozzles the droplet formation also depends on a multitude of factors, both nozzle as well as fluid related. For further detail on spray technology the reader is referred to the work of Lefebrve [17].

In terms of printed electronics, spray deposition has been used for a multitude of device. Both conducting as well as semiconducting devices being produced. Some of these devices include TFTs [18], TCFs [19], chemical/bio sensors [20, 21], OPVs [22] as well as OPDs [23] to name a few. Falco et al. [24] presented a fully spray deposited, TCO free, flexible OPD based

**Figure 4.** Schematic displaying a typical air-assisted atomization nozzle including key elements in a spray deposition setup. The insert displays the point of atomization.

Leigh et al. employed a carbon black filler embedded in a biodegradable polyester to produce an electrically conductive filament [26]. This led to the fabrication of flex sensors as well as embedding it into a 3D printed glove to monitor finger movement. Most commonly, there are three types of processing techniques. Stereolithography (SLA) and Selective Laser Sintering (SLS) use the principle of solidification of a base material while Fused Deposition Modeling (FDM) melts a solid material, which is extruded through a nozzle. **Figure 6** depicts the afore-

Technological Integration in Printed Electronics http://dx.doi.org/10.5772/intechopen.76520 99

In SLS, a pulsed laser source is employed to solidify a base powder that can originate from plastics, resins and metals. Material is fed from a powder reservoir while the sample stage is moved downwards to expose more material to the laser spot. This process is continued until the desired object is finalized. The remaining material is removed and can be reused as such there is very limited waste material. Due to the sensitive interaction at the powder laser interface, precise calibration is required as to not cause unwanted sintering. Conceptually SLA is similar to SLS, employing a laser to crosslink photosensitive resins. Energetically such an approach is significantly advantageous in comparison to SLS. The limiting factor being the base material for SLA while both techniques suffer from a difficulty in further integration in in-line processes. FDM can cover these shortcomings, albeit at a cost in resolution as well as stability. In the FDM the melted filament is extruded through a nozzle and lines are over-

After reviewing the main fabrication techniques for printed electronics, it is clear that each of them have potential to define different electronic components. For example, antennas require thicker metallic layers in order to enhance their performance. Although the desired thickness could be achieved with any of the described technologies, the most efficient one is screen printing because we can achieve tens of μm selecting a coarse mesh with only one turn. The same thickness would need 20 layers in case of using inkjet printing, resulting in a very time consuming process because it is needed to wait until the layer is dried and the area to cover is quite large. In the case of spray deposition, also the stack of several layers would be mandatory. This technique is much faster

than inkjet printing but the pattern definition is poorer in comparison to screen printing.

Another example is the definition of interdigitated electrodes (IDE) for capacitive structures. If the IDE is going to be used to build a capacitive sensor, the distance among consecutive fingers determines the sensitivity of the sensors: the closer they are placed, the higher its sensitivity can be. In this sense, the resolution of the printing technique limits the performance

Therefore, each technique can be more suitable for different electronic components, as highlighted in the previous examples. As the ultimate goal is to fabricate a fully working and efficient electronic system, it is mandatory to properly combine manufacturing techniques to fulfill it. In the last years, some authors have worked in this director, using different printed

techniques to design electronic circuits on flexible substrates.

mentioned 3D processes marking critical components.

lapped and tessellated to create desired structures.

**2. Synergy of printed electronics**

of the device.

**Figure 5.** Summarizing figure displaying spray deposition setup used for the fabrication of fully-sprayed, flexible OPDs, the final device as well as key electrical characteristics (adapted from [24] with authorization).

**Figure 6.** Schematic depictions of common 3D processing techniques of (a) Selective Laser Sintering and (b) Stereolithography and Fused Deposition Modeling.

on a blend of regioregular poly (3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). It demonstrated the feasibility of spray deposition as a method for printed organic electronics especially since all layers were successively deposited by the same technology. **Figure 5** summarizes the work displaying the setup used, the final device as well as key electrical performance.

#### **1.6. 3D printing**

3D printing is a term used to encompass additive manufacturing techniques that aim at fabricating solid objects through the deposition of material. Although only recently having gained broad awareness, due to the surge in available privately affordable devices, 3D printing patents date back to 1986 [25]. Used mostly for rapid prototyping and in research the main fabrication was limited to static objects with limited mechanical features. With the increase research of nano-based materials, 3D printing has moved beyond creating scaffolds for electrical devices with the integration of electrical characteristic being embedded into the 3D structure [26–28]. Leigh et al. employed a carbon black filler embedded in a biodegradable polyester to produce an electrically conductive filament [26]. This led to the fabrication of flex sensors as well as embedding it into a 3D printed glove to monitor finger movement. Most commonly, there are three types of processing techniques. Stereolithography (SLA) and Selective Laser Sintering (SLS) use the principle of solidification of a base material while Fused Deposition Modeling (FDM) melts a solid material, which is extruded through a nozzle. **Figure 6** depicts the aforementioned 3D processes marking critical components.

In SLS, a pulsed laser source is employed to solidify a base powder that can originate from plastics, resins and metals. Material is fed from a powder reservoir while the sample stage is moved downwards to expose more material to the laser spot. This process is continued until the desired object is finalized. The remaining material is removed and can be reused as such there is very limited waste material. Due to the sensitive interaction at the powder laser interface, precise calibration is required as to not cause unwanted sintering. Conceptually SLA is similar to SLS, employing a laser to crosslink photosensitive resins. Energetically such an approach is significantly advantageous in comparison to SLS. The limiting factor being the base material for SLA while both techniques suffer from a difficulty in further integration in in-line processes. FDM can cover these shortcomings, albeit at a cost in resolution as well as stability. In the FDM the melted filament is extruded through a nozzle and lines are overlapped and tessellated to create desired structures.
