**2. Important properties of the soft transparent electrodes**

#### **2.1 Optical transmittance and electrical conductivity**

Preferably, TEs must exhibit both high optical transmittance and high electrical conductance, and these are rather contrary from the physics perspective. It is due to an essential requirement for the electrical conductance of a material is the high charge density, that is restricted via the optical absorption of the free charges. [9] Figure of merit (FoM) is commonly used for evaluation of the overall performance of the transparent electrodes. FoM, which is the proportion of electrical conductivity to optical conductivity (σdc/σopt), and is measured by means of the commonly used expression, as given below: [20, 22, 25, 26].

$$\text{FoM} = \frac{\sigma\_{\text{dc}}}{\sigma\_{\text{opt}}} = \frac{188.5}{\text{R}\_{\text{s}} \left(\frac{1}{\sqrt{\text{T}}} - 1\right)}\tag{1}$$

**281**

**Table 1.**

*literature.*

**Tables 1**–**3**.

**Figure 1.**

**2.2 Mechanical stability**

*minimum industrial standard (red line). [40].*

*Vacuum-Free Fabrication of Transparent Electrodes for Soft Electronics*

these classes, metal-mesh based TEs has higher FoM values, both alone and as part of the hybrid TEs. The detailed FoM values of metal based TEs are presented in

*Comparison of the FoMs of soft TEs (metal NW, metal mesh, and hybrid) and industrial standards. The data was acquired from the literature. [15] The dashed lines represent typical industrial standard (green line) and* 

Mechanically resilient TEs remarkable optoelectronic properties are vital for the development of soft optoelectronic devices as without this, these systems will

**Rsh (**Ω**-□−1) T (%) FoM Applications Reference** 40 85 55 Low-cost TEs [41] 35 84 60 Low-cost TEs [42] 6.2 85 360 Low-cost TEs [43] 10 70 100 GaN-based LEDs [44] 12 82 150 OLEDs [45] 4.5 80 357 Photodetectors [46] 10 89 300 Touch-screens [47] 92 92 40 Touch-screens [48] 3.5 76 366 Low-cost TEs [49] 10 90 350 Large-area TEs [50] 2.5 97.3 4920 High-performance TEs [51] 130.5 92 35 Touch-screens [52] 30 85 80 OSCs [53]

*Optical and electrical performance, and applications of metal NWs based soft TEs published in recent* 

*DOI: http://dx.doi.org/10.5772/intechopen.96311*

Where, T represents the optical transparency value at a wavelength of 550 nm (as it is close to most sensitive wavelength of the human eyes, [39]) and Rs represents the sheet resistance. A larger FOM value discloses a smaller sheet resistance value at a particular optical transmittance value, and vice versa. **Figure 1** presents a comparison of FoMs for metallic soft TEs reported in recent studies. Among

*Vacuum-Free Fabrication of Transparent Electrodes for Soft Electronics DOI: http://dx.doi.org/10.5772/intechopen.96311*

#### **Figure 1.**

*Nanofibers - Synthesis, Properties and Applications*

substitute the TCOs. [15, 16]

soft optoelectronic devices also need decent mechanical deformability [1, 9] in TEs. Currently, the most utilized TEs are based on vacuum-processed TCOs, comprising fluorine-doped tin oxide and indium tin oxide (ITO). [10, 11] Although TCOs based TEs have demonstrated the required optoelectronic performance, several limitations, such as low abundance, [12] film brittleness, [13] low infrared transparency, [14] and failure during high temperature sintering, undermine their appropriateness for utilization in the future soft optoelectronic systems. Thus, researchers have developed novel TE materials and vacuum-free approaches for its fabrication to

Novel intrinsically transparent materials including graphene, [17] carbon nanotubes (CNTs), [18] and conducting polymers [19, 20] have been explored to replace the TCOs. Besides, other promising class of soft TEs designed from metals are widely employed due to their excellent electrical, optical, and mechanical performance. This typically include metal NWs networks [21, 22] and systematic metal meshes, [23–28] and ultra-thin metal films. [29–31] In addition to the advancement of new materials for soft TEs, plenty of research is performed on the development of vacuum-free technologies for the low-cost fabrication of soft TEs. The list of these techniques is mainly consists of spin coating, [32] spray deposition, [33] inkjet printing, [34] screen printing, [35] transfer printing, [36] and slot-die coating. [37] There have been several reviews published over the years, aiming at soft TEs from applications perspective. [1, 11, 38] However, few of them focuses on the soft TEs from the fabrication perspective. In this chapter, latest review of the vacuum-free fabricated TEs for emerging soft electronic devices is presented. The chapter begins with the discussion of key properties of TEs for soft electronics (sections 2). We then introduce the TE materials including metals, carbon materials, and transparent conducting polymers (section 3,4). Finally in section 5, the recent progress on vacuum-free methods that are typically employed for the realization of TEs, discussing their merits and demerits. We hope this chapter will enlighten the readers about the emergent soft TEs to better design and fabricate low-cost soft electronics devices.

**2. Important properties of the soft transparent electrodes**

Preferably, TEs must exhibit both high optical transmittance and high electrical conductance, and these are rather contrary from the physics perspective. It is due to an essential requirement for the electrical conductance of a material is the high charge density, that is restricted via the optical absorption of the free charges. [9] Figure of merit (FoM) is commonly used for evaluation of the overall performance of the transparent electrodes. FoM, which is the proportion of electrical conductivity to optical conductivity (σdc/σopt), and is measured by means of the commonly

= = <sup>−</sup>

s

Where, T represents the optical transparency value at a wavelength of 550 nm (as it is close to most sensitive wavelength of the human eyes, [39]) and Rs represents the sheet resistance. A larger FOM value discloses a smaller sheet resistance value at a particular optical transmittance value, and vice versa. **Figure 1** presents a comparison of FoMs for metallic soft TEs reported in recent studies. Among

(1)

<sup>1</sup> R 1 T

σ

dc opt

188.5 FoM

σ

**2.1 Optical transmittance and electrical conductivity**

used expression, as given below: [20, 22, 25, 26].

**280**

*Comparison of the FoMs of soft TEs (metal NW, metal mesh, and hybrid) and industrial standards. The data was acquired from the literature. [15] The dashed lines represent typical industrial standard (green line) and minimum industrial standard (red line). [40].*

these classes, metal-mesh based TEs has higher FoM values, both alone and as part of the hybrid TEs. The detailed FoM values of metal based TEs are presented in **Tables 1**–**3**.

#### **2.2 Mechanical stability**


130.5 92 35 Touch-screens [52] 30 85 80 OSCs [53]

Mechanically resilient TEs remarkable optoelectronic properties are vital for the development of soft optoelectronic devices as without this, these systems will

#### **Table 1.**

*Optical and electrical performance, and applications of metal NWs based soft TEs published in recent literature.*


#### **Table 2.**

*Optical and electrical performance, and applications of regular metal mesh based soft TEs published in recent literature.*

not be not able to preserve electrical conductivity under significant mechanical deformation. [69] Various approaches are developed to enhance the mechanical stability of the soft TEs. For example, metal meshes are embedded and mechanically anchored into the soft polymer substrates, which significantly enhanced its adhesion with the substrate and as a result improves its mechanical stability under deformation. [15, 16, 86] In addition to the mechanical stability of the TEs, the

**283**

materials. [89, 90]

*Vacuum-Free Fabrication of Transparent Electrodes for Soft Electronics*

intrinsic mechanical stability of the other functional materials are equally impor-

*Optical and electrical performance, and applications of hybrid soft TEs published in recent literature.*

**Rsh (**Ω**-□−1) T (%) FoM Applications Reference** 1 92 5000 High-performance TEs [40] 2 95 4000 High-performance TEs [40] 1.2 80 1330 OSCs [79] 19 92 232 Perovskite Solar Cells [80] 9.1 79 165 OSCs [81] 5 80 325 Long-term Stable TEs [82] 0.6 93 8900 High-performance TEs [40] 0.36 92 12000 E-chromic Devices [83] 0.7 65 1100 Stretchable TEs [55] 11 98 1800 High-performance TEs [84] 3 92 1400 High-performance TEs [84] 60 90 60 PLEDs [85] 11 88 1050 Stretchable TEs [55]

TEs Surface roughness is significant as this considerably influences the morphology and uniformity of the subsequent printed/coated layers. Though, it's hard to define a strict extreme roughness value vital for the effective production of soft electronic devices. Yet, bottom TEs with lower surface roughness value are preferred to minimize the possibility of electrical short circuiting. For instance, the roughness (root-mean-square) of a coated/printed continuous PEDOT:PSS film is normally <10 nm, which is adequately flat for most of the functional thin-films involved in fabrication of electronic devices. But, the surface roughness of metal TEs is much higher (hundreds of nm to few μm). For example, screen printed silver mesh is >2 μm thick, making the subsequent functional layer uniform deposition impossible. [87] To address this, researchers have has embedded the metal-mesh into the polymer substrates to flat the TEs top surface. [16, 69] Similarly, metal NW networks also demonstrate decent FoM as stated above, however, its high roughness resulted in poor device performance. [88] Therefore, multiple approaches have been established to flatten the metal NWs TEs by compacting the unattached networks to a dense structure or filling the openings with supplementary TE

Chemical compatibility of the TEs/functional materials interface is another important concern for TEs. An unsteady interface can cause substandard performance and also fast deprivation of the TEs. For instance, the acidic behavior of PEDOT:PSS TEs can corrode the base ITO layer, causing the diffusion of indium at the TE/active layer boundary. Such erosion might result in critical gap conditions which further caused the degradation of device. [91] To minimize the risk of chemical/electrochemical decay of the sensitive metallic TEs, a traditional method is covering the sensitive metallic materials with a thin-film. [92] This thin-film

tant concerning the successful operations of the soft electronic devices.

*DOI: http://dx.doi.org/10.5772/intechopen.96311*

**2.3 Other surface properties**

**Table 3.**


*Vacuum-Free Fabrication of Transparent Electrodes for Soft Electronics DOI: http://dx.doi.org/10.5772/intechopen.96311*

**Table 3.**

*Nanofibers - Synthesis, Properties and Applications*

**Rsh (**Ω**-□−1) T (%) FoM Applications Reference** 0.036 75 34000 DSSCs/Heaters [15] 4 70 242 OLEDs [54] 15 96 410 Touch-screens [24] 21 85 90 Stretchable TEs [55] 40 80 38 OFETs/ OLEDs/OSCs [56] 22 78 62 OSCs [57] 30 85 75 Touch-screens [23] 3 82 600 Transparent Heaters [58] 8 77 170 OSCs [59] 0.3 70 3200 Large-area TEs [27] 1.7 82 2700 EL displays [60] 0.03 86 80,000 Transparent Heaters [61] 4.8 81 355 Printed TEs [62] 18 76 145 High-durable TEs [26] 5 82 360 Touch-screens [25] 9.8 85.2 237 Touch-screens [63] 6 97 1900 Transparent Heaters [64] 8 94 800 Printed TEs [65] 7 96 12600 Nanofiber based TEs [66] 0.43 97 27000 Wearable TEs [67] 0.07 72 15000 Transparent Heaters [16] 0.13 86 20000 Transparent Heaters [68] 1.32 82 1400 DSSCs [69] 0.84 84 2500 EL displays [70] 3.8 90 900 Wearable Heaters [71] 2.1 88.6 1450 QLEDs [72] 13 87 200 Touch-screens [73] 10 75 120 Solar Cells [74] 11 86 220 Transparent Heaters [75] 4.7 87 550 OLEDs [76] 6.2 90 550 Transparent Heaters [77] 3.9 84 490 Highly Bendable TEs [78]

**282**

**Table 2.**

*literature.*

not be not able to preserve electrical conductivity under significant mechanical deformation. [69] Various approaches are developed to enhance the mechanical stability of the soft TEs. For example, metal meshes are embedded and mechanically anchored into the soft polymer substrates, which significantly enhanced its adhesion with the substrate and as a result improves its mechanical stability under deformation. [15, 16, 86] In addition to the mechanical stability of the TEs, the

*Optical and electrical performance, and applications of regular metal mesh based soft TEs published in recent* 

*Optical and electrical performance, and applications of hybrid soft TEs published in recent literature.*

intrinsic mechanical stability of the other functional materials are equally important concerning the successful operations of the soft electronic devices.

#### **2.3 Other surface properties**

TEs Surface roughness is significant as this considerably influences the morphology and uniformity of the subsequent printed/coated layers. Though, it's hard to define a strict extreme roughness value vital for the effective production of soft electronic devices. Yet, bottom TEs with lower surface roughness value are preferred to minimize the possibility of electrical short circuiting. For instance, the roughness (root-mean-square) of a coated/printed continuous PEDOT:PSS film is normally <10 nm, which is adequately flat for most of the functional thin-films involved in fabrication of electronic devices. But, the surface roughness of metal TEs is much higher (hundreds of nm to few μm). For example, screen printed silver mesh is >2 μm thick, making the subsequent functional layer uniform deposition impossible. [87] To address this, researchers have has embedded the metal-mesh into the polymer substrates to flat the TEs top surface. [16, 69] Similarly, metal NW networks also demonstrate decent FoM as stated above, however, its high roughness resulted in poor device performance. [88] Therefore, multiple approaches have been established to flatten the metal NWs TEs by compacting the unattached networks to a dense structure or filling the openings with supplementary TE materials. [89, 90]

Chemical compatibility of the TEs/functional materials interface is another important concern for TEs. An unsteady interface can cause substandard performance and also fast deprivation of the TEs. For instance, the acidic behavior of PEDOT:PSS TEs can corrode the base ITO layer, causing the diffusion of indium at the TE/active layer boundary. Such erosion might result in critical gap conditions which further caused the degradation of device. [91] To minimize the risk of chemical/electrochemical decay of the sensitive metallic TEs, a traditional method is covering the sensitive metallic materials with a thin-film. [92] This thin-film

can be either from another class of conductor, for example, graphene, [93] and less-sensitive metals, [94] or an insulating material, for instance, poly(methyl methacrylate) (PMMA) and alumina, [95] The insulating film must be ultra-thin (< few nanometers) for proficient charge transport. [96] In addition to roughness and chemical compatibility, surface energy of TEs is also an essential factor to be considered for the efficient performance of the active materials in soft electronic devices. [97]
