**2. Requirements for flexible transparent electrodes in different applications**

Flexible transparent electrodes can be applied in many cases. Different properties are required according to different applications subject to various problems, as shown in **Table 2**. In this section, we will introduce some applications such as touch panels, solar cells, flexible lighting, and flexible sensors.

#### **2.1. Touch panels**

due to the increasing marketing demand for flexible devices and the brittleness and scarce-

Recent studies have suggested carbon nanotubes (CNT) [2, 3], graphene [4] and silver nanowires (AgNWs) [5] as the alternatives. Though CNTs are reported to have good electrical, thermal and mechanical properties, the CNT electrodes show lower electrical conductivity than ITO electrodes due to large contact resistance and extensive bundling of CNTs. Graphene is reported to have high Fermi velocity of 106 m/s and high intrinsic in-plane conductivity. But the large-area production of high-performance graphene films remains a serious issue. Although chemical vapor deposition method has the ability of producing large-area high-

performance graphene, the process costs a lot and needs extremely high temperature.

several transparent electrodes based on different materials.

**Table 1.** Comparison of several transparent electrodes.

Metallic nanowire based electrodes, as the most promising alternative to ITO, have superior optical, electrical and mechanical properties. Both random and regular metallic nanowire networks have received an increasing interest from both academia and industry. Random metallic nanowires can be dispersed in the solvent and be deposited onto the substrates through low-cost solution-based processing [6]. This makes nanowire-based electrodes compatible for high-throughput and large-area production of the next generation flexible optoelectronic devices. Moreover, for regular metallic nanowire based electrodes, called metal mesh, the electrical conductivity and the optical transparency can be easily tuned by changing the geometry parameter of the nanowires. When the period of the metal mesh is in sub-micrometer scale and the line width is close to subwavelength, metal meshes can be considered as bulk materials to estimate the sheet resistance of the films. Various metallic materials, such as gold, silver and copper, are used to achieve different work functions and chemical properties for various applications. Silver, a material with high electrical conductivity and low price to some degree, is considered as the most suitable nanowire material. And the overall performance of AgNW electrodes has already surpassed that of ITO electrodes. **Table 1** shows the comparison of

The present chapter focuses on recent progresses in the fabrication techniques of flexible transparent AgNW electrodes. Firstly, we briefly introduce the requirements of electrical, optical, thermal and mechanical properties for flexible transparent electrodes in different applications. Then synthesis of AgNWs and film-forming techniques of flexible transparent AgNW

**Properties ITO TCO CNT Graphene AgNW Ag mesh** Conductivity ++ ++ — — ++ +++ Transmittance ++ + +++ ++ + +++ Haziness + + ++ ++ — — Flexibility — — +++ +++ +++ — Stability + + + ++ +++ +++ Large-scale — + ++ ++ ++ — Low-cost — — — — +++ —

ness of ITO.

22 Flexible Electronics

Touch Display Research Inc. forecasted that the market of transparent electrodes without ITO will reach \$13 billion by 2023. The surface area of manufactured touch panels will reach more than 80 km<sup>2</sup> in 2025, double of that in 2014, predicted by IDTechEx Ltd. [7]. Companies like Samsung, LG, Apple and Toshiba have all indicated the market trends for flexible displays. Touch screens can be divided into capacitive sensing and resistive sensing by different working principles. When fingers touch the screen, the capacitive sensing works on the change in capacitance instead of the change in resistance as the resistive sensing does. Resistive sensing is low-cost and high-resolution reported by S.H. Ko's team [8, 9]. With the durability and the compatibility of multi-touch features, the capacitive sensing arises many researchers' attention worldwide. Capacitive touch panels can now be divided into single [10–12] and doublesided sensors [13, 14] based on the number of transparent conductive layers. **Figure 1(a)** and **(b)** show the photograph of the working touch panel [10]. The resolution of single-sensor capacitive touch screen is required to be at the millimeter scale while that of double-sided ones is hundreds of micrometers. Not only the distribution but also the orientation and alignment will govern the performance of the AgNW networks. Patterning is also of prime importance for high performance. **Figure 1(c)** exhibits the design of touch sensors and the image is


**Table 2.** Comparison of performance requirements for different applications.

is shown in **Table 3**. Topics concerning the integration of transparent AgNW electrodes into flexible solar cells are as follows: one is the low-cost fabrication for the development of flexible solar cells and another one is the study of plasmonic effects to further control the optoelec-

Fabrication and Applications of Flexible Transparent Electrodes Based on Silver Nanowires

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25

AgNW electrodes can be integrated into LEDs. **Figure 2(a)** shows the schematic of AlGaNbased LEDs with AgNW/ITO electrodes [23]. The active layer of the light emitting devices mostly investigated can be organic materials (OLEDs) or polymer (PLEDs). The emulation of fully rollable lighting panels is time-to-market dependent on our ability to provide not only the active layer but also the interfaces and the transparent electrodes with high flexibility. In this case, it is essential for the transparent electrodes to have no alteration in optoelectronic properties under bending cycles. For conventional ITO-based OLEDs, the luminance and the efficiency of the devices would have a sharp decrease under mechanical stress due to the fracture of the brittle ITO electrodes. Thus the usage of AgNW electrodes with good optoelectronic and mechanical properties seems to be a good strategy to fulfill this demand. Polyvinyl alcohol (PVA) [24], polyacrylate [25], poly(methyl methacrylate) (PMMA) [26], colorless polyimide (cPI) [27] and poly(urethane acrylate) (PUA) [28, 29] are used to produce the transparent electrodes together with AgNWs to improve the performance of electrodes. For instance, AgNW/PMMA OLEDs show high luminous efficiency, the color-independent emission and the nearly perfect Lambertian emission [26], as depicted in **Figure 2(b)**. Many efforts have been done to decrease the current leakage [30, 31]. **Table 4** illustrates the performance comparison of LEDs produced by different researchers. The challenge of keeping the performance of AgNW-based OLEDs unchanged under deformation also arises many

High sensitive and stretchable sensors can be used in both our daily life and large military projects, from the human health monitoring devices to the structural health monitoring of aircrafts and bridges [36–38]. **Figure 3(a)** shows a strain sensor attached to the neck to monitor human activities [39]. And as shown in **Figure 3(b)**, AgNW electrodes can be integrated

**Figure 2.** Light emitting diodes with AgNW electrodes: (a) the schematic of LEDs with AgNW/ITO electrodes [23], and

tronic properties.

**2.3. Flexible lighting**

researchers' attention [28].

(b) the angular dependence property of white OLEDs [26].

**2.4. Flexible sensors**

**Figure 1.** Photographs of touch panels: (a-b) photograph of working capacitive touch panels [10], (c) the patterned AgNW network [12], and (d) healable touch sensor [14].

of patterned AgNW networks respectively [12]. Interestingly, healable touchscreens were produced by Pei and co-workers through embedding AgNWs into the surface of healable polymer substrate [14], as shown in **Figure 1(d)**.

#### **2.2. Solar cells**

Flexible transparent electrodes as front electrodes is a crucial factor in determining photoconversion efficiency of solar cells [15]. High electrical conductivity and high optical transparency of flexible transparent electrodes are required in order to lower the ohmic dissipation of heat and maximize the light absorption in the conversion layer. The band alignment and work function of the electrodes should also be considered. The most commonly used materials for solar cells are doped metal oxides. They are prone to cracking, costly and need hightemperature fabrication. Many researches have proved that AgNW networks are a promising alternative to ITO for both organic solar cells [16–18] and polymer solar cells [19]. AgNW networks have similar photovoltaic performances and excellent bending capacities as ITO and are compatible with solution-processed fabrication. And unlike doped metal oxides, AgNWs have high optical transparency in the IR range, leading to enhanced efficiency and the semitransparency of solar cells. The performance comparison in solar cells using AgNW electrodes


**Table 3.** Performance comparison in solar cells.

is shown in **Table 3**. Topics concerning the integration of transparent AgNW electrodes into flexible solar cells are as follows: one is the low-cost fabrication for the development of flexible solar cells and another one is the study of plasmonic effects to further control the optoelectronic properties.

#### **2.3. Flexible lighting**

of patterned AgNW networks respectively [12]. Interestingly, healable touchscreens were produced by Pei and co-workers through embedding AgNWs into the surface of healable

**Figure 1.** Photographs of touch panels: (a-b) photograph of working capacitive touch panels [10], (c) the patterned

Flexible transparent electrodes as front electrodes is a crucial factor in determining photoconversion efficiency of solar cells [15]. High electrical conductivity and high optical transparency of flexible transparent electrodes are required in order to lower the ohmic dissipation of heat and maximize the light absorption in the conversion layer. The band alignment and work function of the electrodes should also be considered. The most commonly used materials for solar cells are doped metal oxides. They are prone to cracking, costly and need hightemperature fabrication. Many researches have proved that AgNW networks are a promising alternative to ITO for both organic solar cells [16–18] and polymer solar cells [19]. AgNW networks have similar photovoltaic performances and excellent bending capacities as ITO and are compatible with solution-processed fabrication. And unlike doped metal oxides, AgNWs have high optical transparency in the IR range, leading to enhanced efficiency and the semitransparency of solar cells. The performance comparison in solar cells using AgNW electrodes

1.85 −7.22 0.5308 48.475 [20] 6.58 14.29 0.78 59 [1] 3.05 9.191 0.638 0.521 [21] 2.73 8.4 0.58 56.07 [22] 2.66 6.36 1.06 39.59 [19] PCE: power conversion efficiency, Jsc: short-circuit current density, Voc: open-circuit voltage, FF: fill factor.

**) Voc(V) FF(%) Ref**

polymer substrate [14], as shown in **Figure 1(d)**.

AgNW network [12], and (d) healable touch sensor [14].

**2.2. Solar cells**

24 Flexible Electronics

**PCE(%) Jsc(mA/cm2**

**Table 3.** Performance comparison in solar cells.

AgNW electrodes can be integrated into LEDs. **Figure 2(a)** shows the schematic of AlGaNbased LEDs with AgNW/ITO electrodes [23]. The active layer of the light emitting devices mostly investigated can be organic materials (OLEDs) or polymer (PLEDs). The emulation of fully rollable lighting panels is time-to-market dependent on our ability to provide not only the active layer but also the interfaces and the transparent electrodes with high flexibility. In this case, it is essential for the transparent electrodes to have no alteration in optoelectronic properties under bending cycles. For conventional ITO-based OLEDs, the luminance and the efficiency of the devices would have a sharp decrease under mechanical stress due to the fracture of the brittle ITO electrodes. Thus the usage of AgNW electrodes with good optoelectronic and mechanical properties seems to be a good strategy to fulfill this demand. Polyvinyl alcohol (PVA) [24], polyacrylate [25], poly(methyl methacrylate) (PMMA) [26], colorless polyimide (cPI) [27] and poly(urethane acrylate) (PUA) [28, 29] are used to produce the transparent electrodes together with AgNWs to improve the performance of electrodes. For instance, AgNW/PMMA OLEDs show high luminous efficiency, the color-independent emission and the nearly perfect Lambertian emission [26], as depicted in **Figure 2(b)**. Many efforts have been done to decrease the current leakage [30, 31]. **Table 4** illustrates the performance comparison of LEDs produced by different researchers. The challenge of keeping the performance of AgNW-based OLEDs unchanged under deformation also arises many researchers' attention [28].

#### **2.4. Flexible sensors**

High sensitive and stretchable sensors can be used in both our daily life and large military projects, from the human health monitoring devices to the structural health monitoring of aircrafts and bridges [36–38]. **Figure 3(a)** shows a strain sensor attached to the neck to monitor human activities [39]. And as shown in **Figure 3(b)**, AgNW electrodes can be integrated

**Figure 2.** Light emitting diodes with AgNW electrodes: (a) the schematic of LEDs with AgNW/ITO electrodes [23], and (b) the angular dependence property of white OLEDs [26].


in **Figure 3(d)**. In addition to stretchability, the sensitivity can be tuned by controlling the areal density and roughness of AgNW networks [46]. Further optimization of geometry and

Fabrication and Applications of Flexible Transparent Electrodes Based on Silver Nanowires

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Many approaches have been addressed to synthesize AgNWs, which can be mainly divided into two groups: template methods and polyol process [47]. Template methods are classified into two categories, in terms of hard templates and soft templates. Soft templates include polymer film of PVA and DNA chains [48, 49]. Hard templates include silicon wafer and aluminum oxide [50, 51]. Although many literatures have investigated template methods to synthesize AgNWs, these methods are incompatible for large-scale production. The preparation and removal of the templates are time consuming and high cost. Moreover, nanowires synthesized through template

Different from template methods, polyol process provides high yield of nanowires with ideal morphology. As the most promising synthetic procedure, salt-mediated polyol method [52, 53]

nuclei for subsequent growth of AgNWs, as depicted in **Figure 4(a)**. The dimensions of AgNWs can be kinetically controlled by temperature, seeding conditions, and the ratio between PVP

ratio. Increasing the concentration of metal seeds could slightly decrease the diameter of

The aspect ratio of the nanowires is small, unable to meet the requirements for high aspect ratio nanowires. Microwave and UV irradiation have been adopted by researchers to assist the synthesis of AgNWs [56–58]. The controllable methods to fabricate high aspect ratio nanowires have received much attention. Long nanowires with length of over 300 μm were fabricated by Lee et al. [59] using a successive multistep growth method, as shown in **Figure 4(b)**. The fabrication process is time-consuming and complex. Then Andrés et al. [60] demonstrated a rapid synthesis of nanowires with the length reaching 190 μm to overcome this problem.

Apart from the synthesis of AgNWs, coating and printing them onto the flexible plastic substrate is also an essential process in the fabrication of transparent electrodes. The performance of the electrodes varies according to different techniques and devices. The ideal process should meet three requirements: (1) the process should be free from toxic chemicals and costly materials; (2) the process should have a low environmental impact and can be recycled; (3) the process should meet the demand of the large-area, high-efficiency and high-quality production. Solution-processed fabrication can easily be surface scalable. Many solution processes have been reported to produce

, helps the mass synthesis of AgNWs. Metal seeds in the solution served as

. High reaction temperature leads to the formation of nanowires with low aspect

[53], CuCl

S to control the diameter of AgNWs.

has good reproducibility and low cost. The usage of salts, such as NaCl [54], CuCl<sup>2</sup>

**3. Fabrication of flexible transparent electrodes based on silver** 

methods suffer from low aspect ratio, irregular morphology and low yield.

nanowires. Chen et al. [55] adjust the concentration of Na<sup>2</sup>

materials is needed in this field.

**3.1. Controllable synthesis of silver nanowires**

**nanowires**

[53], FeCl<sup>3</sup>

and AgNO<sup>3</sup>

and PtCl<sup>2</sup>

**3.2. Coating techniques**

Vc: turn on voltage, EQE: external quantum efficiency, CE: current efficiency, PE: power efficiency.

**Table 4.** Performance comparison in LEDs.

**Figure 3.** The schematic and photograph of flexible AgNW sensors: (a) photograph of strain sensor attached to the neck [39], (b) AgNW dry electrode for ECG measurements [38], (c) the schematic diagram of strain sensor [39], and (d) the schematic illustration of highly-stretchable nanocomposite generator [45].

into electrocardiogram (ECG) measurements [38]. Performances of flexible sensors concern about the linearity, sensitivity, detecting range, response time, stability and stretchability. Investigations using AgNW electrodes mainly concern about strain sensors [40, 41], pressure sensors [40] and electrochemical sensors [42]. Yao et al. presented wearable sensors based on highly stretchable AgNW electrodes enabling the detection of strain and pressure [40]. The strain sensors produced showed good linearity and reversibility even up to a large strain of 50%. At the same time, the pressure under detection ranged up to 1.2 MPa. Hwang et al. have recently developed a self-powered patchable platform to monitor human activities [39], as shown in **Figure 3(c)**. Usually, the stretchability of the transparent systems has been reported to be among 50–90% [43, 44]. The high stretchability is achieved by compositing AgNWs with a thin layer of elastomer [39, 41]. In particular, Jeong et al. integrated ultra-long AgNWs into an elastic-composite generator which exhibits hyper-stretchability up to 200% [45], as shown in **Figure 3(d)**. In addition to stretchability, the sensitivity can be tuned by controlling the areal density and roughness of AgNW networks [46]. Further optimization of geometry and materials is needed in this field.
