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

### **3.1. Controllable synthesis of silver nanowires**

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

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] has good reproducibility and low cost. The usage of salts, such as NaCl [54], CuCl<sup>2</sup> [53], CuCl [53], FeCl<sup>3</sup> and PtCl<sup>2</sup> , helps the mass synthesis of AgNWs. Metal seeds in the solution served as 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 and AgNO<sup>3</sup> . High reaction temperature leads to the formation of nanowires with low aspect ratio. Increasing the concentration of metal seeds could slightly decrease the diameter of nanowires. Chen et al. [55] adjust the concentration of Na<sup>2</sup> S to control the diameter of AgNWs. 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.

#### **3.2. Coating techniques**

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

**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].

**Category Vc(V) EQE(%) CE(cd/A) PE(lm/W) Ref** OLED NA NA 58.2 NA [32] OLED NA 18.7 68.6 62.8 [33] OLED 6 24.3 49 30.3 [26] PLED 0.6 NA NA NA [34] LED 2.72 NA NA NA [35] OLED 3.6 NA 44.5 35.8 [30]

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

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

26 Flexible Electronics

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

techniques are compatible with low-energy deposition process and without any vacuum equipment. Direct laser ablation [13, 75], shadow mask [11], chemical etching using the photolithogra-

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The processability of AgNW networks by R2R was showed by many researchers due to their compatibility with large-area production [81–83]. The substrate in R2R coating system is required to have mechanical flexibility and in the form of a long sheet. Quite different from other solution-processed coating methods, the R2R process is continuous and is suitable for large-area production. During coating, the substrate is first unwound from a roll and then passed through the coating machine and finally rewound on another roll. Aside from the coating machine, some post-treatment may also be added into the process, such as compressing, heating, UV-curing, chemical welding and drying, as shown in **Figure 6(a)** [83]. Interestingly, Lai's team produced AgNW electrodes combined with moth-eye nanostructures using R2R techniques and greatly enhanced the transmittance [5, 81]. The quality of forming can be influenced by tension, speed, cleaning of the substrate and the removal of static electricity. Also, the pre-treatment and post-treatment can have a great impact on the performance of the coated AgNW films. Many laboratories have developed their own R2R system to study the coating process. **Figure 6(b)**-**(d)** show two laboratory-scale coating system [77, 84]. Hösel et al. have compared the performance of flexible electronics produced by R2R process [85]. The biggest challenge of R2R process is the unification problem [86]. The comparison between different printing methods for large-scale R2R production was reported in Roll-to-Roll Processing

phy process [76] are all the patterning strategies for flexible transparent electrodes.

Technology Assessment by U.S. Department of Energy, as shown in **Table 5** [87].

**Figure 6.** The schematic and equipment of roll-to-roll fabrication process: (a) the schematic [82], (b) photograph of the R2R system [84], (c) a laboratory-scale coating system from solar coating machinery GmbH, Germany [77], and (d)

photograph of the monitoring during coating [77].

*3.2.1. Roll-to-roll techniques*

**Figure 4.** AgNW synthesis: (a) the polyol process of AgNW synthesis [61], and (b) the schematic diagram of a multistep synthesis of ultra-long AgNWs [59].

**Figure 5.** Equipments of different coating techniques: (a) parts of slot-die coating device [77], (b) slot-die coating device [77], (c) Meyer rod coating device [78], (d) electrostatic spray system [79], and (e) pad printer [80].

AgNW electrodes, including Meyer rod coating [5, 33, 62, 63], dip coating [64], spin coating [65–67], drop casting [68], spray coating [69], vacuum filtration [70], roll-to-roll printing [19, 71, 72] and transferring [73, 74]. **Figure 5(a)**-**(e)** show coating devices with different techniques. Most of the techniques are compatible with low-energy deposition process and without any vacuum equipment. Direct laser ablation [13, 75], shadow mask [11], chemical etching using the photolithography process [76] are all the patterning strategies for flexible transparent electrodes.

#### *3.2.1. Roll-to-roll techniques*

AgNW electrodes, including Meyer rod coating [5, 33, 62, 63], dip coating [64], spin coating [65–67], drop casting [68], spray coating [69], vacuum filtration [70], roll-to-roll printing [19, 71, 72] and transferring [73, 74]. **Figure 5(a)**-**(e)** show coating devices with different techniques. Most of the

,

**Figure 5.** Equipments of different coating techniques: (a) parts of slot-die coating device [77], (b) slot-die coating device

[77], (c) Meyer rod coating device [78], (d) electrostatic spray system [79], and (e) pad printer [80].

**Figure 4.** AgNW synthesis: (a) the polyol process of AgNW synthesis [61], and (b) the schematic diagram of a multistep

synthesis of ultra-long AgNWs [59].

28 Flexible Electronics

The processability of AgNW networks by R2R was showed by many researchers due to their compatibility with large-area production [81–83]. The substrate in R2R coating system is required to have mechanical flexibility and in the form of a long sheet. Quite different from other solution-processed coating methods, the R2R process is continuous and is suitable for large-area production. During coating, the substrate is first unwound from a roll and then passed through the coating machine and finally rewound on another roll. Aside from the coating machine, some post-treatment may also be added into the process, such as compressing, heating, UV-curing, chemical welding and drying, as shown in **Figure 6(a)** [83]. Interestingly, Lai's team produced AgNW electrodes combined with moth-eye nanostructures using R2R techniques and greatly enhanced the transmittance [5, 81]. The quality of forming can be influenced by tension, speed, cleaning of the substrate and the removal of static electricity. Also, the pre-treatment and post-treatment can have a great impact on the performance of the coated AgNW films. Many laboratories have developed their own R2R system to study the coating process. **Figure 6(b)**-**(d)** show two laboratory-scale coating system [77, 84]. Hösel et al. have compared the performance of flexible electronics produced by R2R process [85]. The biggest challenge of R2R process is the unification problem [86]. The comparison between different printing methods for large-scale R2R production was reported in Roll-to-Roll Processing Technology Assessment by U.S. Department of Energy, as shown in **Table 5** [87].

**Figure 6.** The schematic and equipment of roll-to-roll fabrication process: (a) the schematic [82], (b) photograph of the R2R system [84], (c) a laboratory-scale coating system from solar coating machinery GmbH, Germany [77], and (d) photograph of the monitoring during coating [77].


(a)-Stopping should be avoided. Risk of registration lost and drying of ink in anilox cylinder. Short run-in length. NA-not available.

**Table 5.** Comparison between different printing methods in terms of their theoretical capacity and practical applicability for large-scale R2R production [87].

#### *3.2.2. Drop casting*

Drop casting is the simplest method to produce flexible transparent electrodes. The equipment needed is only a horizontal work platform. What we need to do is casting the coating solution onto the substrate followed by drying. However, problems exist due to the simple procedure. The thickness of the film is unable to be controlled. The effect of "coffee-ring" may be easily observed causing uneven distribution of nanowires due to the surface tension of the liquid and the self-aggregation of nanowires upon drying.

#### *3.2.3. Spin coating*

Spin coating is an important way to form homogeneous film. As illustrated in **Figure 7(a)**, the substrate is first accelerated to a chosen rotational speed and then the coating solution is applied onto the substrate [86]. Noticeably, most of the coating solution is ejected and only a little of the solution is left on the substrate to form a thin film. **Figure 7(b)**-**(f)** show the spin coating operation and the high speed images with different timing after the first drop [86]. Spin coating is high reproducible. The forming quality of spin coating can be measured by the thickness, morphology and the surface topography of the film coated. All these properties can be tuned by controlling the coating solution, the substrate and the rotational speed. Specially, the molecular weight, viscosity, diffusivity, volatility and concentration of the solutes all have impact on the final forming results.

Third, the patterned electrodes is obtained by filling the screen with coating ink. **Figure 8(a)**- **(c)** show screen printers both in laboratories and factories, while **Figure 8(d)** shows the screen

**Figure 8.** Photographs of screen printers and the coating process: (a-b) pictures of industrial screen printers [80, 86], (c) screen printing of silver nanowires in the laboratory [77], and (d) a close photograph showing screen printing [88].

**Figure 7.** Spin coating: (a) the schematic, (b) photograph of the operation, and (c-f) high-speed images with the timing

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The optimization of the optoelectronic properties has been studied for many years. Since junction resistance plays an important role in the electrical properties of the whole network, decreasing

**4. Performance enhancements of flexible transparent electrodes**

printing process.

**4.1. Optoelectronic properties**

after the first drop of 17, 100, 137 and 180 ms [86].

#### *3.2.4. Screen printing*

Screen printing has a large wet film thickness. The coating ink used needs to have a high viscosity and low volatility. First, the screen should be under tension by being glued to a frame. Second, an emulsion is filled into the screen to obtain the pattern. Here the area of the emulsion should be with no print and the area of the pattern is open waiting for the coating ink. Fabrication and Applications of Flexible Transparent Electrodes Based on Silver Nanowires http://dx.doi.org/10.5772/intechopen.77506 31

**Figure 7.** Spin coating: (a) the schematic, (b) photograph of the operation, and (c-f) high-speed images with the timing after the first drop of 17, 100, 137 and 180 ms [86].

**Figure 8.** Photographs of screen printers and the coating process: (a-b) pictures of industrial screen printers [80, 86], (c) screen printing of silver nanowires in the laboratory [77], and (d) a close photograph showing screen printing [88].

Third, the patterned electrodes is obtained by filling the screen with coating ink. **Figure 8(a)**- **(c)** show screen printers both in laboratories and factories, while **Figure 8(d)** shows the screen printing process.
