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

#### **4.1. Optoelectronic properties**

*3.2.2. Drop casting*

for large-scale R2R production [87].

Flatbed Screen Printing

30 Flexible Electronics

Rotary Screen Printing

Imprint or soft lithography

NA-not available.

*3.2.3. Spin coating*

*3.2.4. Screen printing*

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

(a)-Stopping should be avoided. Risk of registration lost and drying of ink in anilox cylinder. Short run-in length.

**Table 5.** Comparison between different printing methods in terms of their theoretical capacity and practical applicability

**(μm)**

Inkjet Printing Medium 1–5 μm < 50 μm Yes High Limited, materials

Flexography Very high 1–10 μm < 50 μm Yes(a) Medium Very good

Laser ablation Low NA ~10 NA NA Thermal effect

Gravure High NA > 0.07 μm NA NA Very good

Low 5–100 μm 100 μm Yes Low Limited

High 3–500 μm 100 μm Yes(a) Medium Very good

NA 0.1 μm NA NA New technology

**Start/Stop Complexity Applicability**

must be jettable

sensitivity

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.

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.

liquid and the self-aggregation of nanowires upon drying.

**Printing method Speed Wet thickness Resolution** 

High (> 5m/ min)

> 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

the number of junctions and reducing the junction resistance between wires are two main ideas to lower the sheet resistance of the film. In order to decrease the number of junctions, some researchers have studied different approaches to synthesize nanowires with high aspect ratio [59, 60, 89], which have been introduced in Section 3.1. Researchers have also devoted great efforts to decrease the junction resistance between nanowires. Methods such as vacuum filtration [90], graphene coating [91, 92], electrochemical coating [93], modification with graphene oxide (GO) [29] and deposition of particles like Au, ZnO and TiO<sup>2</sup> [94] have been performed in the fabrication of transparent electrodes to reduce the resistance. Liang et al. wrapped the GO sheet around AgNW junctions and obtain a flexible transparent electrodes with the sheet resistance of 14 Ω/sq. and the transmittance of 88%, as shown in **Figure 9(a)** [29]. Many post-treatments such as thermal annealing [24], pressing [95], electrochemical annealing [96], salt treatment [83, 90], plasmonic welding [97], HCl vapor treatment, capillary-force-induced cold welding [63] and high intensity pulsed light technique(HIPL) [98, 99] have also been studied to reduce the junction resistance. **Figure 9(b)** and **(c)** show the obvious changes of AgNW junctions after hot-pressing. Lee et al. [6] demonstrated that annealing of the nanowire network at the temperature of 200°C causes the PVP to flow and partially decompose, leading AgNWs to fuse together. However, thermal annealing needs high temperature and long treatment time. It also cannot be employed with heat-sensitive substrates. Tokuno et al. [100] performed two steps to replace the heat treatment in the fabrication of transparent electrodes. The network was first rinsed with water and ethanol to remove the PVP followed by mechanical pressing to weld the wires. The sheet resistance was reduced from 6.9 × 106

**4.2. Environmental stability**

**Table 6.** Performance comparison in AgNW electrodes.

to 1.8 × 10<sup>4</sup> Ω/sq. and then to 8.6 Ω/sq. However, mechanical pressing may

Fabrication and Applications of Flexible Transparent Electrodes Based on Silver Nanowires

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33

not suitable for delicate substrates. Thus some other different approaches such as joule heating, HIPL, moisture-treating and hybridization with mesoscale wires, have been explored. Song et al. [101] apply the idea that joule heating can weld platinum wires and carbon nanotubes into the metallic nanowire networks. An approach with low additional power and short treatment durations is achieved by current-assisted localized joule heating accompanied by electromigration, as shown in **Figure 9(d)**. The resistance of individual nanowires is also investigated by researchers, such as the utilization of nanowires with large gain size [102] and the hybridization of different scale wires [33, 103]. **Figure 9(e)** shows the procedure used to produce dual-scale nanowire networks [33]. Many investigations also focus on hybridizing AgNWs with other conductive materials. AgNWs were treated as bridges for high resistance grain boundaries of graphene by

Teymouri et al. to obtain highly transparent electrodes, as shown in **Figure 9(f)** [104].

from AgNWs [107]. **Table 6** illustrates the performance comparison in AgNW electrodes.

Though environmental stability seems to be important for future application, few investigations have been reported so far on it compared to the optoelectronic performance. The

**NW dimensions Substrate Rs (Ω/sq) T (%) Ref.** D 20–40 nm, L 20–40 μm Glass/PET 91.3 97.9 [108] D 35 nm, L 25 μm PET ~50 94.5 [1] D 25 nm, L 35 μm PET ~20 86 [5] D 20–90 nm, L 20–150 μm PDMS 179 89.4 [63] D 100 nm, L 100 μm and D 40 nm, L 10 μm Resin 50 90 [33] D 50–90 nm, L 15–25 μm PEN 12 83 [32] D 70 nm, L 8 μm glass 6–21 70–85 [21] D 70 nm, L 10–20 μm and D 85 nm, L 30–60 μm PU 6 68 [98] D 70 nm, L 200 μm No data <30 95 [104] Not available Glass 11 87 [34] D 115 nm, L 20–50 μm PET/PEN 5 92 [83]

Besides the electrical properties, many efforts have been done by researchers to improve the optical transparency of AgNW electrodes. Firstly, the dimensions of AgNWs are optimized for high transmittance. Nanowires networks with large aspect ratio show better optical property [105]. Secondly, different deposition process has been explored. Kim et al. [79] applied the electrostatic spray deposition to obtain electrodes with the transmittance of 92.1%. Thirdly, changing substrates into more transparent materials. Jiang et al. [106] changing the commonly used polyethylene terephthalate (PET) substrate into the flexible resin film and improve the transmittance by nearly 10%. Kim et al. integrated CNT into AgNW electrodes to reduce the haze factor by absorbing the scattered light

**Figure 9.** Different methods to improve the optoelectronic properties of flexible transparent AgNW electrodes: (a) SEM observation of AgNW networks with GO surrounding AgNW junctions [29], (b-c) SEM images of AgNW networks before and after hot-pressing [32], (d) a schematic diagram of electrowelding treatment [101], (e) illustration of the procedure used for the preparation of dual-scale nanowire networks [33], and (f) SEM image of AgNWs bridging graphene grains [104].

reduced from 6.9 × 106 to 1.8 × 10<sup>4</sup> Ω/sq. and then to 8.6 Ω/sq. However, mechanical pressing may not suitable for delicate substrates. Thus some other different approaches such as joule heating, HIPL, moisture-treating and hybridization with mesoscale wires, have been explored. Song et al. [101] apply the idea that joule heating can weld platinum wires and carbon nanotubes into the metallic nanowire networks. An approach with low additional power and short treatment durations is achieved by current-assisted localized joule heating accompanied by electromigration, as shown in **Figure 9(d)**. The resistance of individual nanowires is also investigated by researchers, such as the utilization of nanowires with large gain size [102] and the hybridization of different scale wires [33, 103]. **Figure 9(e)** shows the procedure used to produce dual-scale nanowire networks [33]. Many investigations also focus on hybridizing AgNWs with other conductive materials. AgNWs were treated as bridges for high resistance grain boundaries of graphene by Teymouri et al. to obtain highly transparent electrodes, as shown in **Figure 9(f)** [104].

Besides the electrical properties, many efforts have been done by researchers to improve the optical transparency of AgNW electrodes. Firstly, the dimensions of AgNWs are optimized for high transmittance. Nanowires networks with large aspect ratio show better optical property [105]. Secondly, different deposition process has been explored. Kim et al. [79] applied the electrostatic spray deposition to obtain electrodes with the transmittance of 92.1%. Thirdly, changing substrates into more transparent materials. Jiang et al. [106] changing the commonly used polyethylene terephthalate (PET) substrate into the flexible resin film and improve the transmittance by nearly 10%. Kim et al. integrated CNT into AgNW electrodes to reduce the haze factor by absorbing the scattered light from AgNWs [107]. **Table 6** illustrates the performance comparison in AgNW electrodes. AgNWs resistance

#### **4.2. Environmental stability**

the number of junctions and reducing the junction resistance between wires are two main ideas to lower the sheet resistance of the film. In order to decrease the number of junctions, some researchers have studied different approaches to synthesize nanowires with high aspect ratio [59, 60, 89], which have been introduced in Section 3.1. Researchers have also devoted great efforts to decrease the junction resistance between nanowires. Methods such as vacuum filtration [90], graphene coating [91, 92], electrochemical coating [93], modification with graphene oxide (GO)

tion of transparent electrodes to reduce the resistance. Liang et al. wrapped the GO sheet around AgNW junctions and obtain a flexible transparent electrodes with the sheet resistance of 14 Ω/sq. and the transmittance of 88%, as shown in **Figure 9(a)** [29]. Many post-treatments such as thermal annealing [24], pressing [95], electrochemical annealing [96], salt treatment [83, 90], plasmonic welding [97], HCl vapor treatment, capillary-force-induced cold welding [63] and high intensity pulsed light technique(HIPL) [98, 99] have also been studied to reduce the junction resistance. **Figure 9(b)** and **(c)** show the obvious changes of AgNW junctions after hot-pressing. Lee et al. [6] demonstrated that annealing of the nanowire network at the temperature of 200°C causes the PVP to flow and partially decompose, leading AgNWs to fuse together. However, thermal annealing needs high temperature and long treatment time. It also cannot be employed with heat-sensitive substrates. Tokuno et al. [100] performed two steps to replace the heat treatment in the fabrication of transparent electrodes. The network was first rinsed with water and ethanol to remove the PVP followed by mechanical pressing to weld the wires. The sheet resistance was

**Figure 9.** Different methods to improve the optoelectronic properties of flexible transparent AgNW electrodes: (a) SEM observation of AgNW networks with GO surrounding AgNW junctions [29], (b-c) SEM images of AgNW networks before and after hot-pressing [32], (d) a schematic diagram of electrowelding treatment [101], (e) illustration of the procedure used for the preparation of dual-scale nanowire networks [33], and (f) SEM image of AgNWs bridging

graphene grains [104].

[94] have been performed in the fabrica-

[29] and deposition of particles like Au, ZnO and TiO<sup>2</sup>

32 Flexible Electronics

**NW dimensions Substrate Rs (Ω/sq) T (%) Ref.** D 20–40 nm, L 20–40 μm Glass/PET 91.3 97.9 [108] D 35 nm, L 25 μm PET ~50 94.5 [1] D 25 nm, L 35 μm PET ~20 86 [5] D 20–90 nm, L 20–150 μm PDMS 179 89.4 [63] D 100 nm, L 100 μm and D 40 nm, L 10 μm Resin 50 90 [33] D 50–90 nm, L 15–25 μm PEN 12 83 [32] D 70 nm, L 8 μm glass 6–21 70–85 [21] D 70 nm, L 10–20 μm and D 85 nm, L 30–60 μm PU 6 68 [98] D 70 nm, L 200 μm No data <30 95 [104] Not available Glass 11 87 [34] D 115 nm, L 20–50 μm PET/PEN 5 92 [83]

Though environmental stability seems to be important for future application, few investigations have been reported so far on it compared to the optoelectronic performance. The

**Table 6.** Performance comparison in AgNW electrodes.

pathways, as shown in **Figure 10(b)** [27]. Intense-pulsed-light irradiation and the UV-Ozone treatment are also ways to smooth the film without any severe deterioration in the optical

Fabrication and Applications of Flexible Transparent Electrodes Based on Silver Nanowires

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35

The poor adhesion to substrates, together with the roughness and reduced effective electrical area, hinders the widely application of AgNW electrodes. Strong bonding is essential to avoid detachment of AgNW networks and maintain electrical conductivity at high strain deformation. Modifying substrate surface [74, 116], applying strong conformal pressure, in situ polymerization [24, 25, 94] and surface encapsulation [71] can effectively improve the adhesion. These methods are complex and time consuming, together with changing the properties of the substrates. In recent papers, some new methods have also been put forward. It is a fast and simple method compared with conventional approaches embedding nanowires into polymers [98]. Khan et al. [117] proposed a facile method to make the nanowires a nail-like structure which can be fully embedded in plastic films, greatly enhancing the

The aim of this chapter is to demonstrate the fabrication techniques of flexible transparent AgNW electrodes and the efforts made to enhance the performance. Though AgNW electrodes reported exhibit similar performances to ITO electrodes, there is still a long way to go for future commercialization. Firstly, new synthesis methods for fine-tuning the dimensions of AgNWs are needed. The performances of AgNW electrodes have a close relationship with the dimensions of AgNWs. Secondly, metals other than silver need investigations to reduce the cost of electrodes with similar performances, such as copper. Thirdly, the hybrid materials, such as core-shell Cu-Ni nanowires and sandwich structure, are also of interest. Fourthly, the stability optimization in real environments is lacking now. The evaluation of the intrinsic stability is an important value to prove the possibility of integrating nanowires into future devices. Finally, the toxicity of nanowires needs attention before being integrated into com-

This study was supported by the National Natural Science Foundation of China (51675334).

performance [22, 115].

wire-substrate adhesion.

**5. Conclusion**

mercial devices.

**Acknowledgements**

**Conflict of interest**

There are no conflicts of interest.

**Figure 10.** Schematic of the mechanism for treatments used to improve environmental stability and mechanical properties: (a) the protection mechanism for the nanoparticle coating and sol–gel TiO<sup>2</sup> coating [110], and (b) the conductive pathway enlarging mechanism for the plasma treatment on AgNW-cPI composite electrodes [27].

thermal conductivity of AgNWs and their degradation mechanism is lack of investigations. Mayousse et al. spend over 2 years to study the relationship between the stability of AgNW networks and the temperature, humidity, light, hydrogen sulfide and electrical stress [11]. Khaligh et al. [109] once modeled the random AgNW networks in MATLAB and analyzed the overall circuit with HSPICE. In their work, graphene can slow the degradation of AgNWs and uniform the surface temperature. In this case, the failure mechanism is not the nanowire degradation any more. It is changed into the melting of the substrate. Thus the most commonly used method is to use hybrid materials to improve both thermal and chemical stability. Also, the aging of flexible transparent electrodes under different working conditions is now lack of research. Song et al. compared the environmental stability of nanowires with nanoparticle coating and sol–gel TiO<sup>2</sup> coating [110]. As shown in **Figure 10(a)**, AgNWs can easily react with sulfur ions to form Ag<sup>2</sup> S (dark gray). Nanoparticles can reduce the exposure area but are unable to diminish the whole area. AgNWs with nanoparticles coating can still react with sulfur ions. The sol–gel TiO<sup>2</sup> avoid AgNWs being exposed to sulfur ions and improve the chemical durability of AgNWs.

#### **4.3. Mechanical properties**

The sheet resistance of transparent AgNW electrodes shows negligible increase under bending test, quite different from that of ITO electrodes. AgNWs are able to conform to non-planar surface. They can easily fit to the surface, even the highly roughened surface. Though the flexibility makes AgNW a promising alternative to ITO, the high roughness of the network remains a serious problem which hinders the development of AgNW electrodes. The high roughness would lead to interlayer shorting, high leakage currents, and low quantum efficiency in OLEDs [33]. The buffer layer and the conductive material coating are investigated by many researchers to reduce the roughness of the networks [111–114]. Nevertheless, they would degrade the performance by increasing the driving voltage and the electron–hole imbalance [30]. Burying AgNW into polymer substrate is also a way to reduce the roughness, but the effective electrode areas decreased [33]. In order to overcome this problem, the plasma treatment was applied on AgNW-cPI composite electrodes to enlarge the conductive pathways, as shown in **Figure 10(b)** [27]. Intense-pulsed-light irradiation and the UV-Ozone treatment are also ways to smooth the film without any severe deterioration in the optical performance [22, 115].

The poor adhesion to substrates, together with the roughness and reduced effective electrical area, hinders the widely application of AgNW electrodes. Strong bonding is essential to avoid detachment of AgNW networks and maintain electrical conductivity at high strain deformation. Modifying substrate surface [74, 116], applying strong conformal pressure, in situ polymerization [24, 25, 94] and surface encapsulation [71] can effectively improve the adhesion. These methods are complex and time consuming, together with changing the properties of the substrates. In recent papers, some new methods have also been put forward. It is a fast and simple method compared with conventional approaches embedding nanowires into polymers [98]. Khan et al. [117] proposed a facile method to make the nanowires a nail-like structure which can be fully embedded in plastic films, greatly enhancing the wire-substrate adhesion. and
