# Copper Nanowires for Transparent Electrodes: Properties, Challenges and Applications
## 6 Applications of Copper Nanowires
A simple scheme of the NW network-polymer assembly is shown in Figure 11. Table 2 summarizes the main information on the reported solar cell devices. Figure 10: Scheme of a simple transparent heater. Reprinted with permission from Ref. [70]. Copyright 2016 American Chemical Society. Figure 9: (**a**) Scheme of the blue light LED stack; (**b**) _I-V_ curve of _n_-GaN contact; (**c**) _I-V_ curve of the _p_-GaN contact. Adapted with permission from Ref. [29]. A very popular field of application is in the stretchable and wearable electronics, usually when the Cu NW are coupled with polymers like PU or PDMS. The first demonstration of a flexible touch sensor, based on capacitance change, was given by [PERSON] et al. who fabricated a Cu NW composite with PEDOT:PSS [30]. [PERSON] et al. showed the operation of a touch-screen panel from Cu NWs on PET [59]. Here, while the device operated correctly, the counter-electrode was made of ITO, which may hinder the stability under bending. In the same work, the authors demonstrated a transparent resistive memory, again based on an ITO counter-electrode [59]. The device showed a good ON/OFF ratio of \(10^{3}\). [PERSON] et al. proposed a flexible touch sensor based on their NW-PU composite [35]. Such device is able to produce a current when \"touched\", i.e., a small pressure is applied (Figure 12a). This ability is maintained upon bending, although the response is significantly reduced (Figure 12b) [35]. [PERSON] et al. also demonstrated a transparent tactile sensor that mapped the pressure applied by finger touch or other objects (Figure 13) [53]. [PERSON] et al. demonstrated a wearable sensor using a NW-PDMS composite which is able to detect the movement of a finger joint with fast response [36]. [PERSON] et al. demonstrated the realization of an electrochromic device based on Prussian blue from their NW-RGO composites [49]. Such a device was fabricated depositing the dye between the NW-RGO based electrode and an ITO counter-electrode. The reduction of the Fe\({}^{2+}\)/Fe\({}^{3+}\) mixture, typical of the Prussian blue dye, to Fe\({}^{2+}\) upon applying a 0.6 V voltage reversibly changed the device color from blue to colorless, as shown in Figure 14 [49]. An electrochromic device was also recently demonstrated by [PERSON] et al. using polyaniline which was directly polymerized onto the Cu NW network [62]. Here, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Solar Cell Type** & **Planarization** & **Efficiency** & **Ref** \\ \hline Organic & PEDOT:PSS & 3\% & [72] \\ Organic & PEDOT:PSS & 4.9\% & [42] \\ Organic & PEDOT:PSS & 7.3\% & [68] \\ Polymer & PEDOT:PSS & 4\% & [51] \\ Organic & Acrylic polymer & 2.7\% & [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of solar cell devices made using Cu NWs as transparent electrode. Figure 11: Scheme of a NW network-polymer assembly. different colors were obtained by tuning the voltage. The device also worked for energy storage with a capacitance of 12 mF/cm\({}^{2}\). In summary, this section provided an overview of the applications proposed for Cu NW transparent electrodes. These include LEDs, solar cells, wearable electronics, energy storage and electrochromic devices. Figure 14: Operation of an electrochromic device based on Cu NW-RGO composite. (**a**) As prepared film; (**b**) colored state; (**c**) beached state. Adapted with permission from Ref. [49]. Copyright 2013 American Chemical Society. Figure 13: Transparent flexible tactile sensor. (**a**) touch by fingers; (**b**) touch by an object; (**c**) sensor response to touch by fingers; (**d**) sensor response to touch by an object. Adapted with permission from Ref. [53]. Copyright 2020 American Chemical Society.