# Copper Nanowires for Transparent Electrodes: Properties, Challenges and Applications
## 6 Applications of Copper Nanowires  
Once demonstrated the viability of Cu NW based network as transparent conductors, a number of applications have also been demonstrated by several authors. The first application was shown by [PERSON] et al. who employed Cu NW transparent electrodes in a blue LED as ohmic contacts on _n_- and _p_-type GaN layers (Figure 9a) [29]. The ohmic contact is proved by the _I-V_ curves in Figure 9b,c. The use in a GaN-based LED was also reported by [PERSON] et al. [69]. A LED structure was also demonstrated by [PERSON] et al. who fabricated a Cu NW based phosphorescent organic green LED with stable operation even under flexing [70]. In the same work, the authors proposed the application as transparent heater, working simply by Joule heating by applying a voltage to the ends of a Cu NW pattern, as shown in Figure 10 [70]. A transparent heater was also reported by [PERSON] et al. who used core-shell NWs with ZnO and Al\({}_{2}\)O\({}_{3}\)[47]. Here, the authors employed this device for a defrosting application, reaching heating rate of 14 \({}^{\circ}\)C/s and maximum temperatures of 273 and 204 \({}^{\circ}\)C for Al\({}_{2}\)O\({}_{3}\) and ZnO, respectively [47]. Flexible transparent heaters were similarly demonstrated with high heating uniformity and stable temperature output for up to 5 h by [PERSON] et al. [63; 64]. Flexible transparent heaters were also demonstrated by [PERSON] et al. using a transparent flexible conductive tape, obtained by \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Type** & **Substrate/Matrix** & **Number of Cycles** & **R/R\({}_{0}\)** & **Ref.** \\ \hline Bending & Filtration membrane & 1000 & 1 & [18] \\ Bending & PET & 1000 & 1.33 & [19] \\ Bending & PI & 100 & \(\sim\)1 & [29] \\ Bending & PI & 20,000 & \(\sim\)1 & [60] \\ Folding & PI & 6 & 1.4 & [60] \\ Stretching & Eco-flex & 850 & 1.1 & [60] \\ Stretching & PU & 200 & 1 & [65] \\ Bending & PET & 1280 & 1.5 & [48] \\ Bending & PET & 2000 & \(<\)1.1 & [38] \\ Stretching & PU & 1000 & 2 & [57] \\ Bending & PU & 100 & 1.5 & [57] \\ Bending & PET & 10,000 & \(<\)1.04 & [66] \\ Bending & PU & 1000 & \(<\)1.1 & [39] \\ Bending & PU & 500 & \(<\)1.1 & [67] \\ \hline Bending & PU & 1500 & \(<\)1.05 & [35] \\ Bending & PET/PDMS & 1000 & \(\sim\)1.04 & [40] \\ Stretching & PDMS & 1000 & 1.1 & [40] \\ Twisting & PDMS & 1000 & 1.05 & [40] \\ Bending & PET & 2000 & \(\sim\)1.2 & [62] \\ Bending & PET & 2000 & \(<\)1.05 & [63] \\ Bending & PET & 1000 & \(<\)1.05 & [64] \\ Bending & PET & 1000 & 1.15 & [52] \\ Bending & PET & 500 & 1.33 & [68] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of mechanical tests reported for Cu NWs. PDMS: polydimethylsiloxane. transferring a Cu NW network from glass, where it was firstly deposited, onto a transparent tape [71]. The tape obtained had \(Rs\sim\)20 \(\Omega\)/sq at \(T=80\%\) and temperature up to 143 \({}^{\circ}\)C.  
Another popular application that has been proposed for Cu NW networks is in solar cells. The first demonstration was reported by [PERSON] et al. who reported a small molecule organic solar cell with 3% efficiency [72]. Here, the authors used a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layer for planarization applied on the NW network by spin-coating which, despite its hygroscopic and acidic properties, did not affect the stability of the Cu NWs. [PERSON] et al. reported an organic solar cell based on core-shell Ni-Cu NWs with 4.9% efficiency, again using a PEDOT-PSS layer on top of the NW network [42]. Similarly, [PERSON] et al. fabricated an organic solar cell with a very high efficiency of 7.31% [68]. [PERSON] et al. made a polymer solar cell with 4.04% efficiency using their Cu-graphene core-shell NWs and PEDOT:PSS as planarization layer [51]. This compared to a 1.90% efficiency for pure Cu NWs. [PERSON] et al. reported an organic solar cell based on Cu NW network with 2.67% efficiency, where they used an in-situ polymerization method to make an acrylic polymer layer for planarization [58].