**4. Use of metallic nanowire networks for energy applications**

Among others, photovoltaic systems, light-emitting diodes (LED) or smart windows constitute sustainable green energy technologies which have been intensively studied lately for energy saving and/or for an alternative to fossil fuel energy. Generally speaking the main goals associated with these technologies concern cost reduction, efficiency improvement and use of abundant materials. For photovoltaic and efficient lighting (LED or OLED), the light should either enter a solar cell or exit the LED requiring the use of an efficient transparent electrode for, respectively, collecting or injecting the carriers. Several investigations have shown that MNW network-based transparent electrodes can be efficiently integrated in such energy devices thanks to their electrical and optical properties. Their excellent flexibility constitutes a clear asset for flexible devices and/or when fast (and then low-cost) technologies such as roll-to-roll are used for the industrial fabrication. And the possibility to coat MNWs allows to tune the work function and band alignments and can therefore lead to better integration possibilities.

For solar cells, MNW networks have been mainly tested in organic solar cells. One of the first demonstrations was reported by Leem et al. [55]: these authors used AgNW network as electrode in P3HT/PCBM organic solar cells, and it showed an efficiency of 2.5% which was equivalent to ITO-based devices. And Yang et al. showed that by using fully solution-processed polymer, bulk heterojunction (BHJ) solar cells with anodes composed of AgNWs were successfully fabricated with performances slightly lower than when ITO is used [56]. Interestingly they showed that the BHJ solar cells were highly flexible since the fabricated solar cells exhibited recoverable efficiency even under large bending deformation up to 120°.

AgNW and CuNW can also be efficiently integrated in OLED devices. AgNW networks were the first to be integrated in OLED devices: The obtained electrode was shown to be suitable for the fabrication of high-performance polymer-based LED [57]. A very recent study by Lian et al. reported the use of CuNW-based

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*Metallic Nanowire Percolating Network: From Main Properties to Applications*

composite film for OLED integration [58]. A good electrical conductivity (22 Ω/ sq), high transmittance (88%), low surface roughness and good adherence to the substrate were observed. The good adherence originates from the presence of the polymethyl methacrylate (PMMA) coating on CuNWs. The fabricated CuNW/ PMMA composite film appears to be stable since it can resist air, water and ethanol exposure without electrical deterioration. With this CuNW/PMMA composite film as anode for the OLED, the device performances appear even better than those with

Several articles clearly also showed that MNW network-based transparent electrodes can be efficiently integrated within electrochromic devices. Thanks to the chrono-amperometry curves and the corresponding in situ transmittance curve at 1100 nm for a WO3/AgNW film, it was clearly demonstrated that good performances can be obtained when the fabricated electrochromic device uses AgNW network [59]. Such fabricated films exhibit excellent cycling stability as well as distinct modulation of near-infrared light compared with ITO-based electrochro-

Another energy application which appears promising for MNW integration in energy area concerns supercapacitor. Yuksel et al. [22] fabricated nanocomposite electrochromic supercapacitor electrodes: AgNW network electrodes and a green to transmissive electrochromic polymer (PDOPEQ ). These authors showed that the obtained supercapacitors have a changing colour from vibrant green to transparent with very good characteristics (i.e. specific capacitance of 61.5 F/g at a current density of 0.1 A/g, capacity retention upon 20**,**000 galvanostatic charge-discharge cycles). Such characteristics appear very promising for use of MNW-based trans-

As discussed MNW networks exhibit strong potential to act as efficient transparent electrodes for many applications. Indeed, MNW exhibits high transparency and low electrical resistance levels, which are associated with excellent bendability and good stretchability. This contribution reports briefly the influence of main parameters on the MNW network-based TE, the prevailing parameters being MNW chemical nature and dimensions as well as network density. Still, for approaching an efficient integration into industrial devices such as organic solar cells, efficient light production or smart windows, several other requirements have to be considered. One of the most important ones concerns their stability which appears to be a crucial issue: it can involve either electrical, thermal and mechanical aspects or ageing and chemical degradation. The origin of failure in MNW networks was discussed in this contribution with the following stages: optimization, degradation and breakdown of the MNW network. The breakdown occurs via a localized mechanism thanks to the creation and propagation of a crack. To prevent such instability, encapsulation of MNW network is performed by thin oxide layers: this leads to a drastic enhancement of the MNW networks stability. Moreover such approach shows improved adhesion and much better thermal and electrical stability. Finally this contribution shows that the scientific community has worked in several directions and has demonstrated that MNW network-based transparent electrodes can be integrated in industrial devices such as organic photovoltaic, lightemitting diode, in smart windows or supercapacitors. The prospects concern the replacement of AgNW by cheaper MNW such as CuNW while the stability might be a stronger issue than for AgNW, a better optimization of the many parameters (MNW chemistry and dimensions, coating, etc.) for a given application and to

parent electrodes in electrochromic supercapacitors [22].

*DOI: http://dx.doi.org/10.5772/intechopen.89281*

using ITO anode [58].

mic devices.

**5. Conclusions**

#### *Metallic Nanowire Percolating Network: From Main Properties to Applications DOI: http://dx.doi.org/10.5772/intechopen.89281*

composite film for OLED integration [58]. A good electrical conductivity (22 Ω/ sq), high transmittance (88%), low surface roughness and good adherence to the substrate were observed. The good adherence originates from the presence of the polymethyl methacrylate (PMMA) coating on CuNWs. The fabricated CuNW/ PMMA composite film appears to be stable since it can resist air, water and ethanol exposure without electrical deterioration. With this CuNW/PMMA composite film as anode for the OLED, the device performances appear even better than those with using ITO anode [58].

Several articles clearly also showed that MNW network-based transparent electrodes can be efficiently integrated within electrochromic devices. Thanks to the chrono-amperometry curves and the corresponding in situ transmittance curve at 1100 nm for a WO3/AgNW film, it was clearly demonstrated that good performances can be obtained when the fabricated electrochromic device uses AgNW network [59]. Such fabricated films exhibit excellent cycling stability as well as distinct modulation of near-infrared light compared with ITO-based electrochromic devices.

Another energy application which appears promising for MNW integration in energy area concerns supercapacitor. Yuksel et al. [22] fabricated nanocomposite electrochromic supercapacitor electrodes: AgNW network electrodes and a green to transmissive electrochromic polymer (PDOPEQ ). These authors showed that the obtained supercapacitors have a changing colour from vibrant green to transparent with very good characteristics (i.e. specific capacitance of 61.5 F/g at a current density of 0.1 A/g, capacity retention upon 20**,**000 galvanostatic charge-discharge cycles). Such characteristics appear very promising for use of MNW-based transparent electrodes in electrochromic supercapacitors [22].
