**5. Conclusions**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

high optical-electrical performance and excellent stability [53].

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

can therefore lead to better integration possibilities.

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

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

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

recoverable efficiency even under large bending deformation up to 120°.

coating, the lower the optical transparency.

network transmittance [52].

the deposited ZnO layer, the better the stability. This stability enhancement can be explained as follows: the ZnO oxide coating can (at least partially) hinder silver atomic diffusion through the oxide coating [52], avoiding the spheroidization and/ or electromigration of AgNWs. A compromise in terms of oxide coating thickness has to be considered depending on the target application since the thicker the ZnO

Another example of stability enhancement was reported by Shi et al. [53] who demonstrated that transfer of CVD grown graphene onto CuNW films drastically enhances the stability of the hybrid films over long time scale (up to 180 days), while different ageing conditions were also investigated. Graphene is shown to play a key role for preventing oxygen species permeation which drastically decreases oxidation rate. This allows to obtain stable CuNW networks associated with both

In summary to avoid any degradation or oxidation, metallic nanowires are nowadays often coated by a protective layer for an improved integration. This protective (nanoparticles or thin inorganic) layer could be either metallic [54], based on graphene, polymeric or a transparent oxide [52]. This leads in general to a much enhanced thermal and electrical stability along with a better adhesion, although this is at the expense of optical transmittance decrease. One can also observe that conformal thin oxide coating deposited either by ALD or by spatial ALD appears to be an efficient protecting coating while keeping rather high

**140**

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

make MNW deposition and optimization fast and low-cost enough to be compatible with industrial challenges (for instance, compatible with the very fast roll-to-roll technology).
