**1. Introduction**

Transparent electrodes (TEs) are key components for many industrial devices. TEs indeed do concern applications related to energy field such as photovoltaics or efficient lighting (light emitting diode, LED, or organic-LED, OLED), smart windows or supercapacitors and are therefore associated to rapidly increasing industrial needs. For photovoltaics, the need of TEs concerns, for instance, the front electrode that should be transparent for the sunlight while collecting the photogenerated carriers. For efficient lighting, this is the opposite physical phenomenon: injecting carriers (electrons and holes) by applying a voltage through transparent electrodes to let the generated light exit the LED or OLED device. But TEs are also used in many other applications such as transparent heaters, touch screens, sensors or radio-frequency (RF) devices.

The main TEs investigated in the last decades have been transparent conductive oxides (TCO) [1–4] with the most well-known and used one in the industrial area being indium tin oxide (ITO). And aluminium-doped zinc oxide (AZO) [2] and fluorine-doped tin oxide (FTO) [5] have been also the subject of many studies. While TCO can exhibit good or even very good physical properties, the recent industrial needs have prompted a search of new materials to replace TCO for several applications [6]. Indeed indium, for instance, can be scarce, its deposition often requires vacuum, and TCO by nature are brittle and therefore not compatible with flexible applications. Materials such as carbon nanotubes [7], graphene [8], conducting polymers [9, 10], metallic grids [11] and metallic nanowire networks [12, 13] have been mainly studied for this purpose, and some of them exhibit already promising properties for several applications. In particular, several studies have lately demonstrated that metallic nanowire (MNW) percolating networks can exhibit high electrical conductivity, high optical transparency and high flexibility [12, 14, 15]. The main investigated are silver nanowire (AgNW) and copper nanowire (CuNW). The very high aspect ratio of the nanowires (i.e. length divided by the diameter) allows these networks to achieve very good performances, similar to ITO, however by using much less raw material [12]. Such quantity are often expressed in terms of the so-called areal mass density (*amd*), defined as the required mass of metal (for MNW networks) or indium (for ITO thin layers) per square metre. Their ranges are between 40 and 200 mg.m<sup>−</sup><sup>2</sup> for AgNW or CuNW networks and roughly 750–1050 mg.m<sup>−</sup><sup>2</sup> for ITO thin layers [12]. With rather similar price per unit mass for both In and Ag, replacing ITO by AgNW networks appears to be a cost-effective alternative. Moreover MNW-based TEs exhibit two additional assets: they can be fabricated via solution-based methods, and they present outstanding flexibility (and even good stretchability). These two assets constitute clearly key points for an efficient industrial integration. Another advantage of MNW networks is their high optical transparency in the near-infrared spectrum, especially when compared with TCO: this is of importance for transparent solar cell applications. For those reasons, printed AgNW network-based electrodes have shown a potential as transparent and flexible electrodes in many displays such as solar cells [16–19], OLEDs [20], displays [21], supercapacitors [22], transparent heaters [23–25], radio-frequency antennas [26], antibacterial films [27] or smart windows [28].

In this contribution, we focus on TEs made of AgNWs or CuNWs and will first briefly discuss the role of the nanowire dimensions (both length and diameter) and network density on the physical properties. The network stability will be discussed followed by methods to enhance it, which appears to be a crucial issue for an efficient integration of this technology. Finally, we will briefly discuss the integration of MNW network-based transparent electrodes for energy applications.
