**3. Stability of silver nanowire networks**

The stability of metallic nanowire networks appears as a crucial issue, specifically when such TEs undergo thermal and electrical stress [29, 30]. This concerns nearly all applications, and the stability is related to electrical and thermal stability but also long-term ageing and chemical degradation. Such instability can stem from different physical mechanisms such as diffusion of metallic atoms, electromigration processes during electrical stress or oxidation of silver or copper if networks are in contact with either humid atmosphere and/or in high temperature conditions and/ or under electrical stress.

One of the first investigations of the instability of AgNW networks was reported by Khaligh and Goldthorpe [40]: they showed that when AgNW-based TEs undergo similar electrical currents than those encountered in organic solar cells, the TEs

#### **Figure 4.**

*Electrical failure observations. (a) Time dependence of the electrical resistance of a AgNW network during an electrical ramp of 0.5 V min<sup>−</sup><sup>1</sup> ; the electrical breakdown is observed for voltage larger than 9 volts. (b) Corresponding thermal maps captured in situ with an IR camera at specific times during the experiment depicted in (a) (the electrodes at opposite sides of the specimen are vertical); (c) schematic representation of the mechanisms involved in the crack explaining the crack propagation; (d) scanning electron microscopy observation of the AgNW network location where the crack took place during electrical breakdown.*

can quickly fail within 2 days. They reported that such failure is associated with local Joule heating which deteriorates AgNW network and eventually leads to the network failure. Similar observations were also reported by Chen et al. [41]. Such stability issues are even more pronounced for CuNWs since oxidation (even at low temperature) of CuNWs can occur [42–44].

**Figure 4** exhibits the electrical failure observations. **Figure 4a** shows the typical time dependence of the AgNW network electrical resistance during an electrical ramp with the electrical breakdown observed for voltage larger than 9 volts. At first a slight increase of the resistance is observed: thanks to the Joule effect, the network temperature is slightly increased, and since its behaviour corresponds to a metal, its electrical resistance thus increases. For voltage larger than 9 volts, the network electrical resistance drastically increases: this is associated to the electrical breakdown of the network [45].

**Figure 4b** exhibits in situ thermal maps of the same specimen considered in **Figure 4a** with identical corresponding numbers during the voltage ramp. During the degradation phase (i.e. between (2) and (4)), the heat distribution appears to narrow to a vertical central part of the network parallel to the contact electrodes. At step 4, the accelerated increase in resistance can be associated to the occurrence of a 'thermal' crack which is clearly detectable at the bottom at step 5. A schematic

**139**

**Figure 5.**

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

result of a drastic local heating and/or electromigration.

representation of the involved mechanism is shown **Figure 4c**: the propagation mechanism of the crack is related to the displacement of the local current stress peak, which keeps being constricted to the top extremity of the deteriorated area, leading to a runaway-like destruction phenomenon. Finally as shown by **Figure 4d**, AgNWs located close to the crack do appear fully or partially spheroidized, as a

To mitigate such instability problems, several studies showed that coating of MNW networks with either inorganic nanoparticles or thin films can drastically improve MNW network stability and integration into real devices. One can cite Morgenstern et al. [46] who showed that solution-processed AgNW films coated with ZnO nanoparticles enhance performance of such TEs when integrated in organic solar cells. The observed efficient protection against AgNW oxidation or degradation leads to a photocurrent for the organic solar cell which appears to be enhanced when compared with ITO use [46]. And, Göbelt et al. showed that a thin aluminium-doped zinc oxide layer deposited by atomic layer deposition (ALD) on AgNW leads to similar photovoltaic performances [47], however by using a much lower silver amount. Some other studies also recently reported that the coating with very thin film layers of titanium dioxide (TiO2) [48] and zinc oxide (ZnO) [49] clearly enhances the stability. It is worth noticing that new depositing approaches have been assessed lately, and one of the most promising appears to be the atmospheric pressure spatial atomic layer deposition (AP-SALD) [50, 51]. While maintaining the advantages of ALD (viz., low-temperature deposition, thickness control, high-quality materials, and conformity), it can be much faster (up to 2 orders of magnitude faster) than ALD. Another clear asset is its compatibility with roll-to-roll and open air technology. Lately, our team used AP-SALD [52] to fabricate nanocomposite-based TEs in which AgNWs are protected by a conformal thin oxide layer (ZnO). The AP-SALD method was used to deposit thin layers of 15–30 nm ZnO around the AgNWs with the goal of enhancing the network stability [52]. The ZnO coating improved the adhesion of the AgNW networks to the glass substrate, which is known to be poor for bare AgNW networks. **Figure 5** illustrates the positive effects of a coating on MNW-based TE by showing the evolution of the electrical resistance of bare or ZNO-coated AgNW networks during a voltage ramp of 0.1 V/min. A clear enhancement of electrical stability is observed, reducing the degradation of such TEs. As shown by **Figure 5** and by Khan et al. [52], the thicker

*Electrical failure observations and stability enhancement thanks to coating of MNWs. Variation of electrical resistance for bare and ZnO-coated AgNW networks when subjected to voltage ramps of 0.1 V/min. The bare AgNW network shows failure at around 9 V, whereas the stability of ZnO-coated AgNW networks increases with the increasing ZnO coating thickness to 14 and 18 volts for, respectively, 15 and 30 nm of ZnO coating.*

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

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

representation of the involved mechanism is shown **Figure 4c**: the propagation mechanism of the crack is related to the displacement of the local current stress peak, which keeps being constricted to the top extremity of the deteriorated area, leading to a runaway-like destruction phenomenon. Finally as shown by **Figure 4d**, AgNWs located close to the crack do appear fully or partially spheroidized, as a result of a drastic local heating and/or electromigration.

To mitigate such instability problems, several studies showed that coating of MNW networks with either inorganic nanoparticles or thin films can drastically improve MNW network stability and integration into real devices. One can cite Morgenstern et al. [46] who showed that solution-processed AgNW films coated with ZnO nanoparticles enhance performance of such TEs when integrated in organic solar cells. The observed efficient protection against AgNW oxidation or degradation leads to a photocurrent for the organic solar cell which appears to be enhanced when compared with ITO use [46]. And, Göbelt et al. showed that a thin aluminium-doped zinc oxide layer deposited by atomic layer deposition (ALD) on AgNW leads to similar photovoltaic performances [47], however by using a much lower silver amount. Some other studies also recently reported that the coating with very thin film layers of titanium dioxide (TiO2) [48] and zinc oxide (ZnO) [49] clearly enhances the stability. It is worth noticing that new depositing approaches have been assessed lately, and one of the most promising appears to be the atmospheric pressure spatial atomic layer deposition (AP-SALD) [50, 51]. While maintaining the advantages of ALD (viz., low-temperature deposition, thickness control, high-quality materials, and conformity), it can be much faster (up to 2 orders of magnitude faster) than ALD. Another clear asset is its compatibility with roll-to-roll and open air technology. Lately, our team used AP-SALD [52] to fabricate nanocomposite-based TEs in which AgNWs are protected by a conformal thin oxide layer (ZnO). The AP-SALD method was used to deposit thin layers of 15–30 nm ZnO around the AgNWs with the goal of enhancing the network stability [52]. The ZnO coating improved the adhesion of the AgNW networks to the glass substrate, which is known to be poor for bare AgNW networks. **Figure 5** illustrates the positive effects of a coating on MNW-based TE by showing the evolution of the electrical resistance of bare or ZNO-coated AgNW networks during a voltage ramp of 0.1 V/min. A clear enhancement of electrical stability is observed, reducing the degradation of such TEs. As shown by **Figure 5** and by Khan et al. [52], the thicker

#### **Figure 5.**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

can quickly fail within 2 days. They reported that such failure is associated with local Joule heating which deteriorates AgNW network and eventually leads to the network failure. Similar observations were also reported by Chen et al. [41]. Such stability issues are even more pronounced for CuNWs since oxidation (even at low

*Electrical failure observations. (a) Time dependence of the electrical resistance of a AgNW network during* 

*Corresponding thermal maps captured in situ with an IR camera at specific times during the experiment depicted in (a) (the electrodes at opposite sides of the specimen are vertical); (c) schematic representation of the mechanisms involved in the crack explaining the crack propagation; (d) scanning electron microscopy observation of the AgNW network location where the crack took place during electrical breakdown.*

*; the electrical breakdown is observed for voltage larger than 9 volts. (b)* 

**Figure 4** exhibits the electrical failure observations. **Figure 4a** shows the typical time dependence of the AgNW network electrical resistance during an electrical ramp with the electrical breakdown observed for voltage larger than 9 volts. At first a slight increase of the resistance is observed: thanks to the Joule effect, the network temperature is slightly increased, and since its behaviour corresponds to a metal, its electrical resistance thus increases. For voltage larger than 9 volts, the network electrical resistance drastically increases: this is associated to the electrical break-

**Figure 4b** exhibits in situ thermal maps of the same specimen considered in **Figure 4a** with identical corresponding numbers during the voltage ramp. During the degradation phase (i.e. between (2) and (4)), the heat distribution appears to narrow to a vertical central part of the network parallel to the contact electrodes. At step 4, the accelerated increase in resistance can be associated to the occurrence of a 'thermal' crack which is clearly detectable at the bottom at step 5. A schematic

temperature) of CuNWs can occur [42–44].

down of the network [45].

**138**

**Figure 4.**

*an electrical ramp of 0.5 V min<sup>−</sup><sup>1</sup>*

*Electrical failure observations and stability enhancement thanks to coating of MNWs. Variation of electrical resistance for bare and ZnO-coated AgNW networks when subjected to voltage ramps of 0.1 V/min. The bare AgNW network shows failure at around 9 V, whereas the stability of ZnO-coated AgNW networks increases with the increasing ZnO coating thickness to 14 and 18 volts for, respectively, 15 and 30 nm of ZnO coating.*

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 coating, the lower the optical transparency.

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

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 network transmittance [52].
