The Joule heating problem in silver nanowire transparent electrodes

Abstract

Silver nanowire transparent electrodes have shown considerable potential to replace conventional transparent conductive materials. However, in this report we show that Joule heating is a unique and serious problem with these electrodes. When conducting current densities encountered in organic solar cells, the average surface temperature of indium tin oxide (ITO) and silver nanowire electrodes, both with sheet resistances of 60 ohms/square, remains below 35 °C. However, in contrast to ITO, the temperature in the nanowire electrode is very non-uniform, with some localized points reaching temperatures above 250 °C. These hotspots accelerate nanowire degradation, leading to electrode failure after 5 days of continuous current flow. We show that graphene, a commonly used passivation layer for these electrodes, slows nanowire degradation and creates a more uniform surface temperature under current flow. However, the graphene does not prevent Joule heating in the nanowires and local points of high temperature ultimately shift the failure mechanism from nanowire degradation to melting of the underlying plastic substrate. In this paper, surface temperature mapping, lifetime testing under current flow, post-mortem analysis, and modelling illuminate the behaviour and failure mechanisms of nanowires under extended current flow and provide guidelines for managing Joule heating.

Transparent Heaters: A Review

“…While the first category is supposed to heat homogeneously the whole TH surface, the second category can induce local heating due to its nonhomogeneous nature, if heat does not diffuse along both the active material and/or the substrate quickly enough. [ 54 ] These hotspots can induce local degradation within the TH and are therefore an issue to be dealt with. Das et al investigated scaling in self‐heated percolating networks and concluded that hotspot clustering appears to be a mechanism analogous to crystallization physics.…”

“…This roughly corresponds to an area with more than one hundred nanowires. [ 54 ] A comparable technique used to reveal defects in microelectronic devices is lock‐in thermography (LiT). [ 105 ] It can provide an IR emission intensity and a heat timing map [ 106 ] but does not include any thermal calibration or numerical scale.…”

“…a) 3D temperature profile measured using an IR camera with 250 µm × 250 µm pixels, for AgNW‐rGO after applying an electrical current of 20 mA for 60 s. Reproduced with permission. [ 54 ] Copyright 2017, IOP Publishing. b) LiT image of an AgNW network.…”

“…[ 20 ] However, one of the main issues with MNW networks is their relative low stability under electrical and/or thermal stresses, even at device‐operating conditions, as reported in several studies. [ 46,54,90,170,171 ] This is illustrated in Figure 7e, which displays the evolution of the electrical resistance of an AgNW network during a voltage ramp of 0.5 V min −1 . The electrical resistance exhibits a drastic increase associated to the appearance and propagation of a crack, as revealed by the IR image in the inset (the width of the IR image corresponds to 25 mm).…”

“…For example, this is essential for eye comfort when THs are used in windscreens, while hot spots can potentially be at the origin of the TH degradation. [ 54 ]…”