Composite transparent conducting material using metallic nanowire networks on a flexible substrate

As the race for the development of novel Transparent Conducting Materials (TCMs) compatible with a new generation of flexible electronics continues, the perspectives of Indium Tin Oxide (ITO) in thin film devices are uncertain due to its well-known brittleness and the relatively limited supply of indium. At the same time, ITO is poised to remain the most widespread TCM in the industry for a wide range of applications. In parallel, an extensive amount of research has been devoted to Ag nanowire (AgNW) networks over the past decade, thanks to their high electrical conductivity, optical transparency, and mechanical flexibility.
In this work, we present the design of a new composite material which combines sparse metallic nanowire networks (i.e. with densities below the network critical percolation threshold) and fractured ITO films on flexible polymer substrates. The strong increase of the film sheet resistance due to the formation of linear cracks resulting from mechanical bending along an axis parallel to the substrate was significantly mitigated by the subsequent coverage of the damaged surface with a random network of silver nanowires that can be obtained from a low-cost deposition scheme. Using a complementary approach including formal analytical derivations, Monte-Carlo simulations and electrical circuit representation for the modeling of bridged-percolating nanowire networks based on the Multi-Nodal Representation (MNR), we unveil the key relations between crack linear density, nanowire length and network areal density that ensure electrical percolation through the hybrid system of ITO and AgNW. We show that the sheet resistance of fractured ITO thin films can be decreased by up to 90% via the subsequent deposition of AgNWs with areal densities 100 times lower than those required to reach percolation in conventional nanowire networks to be used as transparent electrodes. This work introduces a novel concept, that we propose to nickname bridge percolation, that leads to the design of a new composite material that has the potential to push back the limits of ITO as a conducting transparent material for flexible applications. The principles of our approach can also be applied for other transparent conducting oxides, such as fluorine-doped tin oxide or aluminum-doped zinc oxide, on any flexible substrate.