Improving mechanical stability and electrical properties of silver nanowire films with a zinc tin oxide overcoat

Silver nanowire (AgNW)-based transparent contacts are promising alternatives to transparent conducting oxides for solar cells owing to their promising optoelectronic properties. However, there is a need to improve issues such as adhesion to substrate, surface roughness, thermal instability, and loose contact between wires. If these issues could be resolved, it should be possible to replace the conducting oxide layers by AgNWs. In the present work, we report electrical, optical, and surface properties of silver nanowires with amorphous zinc tin oxide (ZTO) as overcoat. Two different sizes of silver nanowires have been employed for this study with average diameters of 35 nm (length = 10 μm) and 90 nm (length = 25 μm). Various concentrations of the solution and spin-coating speeds have been used to optimize transmittance and conductivity of bare AgNW films. After deposition, it is possible to improve conductivity by fusing wires through various processes such as thermal annealing. It is observed that thermal annealing at 200°C or less shows improved conductivity. Sheet resistance of 9.8 Ω/sq and transmittance (at 550 nm) of 91% are obtained for 90 nm diameter wires at an annealing temperature of 200°C. AgNW networks of varying sheet resistance and transmittance were over-coated with sputtered ZTO films of 20-, 50-, or 200-nm thickness. Microscope analysis of scratch-tested samples shows that 50-nm-thick ZTO film is sufficient to give robustness to the 90-nm AgNW films with minor changes to conductivity and transmittance.