Stabilizing Silver Window Electrodes for Organic Photovoltaics Using a Mercaptosilane Monolayer

Abstract

A single layer of the bifunctional molecule 3-mercaptopropyltrimethoxysilane is shown to be remarkably effective at improving the stability of optically thin silver film electrodes towards spontaneous morphological change and oxidation by airborne sulfur. Inclusion of this layer in the novel transparent electrode; WO3 (30 nm) / silver (13 nm) / sol-gel ZnO (27 nm), at the silver / ZnO interface improves the efficiency of organic photovoltaic devices using this electrode by 20%, such that the power conversion efficiency is very close to that achievable using a conventional indium-tin oxide glass electrode; 9.6 % ± 0.2 % vs 10.0 % ± 0.3 %, with the advantage that the silver electrode has a sheet resistance one third that of the ITO glass (4 Ohms sq-1). The mercaptosilane monolayer is also shown to retard silver diffusion into the ZnO layer whilst imparting a favorable 400 meV reduction in electrode work function. In addition to its utility inside the device, this molecular layer is shown to be useful for improving the stability of the silver film electrodes in top-illuminated semi-transparent photovoltaics, since it can be deposited directly onto a completed device from the vapor phase.