An upstream cathode system for use with glow discharge deposition apparatus, said apparatus adapted for the production of large area photovoltaic devices. In such apparatus, process gases are commonly introduced into a deposition chamber from a gas manifold disposed on the upstream side of a substrate. As the process gases are drawn across the surface of the substrate, they are continuously disassociated and recombined under the influence of an electromagnetic field developed by a deposition cathode or microwave generator. By providing a precathode system, upstream of the deposition cathode or microwave generator, (1) impurities in the process gases, (2) contaminants from the walls of the deposition chamber and (3) initially disassociated and recombined process gas compositions may be deposited onto and collected from a collection plate disposed upstream of the substrate. In this manner, higher order chains of the process gases are subjected to the upstream electromagnetic field and are therefore more easily broken down by the deposition cathode and deposited onto the substrate in desired lower order chains and compositions so as to provide improved electrical properties.