Abstract of Chapter 4

The electrical contact on CdTe is important for the efficient, long term stable performance of CdTe/CdS solar cells. Since metals usually form a Schottky barrier on p-type CdTe, a surface modification prior to the final metal deposition is required, which is commonly achieved by a wet chemical etching and a subsequent deposition of a buffer layer followed by a metallisation. Chemical etching creates an elemental Te layer on the surface, which is equivalent to p+-doping. The purpose of the thin evaporated buffer layer is to enhance the acceptor concentration reducing the effective barrier height at the back contact and provide a diffusion barrier for the metal. The most commonly used, Cu containing back contacts exhibit low effective barriers due to the doping effect of Cu, but cell performance degrades due to diffusion of Cu along grain boundaries into the CdS.
We investigated alternative back contact materials in terms of stability and efficiency to identify the most suitable back contact. Stability was tested by stressing under different conditions. Cells with Mo metallisation and Sb2Te3 or, to a lesser extent, Sb buffer layers, turned out to be stable, while Al or Au metallisation yielded cells exhibiting degradation. The stable cells had up to 10% efficiency after 9 months of stress testing, which corresponds to 70 years of regular operating conditions.