Forming a low-resistance contact to p-type CdTe is a critical issue for successful commercialization of CdTe-based photovoltaic devices. One solution to this problem has been to incorporate surface pretreatments to facilitate contact formation. In this article, the effects of a nitric-phosphoric (NP) acid pretreatment on material properties and device performance are investigated for polycrystalline CdTe-based devices. We demonstrate that the NP acid pretreatment, when applied to CdTe thin films, forms a thick, highly conductive Te layer on the back surface of the film and on exposed grain boundaries. When etched under optimal conditions, this results in CdS/CdTe devices with reduced series resistance and enhanced performance. On the other hand, we find that the NP etch preferentially etches grain boundaries. Overetching can result in complete device failure by forming shunt paths that extend to the heterointerface. Therefore, carefully controlling the etch concentration and duration is critical to optimizing device performance. [S0734-2101(99)05403-2].