Glancing incidence X-ray diffraction (GIXRD) analysis of crystalline surface phases in cadmium telluride solar cells is correlated with device behavior for different processing conditions. GIXRD sampling depths are calculated for different phases and X-ray sources. Air heat treatment of CdTe films forms the native oxide CdTeO3, while treatment in CdCl2/air vapor forms CdTeO3 and CdO. Air treatment followed by CdCl2/air treatment forms surface oxides, reduces US diffusion into CdTe and improves junction performance. The CdTeO3 surface oxide limits CdCl2 concentration in the CdTe film, yet allows sufficient CdCl2 species for CdTe/CdS junction activation. In devices, residual oxide phases contribute to series resistance and current-voltage retrace hysteresis. These effects are mitigated by oxide removal and formation of a Te layer. Addition of Cu to the contact followed by thermal treatment increases open circuit voltage. Optimal cell performance is obtained for Cu/(Cu+Te) atomic ratio = 0.7. (C) 2003 Elsevier Science B.V. All rights reserved.