The CdTe/Sb2Te3 interface was prepared by co-evaporation on an ultra-thin glass substrate and studied "in-situ" by photoelectron spectroscopy using a tunable synchrotron radiation photon source. The results indicated that the as-deposited antimony telluride was a mixture of Sb-Te phase and Sb2Te3 structure. The band diagram showed a conduction band offset (Delta E-CB) of 0.65 eV at the interface. A thin layer of Sb2Te3 came into being on the surface of CdTe with a subsequent 300 degrees C anneal. The sputter depth profile suggested that the annealed interface was actually constituted by the "substrate CdTe/interface Sb-Cd(+) donors/inner Sb-Te mixture/surface Sb2Te3" structure. The band diagram gave Delta E-CB=1.35 eV for the annealed interface. It was found that there was a thin barrier of 0.2 eV induced by Sb-Cd(+) donors for the hole transportation at the interface. To create a thin p(+) layer between CdTe and the back electrode, a 50 A thick Sb2Te3 layer was used for the back contact to CdTe thin film solar cells. J-V characteristic showed that this thin p+ back contact greatly enhanced the cell performance. (C) 2014 Elsevier B.V. All rights reserved.