The formation of different phases of Cu xTe and their effects on CdTe/CdS solar cells

Abstract

Material studies and device applications of Cu x Te in an NREL-developed CdTe solar cell structured as glass/Cd 2SnO 4/ZnSnO x /CdS/CdTe are presented. The Cu x Te primary back contact was formed by evaporating a Cu layer with various thicknesses at room temperature on HNO 3/H 3PO 4 (NP) solution etched CdTe layer. A post-annealing was then followed. The structural evolution and electrical properties of Cu x Te were investigated. Cu/Te ratio and post-annealing temperature are two processing parameters in this study. The Cu x Te phases are mainly controlled by the Cu/Te ratio. After a post-annealing at a low temperature, such as 100 °C, no Cu x Te phase transformation from its as-deposited phase was observed. A post-annealing treatment at a higher temperature, such as 250 °C, can reveal the stoichiometric Cu x Te phases based on the Cu/Te ratio used in the devices. But a post-annealing at a further higher temperature, such as 400 °C, resulted in a complicated Cu x Te phase appearance. CuTe, Cu 1.4Te, and Cu 2Te are three major phases detected by X-ray diffraction (XRD) for different Cu thickness application annealed at 250 °C. Application of Cu thicker than 60 nm degrades open-circuit voltage ( V oc) and shunting resistance ( R sh), but increases series resistance ( R s). The correlation between device performance and the Cu x Te back contact illustrates that the process used for forming the Cu 2Te back contact failed to produce good fill factor (FF) and also introduced higher barrier height. The best device was observed for a back contact with a mixed Cu 1.4Te and CuTe phases.