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

• Published in: Thin solid films 2007-06, Vol.515 (18), p.7364-7369
• Main Authors: ZHOU, J, WU, X, DUDA, A, TEETER, G, DEMTSU, S. H
• Format: Article
• Language: English
• Subjects: CADMIUM SULFIDE SOLAR CELLS; CADMIUM TELLURIDE SOLAR CELLS; Composition and phase identification; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport phenomena in thin films and low-dimensional structures; Exact sciences and technology; MATERIALS SCIENCE; Other semiconductors; Physics; SOLAR CELLS; SOLAR ENERGY; Solar Energy - Photovoltaics; Specific materials; Structure and morphology; thickness; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Thin film structure and morphology
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2007.03.032

Material studies and device applications of CuxTe in an NREL-developed CdTe solar cell structured as glass/Cd2SnO4/ZnSnOx/CdS/CdTe are presented. The CuxTe primary back contact was formed by evaporating a Cu layer with various thicknesses at room temperature on HNO3/H3PO4 (NP) solution etched CdTe layer. A post-annealing was then followed. The structural evolution and electrical properties of CuxTe were investigated. Cu/Te ratio and post-annealing temperature are two processing parameters in this study. The CuxTe phases are mainly controlled by the Cu/Te ratio. After a post-annealing at a low temperature, such as 100 C, no CuxTe 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 CuxTe 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 CuxTe phase appearance. CuTe, Cu1.4Te, and Cu2Te 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 (Voc) and shunting resistance (Rsh), but increases series resistance (Rs). The correlation between device performance and the CuxTe back contact illustrates that the process used for forming the Cu2Te 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 Cu1.4Te and CuTe phases.