Thin Solid Films 431 – 432 (2003) 249–256 0040-6090/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00266-9 Correlation of surface phases with electrical behavior in thin-film CdTe devices B.E. McCandless *, S.S. Hegedus , R.W. Birkmire , D. Cunningham a, a a b Institute of Energy Conversion, University of Delaware, Newark, DE 19716, USA a BP Solar, 2300 North Watney Way, Fairfield, CA 94533, USA b Abstract 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 CdTeO , while treatment in CdCl yair vapor forms 3 2 CdTeO and CdO. Air treatment followed by CdCl yair treatment forms surface oxides, reduces CdS diffusion into CdTe and 3 2 improves junction performance. The CdTeO surface oxide limits CdCl concentration in the CdTe film, yet allows sufficient 3 2 CdCl species for CdTeyCdS junction activation. In devices, residual oxide phases contribute to series resistance and current– 2 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 Cuy(CuqTe) atomic ratios0.7.  2003 Elsevier Science B.V. All rights reserved. Keywords: Cadmium telluride; X-ray diffraction; Surface composition; Solar cells 1. Introduction This paper examines the relationship between post deposition processing, crystalline surface phases, and device operation in thin film polycrystalline CdTe yCdS solar cells. It is well known that the post-deposition treatments used to process high efficiency CdTe thin- film solar cells significantly modify properties of the CdTe yCdS structure. Progress has been made in corre- lating the resulting bulk structural, chemical and elec- tronic properties with device processing conditions, delineating the propitious effects of high temperature processing, halide vapor treatments, and copper species on device performance w1x and stability w2,3x. The CdTe grain size exerts a controlling influence on chemical and kinetic interactions between the CdTe and CdS films, and the reactive species used to modify them, and with appropriate process modification, cells with efficiency )11% can be obtained using CdTe films with mean lateral grain size from 200 nm to )5 mm w1x. *Corresponding author. Tel.: q1-302-831-6240; fax: q1-302-831- 6226. E-mail address: bem@udel.edu (B.E. McCandless). However, much less has been established with respect to film surface chemistry and grain boundaries, which potentially comprise the most sensitive elements of thin- film devices. Although they may only exert a minor role in cells with thick CdTe films ()5 mm), their properties could predominate as CdTe thickness is reduced to the optical limit needed for absorption, d;1 mm. Further, as commercial development of thin-film CdTe modules seeks to reduce processing step times while maintaining high cell efficiency, the need for establishing basic correlations between surface ygrain boundary effects and device operation and stability is affirmed. A key aspect towards this is determination of the surface phases produced by post-deposition process- ing steps, such as reactive thermal treatments with CdCl and air, and contact formation. 2 In this paper, the effects of post-deposition air and CdCl yair heat treatments on electrodeposited (ED) 2 CdTe are evaluated with respect to surface chemistry, diffusion and device performance. The quantity and distribution of CdTeO and CdO formed during treat- 3 ments are analyzed and correlated with device operation. Back contact processing is evaluated for vapor transport deposited (VTD) CdTe films, and the reaction of Te