Preparation of Cu(In,Ga)Se 2 thin films at low substrate temperatures
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T. Wada Department of Materials Chemistry, Ryukoku University, Seta, Otsu 520-2194, Japan (Received 28 April 2000; accepted 20 October 2000)
Cu(In,Ga)Se2(CIGS) thin films were prepared at substrate temperatures of 350 to 500 °C. The (In,Ga)2Se2 precursor layers were deposited on Mo coated soda-lime glass and then exposed to Cu and Se fluxes to form CIGS films. The surface composition was probed by a real-time composition monitoring method. The CIGS films were characterized by x-ray diffraction, energy dispersive x-ray spectroscopy, secondary ion mass spectroscopy, and atomic force microscopy. The transient formation of a Cu–Se phase with a high thermal emissivity was observed during the deposition of Cu and Se at a substrate temperature of 350 °C. Faster diffusion of In than Ga from the (In,Ga)2Se3 precursor to the newly formed CIGS layer was observed. A growth model for CIGS films during the deposition of Cu and Se onto (In,Ga)2Se3 precursor is proposed. A solar cell using a CIGS film prepared at about 350 °C showed an efficiency of 12.4%.
I. INTRODUCTION
Cu(In,Ga)Se2 (CIGS) has received considerable attention as one of the most promising materials for thin film solar cells. Recently, NREL1 and our group2 have demonstrated efficiencies of over 18% for CIGS solar cells. These high-quality CIGS films were deposited by the “3-stage” process,3 one of the well-known physical vapor deposition (PVD) techniques. In the 3-stage process, the initial stage consists of the formation of an In–Ga–Se precursor layer; at the second stage, deposition of Cu and Se leads to the formation of a Cu-rich CIGS layer. In the final stage, small amounts of In, Ga, and Se are added to the Cu-rich CIGS layer to obtain a slightly (In,Ga)-rich CIGS film. Generally, high-quality CIGS thin films can be grown under Cu-rich conditions at substrate temperatures of about 550 °C. In the Cu–Se binary system,4 a Cu–Se liquid phase exists at temperatures over 523 °C. The Cu–Se liquid segregates to the surface of the Cu-rich CIGS film at about 550 °C. It was believed that the presence of the Cu–Se liquid phase on the surface of Cu-rich CIGS films leads to high quality CIGS crystals.5 In our previous paper, we investigated grain growth and phase changes in the deposition of CIGS films under (In,Ga)-rich conditions corresponding to the second stage of the 3-stage process.6,7 In this work, device-quality CIGS films were prepared without using Cu-rich conditions. The fabricated CIGS solar cell showed an effi394
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J. Mater. Res., Vol. 16, No. 2, Feb 2001 Downloaded: 24 Mar 2015
ciency of 17.5%.7 This result implies that Cu-rich conditions are not necessary for preparing device-quality CIGS films. In the next step of our study, we attempted to prepare device-quality CIGS films at low substrate temperatures. Deposition of CIGS films at low substrate temperatures has practical importance not only for reducing energy usage and lowering production costs for CIGS photovoltaic modules, but it would also allow flexibility in utilizing
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