Effect of Cu Deficiency on CuIn 1-x Ga x Se 2 and High-efficiency Photovoltaic Solar Cells
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Effect of Cu Deficiency on CuIn1-xGaxSe2 and High-efficiency Photovoltaic Solar Cells Sung-Ho Han,1,2 Falah S. Hasoon,1 Joel W. Pankow,1 Allen M. Hermann2, and Dean H. Levi1 1 National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393, U.S.A. 2 Department of Physics, University of Colorado, 2000 Colorado Avenue, Boulder, CO 80303-0390, U.S.A. ABSTRACT We report the results of our studies on the optical and electronic structure of a wide range of polycrystalline thin-film CuIn1-xGaxSe2 (CIGS) alloys. The composition range includes CIS and nearly stoichiometric (slightly Cu-poor) (24.3±0.3 at.% Cu) CIGS with x values located around the value that has the best efficiency (x ~ 0.28). Relative to nearly stoichiometric CIS and CIGS, we find a reduction in the absorption strength in the spectral range 1-3 eV. This reduction can be explained in terms of the predominance of Cu 3d and Se 4p states at the valence band maximum (VBM). In addition, Cu-poor CIS and CIGS materials show an increase in bandgap because the p-d repulsive interaction in Cu-poor CIGS is less than that in nearly stoichiometric CIGS. High efficiency is discussed in terms of optical properties. INTRODUCTION Polycrystalline (px) thin-film chalcopyrite CuIn1-xGaxSe2 (CIGS) is used as an absorber layer for high-efficiency thin-film photovoltaic (PV) solar cells. The efficiency of laboratory px thin-film CIGS solar cells has surpassed 19% [1]. High-efficiency CIGS PV uses slightly Cupoor (23.5–24.5 at.% Cu) CIGS absorber layers [2]. Although there is not a consensus as to why slightly Cu-poor CIGS is necessary for high-efficiency solar cells, recent work has implied an important role for Cu deficiencies at surfaces and GBs [3]. Thus it is important to understand the effects of Cu deficiency on the electronic properties of CIGS. Because the Ga content in highefficiency PV devices is graded, understanding how the role of Cu deficiency changes with the gallium concentration is important to a fundamental understanding of the material properties determining PV device efficiency. Alonso et al. have reported spectroscopic ellipsometric measurements of the pseudo-optical functions of single-crystal CIS and CuGaSe2 (CGS) [4], as well as bulk px-CIGS alloys [5] using the two-phase model [6] for stoichiometric compositions. In this study, through the analysis of the dielectric function, we compare the electronic structure of px-CIS thin films. We also compare the electronic structures between stoichiometric and Cu-poor CIGS with x = 0.2 and 0.4. We compare nearly stoichiometric (slightly Cu-poor) (24.3±0.3 at.% Cu) px-CIGS thin films to investigate the absorption behavior of each material [5]. We determine the bandgaps to compare them between nearly stoichiometric and slightly Cu-poor CIGS.
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EXPERIMENTAL DETAILS Polycrystalline thin-film CIS and CIGS samples were deposited on molybdenum-coated soda-lime glass (SLG) using single-stage co-evaporation. This provides homogeneous CIS/CIGS films that are suitable for ellipsometric analysis
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