Optical Functions of Thin-Film Polycrystalline Chalcopyrite CuIn 1-x Ga x Se 2
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Optical Functions of thin-film polycrystalline chalcopyrite CuIn1-xGaxSe2 Sung-Ho Han1,2, *, Dean H. Levi2, Hamda A. Althani2, Falah S. Hasoon2, Raghu N. Bhattacharya2, Allen M. Hermann1 1 Department of Physics, University of Colorado, Boulder, CO 80309, USA 2 National Renewable Energy Laboratory, Golden, CO 80401, USA
ABSTRACT The highest efficiency CuIn1-xGaxSe2 (CIGS) solar cells use thin-film polycrystalline CIGS absorber layers. We have applied variable angle spectroscopic ellipsometry (VASE) to characterize the dielectric functions of polycrystalline thin films of CIGS with Ga : (In + Ga) ratios ranging from 0.18 to 1.0. The Cu : (In + Ga) ratios in these films are approximately 0.90, which is the ratio that yields the highest efficiency CIGS devices. Spectra were measured over the energy range 0.7 to 5.0 eV at room temperatures. Models used to analyze the ellipsometry data include the full multi-layer structure of the sample, which enables us to report the actual dielectric function rather than the pseudo-dielectric function. We present data on how the critical points change with composition, and compare and contrast our results with measurements of single-crystal and bulk polycrystalline samples reported in the literature. Auger electron spectroscopy, atomic force microscopy, and X-ray diffraction have been used to verify the homogeneity, surface roughness, and phase purity, respectively.
INTRODUCTION Key factors in determining the efficiency of photovoltaic devices are the optical properties of the absorber material. Spectroscopic ellipsometry is an excellent technique for determining the optical functions of interest. Alonso and co-workers have reported ellipsometry measurements of the pseudo optical functions of single-crystal CuInSe2 and CuGaSe2 [1], as well as bulk polycrystalline CuIn1-xGaxSe2 alloys [2]. In those works, they have used a two-phase model to analyze the ellipsometry data. Such a model assumes a semi-infinite bulk sample with an abrupt interface with the ambient, allowing the use of the Fresnel equations to directly invert Ψ and ∆ to obtain the pseudo-dielectric function . Such a treatment is not practical for analysis of thinfilm polycrystalline materials used in real-world CIGS solar cells. To accurately determine the optical properties of these multi-layer thin-film samples one must analyze the ellipsometry data using a full multi-layer model including the effects of the surface roughness and underlying
Table I. Compositions of Ga and Cu with respect to (In + Ga) x Ga/(In + Ga) Cu/(In + Ga)
M2477 0.178 0.898
M2478 0.364 0.921
M2483 0.471 0.888
M2485 0.577 0.871
M2487 0.690 0.808
M2532 1.000 0.896
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molybdenum layer and glass substrate. We have applied these techniques in order to determine the dielectric functions for a
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