Spectroscopic ellipsometric measurements of thin polycrystalline ͑4CuInSe 2 ͒ y ͑CuIn 5 Se 8 ͒ 1−y films reveal that there are important differences in optical properties and electronic structures between ␣-phase CuInSe 2 and Cu-poor CuInSe 2 . We report the optical functions of thin-film polycrystalline ͑4CuInSe 2 ͒ y ͑CuIn 5 Se 8 ͒ 1−y and describe how they change depending on the degree of Cu deficiency. We find a reduction in the absorption strength in the spectral region of 1 -3 eV for Cu-poor CuInSe 2 . This reduction can be explained in terms of density of the Cu 3d states in CuInSe 2 . Cu-poor CuInSe 2 samples show an increase in band gap due to reduced p-d interaction. We find that the reduction in the amplitudes of E 0 ͑A , B , C͒ transitions at the ⌫ point and E 1 ͑A , B͒ transitions at the N point are due to Cu deficiency. Local density approximation calculations with a modeled on-site self-interaction correction of the absorption coefficients of Cu 8 In 8 Se 16 ͑CuInSe 2 ͒, Cu 5 In 9 Se 16 ͑CuIn 3 Se 5 -like͒, and Cu 2 In 10 Se 16 ͑CuIn 5 Se 8 ͒ are in good agreement with those of thin-film polycrystalline CuInSe 2 samples with 24.1, 15.6, and 9.1 at. % Cu, respectively.

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In, Ga, and Se were coevaporated to form precursor films of (Inx,Ga1−x)2Se3. The precursors were then converted to CuInxGa1−xSe2 by exposure to a flux of Cu and Se. The final films were smooth, with tightly packed grains, and had a graded Ga content as a function of film depth. Photovoltaic devices made from these films showed good tolerance in device efficiency to variations in film composition. A device made from these films resulted in the highest total-area efficiency measured for any non-single-crystal, thin-film solar cell, at 15.9%.

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