CuAlxGa1−xSe2 thin films for photovoltaic applications: Optical and compositional analysis

Wide-band gap chalcopyrite semiconductors have a great interest due to their potential application in multi-junction thin film solar cells or as window layers. Polycrystalline CuAlxGa1−xSe2 (CAGS) thin films have been prepared by selenization of evaporated metallic precursor layers on bare and Mo-coated soda lime glass substrates. The optical properties of CAGS films of 2 thicknesses have been analyzed by spectrophotometry in the visible-infrared (VIS-IR) and the compositional characteristics have been studied by energy dispersive analysis of X-rays (EDAX) and X-ray photoelectron spectroscopy (XPS). The optical transmission increases and the band gap energy shifts toward higher values as the Al content increases, which indicates the partial substitution of Ga by Al. The dependence of the band gap with the composition has resulted to be nonlinear and a bowing parameter of b = 0.62 and b = 0.54 for 0.6 μm and 1.1 μm-CAGS samples, respectively, has been obtained. XPS data have shown an Al, Ga and Se composition gradient in depth and a surface strongly oxidized. However, XPS reveals that the Cu composition remains constant in depth and the oxidation is relatively low in bulk increasing slightly in the interface with Mo/SLG. Moreover, samples with high Al content reveal a higher contribution of CuO in depth.

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► Wide band gap CAGS thin films have been obtained by selenization of evaporated metallic precursors.
► Direct nonlinear dependence of the band gap energy with the Al/(Al + Ga) ratio is found.
► The bowing parameter decreases when the CAGS film thickness increases.
► The Cu at% remains constant in depth, together with some Al, Ga and Se gradients.
► Surface is strongly oxidized but the oxidation is relatively low in bulk.