Formation of semitransparent CuAlSe2 thin films grown on transparent conducting oxide substrates by selenization

Wide band-gap semiconductors have been studied for applications as buffer layers in thin film solar cells and as top cell in tandem devices. CuAlSe2 (CAS) thin films were deposited onto bare and two different transparent conducting oxide (TCO)-coated glass substrates, In2O3: Sn (ITO) and ZnO: Al (AZO), by a two stage process consisting on the selenization of metallic precursor layers. Homogeneous and crystalline formation of CAS thin films is not trivial and it is strongly influenced by selenization conditions, type of substrate and the film thicknesses. Under certain conditions, polycrystalline CuAlSe2 thin films with chalcopyrite structure and preferential orientation along the (112) plane were obtained onto bare glass susbtrates. However, formation and crystallization of homogeneous CAS thin films was promoted by transparent conducting oxides (ITO and AZO)-coated glass substrates and take place in a wide range of thicknesses and Se amounts with high degree of reproducibility. TCO-coated substrates promoted larger grains when the CAS compound was formed. The band-gap energy, preferential orientation, crystallite size and the average surface roughness varied depending on the film thickness and type of substrate.