Growth of copper indium sulphide films by thermal evaporation of mixtures of copper sulphide and indium sulphide powders

The physical evaporation of a 1:1 mixture of copper sulphide (CuS) and indium sulphide (In2S3) powders by resistive heating followed by the vacuum annealing of the resulting films at 723 K produces copper indium sulphide (CuInS2) films with about 95% phase purity. Composed of sub-micron sized grains, the films bear stoichiometric or Cu-rich composition and are endowed with p-type conductivity, a band gap of about 1.5 eV and an absorption coefficient of about 4 × 104 cm−1 in visible region. Mechanistically, the formation of CuInS2 films takes place as a result of solid state reaction among Cu7S4, InS and In2S3 in the condensed phase. These intermediate species are produced from the decomposition of CuInS2 formed in the evaporating mixture due to the reaction between CuS and In2S3, and excess CuS. Process simplicity and the absence of a sulphurisation step make this approach attractive for synthesising CuInS2 absorber layers for photovoltaic applications.