Differential calorimetry study of the initial stage of the sulphurisation process of CuInSe2 solar cell materials

Sulphurisation of CuInSe2 solar cell absorber materials is of particular interest for band gap engineering by means of compositional grading. We studied the chalcopyrite formation process of Cu–In–S–Se stacks with varying Se to S ratio from 100% Se (0% S) down to 0% Se (100% S). Differential calorimetry (DSC) was applied as a fundamental characterization tool to track the crystal phase formation process. To verify the evolution of various crystallographic phases, DSC measurements were supplemented by temperature dependent in-situ X-ray diffraction (XRD) experiments of equivalent thin film compositions. For a S content of 25% the system is governed by sulphurisation of Cu and In, with the ternary phase CuIn5(S,Se)8 and the binary phase Cu2(S,Se) playing major roles in the chalcopyrite formation process. It is found that the chalcopyrite formation moves to higher temperatures if the S content is increased.