In situ and ex situ characterisation of thermally induced crystallisation of CuInS2 thin films for solar cell

This work reports a microstructural investigation of thermal processes in polycrystalline CuInS2 films for solar cell by the combination of both ex situ and in situ techniques. Processed films were obtained under Cu excess conditions by sequential Cu and In sputtering onto Mo-coated glass substrates and sulphurised at different temperatures in the range of 400–450 °C. These conditions lead to layers with coexistence of both chalcopyrite (CH) and CuAu polymorphic CuInS2 phases, as well as CuIn5S8. The analysis of the in situ Raman spectra measured during the annealing of the samples at 500 °C has allowed to monitor the existence of a polymorphic transformation of metastable CuAu ordered domains into the equilibrium chalcopyrite structure. In principle, this transformation takes place in the range of 400–500° C and correlates with the presence of a surface Cu2S layer. This agrees with the in situ analysis of the sulphurisation process. However, in some cases, a strongly enhanced CuAu to chalcopyrite transformation has been observed, which is not related to the presence of the Cu–S binary phase. The origin of this anomalous behaviour is still unclear, and seems related to the microstructure of the metallic precursors before sulphurisation. Finally, ex situ X-ray diffraction (XRD) and in-depth Auger electron spectroscopy (AES) composition measurements have allowed to characterise the CuIn5S8 to CuInS2 transformation which takes place during annealing.