Valence band splitting in Cu2(Sn,Ge,Si)S3: Effect on optical absorption spectra

We perform a detailed analysis of the valence band splitting (VBS) effect on the absorption spectra of monoclinic Cu2(Sn,Ge,Si)S3 combining theory and experiment. We calculate the imaginary part of the dielectric function for all three compounds using hybrid functionals and maximally localized Wannier functions in remarkably dense k ‐meshes to ensure an accurate description of the low energy spectral regime. We find that the VBS will affect the absorption spectra of these materials leading to multiple absorption onsets. Our experimental spectra on Cu2(Sn,Ge)S3, analysed using both Tauc plots and inflection points, verify this prediction. A good agreement between theory and experiment in terms of VBS values is recorded. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) The ternary semiconductors Cu2(Sn, Ge, Si)S3 have lately attracted interest as photovoltaic material and have multiple absorption onsets near the band edge. These onsets stem from the splitting of the valence band, due to their reduced symmetry, i.e. monoclinic phase. Theoretical values of the valence band splitting are compared with experimentally determined values from Tauc plots and inflection points and are shown to be in good agreement.