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 Cu-2(Sn, Ge, Si)S-3 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 Cu-2(Sn, Ge)S-3, 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. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim