Valence band offset at the CdS/Cu2ZnSnS4 interface probed by x-ray photoelectron spectroscopy

The valence band offset (VBO) at the interface CdS/Cu2ZnSnS4 was investigated by x-ray photoelectron spectroscopy (XPS). The VBO was measured by two different procedures: an indirect method involving the measurements of the core levels together with the XPS bulk valence band (VB) spectra and a direct method involving the analysis of XPS VB spectra at the interface. The indirect method resulted in a VBO value of (-1.20 +/- 0.14) eV while the direct method returned a similar value of (-1.24 +/- 0.06) eV but affected by a lower uncertainty. The conduction band offset (CBO) was calculated from the measured VBO values. These two measured values of the VBO allowed us to calculate the CBO, giving (-0.30 +/- 0.14) eV and (-0.34 +/- 0.06) eV, respectively. These values show that the CBO has a cliff-like behaviour which could be one of the reasons for the V-oc limitation in the CdS/CZTS solar cells.