Valence band offsets at Cu(In,Ga)Se2/Zn(O,S) interfaces

The energy band alignment at interfaces between Cu-chalcopyrites and Zn(O,S) buffer layers, which are important for thin-film solar cells, are considered. Valence band offsets derived from X-ray photoelectron spectroscopy for Cu(In,Ga) Se-2 absorber layers with CdS and Zn(O,S) compounds are compared to theoretical predictions. It is shown that the valence band offsets at Cu(In, Ga)Se-2/Zn(O,S) interfaces approximately follow the theoretical prediction and vary significantly from sample to sample. The integral sulfide content of chemical bath deposited Zn(O,S) is reproducibly found to be 50-70%, fortuitously resulting in a conduction band offset suitable for solar cell applications with Cu(In,Ga)Se-2 absorber materials. The observed variation in offset can neither be explained by variation of the Cu content in the Cu(In, Ga)Se-2 near the interface nor by local variation of the chemical composition. Fermi level pinning induced by high defect concentrations is a possible origin of the variation of band offset. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim