Band alignment at the CdS∕Cu(In,Ga)S2 interface in thin-film solar cells

Abstract

The band alignment at the CdS∕Cu(In,Ga)S2 interface in thin-film solar cells on a stainless steel substrate was investigated using photoelectron spectroscopy and inverse photoemission. By combining both techniques, the conduction and valence band offsets were independently determined. We find an unfavorable conduction band offset of −0.45 (±0.15) eV, accounting for the generally observed low open-circuit voltage and indicating the great importance of the buffer∕absorber conduction band offset for such devices. The surface band gap of the Cu(In,Ga)S2 absorber is 1.76 (±0.15) eV, being increased with respect to the expected bulk value by a copper depletion near the surface.