Band offsets at the CdS/CuInSe2 heterojunction

Abstract

The traditional explanation for the successful electron-hole separation in CdS/CuInSe2 solar cells rests on the assumption of a type-II band lineup: The conduction-band minimum is assumed to be on the CdS window while the valence-band maximum is assumed to be localized on the CuInSe2 absorber. This picture of negative conduction-band offset ΔEc<0 was supported by the electron affinity rule, but was sharply contradicted by the more recent photoemission experiments of Nelson et al. for CdS/CuInSe2 yielding ΔEc=+1.08 eV. Our first principles calculations yield for CdS/CuInSe2 ΔEc=+0.31 eV, hence, a type-I band alignment. We challenge the published experimental value as being in error and point to the need of revising current solar cell device models that assume ΔEc<0.