Band-lineup and junction formation between Zn-VI (VI = O, S, Se) and epitaxial CuInSe2

The crucial interface in chalcopyrite-based solar cells is the one between Cu(In,Ga)Se2 absorber and CdS buffer layer, where the p/n junction is situated and which should provide a beneficial energetic lineup among absorber and ZnO window. Due to its toxicity, Cd-free buffer layers are desirable. Single-crystalline CuInSe2(112) and (001) films were grown by molecular beam epitaxy as well-defined model systems to study the band alignment with alternative buffer layer materials. The Zn-VI layers were deposited stepwise with intermediate analysis by combined XPS/UPS and LEED, completely under UHV conditions. ZnO deposition by Metal-Organic MBE leads to the formation of an ultra-thin intrinsic ZnSe buffer layer (1-2 nm thickness), consistent with our findings for CuInS2(112). The valence band offset for the intrinsic buffer layer of 0.7 eV is conform with our result for the bulk and agrees well with theory. On CuInSe2(112) substrates, ZnO grows in registry with ZnSe(111) with its own lattice constant in the (0001) direction, as confirmed by the LEED pattern. From our measurements, the band alignment is largely independent on orientation.