Chemical Interface Structures in CdS/RbInSe2/Cu(In,Ga)Se2 Thin-Film Solar Cell Stacks

Performance-enhancing heavy alkali-based post-deposition treatments (PDT) of Cu(In,Ga)Se-2 (CIGSe) thin-film solar cells absorbers often induce the formation of a Rb- In-Se phase on the CIGSe absorber. Co-evaporation of an interfacial RbInSe2 (RISe) layer between buffer and absorber can also benefit cell performance. A detailed analysis of the chemical interface structures in CdS/RISe/CIGSe layer stacks is performed using hard X-ray photoelectron spectroscopy (HAXPES). For comparison, stacks without RISe and based on RbF PDT CIGSe absorbers are also studied. When aiming for the direct co-evaporation of a RISe layer on the CIGSe absorber, the formation of an additional In-Se phase is found. For the RbF PDT CIGSe absorbers, the study only finds small amounts of Rb and no indication for a RISe layer formation. Examining layer stacks prepared via additional chemical bath deposition (CBD) of CdS reveals a clear impact of the presence of Rb (or of Rb-containing species) on the CIGSe surface. In these cases, an increase of the induction/coalescence period is found at the beginning of the CBD buffer layer growth process and the formation of Cd & horbar;Se bonds; thereafter, a more compact CdS layer growth is observed.