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

Heavy alkali – based post-deposition treatments (PDT) of Cu(In,Ga)Se2 (CIGSe) thin-film solar cells absorbers often result in an performance enhancement. Employing an RbF PDT in some cases induces the formation of an Rb-In-Se phase on the CIGSe surface. Mimicking this effect, recently the direct deposition of an interfacial RbInSe2 (RISe) layer between buffer and absorber was suggested and realized by co-evaporation; also benefitting cell performance. In order to clarify the beneficial effect, we performed a detailed analysis of the chemical interface structures in CdS/RISe/CIGSe layer stacks using hard X-ray photoelectron spectroscopy (HAXPES). When aiming for the direct co-evaporation of a RISe layer on the CIGSe absorber, we find the formation of an additional In-Se phase. For the RbF PDT CIGSe absorbers, we only find 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 on the CIGSe surface. In these cases, we find an increase of the induction/coalescence period at the beginning of the CBD buffer layer growth process and the formation of Cd-Se bonds; thereafter, a more compact CdS layer growth is observed.