AbstractPerformance‐enhancing heavy alkali‐based post‐deposition treatments (PDT) of Cu(In,Ga)Se₂ (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─Se bonds; thereafter, a more compact CdS layer growth is observed.