Impact of CIGS Absorber and Sputtered InxSy:Na Buffer Composition on Solar Cell Performance

We report on sodium containing indium sulfide (In x S y :Na) buffer layer in combination with a Cu(In,Ga)Se 2 (CIGS) absorber and investigate the mutual interaction and influence of them on the thin‐film solar cell device performance. We examine a variety of absorber layers including CIGS with RbF post‐deposition treatment (PDT), CIGS without PDT, and Ag‐alloyed CIGS without PDT, each with three different copper concentrations. All absorber layers are prepared by in‐line coevaporation of the elements using a multistage industrially relevant process. The In x S y :Na buffer layers are deposited by magnetron sputtering from three different indium sulfide targets containing 0 mol%, 2 mol%, and 10 mol% NaF. Devices in which the In x S y :Na layer is combined with CIGS with RbF‐PDT have the highest power conversion efficiencies. The presence of sodium in In x S y can contribute to a higher cell efficiency depending on the quality of the absorber used. Sodium likely has a positive effect if the alkali doping in the absorber is insufficient and can be compensated by the sodium supplied from the buffer. We demonstrate cell efficiencies up to 19.1% with a sodium‐free In 2 S 3 buffer combined with a high‐quality RbF‐PDT CIGS absorber with a comparably high copper content.