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

We report on sodium containing indium sulfide (In_xS_y:Na) buffer layer in combination with a Cu(In,Ga)Se₂ (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_xS_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_xS_y:Na layer is combined with CIGS with RbF-PDT have the highest power conversion efficiencies. The presence of sodium in In_xS_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 In₂S₃ buffer combined with a high-quality RbF-PDT CIGS absorber with a comparably high copper content.