Effect of Cu Content on Post‐Sulfurization of Cu(In,Ga)Se 2 Films and Corresponding Solar Cell Performance

Herein, the effect of the initial copper content of co‐evaporated Cu(In 1− x ,Ga x )Se 2 (CIGS) absorber films on the impact of a post‐annealing step in elemental sulfur atmosphere is studied. The Cu concentration is varied over a wide range ([Cu]/[III] = CGI = 0.57–1.23), allowing to identify composition‐dependent trends in phase formation, chemical rearrangements, and solar cell performance after sulfurization. For all samples, a ternary CuInS 2 layer forms at the surface. In addition, sulfur 1) is incorporated in randomly distributed CuIn(S,Se) 2 mixed crystals underneath CuInS 2 ; 2) diffuses into multidimensional defects (e.g., dislocations and grain boundaries); and 3) is bound in Na–In–S surface plates. It is found that Cu‐poor absorber composition (CGI ≤ 0.82) favors CuInS 2 growth as compared with close‐stoichiometric CIGS films, driven by a faster diffusion of Cu toward the surface. For Cu‐rich absorbers (CGI > 1), Se—S exchange is significantly accelerated, presumably by the presence of Cu 2− x Se phases reacting to Cu 2− x S and eventually catalyzing CuInS 2 formation. Finally, open‐circuit voltage ( V OC ), fill factor (FF), and efficiency ( η ) of corresponding solar cells increase after sulfurization with increasing CGI until stoichiometry is reached. The result is explained by a mitigated Cu depletion of the absorber bulk after sulfurization for close‐stoichiometric CIGS.