Effect of Cu Content on Post-Sulfurization of Cu(In,Ga)Se_2 Films and Corresponding Solar Cell Performance

摘要

Herein, the effect of the initial copper content of co-evaporated Cu(In1-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 CuInS2 layer forms at the surface. In addition, sulfur 1) is incorporated in randomly distributed CuIn(S,Se)(2) mixed crystals underneath CuInS2; 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 CuInS2 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 Cu2-xSe phases reacting to Cu2-xS and eventually catalyzing CuInS2 formation. Finally, open-circuit voltage (V-OC), fill factor (FF), and efficiency (eta) 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.