Ga Grading Induced Cu Compositional Effects on Solution-Processed Cu(In,Ga)(S,Se)₂ Thin-Film Solar Cells

This study investigates the effect of introducing Ga-free layers at various positions within metal oxide precursor films to control band gap grading in Cu(In,Ga)(S,Se)2 (CIGSSe) absorber films during solution processing. Unexpected changes in the Cu/(Ga+In) (CGI) composition ratio were observed concomitant with Ga/(Ga+In) (GGI) variations, which affects the band gap profile, morphological structure, and solar cell device performance. Specifically, as the Ga-free layer approached the surface, the open-circuit voltage (VOC) decreased and the short-circuit current density (JSC) increased, consistent with the predicted band gap trends. Larger grain sizes and increased void formation affected recombination and fill factor characteristics. The film with the Ga-free layer positioned in the middle exhibited the most favorable VOC and JSC in terms of efficiency. The reduction in recombination, driven by decreased voids and the formation of large grains, resulted in the best solar cell performance. This study highlights that depth-dependent GGI distribution directly affects CGI, emphasizing the importance of precise GGI control at the precursor stage to obtain more efficient thin-film solar cells.