Numerical simulation and buffer layer optimization for Cd-free Cu(In, Ga)Se2 solar cells using SCAPS-1D

Copper indium gallium selenium (Cu(In, Ga)Se₂, CIGS) thin-film solar cells have garnered significant attention as a promising solution to address the global energy crisis. However, the conventional cadmium sulfide (CdS) buffer layer presents inherent limitations, including optical absorption losses in the short-wavelength spectrum and environmental toxicity concerns. This study systematically evaluates three eco-friendly wide-bandgap alternatives, such as gallium selenide (Ga₂Se₃), zinc magnesium oxide (Zn_0.8Mg_0.2O), and zinc oxysulfide (Zn(O, S)), through comprehensive numerical simulations using SCAPS-1D. Initial comparative analysis reveals that Zn_0.8Mg_0.2O and Zn(O, S) demonstrate superior interfacial properties, particularly through their favorable conduction band alignment that effectively minimizes carrier recombination barriers at the buffer/absorber interface. Extended parametric studies on buffer layer thickness, operating temperature, doping concentration, sulfur composition in Zn(O, S), and the gallium ratio (Ga/(Ga + In)) in CIGS further establish Zn(O, S) as the most promising candidate. Optimized devices with Zn(O, S) buffers achieve enhanced carrier collection efficiency owing to their optimal band alignment and reduced interface defect states. This computational investigation provides critical insights for developing high-performance, cadmium-free CIGS photovoltaic devices through buffer layer engineering.