Effect of Ga/(In+Ga) ratio on the Efficiency of Cu(InxGa1-x)Se2 (CIGS) Thin-Film Solar Cells

Abstract

Copper indium gallium diselenide (Cu(In_xGa_1-x)Se₂, CIGS)  continues to attract significant interest as an absorber layer in thin-film solar cells owing to its high absorption coefficient, long carrier diffusion length, and tunable bandgap. A key factor governing its performance is the Ga/(In+Ga) ratio, which directly influences the electronic structure, defect formation, and carrier transport within the absorber. In this study, numerical simulations were carried out using SCAPS-1D to systematically investigate the effect of Ga incorporation across the full compositional range (x = 0–1). Composition-dependent models were employed to account for bandgap variation, electron affinity, acceptor density, defect states, and density of states. The results demonstrate that gallium incorporation significantly modifies device behavior. Increasing Ga content leads to bandgap widening, which enhances the open-circuit voltage (V_OC) but reduces the short-circuit current density (J_SC) due to diminished absorption in the longer wavelength region. Conversely, indium-rich compositions exhibit stronger photocurrent generation but suffer from lower voltages. The fill factor (FF) exhibits a maximum of ~61% at Ga fraction x = 0.6, while the overall device efficiency follows a bell-shaped dependence, peaking at 11.7% for x = 0.4. These results reveal a trade-off between voltage and current that must be carefully balanced through absorber composition engineering. Overall, this study identifies the optimal Ga/(In+Ga) ratio to lie within the range of 0.3–0.6, where a balance between Jsc​, V_OC and FF yield the best photovoltaic response. The insights provided here can serve as practical guidelines for tailoring CIGS absorber layers to achieve improved device efficiency.