Highly Efficient Bifacial Narrow Bandgap Ag-CuInSe2 Solar Cells on ITO

Bifacial CuInSe₂ (CISe) solar cells hold significant promise for various applications but are constrained by relatively low power conversion efficiencies. This study boosts performance through reducing CISe absorber deposition temperature and using low-Ga back grading for an optimum gallium-to-indium ratio (Ga/(Ga+In); GGI) profile. Low deposition temperatures reduced ITO back contact thermal degradation, while low Ga concentration reduced GaO_X formation and CISe/ITO charge recombination. Ag incorporation significantly improved key photovoltaic parameters, including open-circuit voltage (V_OC) and fill factor (FF), while reducing Cu_2−XSe secondary phase formation. This approach enables high-quality CISe growth below 420 °C-substantially lower than conventional temperatures. The study achieves record efficiency in the narrow bandgap CISe category, with Ag-alloyed devices demonstrating a champion rear-side efficiency of 8.44% at 390 °C, and a front-side efficiency of 15.30% at 420 °C. Under the assumption of double-sided total 2.0 solar illumination in an albedo environment, a champion bifacial power generation density (BPGD) of 23.1 mWcm⁻² is achieved. Results indicate that lower deposition temperatures enhance rear-side performance, highlighting the role of low-temperature processing, low Ga doping, and Ag alloying in suppressing carrier recombination losses in CISe solar cells.