Design and optimization of emerging quaternary Cu-Ag-Bi-I-based solar cells with SCAPS-1D to address energy level mismatches

Quaternary Cu-Ag-Bi-I compounds (CABI) have gained attention as potential alternatives to lead halide perovskites (LHPs) in solar cells due to their low toxicity, excellent stability, and promising photovoltaic (PV) properties. However, the highest reported power conversion efficiency (PCE) of CABI-based solar cells (CSCs) has not exceeded 3 %, primarily due to energy band alignment mismatches. In this work, we used SCAPS-1D to simulate the current state-of-the-art CSC, which holds the efficiency record to date, and performed a series of optimizations to address the energy band alignment mismatches. Through loss analysis based on VOC, JSC, and FF, combined with a figure of merit (FoM) approach to quantify losses, we sequentially optimized the electron transport materials (ETMs), hole transport materials (HTMs), rear electrode materials (REMs), and the thickness of each semiconductor layer. Notably, we introduced dual interface modification layers (IMLs), a MoO3/CuI bilayer, to significantly reduce the energy barrier at the HTL/RE interface. When the energy barrier at the HTL/RE interface was reduced to or below 0.38 eV, non-radiative and resistive losses were minimized, leading to saturation of VOC and FF at their maximum values. The optimized ETM, HTM, and REM were ZnSe, CuAlO2, and Au, respectively. The optimized device achieved energy band offsets of 0 eV for the electron transport layer (ETL)/CABI interface, 0.16 eV for  the CABI/hole transport layer (HTL) interface, and 0.12 eV for the HTL/rear electrode (RE) interface, which collectively reduced recombination and resistive losses. The optimized band alignment resulted in remarkable performance enhancements: VOC surged by 122 % to 1.281 V, JSC increased by 83 % to 19.78 mA/cm2, and FF improved by 78 % to 79.52 %. These synergistic improvements propelled the final optimized device (FTO/ZnSe/CABI/CuAlO2/MoO3/CuI/Au) to achieve a record PCE of 20.14 %–representing a 624 % leap from the initial PCE baseline (2.78 %). This significant enhancement highlights the remarkable PV potential of CABI and emphasizes the importance of energy band alignment matching in device optimization. The effectiveness of our strategies in CSCs suggests their potential applicability to other solar cells with energy band alignment mismatches, providing a generalizable framework for efficiency optimization.