Design and Optimization of Cs2AgBiBr6/AgBiS2 Two-Terminal Tandem Solar Cells for High-Efficiency Lead-Free Photovoltaics

Abstract The ternary chalcogenide AgBiS2 has emerged as a promising lead-free photovoltaic absorber due to its high optical absorption, tunable bandgap (∼1.3 eV), and nontoxic composition. In this work, we present a comprehensive numerical study of AgBiS2-based thin-film solar cells using a one-dimensional solar cell capacitance simulator (SCAPS-1D). The influence of absorber thickness, defect density, bandgap variation, and charge transport layers on device performance is systematically analyzed. The optimized single-junction architecture─FTO/TiO2/AgBiS2/Cu2O─achieves a simulated power conversion efficiency (PCE) of 13.98% under AM1.5G illumination. Because of its high absorption, a two-terminal tandem structure combining AgBiS2 as the bottom cell with Cs2AgBiBr6 (top cell) is also modeled, achieving a PCE of 23.47% after current matching. Simulations reveal that AgBiS2 thickness and defect density critically impact both Jsc and Voc, while Cu2O and TiO2 provide optimal band alignment and charge extraction. The results demonstrate the potential of AgBiS2 as an efficient, environmentally benign absorber for next-generation tandem photovoltaics and establish theoretical design guidelines for future experimental development.