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

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 J sc and V oc, 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.