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

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.