DFT and SCAPS-1D Simulation of Single-Layer and Bilayer Perovskite Solar Cells: Ca₃BiI₃ and Sr₃BiI₃

Abstract This study investigates the structural, electronic, elastic, and optical properties of the inorganic perovskites Ca3BiI3 and Sr3BiI3 using Density Functional Theory (DFT) with the Wien2k code. The optimized lattice parameters are 6.38 Å for Ca3BiI3 and 6.69 Å for Sr3BiI3. The calculated band gaps using the GGA-PBE functional are 1.36 eV for Ca3BiI3 and 1.30 eV for Sr3BiI3, which increase to 1.79 eV and 1.61 eV, respectively, after applying the TB-mBJ correction. Both materials are dynamically stable, confirmed through formation energy and phonon dispersion analysis, and mechanically stable based on Born criteria. Optical analysis shows strong absorption in the visible range, with Ca3BiI3 slightly outperforming Sr3BiI3, making both candidates for photovoltaic applications. In the second part, we simulate the performance of these perovskites in single-layer and bilayer solar cells, with Ca3BiI3 as the top absorber and Sr3BiI3 as the bottom. By optimizing the bilayer device, we achieve an efficiency of up to 28.37%, with a VOC of 1.36 V, a fill factor (FF) of 89.78%, and a short-circuit current density (JSC) of 23.14 mA/cm².