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 Ca 3 BiI 3 and Sr 3 BiI 3 using density functional theory (DFT) with the Wien2k code. The optimized lattice parameters are 6.38 Å for Ca 3 BiI 3 and 6.69 Å for Sr 3 BiI 3 . The calculated band gaps using the generalized gradient approximation-Perdew–Burke–Ernzerhof functional are 1.36 eV for Ca 3 BiI 3 and 1.30 eV for Sr 3 BiI 3 , 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 Ca 3 BiI 3 slightly outperforming Sr 3 BiI 3 , 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 Ca 3 BiI 3 as the top absorber and Sr 3 BiI 3 as the bottom. By optimizing the bilayer device, we achieve an efficiency of up to 28.37%, with a V OC of 1.36 V, a fill factor of 89.78%, and a short-circuit current density ( J SC ) of 23.14 mA cm −2 .