Theoretical design of A/B-site co-doped CsPbI₃ perovskites for enhanced stability and photovoltaic performance

Lead toxicity and phase instability are major challenges for CsPbI₃ perovskite photovoltaics. To address these issues, we employ a combined theoretical approach using density functional theory (DFT) and SCAPS-1D simulations to investigate Sn/Mg (B-site, 12.5%) and Rb (A-site, 50%) co-doping strategies. Sn and Mg substitutions both shorten the central B-I bond lengths but modulate bonding character through distinct mechanisms: Sn enhances Sn-5p/I-5p covalent hybridization, while Mg strengthens ionic character. Both mechanisms contribute to suppressing halide vacancy formation and improving structural stability. Electronic structure calculations yield band gaps ranging from 1.23 to 1.45 eV (raw DFT band gaps), all within the ideal range for single-junction solar cells. Notably, Sn doping preserves a direct band gap and introduces intermediate states, while Mg doping induces an indirect transition and exhibits p-type semiconducting behavior. Subsequent Rb incorporation further fine-tunes the band edges. Optically, all compounds show strong visible-light absorption, with pristine and Sn, Rb co-doped CsPbI₃ being particularly efficient for photon harvesting. Furthermore, Mg, Rb co-doped and pristine samples exhibit superior infrared dielectric response, which aids in suppressing carrier recombination. With the corrected band gaps, device simulation results identify Rb, Sn co-doped CsPbI₃ as the optimal absorber material. After optimizing absorber layer thickness and defect density, the corresponding hole-transport-layer-free solar cell achieves a predicted power conversion efficiency of 24.49%. This work demonstrates that strategic A- and B-site co-doping can effectively balance structural stability, optoelectronic properties, and device performance, providing a viable pathway for developing high-efficiency, lead-reduced inorganic perovskite solar cells.