Enhancing the Performance and Stability of FAPbI₃ Perovskite X‐ray Detectors via Bi‐Doping

Perovskite materials have demonstrated significant potential in X‐ray detection due to their high atomic number and robust X‐ray absorption capacity, especially organic–inorganic hybrid perovskites. Among these, CH(NH 2 ) 2 PbI 3 (FAPbI 3 ) stands out for its narrow bandgap and robust absorption properties. However, FAPbI 3 undergoes a rapid phase transition from a black octahedral perovskite phase to a yellow non‐perovskite phase under environmental conditions. This work addresses this issue by proposing the doping of Bi elements with mixed valence at the Pb site, verified through density functional theory simulations. The results indicate that Bi doping increases the formation of FA ionic vacancies, enhancing degradation energy and stabilizing the perovskite phase. FAPb 0.99 Bi 0.01 I 3 exhibits higher ion‐migration activation energy (0.75 eV), carrier mobility (6.88 × 10 −3 cm 2 V −1 ), and carrier lifetime compared to FAPbI 3 . Additionally, Bi doping reduces FAPbI 3 perovskite crystal defects, inhibits ion migration, and increases resistivity. These improvements confirm the feasibility of B‐site non‐homovalent doping in perovskite X‐Ray detector applications.