Synergistic Interface Modification and Crystal Growth Regulation via a Fluorinated Sulfonate Multifunctional Buried Additive for High‐Efficiency CsPbI₃ Perovskite Solar Cells Exceeding 21%

Abstract All inorganic CsPbI₃ perovskites have attracted significant attention due to their excellent thermal stability and ideal bandgap characteristics. However, interfacial defects at the perovskite/electron transport layer (ETL) interface and uncontrolled crystallization processes of the perovskite remain critical bottlenecks for advancing device performance. Herein, a multifunctional buried interface‐modifying additive, pentafluoroaniline trifluoromethane sulfonate (PFAT), is employed. Analytical results confirm that PFAT can effectively anchor at the TiO₂/perovskite interface while passivating defects in both layers, thereby suppressing interfacial recombination losses. Furthermore, this modification can reduce the surface energy of the CsPbI₃ crystal plane, promoting perovskite crystallization and yielding films with enhanced crystallinity. To strengthen PFAT‐perovskite interactions, a PFAT‐PbI₂ hybrid solution (PFATLI) is synthesized for interfacial modification. Consequently, the optimized PFATLI‐modified device achieved a power conversion efficiency (PCE) of 21.36%, an open‐circuit voltage (V_OC) of 1.23 V, and a fill factor (FF) of 83.44%. For the larger‐area devices with an active area of 1 cm², the PCE reached 17.41%, while under the weak illumination conditions, the PCE further increased to 41.27%. After 800 h of storage in an ambient environment with 5% relative humidity (RH) at room temperature (RT), the unencapsulated device retained 87.27% of its initial efficiency.