Chemically Tailored Organic Ammonium Salts for Integrated Regulation of CsPbI 3 Perovskite Solar Cells

Understanding and chemically tailoring the interfacial properties is essential for improving both efficiency and stability of perovskite solar cells (PSCs). All‐inorganic cesium‐based perovskites have emerged as promising candidates for thermally stable PSCs, however, their poor phase stability and high density of surface defects continue to impede device performance. Herein, we introduce functionalized halogenated phenethylammonium iodide (X‐PEAI, X = H, F, Cl, Br) as modifiers, and a synergistic optimization of the perovskite bulk and interface is achieved through an integrated regulation strategy. It is found that Cl‐PEAI with a strong dipole moment, achieves the optimal regulatory effect. It not only improves the film morphology but also effectively passivates the defect states through strong Lewis acid‐base interactions. In addition, it also introduces an additional dipole layer at the interface, which enhances the carrier transport effect. Consequently, Cl‐PEAI‐treated devices deliver a champion power conversion efficiency (PCE) of 19.53% and retain 92.9% of their initial efficiency after 720 h of ambient storage, thereby underscoring the potential of rational ligand design within this specific ammonium salt category for advancing stable, high‐performance all‐inorganic PSCs.