Vertically Graded Dual Salts to Suppress Interfacial Mobile-Ion Accumulation for Stable Perovskite Photovoltaics

Suppression of mobile-ion accumulation at the perovskite interface is critical for perovskite photovoltaic (PV) stability. Post-treatment with fluorinated aromatic ammonium salts can suppress ion migration at perovskite interfaces. However, these salts damage underlying three-dimensional (3D) perovskites during device operation, inducing interfacial mobile-ion accumulation. Although these cations form gradient low-dimensional perovskites for enhanced quality of bulk quasi-2D films, constructing an ultrathin gradient structure by post-treatment remains challenging. Here, we demonstrate a preferential spatial arrangement of dual fluorinated ammonium cations at the FA0.95Cs0.05PbI3 perovskite interface, which suppresses the interfacial mobile-ion accumulation. This arrangement includes both long-chain cations on the topmost surface and short-chain cations preferentially in the sub-surface region. This unique surface structure suppresses mobile-ion accumulation at the LD/3D heterostructure. Our n–i–p perovskite PV achieves an efficiency of 26.47% (certified 25.95%). Moreover, the resulting perovskite PVs exhibit an operational stability of 3600 h at 55 °C and a thermal stability of 3500 h at 85 °C.