Nanoscopic cation heterogeneity dictates the ambient stability of halide perovskites

Ambient instability in formamidinium-caesium mixed-cation perovskites limits the efficiency and long-term durability of solar cells, yet its nanoscopic origin remains unclear. Using quasi-in situ scanning transmission electron microscopy, we identify Cs-enriched grain triple junctions as intrinsic weak points under ambient exposure, where Cs⁺ segregation and phase separation initiate. This localized instability induces structural disordering and δ-CsPbI3 crystallization, which propagates into neighboring grains, progressively degrading the perovskite matrix and interfacial optoelectronic properties. To suppress cation heterogeneity, we introduce N-Octylsulfobetaine, which regulates crystallization kinetics, promotes homogeneous cation distribution, and passivates grain boundaries, thereby enhancing resistance to water and oxygen intrusion. The resulting n-i-p perovskite solar cells achieve a champion power conversion efficiency of 26.37% and retain 96.47% of initial performance after 2,200 h under ISOS-L-1l protocol. Our findings elucidate the nanoscopic ambient degradation pathways in FA-Cs perovskites, establishing a promising strategy for achieving robust perovskite photovoltaics combining high efficiency with long-term durability. The nanoscopic origin of the ambient instability in mixed-cation perovskites is unclear. Wang et al. identify Cs-enriched grain junctions as weak points and introduce N-Octylsulfobetaine to regulate crystallization kinetics. The resulting device achieves a power conversion efficiency of 26.37%.