Oleylamine‐Formamidinium Coherent Structure Boosts the Synergistic Enhancement of Efficiency and Stability in Perovskite Solar Cells

Interface optimization is an indispensable key step for planar high‐efficiency and stable perovskite solar cells (PSCs). However, existing interface optimization strategies mostly focus on defect passivation or energy level alignment on the upper surface of perovskite, making it difficult to construct a dense and stable protective layer on the perovskite surface to effectively resist structural degradation caused by moisture intrusion and ion migration. In this study, the perovskite surface was post‐treated with an oleylamine (OAm)/chlorobenzene (CB) solution. OAm partially replaced the formamidinium (FA + ) cations on the perovskite surface through protonation/deprotonation reactions and formed a robust coherent structure, where OAm + and FA + coexist on the surface of perovskite. This structure significantly improved the stability of PSCs without inducing phase transition or additional interfaces. Based on this strategy, p‐i‐n type PSCs achieved a power conversion efficiency (PCE) of 25.7% (steady‐state output of 25.5%). As being continuous exposure to 85°C/85% relative humidity (R.H.) conditions, the photovoltaic devices still maintained more than 85% of their initial efficiency for 1200 h, and 80% for over 2000 h, demonstrating a significant improvement in stability. This study reveals how surface properties affect perovskite stability and introduces a new approach to engineering durable, high‐performance perovskite interfaces.