Anion‐Engineered Reinforcement of Hydrogen Bonding toward Homogeneous and Stable Perovskite Solar Cells

ABSTRACT Achieving homogeneous and stable perovskite films is pivotal for advancing photovoltaics toward commercialization. Toward this goal, a targeted anion engineering strategy is developed, utilizing 4‐guanidinobenzoic acid methanesulfonate (GUAMS) and contrasting it with its chloride analogue. Theoretical and multifaceted spectroscopic analyses collectively demonstrate that the engineered methanesulfonate anion uniquely fulfills a dual role: effectively passivating undercoordinated Pb 2+ defects while concurrently serving as a hydrogen‐bond bridge with formamidinium cations—a multifunctionality absent in the chloride anion. This anion‐driven synergy, coupled with the guanidinium cation's crystallization‐templating ability, directs the formation of homogeneous and robust perovskite films. Their improved structural integrity is corroborated by the uniform distribution of the additive, revealed via time‐of‐flight secondary ion mass spectrometry, and by the enhanced electronic homogeneity, confirmed by Kelvin probe force microscopy. Consequently, p‐i‐n perovskite solar cells achieve a champion efficiency of 26.11% with enhanced stability, retaining 95.2% of their initial efficiency after 500 h of thermal aging at 65°C and 80% after 1200 h of maximum power point tracking under continuous illumination at 55°C. The strategy also enables flexible devices with an efficiency of 24.49%. These results collectively establish anion‐functionality engineering as a critical design principle for high‐performance and durable perovskite optoelectronics.