<p>Two-dimensional surface passivation has been shown to be useful for achieving state-of-the-art perovskite optoelectronics, and the microstructural and phase heterogeneities of two-dimensional perovskite passivators can influence their roles. However, the synthesis of co-homogenized, stable microstructure and phase in such passivators remains challenging. Herein we leverage a [6,6]-phenyl-C₆₁-butyric acid methyl ester molecular interlayer to mediate the reaction of the two-dimensional passivator and perovskite, leading to a uniform purer-phase two-dimensional perovskite capping layer. This interlayer mitigates the grain-boundary etching encountered in conventional approaches, creating molecular passivation directly onto the perovskite surface. The inverted perovskite solar cells made with the interlayer feature a laminate-structured perovskite heterointerface at the electron-extracting side, which contributes to improved charge energetics and film stability, owing to the regulated band alignment and laminate-layer protection, respectively. Power conversion efficiencies up to 25.97% are achieved, together with enhanced device stabilities under protocols standardized by the International Summit on Organic Photovoltaic Stability, showing <i>T</i>₉₀ lifetimes (the time at which they maintain 90% of their efficiency) of over 1,000 h in both the damp-heat test (85 °C, 85% relative humidity) and maximum power point tracking under one-sun illumination. Lattice-resolved insights are provided to link the microstructure to device performance, shedding light on the significance of passivator-microstructure uniformity and reliability on the performance of perovskite optoelectronics.</p><p></p>