Origins and Chemical Nature of the Amorphous Buried Interface in Perovskite Solar Cells

The buried interface between perovskite and charge transport layer plays a crucial role in determining the performance and long-term stability of perovskite solar cells. Self-assembled monolayers (SAMs) have been widely adopted to adjust interfacial alignment and enhance device efficiency, however, their impact on the stability has largely been overlooked. In this study, we reveal that instability at the buried interface originates from the accumulation of organic cations within chemically heterogeneous, amorphous interfacial regions. The unanchored SAM molecules, which retain the ─OH groups of the active phosphonic acid moieties, attract the organic cations, thereby triggering acid–base interactions that destabilize the interface. By introducing a surface polishing strategy to eliminate amorphous components and restore interfacial stoichiometry, the stability of the buried interface is significantly enhanced. As a result, modulation of the buried interface leads to a power conversion efficiency (PCE) increase from 23.43% to 26.54% (certified 26.13%). At the module level, the PCE is enhanced to 22.85% (aperture area of 64 cm²). Furthermore, the target device retains 94% of its initial efficiency after 1200 h of maximum power point tracking under 1-sun illumination at 50°C. Notably, the improved buried interface leads to substantial enhancements under ultraviolet irradiation and laser illumination.