Dual‐Function Interface Engineering for δ‐Phase‐Free Crystallization Toward Efficient and Stable FAPbI ₃ Perovskite Solar Cells

The presence of non‐perovskite δ‐phase at the buried interface between the perovskite active layer and the charge transport layer significantly hinders the commercialization of perovskite solar cells. This is primarily due to the difficulty in detecting it during the initial device fabrication. This unwanted phase acts as a nucleation site for degradation and is notoriously difficult to eliminate once formed. In this work, we uncovered its origin in the initial crystallization process: the inherent surface oxygen‐step structure and oxygen vacancies on the SnO 2 interfacial layer are responsible for the formation of δ‐FAPbI 3 at the buried interface, even in fresh films. To counteract this, we implemented a dual‐function interface engineering strategy by incorporating phenethylammonium chloride (PEACl) onto the SnO 2 layer. This strategy effectively passivated the SnO 2 surface and precisely controlled the growth of the desired FAPbI 3 α‐phase on top, thereby preventing δ‐phase formation at the buried interface. Consequently, this treatment successfully suppressed nonradiative recombination and enhanced carrier extraction, leading to power conversion efficiencies (PCEs) as high as 25.6% with enhanced stability under illumination and humidity. Furthermore, flexible devices incorporating our buried interface treatment exhibited promising mechanical stability, retaining 83% of their initial PCE after 21 000 bending cycles.