Abstract
The long-term stability of polycrystalline perovskite solar cells (PSCs) remains a major hurdle despite their high power conversion efficiencies (PCEs), underscoring the importance of eliminating all potential photodegradation pathways. While efforts have been devoted to address bulk and interfacial defect-induced photodegradation, those correlate to the microscale inter-grain carrier reactions intrinsic to the polycrystalline nature of PSCs remain unsolved. Here we uncover a defect-dependent asymmetric inter-grain recombination pathway that governs photodegradation in mixed-facet PSCs. By resolving the nature and evolution of facet-specific defects for the first time, we reveal that hetero-facet grain boundaries induce non-equilibrium hole accumulation at (111)-facet grain boundaries that unexpectedly accelerates their photodegradation, despite their superior intrinsic photostability. By introducing 3-fluorophenylethylammonium bromide that targets identified formamidinium vacancy defects at the (111)-facet grain boundaries, we homogenize the inter-grain recombination, yielding PSCs with PCEs approaching 26.55% and T90 lifetimes over 2200 h under continuous 1-sun illumination.