Synergistic Passivation Enables 22.76%‐Efficiency Quasi‐2D Perovskite Solar Cells

ABSTRACT Quasi‐two‐dimensional (quasi‐2D) perovskite solar cells (PSCs) offer intrinsically enhanced environmental stability owing to their hydrophobic spacer layers, but the associated uncoordinated sites and ionic vacancies act as severe non‐radiative recombination centers that limit further efficiency gains. Here, we introduce acetic anhydride (Ac 2 O) as a multifunctional additive for quasi‐2D alternating‐cation‐interlayer (ACI) perovskites. Spectroscopic analysis and density‐functional theory calculations reveal that the carbonyl groups in Ac 2 O simultaneously to undercoordinated Pb 2+ ions and form hydrogen bonds with guanidinium cations (GA + ). This synergistic interaction slows down crystallization, yielding films with improved crystallinity, enlarged grain size, and markedly reduced defect‐state density. In parallel, Ac 2 O‐induced passivation shifts the Fermi level upward, strengthening the built‐in electric field and thereby promoting faster charge separation and extraction. As a result, the optimized quasi‐2D PSCs achieve a champion power conversion efficiency of 22.76% (certified 21.69%), representing a record among reported quasi‐2D PSCs. Encapsulated devices retain 96% of their initial efficiency after 2000 h of continuous maximum‐power‐point operation, underscoring the effectiveness of this simple additive strategy in simultaneously advancing both efficiency and operational stability in quasi‐2D perovskite photovoltaics.