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

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 (Ac₂O) 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 Ac₂O simultaneously to undercoordinated Pb²⁺ 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, Ac₂O-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.