Flexible Ammonium Salts Induce Dipole Field-Effect Passivation in Inverted Perovskite Solar Cells.

Ammonium salt-based top-interface passivation is effective for inverted perovskite solar cells, yet how the head-group conformation governs the passivation mechanism remains unresolved. Herein, using 9H-fluoren-9-amine hydrochloride (FAMACl) and aminodiphenylmethane hydrochloride (DPMACl) as a comparative pair, we reveal that structural differences in the head groups drive fundamentally distinct passivation pathways. The rigid, conjugated structure of FAMACl enables strong chemical passivation through coordination with undercoordinated Pb2+ defects, but introduces unfavorable energy level alignment that limits efficiency gains. In contrast, the flexible DPMACl architecture generates a substantial molecular dipole moment, which induces a strong interfacial electric field that optimizes energy level alignment and suppresses nonradiative recombination via field-effect passivation, delivering a champion power conversion efficiency of 26.02%. This work establishes a direct link between the conformational flexibility of ammonium head-groups and their passivation functionality, offering a new molecular design principle for high-performance perovskite photovoltaics.