Molecular Modulation of Defects and Ionic Dynamics in Triple‐Cation Perovskite Solar Cells

Minimizing defect states and ionic migration in perovskite absorbers is crucial for improving the photovoltaic performance of perovskite solar cells (PSCs). Herein, we introduce two benzylic bromide derivatives, benzyl bromide (BBr) and p‐nitrobenzyl bromide (p‐NBBr), as novel molecular additives in triple‐cation (CsFAMA) perovskite precursor solutions. We found that the incorporation of these small molecules modifies the local chemical environments of Pb‐ and I‐containing species and suppresses the formation of metallic Pb 0 , indicating effective mitigation of defect‐related surface species. In particular, p‐NBBr regulates crystallization, promotes preferential crystal orientation along the (100) plane, and improves the surface morphology and interfacial quality of the perovskite films, enabling more efficient charge extraction. As a result, p‐NBBr‐treated PSCs achieved a power conversion efficiency of 18.85%, with an increased open‐circuit voltage ( V oc ) of 1.08V and markedly reduced hysteresis, typically associated with ion migration processes that induce defect formation and interfacial charge accumulation. Electrochemical impedance spectroscopy was applied to probe the charge transport dynamics of the devices. The analysis revealed reduced interfacial recombination losses and prolonged carrier lifetimes in the treated devices, confirming improved photovoltaic performance in the treated devices. These findings demonstrate the potential of molecular additive engineering for defect management and performance enhancement in efficient and stable PSCs.