Regioisomerism‐Guided Molecular Passivation for 26.05%‐Efficient Perovskite Solar Cells

Defect‐mediated nonradiative recombination remains a major barrier to efficient and stable perovskite solar cells (PSCs). Although considerable advances have been made with molecular additives, how the spatial arrangement of functional groups affects device performance remains largely unexplored. Here, we present a regioisomerism‐guided molecular design strategy using fluorinated aniline as a well‐defined model system to incorporate into the lead iodide (PbI 2 ) precursor solution during two‐step sequential perovskite deposition. By systematically comparing ortho‐, meta‐, and para‐fluoroaniline, we demonstrate that the ortho isomer, 2‐fluoroaniline (2‐FA), affords the most effective regulation of defect states and device performance. This superiority arises from the formation of an intramolecular hydrogen bond, which redistributes the molecular electrostatic potential and confers stable bidentate coordination with the perovskite lattice. Additionally, 2‐FA facilitates high‐quality α‐FAPbI 3 films and aligns the energy level favorably with the hole transport layer, yielding a champion efficiency of 26.05% for the n‐i‐p PSCs. Moreover, the 2‐FA‐containing device exhibits superior operational stability. This regioisomerism‐guided design works universally across n‐i‐p and p‐i‐n architectures, irrespective of one‐step or two‐step fabrication. Our work establishes regioisomerism as a simple yet powerful molecular design principle for molecular engineering and provides new insight into the rational development of efficient and stable PSCs.