Dynamic Stress‐Regulating Molecular Networks Directing Oriented Growth in FAPbI ₃ Perovskites

ABSTRACT Metal halide perovskite solar cells (PSCs) combine high efficiency with low‐cost processing, yet their commercialization is hindered by residual tensile strain and defect‐driven instability. In formamidinium‐based FAPbI 3 perovskite, thermal expansion mismatch amplifies lattice stress, accelerating defect formation and non‐radiative recombination. Here, we introduce a dynamic stress‐regulating molecular additive, 4‐vinylpiperidine (VPD), which integrates strong Lewis basicity with a polymerizable vinyl group. During annealing, VPD simultaneously coordinates with Pb 2+ ions and polymerizes in situ into a cross‐linked network, suppressing PbI 2 formation, reprogramming tensile strain into compressive stress, and directing oriented (100) crystal growth. These synergistic effects yield perovskite films with reduced trap density, prolonged carrier lifetimes, and optimized energy‐level alignment. Consequently, VPD‐modified PSCs achieve a power conversion efficiency of 25.50% and retain 95% of their initial performance after 500 h of continuous operation. This work establishes a generalizable molecular design strategy that couples defect passivation with strain engineering, advancing both efficiency and In situ polymerization, Lewis Base Passivation, Perovskite Solar Cells, Strain‐regulating durability in perovskite photovoltaics.