Synergistic Crystallization Control and Defect Passivation via Bifunctional Hydrogen‐Bonding Networks in Methylammonium‐Free Perovskite Solar Cells

ABSTRACT Methylammonium (MA)‐free Cs X FA 1‐X PbI 3 perovskite solar cells (PSCs), despite their commercial promise stemming from enhanced stability, face the dual challenge of controlling crystallization and achieving stable defect passivation. Here, we introduce 4‐hydroxybenzamidine hydrochloride (4OHFACl) as a bifunctional additive to address these challenges. The amidinium and hydroxyl groups in 4OHFACl form hydrogen‐bonding with I − and [PbI 6 ] 4− in perovskite precursors, regulating the colloidal properties of the precursor solution to optimize nucleation and crystallization kinetics. This enables a synergistic process of rapid nucleation followed by slow crystal growth, yielding high‐quality perovskite films with enlarged grains and low residual strain. Owing to its higher pKa, the amidinium group also provides stable defect passivation, effectively suppressing non‐radiative recombination. Consequently, 4OHFACl‐modified PSCs achieve a power conversion efficiency (PCE) of 26.16% along with enhanced operational stability, retaining 90% of their original efficiency after 900 h of continuous illumination. Moreover, this approach exhibits excellent scalability, enabling a 66 cm 2 (aperture area) perovskite solar module to attain a champion PCE of 21.15%, underscoring its potential for practical large‐area fabrication. This work demonstrates a new strategy to couple precursor coordination chemistry with defect passivation, enabling concurrent control of crystallization kinetics and defect energetics for stable perovskite solar cells.