Lead halide perovskite solar cells have attracted considerable attention owing to their excellent optoelectronic properties and solution processability. However, uncontrolled crystallization during film formation often leads to compositional inhomogeneity and defects at surfaces and grain boundaries, limiting device efficiency and long-term stability. Although Lewis base additives have been widely employed to regulate crystallization and passivate defects, the effects of their molecular structures and functional groups on crystallization pathways and defect chemistry remain insufficiently understood.

Herein, we systematically investigate the role of functional-group-dependent chemical interactions by comparing two structurally similar phosphine-based Lewis bases, tri(p-tolyl)phosphine (pTol) and tris(4-methoxyphenyl)phosphine (pMeO). Both molecules coordinate with Pb²⁺ through phosphorus lone pairs, regulating precursor chemistry and suppressing undesirable intermediate-phase formation. Notably, pMeO exhibits an additional hydrogen-bonding interaction with organic ammonium cations through its spatially accessible methoxy groups, enabling synergistic dual-mode interactions. Through comprehensive chemical, crystallographic, and optoelectronic analyses, we demonstrate that these distinct interaction modes lead to different crystallization behaviors and defect passivation efficiencies. Compared with pTol, pMeO more effectively regulates crystallization, promoting homogeneous grain growth and suppressing compositional inhomogeneity and grain-boundary defects. Furthermore, the dual-mode interactions contribute to reduced trap density, suppressed nonradiative recombination, and inhibited ion migration, resulting in improved photovoltaic performance and operational stability.

Consequently, pMeO-incorporated perovskite solar cells achieve a power conversion efficiency of 25.46%, together with enhanced thermal and moisture stability. These findings highlight that the effectiveness of molecular additives is governed not only by their Lewis basicity but also by the nature and spatial accessibility of their functional groups. This study provides mechanistic insights into the relationship between molecular structure, chemical interactions, crystallization dynamics, and defect passivation, offering a rational molecular design strategy for developing multifunctional additives toward efficient and stable perovskite solar cells.