Molecular basis of ligand-regulated precursor evolution in hybrid perovskite crystallization: a theoretical study

Ligand-assisted precursor engineering has emerged as an effective strategy to regulate the crystallization of metal halide perovskites. However, molecular-level design principles governing ligand-precursor interactions during early precursor evolution remain poorly understood. Here, using formamidinium lead iodide (FAPbI₃) as a model system, we establish an electronic-structure-driven framework to elucidate how functional ligands modify Pb-I precursor coordination through the interplay of lone-pair donation and π-electron distribution. Density functional theory (DFT) calculations were employed to systematically investigate a series of mono- and bifunctional ligands, including phenethylammonium chloride (PEA), 3-phenylpropionic acid (PPA), L-phenylalanine hydrochloride (PhA), and their conjugation-engineered derivatives. Enthalpic analysis shows that strong ligand-PbI₂ coordination is competitive with formation of the ligand-free FAI-PbI₂ adduct, while subsequent FAI association with ligand-bound PbI₂ yields stable ternary precursor complexes. Charge distribution, electrostatic potential mapping, interaction region analysis, and energy decomposition collectively demonstrate that moderate π participation can enhance ligand-PbI₂ binding through dispersion interactions, whereas direct π conjugation can weaken overall coordination by altering donor-site electronic structure and/or reducing favorable dispersion interactions through structural rigidification. Notably, bifunctional ligands exhibit multidentate coordination that provides enhanced enthalpic stabilization of ligand-bound precursor complexes. These findings provide molecular-level insights into ligand-regulated perovskite precursor evolution and offer rational guidelines for designing functional additives toward high-quality perovskite films.