Enhancing Surface Termination and Stability of Hybrid Halide Perovskites via Phosphonic Acid Passivation

Surface passivation at hybrid halide perovskite interfaces is critical for suppressing nonradiative recombination and improving operational stability, but the molecular-scale mechanisms remain incompletely understood. Here, we use density functional theory to investigate the adsorption of phenylphosphonic acid (PPA) and 2-carboxyethylphosphonic acid (CEPA) on representative surfaces CH₃NH₃PbI₃ (MAPbI₃) using periodic slab models. Both acids bind exergonically to all terminations considered, with CEPA consistently exhibiting stronger adsorption than that of PPA as a result of additional hydrogen bonding enabled by its carboxyl group. Adsorption becomes more favorable with increasing exposure to the undercoordinated surface Pb sites, consistent with Pb-O coordination as the dominant anchoring motif. Charge-density-difference analysis and atom-resolved charge partitioning reveal substantial interfacial charge redistribution, while local density-of-states calculations show adsorption-induced hybridization near the valence-band edge without introducing localized midgap states. CEPA also produces larger adsorption-induced interfacial dipoles, raising the (VBM-referenced) work-function upper bound to values above 5.9 eV. In general, these results support phosphonic acidsparticularly CEPAas effective molecular passivators that stabilize MAPbI₃ surfaces and tune interfacial energetics relevant to perovskite optoelectronic devices.