Effect of Intramolecular Geometry in Phosphonic Acid on Surface Passivation and Lead Immobilization in Perovskite Solar Cells

Post-treatment with lead halide perovskite films effectively passivates surface defects and immobilizes lead. We report how the intramolecular geometry of phosphonic acids influences photovoltaic performance and lead retention in perovskite solar cells (PSCs) using phenyl phosphonic acid (PPA) and benzyl phosphonic acid (BPA). Although the phenyl group in PPA possesses a weak electron-withdrawing nature compared to the moderately electron-donating benzyl group, PPA exhibits stronger interactions between its phosphonic anchoring group and undercoordinated Pb²⁺ through coordination bonding, as well as with iodide via hydrogen bonding. These effects originate from the higher dipole moment of PPA, resulting from the collinear alignment between the phenyl ring and phosphonic group. Consequently, PPA-treated perovskite films show longer carrier lifetimes and lower trap densities than the BPA-treated ones. The power conversion efficiency increases from 22.75% to 24.61% after PPA treatment, and Pb²⁺ leakage is markedly reduced owing to stronger PPA-perovskite interactions.