Phosphonic Acid Molecular Regulation of Frenkel Defects for Suppressing Nonradiative Recombination in FAPbI3 Perovskites

Surface Frenkel defects are an important source of nonradiative carrier losses in formamidinium lead iodide (FAPbI3) perovskites; however, the microscopic mechanisms by which molecular geometry regulates defect-mediated excited-state dynamics remain poorly understood. Here, we combine first-principles calculations with nonadiabatic molecular dynamics to investigate the influence of benzylphosphonic acid (BPA) and phenylphosphonic acid (PPA) on the electronic structure, quantum coherence, and carrier recombination associated with Frenkel defects on the FAPbI3(100) surface. An integrated PYXAID-SDM framework incorporating fewest-switches surface hopping (FSSH) simplified decay of mixing (SDM), and decoherence-induced surface hopping (DISH) is employed to evaluate the influence of different decoherence treatments on carrier dynamics. Both phosphonic acids reconstruct the local Pb-I coordination environment through strong Pb-O coordination and hydrogen-bond interactions, reducing nonadiabatic coupling while simultaneously accelerating electronic decoherence. Consequently, nonradiative electron-hole recombination is significantly suppressed, with PPA consistently producing the weakest nonadiabatic coupling and the longest carrier lifetime among all systems investigated. Climbing-image nudged elastic band calculations further demonstrate that molecular adsorption markedly increases the Frenkel-defect migration barrier, indicating enhanced kinetic stability. These results reveal how molecular geometry simultaneously regulates defect stability and excited-state carrier dynamics, providing atomistic insights into rational molecular design for suppressing nonradiative recombination in metal halide perovskites.