Tailoring Hot Carrier Cooling Dynamics via Targeted Vacancy Design in FAPbI₃ for High-Efficiency Photovoltaics

Hot carrier (HC) cooling represents a dominant nonradiative loss pathway that ultimately constrains the efficiency of perovskite solar cells (PSCs). Despite the ubiquity of intrinsic vacancy defects in halide perovskites, their mechanistic influence on HC relaxation dynamics has remained elusive and is often overlooked, largely because the ultrafast time scales, intricate defect-phonon interactions, and subtle band-edge perturbations make these effects difficult to isolate and quantify. Here, first-principles calculations coupled with nonadiabatic molecular dynamics (NAMD) are employed to systematically assess how intrinsic vacancy defects affect HC cooling behavior in FAPbI₃. We demonstrate that while vacancy defects significantly modulate the bandgap of FAPbI₃, the carrier relaxation rate does not scale directly with the gap magnitude. Notably, iodine and formamidinium vacancies selectively hinder the cooling of hot electrons and hot holes, respectively. This defect-mediated suppression stems from two synergistic mechanisms: weakened electron-phonon (e-ph) coupling and a transition of carrier relaxation pathways from fast, direct relaxation to slower, stepwise processes. These effects synergistically prolong the HC lifetimes and mitigate energy dissipation during the cooling process. Our findings establish a defect-type-specific framework for tuning HC dynamics and highlight defect engineering as a powerful strategy to enhance hot carrier utilization in next-generation high-efficiency photovoltaic devices.