Dimerization of Organic Cations Triggered by Self-Interstitials Enhances the Stability and Carrier Lifetime of FAPbI 3 Perovskites

Intrinsic structural instability and nonradiative recombination limit the long-term performance of formamidinium lead iodide (FAPbI3). Using ab initio nonadiabatic molecular dynamics, we identify a novel FA dimerization mechanism and its impact on structural stability and carrier dynamics. FA dimerization proceeds via a self-interstitial mechanism and formation of noncovalent parallel dimers within a single A-site cavity. This configuration strengthens hydrogen bonding with surrounding [PbI6]4– octahedra, promotes octahedral tilting, suppresses low-frequency Pb–I vibrations, and reduces nonadiabatic coupling, resulting in ∼50% longer carrier lifetimes relative to that of the nondimerized system. In contrast, FA deprotonation generates deep trap states that dramatically accelerate nonradiative recombination. Importantly, FA dimerization in defective lattices activates nitrogen lone pairs, inducing C–C covalent and N–Pb coordination bonding. These interactions eliminate midgap states, restore hydrogen bonding, suppress dynamic disorder, and reestablish long carrier lifetimes. Together, these results establish FA dimerization as an intrinsic stabilization and defect-passivation mechanism in perovskite optoelectronic materials.