Long Carrier Lifetimes in PbI₂-Rich Perovskites Rationalized by Ab Initio Nonadiabatic Molecular Dynamics

Hybrid organic-inorganic perovskites have attracted considerable interest due to their impressive performance in solar energy applications. Many experiments show that a slight excess of PbI₂ significantly enhances the properties of the most studied CH₃NH₃PbI₃ compound. We use real-time time-dependent density functional theory and nonadiabatic molecular dynamics to demonstrate that the effect arises due to decreased electron-phonon interactions responsible for nonradiative charge recombination. The fast organic CH₃NH₃ ⁺ (MA) cations, present on surfaces of stochiometric and MAI-rich perovskites, are particularly mobile and introduce high-frequency phonons and strong electric fields that couple to the charge carriers and create large nonadiabatic coupling. Excess PbI₂ decreases MA surface coverage, reduces the nonadiabatic coupling by up to an order of magnitude, and extends the charge carrier lifetime. Generally, charges in perovskites are long-lived because the nonadiabatic coupling is very small, less than 1 meV, and quantum coherence formed during charge recombination is short, less than 10 fs. Our results rationalize why decreasing the concentration of the organic cations on perovskite surfaces can suppress nonradiative charge carrier recombination and improve material performance.