Advancing organohalide perovskite solar cells requires understanding of carrier dynamics. Electron–hole recombination is a particularly important process because it constitutes a major pathway of energy and current losses. Grain boundaries (GBs) are common in methylammonium lead iodine CH₃NH₃PbI₃ (MAPbI₃) perovskite polycrystalline films. First-principles calculations have suggested that GBs have little effect on the recombination; however, experiments defy this prediction. Using nonadiabatic (NA) molecular dynamics combined with time-domain density functional theory, we show that GBs notably accelerate the electron–hole recombination in MAPbI₃. First, GBs enhance the electron–phonon NA coupling by localizing and contributing to the electron and hole wave functions and by creating additional phonon modes that couple to the electronic degrees of freedom. Second, GBs decrease the MAPbI₃ bandgap, reducing the number of vibrational quanta needed to accommodate the electronic energy loss. Third, the phonon-induced loss of electronic coherence remains largely unchanged and not accelerated, as one may expect from increased electron–phonon coupling. Further, replacing iodines by chlorines at GBs reduces the electron–hole recombination. By pushing the highest occupied molecular orbital (HOMO) density away from the boundary, chlorines restore the NA coupling close to the value observed in pristine MAPbI₃. By introducing higher-frequency phonons and increasing fluctuation of the electronic gap, chlorines shorten electronic coherence. Both factors compete successfully with the reduced bandgap relative to pristine MAPbI₃ and favor long excited-state lifetimes. The simulations show excellent agreement with experiment and characterize how GBs and chlorine dopants affect electron–hole recombination in perovskite solar cells. The simulations suggest a route to increased photon-to-electron conversion efficiencies through rational GB passivation.

We present a time-domain ab initio study of electron–hole recombination in pristine MAPbI₃, and compare it to the trap mediated recombination in MAPbI₃ with the iodine interstitial defect. Nonadiabatic molecular dynamics combined with time-domain density functional theory show that the iodine interstitial defect creates a subgap state capable of trapping both electrons and holes. Hole trapping occurs much faster than electron trapping or electron–hole recombination. The trapped hole survives for hundreds of nanoseconds, because, rather surprisingly, recombination of electrons with the trapped hole takes several times longer than recombination of electrons with holes in the valence band. Because the hole trap is relatively shallow, the hole can escape into the valence band prior to recombining with the electron. The differences are rationalized by variation in nonadiabatic electron–phonon couplings, phonon-induced pure-dephasing times and electronic energy gaps. The time-domain atomistic simulations contribute to understanding of the experimentally known defect-tolerance of perovskite solar cells, which is of great importance to the solar cell performance.