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.

Lead halide perovskite solar cells (PSCs) have shown unprecedented development in efficiency and progressed relentlessly in improving stability. All the achievements have been accompanied by diverse passivation strategies to circumvent the pervasive defects in perovskite materials, which play crucial roles in the process of charge recombination, ion migration, and component degradation. Among the tremendous efforts made to solve these issues and achieve high-performance PSCs, we classify and review both well-established and burgeoning passivation strategies to provide further guidance for the passivation protocols in PSCs, including chemical passivation to eliminate defects by the formation of chemical bonds, physical passivation to eliminate defects by strain relaxation or physical treatments, energetic passivation to improve the stability toward light and oxygen, and field-effect passivation to regulate the interfacial carrier behavior. The subtle but non-trivial consequences from various passivation strategies need advanced characterization techniques combining synchrotron-based X-ray analysis, capacitance-based measurements, spatially resolved imaging, fluorescent molecular probe, Kelvin probe force microscope, etc., to scrutinize the mechanisms. In the end, challenges and prospective research directions on advancing these passivation strategies are proposed. Judicious combinations among chemical, physical, energetic, and field-effect passivation deserve more attention for future high-efficiency and stable perovskite photovoltaics.

Ultrafast charge recombination in hematite (α-Fe₂O₃) severely limits its applications in solar energy conversion and utilization, for instance, in photoelectrochemical water splitting. We report the first time-domain ab initio study of charge relaxation dynamics in α-Fe₂O₃ with and without the oxygen vacancy (O_v) defect, using non-adiabatic molecular dynamics implemented within time-dependent density functional theory. The simulations show that the hole trapping is the rate-limiting step in the electron–hole recombination process for both neutral and ionized O_v systems. The electron trapping is fast, and the trapped electron are relatively long-lived. A similar asymmetry is found for the relaxation of free charge carriers: relaxation of photoholes in the valence band is slower than relaxation of photoelectrons in the conduction band. The slower dynamics of holes offers an advantage to water oxidation at α-Fe₂O₃ photoanodes. Notably, the neutral O_v defect accelerates significantly the charge recombination rate, by about a factor of 30 compared to the ideal lattice, due to the stronger electron-vibrational coupling at the defect. However, the recombination rate in the ionized O_v defect is decreased by a factor of 10 with respect to the neutral defect, likely due to expansion of the local iron shell around the O_v site. The O_v defect ionization in α-Fe₂O₃ photoanodes is important for increasing both electrical conductivity and charge carrier lifetimes. The simulations reproduce well the time scales for the hot carrier cooling, trapping and recombination available from transient spectroscopy experiments, and suggest two alternative mechanisms for the O_v-assisted electron–hole recombination. The study provides a detailed atomistic understanding of carrier dynamics in hematite, and rationalizes the experimentally reported activation of α-Fe₂O₃ photoanodes by incorporation of O_v defects.