A Conical Intersection Perspective on the Low Nonradiative Recombination Rate in Lead Halide Perovskites

The utility of optoelectronic materials can be greatly reduced by the presence of efficient pathways for nonradiative recombination (NRR). Lead halide perovskites have garnered much attention in recent years as materials for solar energy conversion, because they readily absorb visible light, are easy to synthesize, and have a low propensity for NRR. Here we report a theoretical study of the pathways for NRR in an archetypal lead halide perovskite: CsPbBr₃. Specifically, we identified a set of conical intersection (CIs) in both a molecule-sized cluster model (Cs₄PbBr₆) and nanoparticle model (Cs₁₂Pb₄Br₂₀) of the CsPbBr₃ surface. The energies of the minimal energy CIs, corrected for both dynamical electron correlation and spin-orbit coupling, are well above the bulk band gap of CsPbBr₃, suggesting that these intersections do not provide efficient pathways for NRR in this material. Analysis of the electronic structure at these intersections suggests that the ionic nature of the bonds in CsPbBr₃ may play a role in the high energy of these CIs. The lowest-energy intersections all involve charge transfer over long distances, whether it be across a dissociated bond or between neighboring unit cells.