Interplay between Localized and Free Charge Carriers Can Explain Hot Fluorescence in the CH₃NH₃PbBr₃ Perovskite:...

Delayed high-energy fluorescence observed experimentally in methylammonium lead bromine CH₃NH₃PbBr₃ (MAPbBr₃) demonstrates long-lived energetic charge carriers with extremely high mobilities that can be used to enhance photon-to-electron conversion efficiency of perovskite solar cells. It has been suggested that hot fluorescence is associated with reorientational motions of the MA molecules. We support this hypothesis by time-domain ab initio quantum dynamics calculations showing that reorientation of the MA molecules can affect strongly the perovskite emission energy and lifetime. We demonstrate MAPbBr₃ structures differing in the MA orientations and exhibiting the same emission properties as in the experiments. The higher bandgap structures responsible for hot fluorescence support delocalized wave functions that can be interpreted as free charge carriers. The lower energy structures exhibit localized polaron-like electrons and holes, and a significantly longer electron-hole recombination time, in agreement with experiment. The fluorescence lifetimes differ owing to variation in the nonadiabatic coupling between the emitting and ground states, stemming from charge carrier localization. Loss of coherence due to elastic electron-phonon scattering is similar in the two cases. The simulations provide a detailed atomistic understanding of excited-state dynamics in MAPbBr₃ and show how structural transformations can rationalize the experimentally reported hot fluorescence in MAPbBr₃. Other localized structures involving inorganic lattice distortions, defects, domain boundaries, ion diffusion, electric ordering, etc., can be invoked with the proposed two-emitter interpretation of hot and regular luminescence.