We have imaged the emissions from excitons and free charges in a methylammonium lead bromide perovskite (MAPbBr3) crystal. In a direct band gap semiconductor, dynamics of excited electrons and holes in hybrid lead-halide perovskites is rather complex because of the formation of excitons and the presence of traps and structural inhomogeneities. A recent report by Nah et al. (Nah, S.; Spokoyny, B.; Stoumpos, C.; Soe, C. M. M.; Kanatzidis, M.; Harel, E. Nat. Photonics 2017, 11, 285-288) has identified spatially segregated populations of excitons and free charges in perovskites. Understanding the cause of segregation and how such distributions contribute to the photoresponse has remained a challenge. Here, we have used phase-modulated two-photon photoluminescence microspectroscopy to separately quantify the emissions from excitons and free charges in a MAPbBr3 crystal. Our results show that while most of the emission at room temperature is from the recombination of free charges, there are also localized spots where the emissions from excitons dominate. By analyzing the enhancement of the emissions at low temperatures, we show that the trap-mediated nonradiative recombinations are suppressed by local deformations in the band structure. The deformations that are induced by local strains in the crystal prevent excitons and free charges from reaching the traps. Our results indicate that strain engineering through structural designs can improve the performance of perovskite light-emitting diodes.
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