Influence of Exciton Diffusivity and Nanocrystal Size on Carrier Dynamics in CsPbBr3 Perovskites

Halogenated solvents like chloroform and dichloromethane are known electron acceptors of photoexcited CsPbBr₃ perovskite nanocrystals. However, CsPbBr₃ perovskites possess moderately good emission yields in these electron-accepting solvents in contrast to those of normal semiconductors. Herein, we investigate the reason for the retention of the significant emission yield of CsPbBr₃ in chloroform. For this purpose, we synthesize CsPbBr₃ of three different sizes, and the emission yields are measured in an accepting solvent, chloroform, and compared with the emission in a nonelectron-accepting solvent, hexane. Interestingly, the change in the emission quantum yield and lifetime is maximum for smaller-sized CsPbBr₃ crystals, and the change decreases with an increase in size. During photoexcitation, excitons can be formed anywhere in the crystal, while electron transfer occurs at the surface. Before electron transfer, most excitons recombine radiatively owing to their lower exciton diffusivity and emission lifetime. Thus, in large CsPbBr₃ crystals, the majority of excitons recombine radiatively compared to the smaller-sized nanocrystals, which results in maximum emission quenching in small CsPbBr₃ crystals. The aspects discussed here can provide directions to modulate light-induced charge transfer processes, which can improve the efficiency of light energy harvesting using lead halide perovskites.