Thermally Activated Exciton Transition Induced Long Photoluminescence Lifetime in Large CsPbBr ₃ Nanocrystals

Large CsPbBr₃ nanocrystals show unique photoluminescence properties of an ultralong lifetime, supernarrow line width, and high exciton efficiency; however, the mechanism of the ultralong photoluminescence lifetime is still not clear. In this work, we investigated the size- and temperature-dependent photoluminescence properties of CsPbBr₃ nanocrystals with steady and transient photoluminescence spectra. At cryogenic temperature, small CsPbBr₃ nanocrystals (8.6 and 10.6 nm) show a nearly single-peak emission of confined excitons, while large CsPbBr₃ nanocrystals (12.9, 17.5, and 25.8 nm) exhibit two band emissions with a short component lifetime (several nanoseconds) of free excitons and a long component lifetime (hundreds of nanoseconds) of bound excitons. For large CsPbBr₃ nanocrystals, the ratio of photoluminescence emission between bound and free excitons varied from 7:3 to 3:7 with increasing temperature. Based on the experimental results, we propose a mechanism of thermally activated transition between bound and free excitons to explain the ultralong photoluminescence lifetime in large CsPbBr₃ nanocrystals. In all, this understanding may exploit these thermally activated transition effects to design advanced quantum emitters.