Charge Trapping and Detrapping in CsPbBr<sub>3</sub> Perovskite Nanocrystals: Implications for Photovoltaic and Photocatalysis\nApplications

Perovskite nanocrystals (PNCs) have\nemerged as a promising platform\nfor the development of highly efficient nanomaterial-based lighting\ndevices. Although experiments and calculations have shown that defect\ndensities are on the order of 10<sup>11</sup>–10<sup>16</sup> cm<sup>–3</sup> in bulk perovskites, this is not true for\nnanomaterials because they present higher defect densities. To understand\nthe origin and relative contribution of those defects, we employed\ntransient absorption spectroscopy and time-resolved photoluminescence.\nWe scrutinize the exciton dynamics in PNCs for over 7 orders of magnitude\nin time, and we conclude that, in the ensemble of CsPbBr<sub>3</sub> PNCs, excited carriers can take different paths to return to the\nfundamental level: electron traps (hundreds of picoseconds), surface\ntraps (2–5 ns), direct radiative recombination (∼10\nns), and delayed emission. Our measurements revealed that the excitonic\ncomponent of PNCs tends to decrease with increasing nanocrystal (NC)\nsize. On the other hand, we observed that the electron trap decay\namplitude correlates with the relative delayed emission contribution,\nsuggesting that the recombination of detrapped species comes from\nelectron-trapping sites. Besides, the relative contribution of delayed\nemission is size-dependent and increases with the NC size, achieving\nabout 80% of reversibility. Although the excitonic contribution of\nlarger NCs is lower compared to the smaller ones, the results suggest\nthat electron traps are reversible and do not decrease the photoluminescence\nquantum yield of the NCs. These results can be useful for applications\nthat involve charge-carrier extraction, such as photovoltaics and\nphotocatalysis.