Addressing the Charge Carrier Trapping Mechanism in CsPbBr₃ NCs

Perovskite nanocrystals (PNC) have been studied in recent years because of their interesting optical properties, which include high photoluminescence quantum yield (PLQY). However, despite the large number of reports on the effects of quantum confinement on the optical and electronic properties in PNCs, the literature still lacks information regarding the size dependence of the charge carrier trapping rate. To solve this puzzle, we investigate exciton dynamics for PNCs, with sizes ranging from 5 to 12 nm, using transient absorption spectroscopy (fs-TAS) and time-resolved photoluminescence (TR-PL) measurements. By combining both techniques, we were able to cover the dynamics with six orders of magnitude in time. Charge carrier trapping was observed to occur only on the nanosecond timescale. By considering the charge carrier trapping, and radiative recombination rates, we were able to quantify PLQY for these PNCs. The results show that PLQY increases with PNC size, which can be due to the reduction of the surface-volume ratio. We also observed that after carrier trapping and excitonic recombination, the PL decay is dominated by a very slow decay emission channel, which is related to detrapping. Our findings open new routes to take charge-carrier trapping mechanisms into account and precisely measure the PLQY of nano-structured materials and evaluate the effectiveness of surface passivation methods as well as investigate the basic phenomena in such materials.<br/><b>Acknowledgments:</b><br/>The authors gratefully acknowledge the financial support from the São Paulo Research Foundation (FAPESP).