Suppression of Trion Formation in CsPbBr3 Perovskite Nanocrystals by Postsynthetic Surface Modification

Lead halide perovskite nanocrystals (NCs) are one of the most anticipated and promising materials for light-emitting diodes and lasers because of their high photoluminescence quantum yields (PLQYs). However, the formation of trions (charged excitons) in the NCs reduces their PLQYs. Here, we clarify the trion formation mechanism in perovskite CsPbBr3 NCs by analyzing the excitation fluence dependence of transient absorption signals. Under weak photoexcitation, trions are formed by charge carrier trapping at surface states. In contrast, biexciton Auger recombination dominates the trion formation under strong photoexcitation. We found that the postsynthetic surface treatment suppresses the extrinsic surface-related formation of trions. The thorough understanding of the trion formation mechanisms is essential for the PLQY improvement of perovskite NCs and helps to reduce ionization of NCs in solid-state devices.