Suppression of Thermally Assisted Photoluminescence Quenching in CsPbBr3 Nanocrystals via Surface Engineering: Implications for Optoelectronic Devices

Lead halide perovskite (LHP) nanocrystals (NCs) are witnessing tremendous success in LED and display technology applications for their defect tolerance properties and near-unity photoluminescence quantum yield (PLQY). Despite this success, thermally assisted PL quenching in LHP NCs is a major obstacle during the operation of devices at a higher temperature. To overcome this obstacle, we have introduced an effective washing-assisted short-chain capping ligand engineering by 4-fluorobenzylammonium bromide (4FBABr) for the first time to obtain the CPB-4FBABr NC, which is simple and cost-effective. We observed a retention of about 74% PL intensity for the CPB-4FBABr NC at 373 K compared to its PL intensity at 283 K, whereas for the CPB-parent NC, only 2% of the initial PL intensity was found to be retained in the same temperature range. We argued the importance of optimum washing prior to surface engineering in this report. We also highlighted that the F-atom in 4FBABr enhances the positive charge density on the N atom of the ammonium headgroup, facilitating exceptional ligand-NC binding, while the bromide ion eliminates detrimental halide vacancy-related trap states. These two factors synergistically enhanced the heat tolerance of the engineered NC. Time-resolved PL (TRPL) measurements shed more light on the charge carrier dynamics, where shallow trap state-mediated charge carrier trapping and detrapping in the engineered NC reduced the permanent loss of excitons under extreme thermal conditions. An insight into reversibility during the cooling cycle (PL recovery by lowering the temperature) has also been established by TRPL measurements. Our findings underscore the critical role of washing before postsynthetic treatment and the effectiveness of using short-chain fluorinated aryl ammonium ligands in suppressing the thermally assisted PL quenching. This investigation will be highly beneficial to fabricating high-temperature stable perovskite-based LEDs.