Luminescence and Stability Enhancement of Inorganic Perovskite Nanocrystals via Selective Surface Ligand Binding

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as one of the most promising light-emitting materials for optoelectronic devices with outstanding performance. However, the facile detachment of surface capping organic ligands from these NCs leads to very poor colloidal stability and durability. This is mainly due to the weak interfacial interactions between the inorganic perovskite core and ligands, high density of surface defect states, and aggregation of NCs. Here, using a combination of time-resolved laser spectroscopy and density functional theory (DFT) calculations, we explored the major impact of surface orientations and terminations for both CsPbBr3 and Cs4PbBr6 NCs not only on the interfacial binding affinities with organic ligands but also on surface defect formation and NC aggregation. By rationalizing that surface trap states are responsible for the decrease in photoluminescence (PL) upon fabrication and purification, we propose a powerful ligand-engineering strategy for eliminating these trap states and preventing the aggregation of CsPbBr3 and Cs4PbBr6 NCs. Interestingly, we find that the surface orientation and dimensionality determine the degree of interfacial interactions between the inorganic perovskite core and ligands and subsequently control the overall PL intensity and NC stability. Our results demonstrate that a treatment of as-synthesized CsPbBr3 NCs consisting of the addition of extra oleylammonium bromide (OAmBr) as a capping ligand, allows the CsPbBr3 NCs to retain their green emission with increased PL intensity and quantum yields and improves colloidal durability. On the other hand, the ultraviolet emissions of Cs4PbBr6 NCs are effectively increased upon addition of extra cesium oleate (CsOL) as the trap states induced by surface cesium ions are largely reduced by the formation of Cs-O bonds. Our work provides a robust and adequate ligand engineering approach to significantly enhance the optical behavior of perovskite NCs with different dimensionalities and various compositions and to achieve more efficient and stable light-harvesting devices.