Design Principles for Surface-Passivating Ligands of Cesium Lead Halide Perovskite Nanocrystals in the Strongly Quantum-Confined Regime

Passivation of surface defects of cesium lead halide (CsPbX₃, X = Cl, Br, I) nanocrystals is crucial to improving their stability and photoluminescence for further optoelectronic applications. Many ligands have been examined for surface passivation; however, a fundamental understanding of the origin of improved photoluminescence quantum yield (PLQY) is still not available. Here, we systematically investigated various commercially available ligands with combined computational and experimental studies. We observe a volcano trend between the calculated ligand binding energy and the experimental PLQY for CsPbBr₃ nanocrystals in the quantum-confined regime, highlighting the potential deleterious effect of strongly bound ligands compared to those with similar binding energy to the halide species. Further electronic structure analysis reveals that the strongly bound ligands are limited by the charge distributions at the interface. With this, we establish a two-parameter descriptor for PLQY, comprising binding energy and Bader charge of the ligands. We further applied this descriptor to CsPbCl₃ and CsPbI₃ nanocrystals, and our computational predictions on photoluminescence were successfully validated by the experiments.