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 the stability and photoluminescence of these materials for further optoelectronic applications. Many ligands have been examined for surface passivation; however, a ligand design principle for improved photoluminescence quantum yield (PLQY) is still not available. Here, we report a combined computational and experimental study to systematically investigate 27 commercially available ligands and develop foundational guidelines. Using first-principles density functional theory, we calculated the binding energy of the ligands on the CsPbBr₃ nanocrystal. We find a volcano relationship between ligand binding energy and the experimental PLQY, which reveals the negative impact of overly strong binding energy. We further perform electronic structure analysis and time-resolved optical spectroscopy to reveal that these strong-binding ligands can withdraw more electrons from the surface and induce trap states within the bandgap. With this, we develop a design principle for the PLQY of CsPbBr₃ nanocrystals, highlighting the importance of the ligand binding energy comparable to that of the native halide species. We further applied this design principle to quantum-confined CsPbCl₃ and CsPbI₃ nanocrystals, and our computational predictions have been successfully validated by experiments.