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.