Control of Charge Carrier Separation and Recombination in Quantum Dots by Ligand Binding Mode: Time-Domain Ab Initio Study

Point defects and nonstoichiometric surfaces are common in quantum dots (QDs). They influence the QD performance in optoelectronic devices by trapping charges, creating charge-separated states, and altering charge carrier lifetimes. We demonstrate by ab initio quantum dynamics simulation that the location of ligands relative to defect sites has a strong quantitative and even qualitative influence on the charge carrier dynamics. Considering a Cd vacancy in a CdS QD and a phenothiazine (PTZ) ligand, we show that when the ligand is located far from the vacancy, photogenerated holes are transiently trapped by the vacancy and transfer to PTZ. This delays the formation of the charge-separated state (CdS–-PTZ+). However, the charge-separated state is long-lived. In comparison, if the Cd vacancy and the PTZ molecule are in proximity of each other, the molecule passivates the defect. The charge separation becomes much less pronounced, and the charge recombination is accelerated even compared with the pristine CdS QD. The former situation is favorable for solar energy and catalytic applications of QDs, while the latter is advantageous for light emission. We suggest that two types of ligands may be required in general: one passivating the defect and the other acting as a charge acceptor. The atomistic description of how the photoinduced dynamics of charge carriers in QD-ligand complexes is influenced by defect and ligand location and can efficiently compete with electron–phonon energy dissipation, even in the presence of defects, suggests strategies for tuning QD performance through defect and ligand engineering.