Investigation of carrier dynamics of QDs using kinetic model and ultrafast spectroscopy

The study of carrier dynamics of QDs is extremely important for developing efficient light-harvesting systems. Here, we investigate size-dependent hole transfer from CdSe QDs to 4-aminothiophenol (4-ATPh) ligand using ultrafast spectroscopy. The photoluminescence (PL) quenching is found to be 99% and 77% for 4-ATPh ligand capped 2.9 nm and 4.3 nm QDs, respectively. We propose a stochastic model for analyzing time-resolved fluorescence decay curves of QDs to estimate the average number of ligands around QDs and it is found that the larger number of ligands are attached with smaller QDs. The analysis of TA spectroscopy data reveals that the kinetics of hole transfer from 2.9 nm QDs to ligand is faster than 4.3 nm CdSe QDs, depending on the offset between valence band (VB) of the CdSe QD and the HOMO of 4-ATPh ligands, and the average number of 4-ATPh ligands attached to each CdSe QD. The fundamental study of ligand-induced charge transfer processes in QDs is important for QD-based solar cell applications. • Quantum dot (QD)-based light-harvesting systems are found to be potentially interesting for solar cell and optoelectronic device applications because of efficient charge carrier generation right after photoexcitation. • We have investigated the ligand-induced hole transfer process from QDs to ligands which depends on the size of QDs and the number of ligands on the surface of QDs using steady-state and time-resolved ultrafast spectroscopic techniques. • A stochastic kinetic model is proposed to understand the influence of the size of QDs on the number of ligands and trap states. • The analysis of TA spectroscopy data reveal that the bleaching recovery dynamics of the smaller sized CdSe QDs becomes slower in presence of the hole-withdrawing ligand, 4-ATPh. • The fundamental understanding of ligand-induced charge transfer processes in QDs is important for QD-based solar cell applications.