Accurately characterizing the energy level alignment of the excited state of semiconductor quantum dots (QDs) and ligands is a prerequisite for using QDs as an absorber in photovoltaic devices. The ligands can act as either electron or hole scavengers and thus are significant players in terms of optimizing charge injection and transfer processes to increase photo-induced current in these devices. In addition, a better understanding of the surface interaction between QDs and ligands and its relation to surface chemistry will allow us to have greater control of photophysical properties. In this report, we aim to study interfacial interactions as they affect the optical and electronic properties between QDs and 4 different ligands, which have a variety of energetic offsets with respect to the QD. Specifically, tri-n-octylphosphine oxide and hexadecylamine capping CdSe QDs are replaced by 4 different short thiol and amine ligands by ligand exchange treatment and the corresponding efficiency of the ligand exchange processes of the different linkers is about 50-55%, as confirmed by FTIR. Using steady-state and time-resolved photoluminescence (PL) optical spectroscopy, we have studied the effects of ligand choice on PL emission peak shift and PL quenching; the corresponding rates of photo-induced filling of traps and emission decay kinetics are found to depend on the new band offsets from the CdSe QD-ligand complex. The predicted influence of energy level offsets of the ligands on charge separation is reflected in our optical data. These data provide insight into the utility of each ligand for our QD-heterostructure systems.

Effect of energy band offsets by various ligands on the photophysical properties of CdSe quantum dots

247th National Meeting (2014)

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