Semiconductor quantum dots (QDs) are prime candidates as harvesters of light and donors of excited charge carriers for solar energy conversion. The unique properties of QDs can be exploited to generate desirable energetic offsets to promote interfacial charge transfer between QDs. Our group’s recent efforts have established the validity of utilizing carbodiimide-mediated coupling chemistry to selectively tether two QDs through the formation of an amide bond between the terminal functional groups of capping ligands [1]. We previously reported on excited-state hole transfer in colloidal CdS/CdSe QD heterostructures, which exhibit quasi-type-I interfacial energetic offsets [1]. Type-I energetics rely on the excitation of one QD component, which results in unidirectional charge transfer and hindered charge separation.

This presentation reports on our efforts to improve and expand upon our previous work in two ways. First, we synthesized and characterized covalently tethered colloidal CdSe/CdTe QD heterostructures via formation of amide bonds. These heterostructures exhibit type-II energetics that promote interfacial charge separation, irrespective of which constituent QD is initially excited, and afford enhanced control over the thermodynamic driving forces for charge transfer. Within these heterostructures, photogenerated electrons are transferred from CdTe to CdSe, and photogenerated holes are transferred from CdSe to CdTe, on timescales of 10^-8 s. Second, we prepared ternary CdSe/CdTe heterostructures by immobilizing a covalently-linked bilayer of these QDs on a metal oxide substrate. When compared to colloidal heterostructures, thin films consisting of QDs adsorbed to a metal oxide substrate, introduce the possibility of an additional stepwise excited-state charge transfer process. We hypothesized that a stepwise process such as this should facilitate extended spatial separation of charge carriers and longer charge-separated state lifetimes, such that energy is harvested more efficiently and desirable processes can more effectively compete with recombination. Dynamic quenching of emission was observed in heterostructure-modified thin films, consistent with excited-state charge transfer. Rate constants for photoinduced electron and hole transfer between QDs are on the order of 10⁸ s^-1 and 10⁷ s^-1_, respectively.

The bidirectional interfacial charge transfer within these type-II QD heterostructures, both in dispersion and within films, further reveals the potential of this system for use in light harvesting and solar energy conversion. This presentation will highlight these recent results as well as our ongoing time-resolved spectroscopic characterization of photoinduced charge transfer in CdSe/CdTe QD heterostructures.