Multi-exciton dissociation (MED) is an important charge multiplication mechanism for quantum dot (QD) based energy applications. To achieve high efficiency via MED process, charge transfer rates need to outcompete the ultrafast exciton annihilation rates. In model systems composed of QDs and molecular acceptors, the electron transfer dynamics in MED process have been well studied, whereas measurements and knowledge on the much slower hole transfer dynamics are insufficient. Here we have designed a QD-dye system where a new redox-active dye anchored on the QD surface is utilized in transient absorption measurements to monitor the hole transfer kinetics from a QD to its ligand shell. The hole transfer efficiencies and kinetics in CdSe, CdS, and CdSe@CdS quantum dots with different Auger recombination rates, driving forces and the electronic couplings between ligands and QDs were investigated and compared. It is revealed that the hole trap states near the valence band (VB) edge play a vital role in the competition between hole transfer kinetics and the fast Auger recombination dynamics. The observed kinetics can be modelled well by a two-stage trap-mediated transfer mechanism in which forward and backward transfer between VB, trap states and ligand levels are considered. The results here highlight the beneficial role of trap states in engineering hole transfer pathways in QD-based systems under multi-excitonic conditions.

Probing multi-excitonic hole transfer dynamics in surface-functionalized quantum dots with ultrafast transient absorption spectroscopy

ACS Fall 2020 Virtual Meeting

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