Untangling the ultrafast charge dynamics in colloidal quantum dots – charge trapping, charge transfer, and multiple exciton generation

Abstract

In this thesis the charge dynamics in novel colloidal quantum dots (QDs) are investigated using ultrafast transient absorption spectroscopy, and other complementary spectroscopic techniques, to examine the mechanisms and rates of charge carrier relaxation on sub-nanosecond timescales. A deeper understanding of specific charge relaxation processes in various QD systems is achieved, with implications for the optimisation and understanding of quantum dot applications, particularly for the enhancement of QD sensitised photovoltaics. Surface mediated charge trapping is a process which reduces the efficiency of QD based devices and complicates the analysis of QD charge dynamics. We compare the dynamics in CdTe QDs before and after surface treatment with chloride ions which we show to be capable of completely eliminating trapping, evidenced by attaining near unity photoluminescence quantum yields. This unique comparison enabled isolation of the effects of traps which has not been achieved by any other studies. Both hot and cold electron trapping processes are identified, which if unrecognised in other QD studies could lead to significant misinterpretation of results. Charge transfer in quinone conjugated CdTe/CdS QDs is investigated for a range of average acceptor to quantum dot ratios. The charge transfer process competes with radiative recombination, resulting in efficient quenching of the PL intensity with promising applications in bio-sensors. A complex behaviour in the charge dynamics is discovered which suggests a relationship between charge transfer and surface mediated charge trapping. We report the first measurement of multiple exciton generation (MEG) in quasi-Type II core/shell InP/CdS QDs. The quantum yield of MEG is found to be 1.22 ± 0.01 for an excitation photon energy equivalent to three times the band gap. This is comparable to Type I InP core/shell QDs, but the quasi-Type II structure brings several effects which have the potential to enhance device performance. The enhancement to solar cell efficiency expected from such effects are demonstrated using the Detailed Balance model.

Date of Award	31 Dec 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Wendy Flavell (Supervisor) & David Binks (Supervisor)