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Science and Engineering of Spin Triplet Excitons in Luminescent Molecular Semiconductors


Type

Thesis

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Authors

Abstract

Luminescent molecular semiconductors exhibit many properties that are attractive for optoelectronic devices, such as organic light-emitting diodes (OLEDs). Maximising the luminescence from OLEDs requires careful management of spin-triplet excitons, which are typically non-emissive. For successful management, the formation and harvesting dynamics of triplet excitons must be probed directly. In this thesis, we employ optical and spin-sensitive probes to reveal the dynamics of triplet excitons in a selection of luminescent molecular semiconductors. The findings and frameworks developed in this thesis allow for informed engineering of triplet excitons in optoelectronic devices.

After introducing the theoretical and experimental background relevant to the conversion of non-emissive triplet excitons to emissive singlet excitons, we present an optical study of thermally activated delayed fluorescence (TADF) in the near infrared emitter CAT-1. We demonstrate TADF at a lower energy than had before been realised, verifying a simple molecular design concept that pushes the boundary of TADF.

We then begin spin-sensitive studies of TADF using transient electron spin resonance (trESR). We directly resolve the interaction of multiple triplet excitons in the spin conversion process, revealing the spin-vibronic mechanism of TADF in isolated molecules in dilute solution. To probe the operational environment of OLEDs, we next perform trESR on TADF thin films, finding that intermolecular interactions give rise to the involvement of additional triplet excitons in the spin conversion process.

Finally, we investigate an alternative method of enhancing spin conversion in luminescent molecular semiconductors: through radical--chromophore interactions. Optical and trESR studies demonstrate that an organic radical can enhance the rate of conversion ten-fold whilst also producing well-defined electron spin polarisation. Controlling spin polarisation has wider implications for quantum information science.

Description

Date

2022-06-29

Advisors

Friend, Richard
Bronstein, Hugo

Keywords

TADF, triplet exciton, thermally activated delayed fluorescence, electron spin resonance, transient electron spin resonance, organic semiconductors, molecular electronics, optoelectronics

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
EPSRC (1948660)
Engineering and Physical Sciences Research Council (1948660)
Leathersellers' Graduate Scholarship (Fitzwilliam College) E D Davies Scholarship (Fitzwilliam College)

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