Measurements and modelling of the kinetics of thermally activated delayed fluorescence

Abstract

Efficient organic light-emitting diodes (OLEDs) enable the commercialisation of energy efficient and high contrast displays, and are being exploited increasingly in telecommunication and biomedical applications. In an efficient OLED all electrically generated singlet and triplet excitons recombine to emit light. In recent years, the exploitation of emitters that show thermally activated delayed fluorescence (TADF) has proven to be a complementary strategy to phosphors that leads to devices of comparable efficiencies without relying on scarce metals in the emitter design.

In this thesis, the kinetics of TADF are explored. First, two families of novel emitters are discussed by investigating their photophysics. One series incorporates a novel pyridazine acceptor whilst the other series makes use of a through-space conjugated donor design using a [2,2]paracyclophane. In both families of emitters, the emitter with the largest donor strength showed efficient triplet up-conversion via TADF in OLEDs.

The kinetics of TADF can be calculated from a bimolecular fit of the transient photo- luminescence (PL). However, in the literature, different assumptions are used to calculate rate constants from these data. Therefore, a three-level model was analysed to understand the limiting cases employed in literature.

The transient PL of TADF is characterised by two decay regimes that stretch over multiple orders of magnitude in time and intensity. This makes recording both regimes in one measurement challenging. To solve this, first improvements and implementations of existing techniques are discussed. In order to remove the need to measure over a large dynamic range of intensities a novel measurement technique was developed which allows the extraction of kinetic parameters of TADF by recording the emission during a step-function excitation.

Finally, a figure of merit based on the TADF kinetics under constant electrical excitation was developed to evaluate new TADF emitters for use in OLED with low efficiency roll-off.

Date of Award	29 Nov 2023
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Ifor David William Samuel (Supervisor) & Eli Zysman-Colman (Supervisor)