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D-A3 TADF Emitters: The Role of the Density of States for Achieving Faster Triplet Harvesting Rates

Lookup NU author(s): Dr Julien Eng, Dr Jerry Hagon, Dr Thomas Penfold

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This is the authors' accepted manuscript of an article that has been published in its final definitive form by Royal Society of Chemistry, 2019.

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Abstract

The triplet harvesting rate in purely organic materials based upon Thermally Activated Delayed Fluorescence (TADF) is critically important for achieving high performance Organic Light Emitting Diodes (OLEDs). One of the major challenges that must be overcome in these materials is the weak spin orbit coupling which can lead to triplet harvesting rates that extend into the millisecond range. This causes poor roll-off in device efficiency at higher current densities. In this work, we study the excited state properties of a new TADF design strategy based upon the D-A3 approach. Using TAT-3DBTO2, composed of a rigid triazatruxene donor core with three dibenzothiophene-S,S-dioxide peripheral acceptors, we explain how exploiting the high density of excited states and low lying conical intersections associated with this structure can achieve high-rates of triplet harvesting. However, we also demonstrate that excited state structural changes reduce the sym- metry of the molecule and work against high triplet harvesting rates by causing localisation of the excited state electronic structure and non-equivalence between the acceptors. Consequently, the fast initial reverse intersystem crossing (rISC) channels are replaced by undesirable slower rISC channels. Our results imply that it will be possible to engineer molecules where undesirable decay pathways are removed giving new perspectives for designing functional TADF molecules.


Publication metadata

Author(s): Eng J, Hagon J, Penfold TJ

Publication type: Article

Publication status: Published

Journal: Journal of Materials Chemistry C

Year: 2019

Pages: ePub ahead of print

Print publication date: 09/07/2019

Online publication date: 09/07/2019

Acceptance date: 09/07/2019

Date deposited: 10/07/2019

ISSN (print): 2050-7526

ISSN (electronic): 2050-7534

Publisher: Royal Society of Chemistry

URL: https://doi.org/10.1039/C9TC02561H

DOI: 10.1039/C9TC02561H


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