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Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

Alexander J. Gillett, Claire Tonnelé, Giacomo Londi, Gaetano Ricci, Manon Catherin, Darcy M. L. Unson, David Casanova, Frédéric Castet, Yoann Olivier, Weimin M. Chen, Elena Zaborova, Emrys Evans, Bluebell H. Drummond, Patrick J. Conaghan, Lin-Song Cui, Neil C. Greenham, Yuttapoom Puttisong, Frédéric Fages, David Beljonne, Richard H. Friend

Nature Communications, Volume: 12, Issue: 1, Start page: 6640

Swansea University Author: Emrys Evans

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Abstract

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a stron...

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Published in: Nature Communications
ISSN: 2041-1723
Published: Springer Science and Business Media LLC 2021
Online Access: Check full text

URI: https://cronfa.swan.ac.uk/Record/cronfa58730
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Abstract: Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm−1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters.
College: College of Science
Funders: EC | EU Framework Programme for Research and Innovation H2020 | H2020 Priority Excellent Science | H2020 European Research Council (H2020 Excellent Science - European Research Council) Grant: 670405 Identifier: doi https://doi.org/10.13039/100010663 RCUK | Engineering and Physical Sciences Research Council (EPSRC) Grant: EP/M01083X/1 Grant: EP/M005143/1 Identifier: doi https://doi.org/10.13039/501100000266 Simons Foundation Grant: 601946 Identifier: doi https://doi.org/10.13039/100000893
Issue: 1
Start Page: 6640