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Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors
Nature Communications, Volume: 12, Issue: 1, Start page: 6640
Swansea University Author: Emrys Evans
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DOI (Published version): 10.1038/s41467-021-26689-8
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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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.
Faculty of Science and Engineering
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