Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states

Abstract

A spin-flip from a triplet to a singlet excited state, that is, reverse intersystem crossing (RISC), is an attractive route for improving light emission in organic light-emitting diodes, as shown by devices using thermally activated delayed fluorescence (TADF). However, device stability and efficiency roll-off remain challenging issues that originate from a slow RISC rate (kRISC). Here, we report a TADF molecule with multiple donor units that form charge-resonance-type hybrid triplet states leading to a small singlet–triplet energy splitting, large spin–orbit couplings, and a dense manifold of triplet states energetically close to the singlets. The kRISC in our TADF molecule is as fast as 1.5 × 107 s−1, a value some two orders of magnitude higher than typical TADF emitters. Organic light-emitting diodes based on this molecule exhibit good stability (estimated T90 about 600 h for 1,000 cd m−2), high maximum external quantum efficiency (>29.3%) and low efficiency roll-off (<2.3% at 1,000 cd m−2).

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Fig. 1: Chemical structures and results of quantum-chemical calculations for 5Cz-TRZ, mCz-TRZ, TmCz-TRZ and DACT-II.
Fig. 2: Photophysical characterization of 5Cz-TRZ.
Fig. 3: Device structures and materials.
Fig. 4: Device performance of OLEDs.

Data availability

The data that support the plots within this paper and other findings of this study are available in the University of Cambridge Repository (https://doi.org/10.17863/CAM.52923). Related research results are available from the corresponding authors upon reasonable request.

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Acknowledgements

L.-S.C., A.J.G., E.W.E. and R.H.F. acknowledge the Engineering and Physical Sciences Research Council (EPSRC) for funding (EP/M01083X/1, EP/M005143/1). The Centre for Advanced ESR (CAESR) is supported by UK EPSRC (EP/L011972/1). We thank Diamond Light Source (UK) for synchrotron beamtime on I19 (CY21497). X.-K.C. and J.-L.B. acknowledge support from the Georgia Institute of Technology, Georgia Research Alliance, Vasser-Woolley Foundation and Kyulux. H.Y., Z.-S.L., H.N. and C.A acknowledge the Japan Science and Technology Agency, ERATO, Adachi Molecular Exciton Engineering Project for funding (JPMJER1305). Y.L. acknowledges a stipend from the Chinese Scholarship Council. S.-F.Z. acknowledges financial support by Guangxi Department of Science and Technology (no. AD19110030), Department of Education (no. 2019KY0394) and the start-up funds provided by Guangxi University of Science and Technology. E.W.E also thanks the Leverhulme Trust for funding (ECF-2019-054). We thank J. Y. Lee for providing the control TADF materials and C. Zhong for discussions.

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Contributions

L.-S.C. designed the molecules and carried out device fabrication and measurements. A.J.G. conducted the transient absorption experiments. S.-F.Z. performed the theoretical calculations under the supervision of X.-K.C. Y.L. performed the molecular orientation and optical simulations under the supervision of S.R. H.Y. and Z.-S.L carried out steady-state and time-resolved photophysical properties under the supervision of C.A. E.W.E. and W.K.M conducted the transient electron spin resonance measurements and analysed the results. T.K.R. performed single-crystal X-ray diffraction and analysed the results. H.N. participated in the discussion of the photophysical mechanism. L.-S.C., A.J.G., X.-K.C., J.-L.B. and R.H.F. analysed the data and wrote the manuscript. All authors discussed the progress of research and reviewed the manuscript.

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Correspondence to Lin-Song Cui or Xian-Kai Chen or Chihaya Adachi or Richard H. Friend.

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Supplementary Information

Supplementary Figs. 1–26, Scheme 1 and Tables 1–7.

Crystallographic Data 1

Crystallographic Data of 5Cz-TRZ.

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Cui, L., Gillett, A.J., Zhang, S. et al. Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states. Nat. Photonics 14, 636–642 (2020). https://doi.org/10.1038/s41566-020-0668-z

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