Letter | Published:

Efficient radical-based light-emitting diodes with doublet emission

Naturevolume 563pages536540 (2018) | Download Citation

Abstract

Organic light-emitting diodes (OLEDs)1,2,3,4,5, quantum-dot-based LEDs6,7,8,9,10, perovskite-based LEDs11,12,13 and micro-LEDs14,15 have been championed to fabricate lightweight and flexible units for next-generation displays and active lighting. Although there are already some high-end commercial products based on OLEDs, costs must decrease whilst maintaining high operational efficiencies for the technology to realise wider impact.  Here we demonstrate efficient action of radical-based OLEDs16, whose emission originates from a spin doublet, rather than a singlet or triplet exciton. While the emission process is still spin-allowed in these OLEDs, the efficiency limitations imposed by triplet excitons are circumvented for doublets. Using a luminescent radical emitter, we demonstrate an OLED with maximum external quantum efficiency of 27 per cent at a wavelength of 710 nanometres—the highest reported value for deep-red and infrared LEDs. For a standard closed-shell organic semiconductor, holes and electrons occupy the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs), respectively, and recombine to form singlet or triplet excitons. Radical emitters have a singly occupied molecular orbital (SOMO) in the ground state, giving an overall spin-1/2 doublet. If—as expected on energetic grounds—both electrons and holes occupy this SOMO level, recombination returns the system to the ground state, giving no light emission. However, in our very efficient OLEDs, we achieve selective hole injection into the HOMO and electron injection to the SOMO to form the fluorescent doublet excited state with near-unity internal quantum efficiency.

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Data availability

The datasets collected and analysed in this work are available at https://doi.org/10.17863/CAM.31543.

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Acknowledgements

X.A., S.D., H.G., Y.C. and F.L. are grateful for the financial support received from the National Key R&D Program of China (grant number 2016YFB0401001), the National Natural Science Foundation of China (grant numbers 51673080 and 91233113) and the National Key Basic Research and Development Program of China (973 programme, grant number 2015CB655003). E.W.E., A.J.G. and R.H.F. thank the EPSRC for funding (EP/M01083X/1, EP/M005143/1). T.J.H.H. thanks Jesus College, Cambridge for a Research Fellowship. F.L. is an academic visitor at the Cavendish Laboratory, Cambridge and is supported by the China Scholarship Council (CSC) and the Talents Cultivation Program (Jilin University, China).

Reviewer information

Nature thanks T. Kusamoto and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Author notes

  1. These authors contributed equally: Xin Ai, Emrys W. Evans, Shengzhi Dong

Affiliations

  1. State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, China

    • Xin Ai
    • , Shengzhi Dong
    • , Haoqing Guo
    • , Yingxin Chen
    •  & Feng Li
  2. Cavendish Laboratory, University of Cambridge, Cambridge, UK

    • Emrys W. Evans
    • , Alexander J. Gillett
    • , Timothy J. H. Hele
    • , Richard H. Friend
    •  & Feng Li

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Contributions

X.A., S.D. and H.G. designed and synthesized the luminescent radicals and performed the steady-state spectroscopy. E.W.E. performed the transient-photoluminescence measurements and the quantum chemical calculations. T.J.H.H devised the group theory treatment. A.J.G. conducted the transient-absorption spectroscopy measurements. X.A., Y.C. and F.L. optimized the devices. E.W.E., R.H.F. and F.L. initiated, designed and supervised the work. E.W.E., R.H.F. and F.L. wrote the manuscript, with input from all authors.

Competing interests

The authors declare no competing interests.

Corresponding authors

Correspondence to Richard H. Friend or Feng Li.

Extended data figures and tables

  1. Extended Data Fig. 1 Thermal stability and electron paramagnetic resonance measurements of TTM-3NCz and TTM-3PCz.

    a, Thermogravimetric analysis measurements show thermal decomposition temperatures of 362 °C (TTM-3NCz) and 367 °C (TTM-3PCz). b, EPR spectra for solid samples at room temperature. ESR, electron spin resonance; B, magnetic field.

  2. Extended Data Fig. 2 Electrochemical properties and stability of TTM-3NCz and TTM-3PCz.

    a, c, Cyclic voltammograms of TTM-3NCz (a) and TTM-3PCz (c) in CH2Cl2. For both TTM-3NCz and TTM-3PCz, the average of the cathodic and anodic potentials gives a reduction potential of −1.1 V, first oxidation potential of +0.4 V and second oxidation potential of +0.9 V. b, d, Multi-cycle (20 cycles) cyclic voltammetry measurements of TTM-3NCz (b) and TTM-3PCz in CH2Cl2 (d). A ferrocence cation/ferrocence (Fc+/Fc) reference redox couple was used for the measurements.

  3. Extended Data Fig. 3 Photostability of TTM and TTM-3NCz.

    Luminescence intensity (l) of TTM-3NCz and TTM solutions (10 µM, cyclohexane) as a function of time. A pulsed laser of 355 nm with an energy density of 315 kW cm−2 (pulse width, 8 ns; frequency, 10 Hz) was used under ambient conditions.

  4. Extended Data Fig. 4 Device reproducibility for TTM-3NCz.

    a, EQE–current density plots for five TTM-3NCz devices. The peak EQEmax values are: 25% (device 1), 27% (device 2), 20% (device 3), 24% (device 4) and 16% (device 5). The EQE at 1 mA cm−2 is: 8% (device 1), 10% (device 2), 7% (device 3), 9% (device 4) and 7% (device 5). b, Radiance–voltage plots for the same five TTM-3NCz devices. The radiance levels for EQEmax are indicated, which can be distinguished from the noise. c, Current density–voltage plots for the same five TTM-3NCz devices.

  5. Extended Data Fig. 5 Device stability for TTM-3NCz.

    Electroluminescence spectra for TTM-3NCz devices operated at current densities of 0.006–1.6 mA cm−2. There is no current dependence.

Supplementary information

  1. Supplementary Information

    This file contains further experimental details; Supplementary Text: including synthesis and MO diagram derivation; Supplementary Figures: 1–2) TA data, 3–4) MO diagrams and basis set, 5–6) DFT orbitals and difference density plots; and Supplementary Tables: 1) TDDFT orbital contributions, 2) red LED literature.

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DOI

https://doi.org/10.1038/s41586-018-0695-9

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