Overcoming the energy gap law in near-infrared OLEDs by exciton–vibration decoupling

Abstract

The development of high-performance near-infrared organic light-emitting diodes is hindered by strong non-radiative processes as governed by the energy gap law. Here, we show that exciton delocalization, which serves to decouple the exciton band from highly vibrational ladders in the S0 ground state, can bring substantial enhancements in the photoluminescence quantum yield of emitters, bypassing the energy gap law. Experimental proof is provided by the design and synthesis of a series of new Pt(ii) complexes with a delocalization length of 5–9 molecules that emit at 866–960 nm with a photoluminescence quantum yield of 5–12% in solid films. The corresponding near-infrared organic light-emitting diodes emit light with a 930 nm peak wavelength and a high external quantum efficiency up to 2.14% and a radiance of 41.6 W sr−1 m−2. Both theoretical and experimental results confirm the exciton–vibration decoupling strategy, which should be broadly applicable to other well-aligned molecular solids.

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Fig. 1: Partitioning of reorganization energy by exciton delocalization.
Fig. 2: Molecular structures and photophysical properties.
Fig. 3: Analysis of GIXD results.
Fig. 4: Probing delocalization length from two-exciton signals through time-resolved step-scan Fourier transform UV–vis spectroscopy.
Fig. 5: Device characteristics of optimized NIR OLEDs.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This research was supported by funding from the Ministry of Science and Technology (MOST), the Featured Areas Research Program within the framework of the Higher Education Sprout Project administered by the Ministry of Education (MOE), National Taiwan University, Soochow University and the Research Grant Council and City University of Hong Kong. We are also grateful to the National Center for High-Performance Computing (NCHC) and National Synchrotron Radiation Research Center (NSRRC) for computer time and facilities, respectively.

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Y.-C.W., D.-G.C., K.-H.C., C.-W.W. and S.-H.L. performed optical measurements, simulations and calculations. S.F.W., W.-H.C. and J.-L.L. conducted the synthesis and characterization of Pt(ii) complexes. Y.H., L.-S.L. and W.-Y.H. executed OLED fabrications and analysed data. Y.-C.W. and P.-T.Chou developed the theoretical approach and prepared the manuscript. Y.C. designed the Pt(ii) complexes and interpreted the spectroscopic data. T.-H.W., P.-T.Chen and H.-F.H. performed GIXD experiments and data analysis. All authors discussed the results and contributed to the manuscript.

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Correspondence to Liang-Sheng Liao or Yun Chi or Pi-Tai Chou.

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Supplementary Information

Supplementary Figs. 1–11 and Tables 1–5.

Crystallographic Data

Crystal structure of 4dMp.

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Wei, Y., Wang, S.F., Hu, Y. et al. Overcoming the energy gap law in near-infrared OLEDs by exciton–vibration decoupling. Nat. Photonics (2020). https://doi.org/10.1038/s41566-020-0653-6

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