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Stable blue phosphorescent organic LEDs that use polariton-enhanced Purcell effects

Nature volume 626, pages 300–305 (2024)Cite this article

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Abstract

Phosphorescent organic light-emitting diodes (PHOLEDs) feature high efficiency1,2, brightness and colour tunability suitable for both display and lighting applications3. However, overcoming the short operational lifetime of blue PHOLEDs remains one of the most challenging high-value problems in the field of organic electronics. Their short lifetimes originate from the annihilation of high-energy, long-lived blue triplets that leads to molecular dissociation4,5,6,7. The Purcell effect, the enhancement of the radiative decay rate in a microcavity, can reduce the triplet density and, hence, the probability of destructive high-energy triplet–polaron annihilation (TPA)5,6 and triplet–triplet annihilation (TTA) events4,5,7,8. Here we introduce the polariton-enhanced Purcell effect in blue PHOLEDs. We find that plasmon–exciton polaritons9 (PEPs) substantially increase the strength of the Purcell effect and achieve an average Purcell factor (PF) of 2.4 ± 0.2 over a 50-nm-thick emission layer (EML) in a blue PHOLED. A 5.3-fold improvement in LT90 (the time for the PHOLED luminance to decay to 90% of its initial value) of a cyan-emitting Ir-complex device is achieved compared with its use in a conventional PHOLED. Shifting the chromaticity coordinates to (0.14, 0.14) and (0.15, 0.20) into the deep blue, the Purcell-enhanced devices achieve 10–14 times improvement over similarly deep-blue PHOLEDs, with one structure reaching the longest Ir-complex device lifetime of LT90 = 140 ± 20 h reported so far10,11,12,13,14,15,16,17,18,19,20,21. The polariton-enhanced Purcell effect and microcavity engineering provide new possibilities for extending deep-blue PHOLED lifetimes.

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Fig. 1: The PEP-enhanced Purcell effect.
Fig. 2: Polariton dispersion engineering.
Fig. 3: Optical engineering of blue PHOLEDs.
Fig. 4: Ir(dmp)3 device performance.
Fig. 5: Ir(dmp)3 device performance summary.

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Data availability

The data that support the findings of this study are available from the corresponding author on request.

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Acknowledgements

We thank J.-Y. Lee from Sungkyunkwan University for providing SiTrzCz2 and Ir(cb)3 and J. Kim from Yonsei University for helpful discussions. This material is based on work partially supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under award number DE-EE0009688. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise does not necessarily constitute or imply its endorsement, recommendation or favouring by the United States Government or any agency thereof. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. We also thank Universal Display Corp. for partial financial support of this research.

Author information

Authors and Affiliations

  1. Department of Physics, University of Michigan, Ann Arbor, MI, USA

    Haonan Zhao, Claire E. Arneson & Stephen R. Forrest

  2. Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, USA

    Dejiu Fan & Stephen R. Forrest

  3. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, USA

    Stephen R. Forrest

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  1. Haonan Zhao
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Contributions

H.Z. planned the experiments, built the experimental setup and conducted the experiments and simulations. D.F. helped with fabrication. C.E.A. helped with obtaining the optical data. S.R.F. planned and supervised the project and analysed the data. All authors contributed to the preparation of the manuscript and analysed the data.

Corresponding author

Correspondence to Stephen R. Forrest.

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Competing interests

S.R.F. has an equity interest in Universal Display Corp. This apparent conflict is under management by the University of Michigan Office of Research. Also, the University of Michigan has a royalty-bearing license agreement with Universal Display Corp.

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Extended data figures and tables

Extended Data Fig. 1 Molecular structural formulae of organic materials used in the EML, ETL and hosts.

The molecules are all deposited from vacuum along with other molecules forming the complete OLED structures, whose complete chemical nomenclatures are provided in Methods.

Extended Data Fig. 2 Angle-resolved TM-mode reflectance of Al/BPyTP2.

The measurement angles are increased from θ = 51° to 87° by 3° increments, indicated by solid lines. The dashed black lines provide guides to the dispersion of LP, MP and UP branches. The extinction coefficient of BPyTP2 is shown by the filled pink area. The PEP dispersion in Fig. 2a is fitted to the local minima. Inset, ellipsometry measurement setup. 20-nm-thick Al/20 nm BPyTP2 is coated on a prism coupler.

Extended Data Fig. 3 Device structures used in this study.

C, H and F label the cathode and anode structures and 1–6 label the organic structures sandwiched between the electrodes.

Extended Data Fig. 4 Simulated and measured PFs for the devices studied.

a, ODoS distribution simulated using dyadic Green’s functions31,32 for structure C with a 15-nm-thick SF3Trz ETL and structure F with a 15-nm-thick BPyTP2 ETL. The triplet dipole position is in the middle of the EML with an isotropic orientation relative to the film plane. Dashed white lines separate regions of different optical modes. Structure F has a low-Q Ag/DBR cavity mode at kx/k0 < 1. The relative ODoS intensity is labelled by the colour bar on the right. The ODoS for structure C is multiplied by five for comparison. b, Simulated PF versus triplet emitter position for structures C and F for different cathode/ETL combinations. The horizontal and vertical dipoles are labelled by dash blue and red lines, respectively. The PF for isotropic dipoles is averaged over 67% horizontal (in-plane) and 33% vertical (orange line) dipoles. The measured PF of horizontal dipoles is labelled by diamonds at 45 nm from the cathode, corresponding to the centre of the 50-nm-thick EML. The simulated centre wavelength is 465 nm. c, Simulated PF of isotropic dipoles in half cavities H versus wavelength overlapped with the Ir(dmp)3 PL spectrum. The exciton is located at 36 nm from the interfaces of Ag/BPyTP2, Al/BPyTP2, Ag/SF3Trz and Al/SF3Trz. d, Measured normalized decay rate, kr/kr,0 overlaid with the simulated PFs for horizontal (PFhor) and isotropic (PFiso) dipoles using the Green’s function method31,32. Here kr,0 is the decay rate of the bare EML.

Extended Data Fig. 5 Ir(dmp)3 device energy levels and performance.

a, Device structure and frontier orbital levels (in eV). b, EQE versus J of C1–C3, H1–H3 and F1–F3. EQE data are averaged from at least from two different batches with two devices in each batch. c, EL spectra of C1– C3, H1–H3 and F1–F3 at J = 10 mA cm−2. Inset, chromaticity coordinates of the devices. The arrow indicates the blueshift from the control to the full-cavity devices. d, Current–voltage (J–V, left axis) and luminance (right axis) characteristics of C1–C3, H1–H3 and F1–F3. e, Device operational lifetime for C1–C3, H1–H3 and F1–F3 at J = 7 mA cm−2. For C1–C3, H1–H3 and F1, lifetime data are shown from two devices in different batches. For F2 and F3, lifetime data are shown from three devices in different batches. Shaded area indicates the 95% confidence interval. Solid black lines are fits to a stretched exponential: L(t)/L0 = exp[−(t/t0)β] for C and F devices, in which L0 is the initial luminance and t0 and β are parameters (see Extended Data Table 2).

Extended Data Fig. 6 Ir(cb)3 device energy levels and performance.

a, Device structure and frontier orbital levels (in eV). b, Device operational lifetime at J = 5 mA cm−2. Errors are indicated in Extended Data Table 1. Average from three devices for each curve. Solid black lines are fits to a stretched exponential: L(t)/L0 = exp[−(t/t0)β], in which L0 is the initial luminance and t0 and β are parameters (see Extended Data Table 2). c, EL spectra of C4–C6 and H5 and H6 at J = 10 mA cm−2. d, EQE versus J of C4, C5 and C6 and H5 and H6. Errors are indicated in Extended Data Table 1. Average from three devices for each curve. e, J–V (left axis) and luminance (right axis) characteristics of C4–C6 and H5 and H6.

Extended Data Table 1 Summary of Ir(dmp)3 and Ir(cb)3 device performance
Full size table
Extended Data Table 2 Summary of stretched exponential model for Ir(dmp)3 and Ir(cb)3 devices
Full size table

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Zhao, H., Arneson, C.E., Fan, D. et al. Stable blue phosphorescent organic LEDs that use polariton-enhanced Purcell effects. Nature 626, 300–305 (2024). https://doi.org/10.1038/s41586-023-06976-8

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