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Understanding and predicting the orientation of heteroleptic phosphors in organic light-emitting materials

Nature Materials volume 15, pages 85–91 (2016)Cite this article

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Abstract

Controlling the alignment of the emitting molecules used as dopants in organic light-emitting diodes is an effective strategy to improve the outcoupling efficiency of these devices. To explore the mechanism behind the orientation of dopants in films of organic host materials, we synthesized a coumarin-based ligand that was cyclometalated onto an iridium core to form three phosphorescent heteroleptic molecules, (bppo)2Ir(acac), (bppo)2Ir(ppy) and (ppy)2Ir(bppo) (bppo represents benzopyranopyridinone, ppy represents 2-phenylpyridinate, and acac represents acetylacetonate). Each emitter was doped into a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl host layer, and the resultant orientation of their transition dipole moment vectors was measured by angle-dependent p-polarized photoluminescent emission spectroscopy. In solid films, (bppo)2Ir(acac) is found to have a largely horizontal transition dipole vector orientation relative to the substrate, whereas (ppy)2Ir(bppo) and (bppo)2Ir(ppy) are isotropic. We propose that the inherent asymmetry at the surface of the growing film promotes dopant alignment in these otherwise amorphous films. Modelling the net orientation of the transition dipole moments of these materials yields general design rules for further improving horizontal orientation.

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Figure 1: Material structure and properties.
Figure 2: Polarized emission spectra.
Figure 3: Electrostatic surfaces.
Figure 4: Molecular orientation.
Figure 5: Oriented homoleptic phosphors.
Figure 6: Ideal molecular orientation.

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Acknowledgements

The research described here was carried out with the support of the Universal Display Corporation, The Humboldt Foundation, Bavaria California Technology Center (BaCaTeC) and Deutsche Forschungsgemeinschaft (DFG Br 1728/16-1). C.M. acknowledges financial support by Bayerische Forschungsstiftung.

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Authors and Affiliations

  1. Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA

    Matthew J. Jurow, Peter I. Djurovich & Mark E. Thompson

  2. Institute of Physics, University of Augsburg, 86135 Augsburg, Germany

    Christian Mayr, Tobias D. Schmidt, Thomas Lampe & Wolfgang Brütting

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Contributions

M.J.J. prepared materials and samples, designed experiments and prepared the manuscript; C.M. measured molecular orientation; T.D.S. designed and prepared mathematical models; T.L. designed and prepared mathematical models; P.I.D., W.B. and M.E.T. designed and assisted with experiments and the manuscript.

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Correspondence to Mark E. Thompson.

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M.E.T. has a competing interest in the Universal Display Corporation.

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Jurow, M., Mayr, C., Schmidt, T. et al. Understanding and predicting the orientation of heteroleptic phosphors in organic light-emitting materials. Nature Mater 15, 85–91 (2016). https://doi.org/10.1038/nmat4428

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