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Management of singlet and triplet excitons for efficient white organic light-emitting devices

Abstract

Lighting accounts for approximately 22 per cent of the electricity consumed in buildings in the United States, with 40 per cent of that amount consumed by inefficient (15 lm W-1) incandescent lamps1,2. This has generated increased interest in the use of white electroluminescent organic light-emitting devices, owing to their potential for significantly improved efficiency over incandescent sources combined with low-cost, high-throughput manufacturability. The most impressive characteristics of such devices reported to date have been achieved in all-phosphor-doped devices, which have the potential for 100 per cent internal quantum efficiency2: the phosphorescent molecules harness the triplet excitons that constitute three-quarters of the bound electron–hole pairs that form during charge injection, and which (unlike the remaining singlet excitons) would otherwise recombine non-radiatively. Here we introduce a different device concept that exploits a blue fluorescent molecule in exchange for a phosphorescent dopant, in combination with green and red phosphor dopants, to yield high power efficiency and stable colour balance, while maintaining the potential for unity internal quantum efficiency. Two distinct modes of energy transfer within this device serve to channel nearly all of the triplet energy to the phosphorescent dopants, retaining the singlet energy exclusively on the blue fluorescent dopant. Additionally, eliminating the exchange energy loss to the blue fluorophore allows for roughly 20 per cent increased power efficiency compared to a fully phosphorescent device. Our device challenges incandescent sources by exhibiting total external quantum and power efficiencies that peak at 18.7 ± 0.5 per cent and 37.6 ± 0.6 lm W-1, respectively, decreasing to 18.4 ± 0.5 per cent and 23.8 ± 0.5 lm W-1 at a high luminance of 500 cd m-2.

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Figure 1: Proposed energy transfer mechanisms in the fluorescent/phosphorescent WOLED.
Figure 2: Un-normalized electroluminescence spectra of three device structures shown in the inset.
Figure 3: Performance characteristics of the fluorescent/phosphorescent WOLED.
Figure 4: Triplet diffusion profile and reduced efficiency roll-off at high currents.

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Acknowledgements

H.K. is currently on leave from Sanyo Electric Co., Ltd., Osaka, Japan. The authors thank the Department of Energy and Universal Display Corp. for partial support of this work, as well as R. Holmes and B. W. D'Andrade for discussions.

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Correspondence to Stephen R. Forrest.

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Supplementary Data

This file includes three figures and a table, and (1) a comparison between selected electrophosphorescent white organic light emitting device (WOLED) architectures with their corresponding performance characteristics; (2) Luminescence characteristics of the WOLED presented in the manuscript; and (3) Discussion of non-radiative exciton transfer with comparison between devices related to the WOLED architecture presented in the manuscript. (DOC 822 kb)

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Sun, Y., Giebink, N., Kanno, H. et al. Management of singlet and triplet excitons for efficient white organic light-emitting devices. Nature 440, 908–912 (2006). https://doi.org/10.1038/nature04645

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