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Extrafluorescent electroluminescence in organic light-emitting devices

Abstract

Organic light-emitting devices (OLEDs) are a promising technology for flat-panel displays and solid-state lighting. While OLED efficiencies have increased dramatically in recent years, further progress is complicated by the fact that the vast majority of organic materials are fluorescent and therefore emit only from molecular excited states (‘excitons’) with spin 0, or ‘singlet’ spin symmetry. Here, we demonstrate the ability to manipulate the fraction of excitons which form as singlets in fluorescent materials by altering the OLED structure. We insert a mixing layer that affects only charge-transfer (CT) states, which are the precursors to excitons. As a result, we triple the singlet fraction and the efficiency of the red fluorophore DCM2. We term fluorescence enhanced by CT spin mixing ‘extrafluorescence’, and show that its origin is in part an inversion of the usual energetic ordering of the singlet and triplet CT states.

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Figure 1: The spin states of excitons and their CT-state precursors treated with a rate model and a density functional theory calculation.
Figure 2: A comparison of the external quantum efficiency of an Alq3 OLED with mixing (doped with PtOEP) and without mixing (doped with OEP).
Figure 3: The PtOEP EL efficiency of the OLED of Fig. 2, doped with PtOEP.
Figure 4: The structure and performance of the X-OLED and its control.
Figure 5: Current–voltage characteristics and EL spectra at 5 V (inset) of the X-OLED (blue) and its control (black).
Figure 6: Transient responses of the X-OLED and its control to a 16 V drive.

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Acknowledgements

This work was supported by a gift from 3M, the Natural Sciences and Engineering Research Council of Canada and a National Science Foundation Career Award CHE-0547877. We also thank S. Kooi for his assistance with the streak camera measurements.

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Correspondence to T. Van Voorhis or M. A. Baldo.

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Segal, M., Singh, M., Rivoire, K. et al. Extrafluorescent electroluminescence in organic light-emitting devices. Nature Mater 6, 374–378 (2007). https://doi.org/10.1038/nmat1885

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