The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules1 to those using phosphorescent molecules2,3. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio1; the use of phosphorescent metal–organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency2,3. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design4, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 106 decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs3.
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This work was supported by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. H.U. acknowledges a Grand-in-Aid for JSPS Fellows. We thank H. Nakanotani, J.-i. Nishide, and H. Miyazaki for their assistance with this research. We also thank K. Tokumaru, H. Sasabe, W. Potscavage and M. Gábor for their assistance with preparation of this manuscript.
The authors declare no competing financial interests.
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Uoyama, H., Goushi, K., Shizu, K. et al. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 (2012). https://doi.org/10.1038/nature11687
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