One successful way of enhancing the efficiency of organic light-emitting diodes (OLEDs) is to incorporate additional phosphorescent metal-organic molecules that are powered by the normally non-emitting 'triplet' excitons (triplet excitons typically account for 75% of the injected charge carriers). Now Hiroki Uoyama and colleagues describe an alternative strategy in which the electronic properties of the organic host material are tuned by molecular design to achieve the same net result without the need for adding phosphorescent entities. The new method makes use of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, so that triplet excitons are efficiently converted into states that can contribute effectively to the overall emissions. Their devices reach levels of efficiency in excess of 19%, comparable to those of phosphorescence-based OLEDs.
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