Highly efficient organic light-emitting diodes from delayed fluorescence

A class of metal-free organic electroluminescent molecules is designed in which both singlet and triplet excitons contribute to light emission, leading to an intrinsic fluorescence efficiency greater than 90 per cent and an external electroluminescence efficiency comparable to that achieved in high-efficiency phosphorescence-based organic light-emitting diodes. 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. 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.