Center for Organic Photonics and Electronics Research, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan.
Nature. 2012 Dec 13;492(7428):234-8. doi: 10.1038/nature11687.
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 molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. 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 design, 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 10(6) 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 OLEDs.
在过去的二十年中，分子设计所赋予的固有灵活性加速了各种有机半导体的发展。特别是，在有机发光二极管（OLED）材料的开发方面取得了重大进展，从早期基于荧光分子的器件发展到使用磷光分子的器件。在 OLED 中，电注入的电荷载流子以 1:3 的比例复合形成单重态和三重态激子；磷光金属有机配合物的使用利用了通常非辐射的三重态激子，从而提高了整体电致发光效率。在这里，我们报告了一类无金属有机电致发光分子，其设计将单重态和三重态激发态之间的能隙最小化，从而促进高效的自旋上转换，将非辐射三重态高效转化为辐射单重态，同时保持高辐射衰减率，超过每秒 10^6 次衰减。换句话说，这些分子通过荧光衰减通道利用单重态和三重态激子来发光，导致固有荧光效率超过 90%，并且具有非常高的外部电致发光效率，超过 19%，与高效率基于磷光的 OLED 相当。
