Sub-Second Long Lifetime Triplet Exciton Reservoir for Highly Efficient and Stable Organic Light-Emitting Diode.

In organic light-emitting diode (OLED), achieving high efficiency requires effective triplet exciton confinement by carrier-transporting materials, which typically have higher triplet energy (E T ) than the emitter, leading to poor stability. Here we report an electron-transporting material (ETM), whose E T is 0.32°eV lower than that of the emitter. In devices, it surprisingly exhibits strong confinement effect and generates excellent efficiency. Additionally, the device operational lifetime is 4.9 times longer than the device with a standard ETM, TPBi (whose E T 0.36°eV higher than the emitter). This anomalous finding is ascribed to the exceptionally long triplet state lifetime (∼0.2°s) of the ETM. We name it as long-lifetime triplet exciton reservoir (LTER) effect. The systematic analysis revealed that the long triplet lifetime of ETM can compensate the requirement for high E T with the help of endothermic energy transfer. Such combination of low E T and long lifetime provide equivalent exciton confinement effect and high molecular stability simultaneously. It offers a novel molecular design paradigm for breaking the dilemma between high efficiency and prolonged operational lifetime in OLEDs. This article is protected by copyright. All rights reserved.