Efficient Quantum-Dot Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence Emitters as Exciton Harvesters.

Utilization of triplet excitons plays a key role in obtaining highly efficient quantum-dot light-emitting diodes (QD-LEDs). However, to date, only phosphorescent materials have been implemented to harvest triplet excitons in QD-LEDs. In this work, we introduced a thermally activated delayed fluorescence (TADF) emitter, 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN), doped into poly(N-vinylcarbazole) (PVK) as an exciton harvester in red QD-LEDs by solution processing. As a result, electrons leaking to the PVK layer will be trapped by 2CzPN to form long-lifetime TADF excitons in the 2CzPN:PVK layer, and then this harvested exciton energy can be effectively transferred to the adjacent QDs by the Förster resonance energy-transfer process. The fabricated red CdSe/CdS core/shell QD-LEDs show a maximum luminescence efficiency of 17.33 cd/A and longer lifetime. Our results demonstrate that the TADF sensitizer would be a promising candidate to develop highly efficient QD-LEDs.