Chiral luminescent materials with special structures of levorotation and dextrorotation can emit circularly polarized luminescence (CPL), which have significant potential applications in photoelectric devices, biosensing, and optical information storage. Circularly polarized organic light-emitting diodes (CP-OLEDs) by using chiral luminescent material as the emitting core, which would be particularly favorable for the direct generation of CP light in terms of facile device architectures, high efficiencies, and tunable emissions, have great value for facilitating the development of the next generation of display technologies, especially for 3D display. Though the first report on CP-OLEDs dates from 1997 and considerable efforts have been made to develop various kinds of chiral emitters for fabricating CP-OLEDs, the development of CP-OLEDs is quite sluggish and there are less than 20 examples for CP-OLEDs to 2019. Meanwhile, such CP-devices have suffered from low brightness, insufficient efficiency, serious efficiency roll-off as well as unstable asymmetry g factor, which can hardly be used in commercial application.
The Zheng group has been engaged in the development of efficient phosphorescent metal complexes and thermally activated delayed fluorescence (TADF) materials for achieving high-performance OLEDs. Recently, they developed a pair of chiral platinum complexes and their CP-OLEDs with a maximum brightness of 11590 cd m-2, a maximum external quantum efficiency (EQEmax) up to 18.81 %. To suppress the effect of reverse CPEL signal from the cathode reflection, the further implementation of semitransparent aluminum/silver cathode successfully boosts up the g value by over three times to 5.1×10-3.(Chem. Eur. J. 2019, 25, 5672, Ph. D candidate, Zhi-Ping Yan as the first author). Subsequently, a pair of octahydro-binaphthyl (OBN)-based chiral emitting enantiomers was developed by ingeniously merging the chiral source and the TADF-based luminophore skeleton of cyan and diphenylamine groups, which demonstrates a high photoluminescence quantum yield (PLQY) of 82% with aggregation-induced emission (AIE) characteristic. The CP-OLEDs show high brightness over 25000 cd cm-2, high efficiency with EQEmax of 12.4%, and negligible efficiency roll-off. (J. Mater. Chem. C, 2019, 7, 7045, research fellow, Zheng-Guang Wu as the first author).
When the diphenylamine group was replaced by carbazole, the novel CP-TADF molecules display the better performance with a PLQY of 92% and a small ΔEST of 0.037 eV. The CP-OLED displays extraordinary high efficiencies with a maximum current efficiency of 93.7 cd A−1, EQEmax of 32.6% and a maximum power efficiency of 59.3 lm W−1 as well as extremely low efficiency roll-offs. For example, from the peak value (EQEmax of 32.6%) to the brightness of 1000, 3000, and 5000 cd m−2, the device can still maintain the EQE at 31.7%, 31.6%, and 30.6%, respectively, which are among the best results achieved from TADF-based OLEDs. Notably, the CP-OLEDs display obvious CPEL signals with opposing gEL of +2.30 × 10−3 and −1.94 × 10−3, respectively. This work entitled “Chiral Octahydro-binaphthol Compound-based Thermally Activated Delayed Fluorescence Materials for Circularly Polarized Electroluminescence with Superior EQE of 32.6% and Extremely Low Efficiency Roll-off” has been published in Adv. Mater.2019, 1900524, Zheng-Guang Wu as the first author and You-Xuan Zheng as the corresponding author.
Thanks to professor Jing-Lin Zuo, Yi Wang and Yi Pan for their valuable advice to this work.
This work was supported by the National Natural Science Foundation of China (51773088).