Organic light-emitting diodes (OLEDs) is a new generation of display technology, which has been widely used in mobile phones, televisions and other fields. As the core luminescent materials, the commercial green and red luminescent materials are iridium complexes. However, the synthetic conditions of iridium complexes are relatively harsh, which need to be heated for a long time above 100 oC, then purified, and the vacuum sublimation rate is also limited. If the complexes can be synthesized quickly and efficiently at room temperature, and its sublimation rate can be increased, the production cost of materials and devices will be greatly reduced, and the industrialization process will be promoted.
Recently, You-Xuan Zheng and Jing-Lin Zuo research team introduced S atom into the ancillary ligands. Due to the large radius and deformation of sulfur atom, it was easy to combine with transition metal, realizing the rapid synthesis of iridium complexes at room temperature and ultra-high vacuum sublimation rate, and showing good OLEDs performances.
Tetraphenylimidodiphosphinate and its derivatives were a kind of ligands developed by our team in the early stage, and the iridium complexes prepared by them showed good device performances (Adv. Mater., 2011, 23, 4041, etc.). Recently, the team prepared thiotetraphenylimidodiphosphinate ligands by replacing O atom with S atom. Through the modification of the main ligands, three pure green iridium complexes were obtained at room temperature within 5 minutes. The synthesis yields and the sublimation yields were over 70% and 90%, respectively, and the photoluminescent quantum yields (PLQYs) were also up to 98%. The external quantum efficiency (EQE) of the device prepared in cooperation with Dr. Cheng Gang and the Academician Che Chi-Ming of the University of Hong Kong has reached 26.52%, and the efficiency roll-off of the device was very slow, showing a good application prospect. This achievement was published in ACS Appl. Mater. Interfaces (2019, 11, 7184-7191). Liang Xiao, a doctoral student, and Zhang Feng, an engineer from the MaAnShan High-Tech Research Institute of Nanjing University were the co-first authors. These ligands could also be widely used in iridium complexes emitting different colors, such as high efficient blue-green light.
At the same time, when diisopropylamine, diphenylamine or carbazole reacted with carbon disulfide in strong alkaline condition, the thiocarbamate derivatives were obtained and used as ancillary ligands in red iridium complexes. Similarly, at room temperature within 5 minutes, more than 70% of the synthesis yields and more than 90% of the vacuum sublimation rate could be obtained. The theoretical calculation of Gibbs free energy showed that the formation of Ir-S bond was thermodynamically favorable process, and there was no stable transition state. The synthesis of all complexes was exothermic, which was conducive to the rapid synthesis of complexes.
The obtained complexes showed pure red emission and the bipolar properties, and the PLQY was as high as 93%. The maximum luminance of the device is more than 60000 cd/m2, and the EQE of the device reached 30.54%. When the luminance reached 1000 cd/m2, the EQE could still be kept as 26.79%, which was one of the highest performances ever reported. The results titled with Rapid room temperature synthesis of red iridium(III) complexes containing four membered Ir-S-C-S chelating ring for highly efficient OLEDs with EQE over 30%, were published in Chem. Sci. (2019, 10, 3535 – 3542). Guang-Zhao Lu and Ning Su are the co-first authors, and Hui-Qing Yang helped to carried out theoretical calculation of complexes, Zhu Qi carried out device fabrication. Prof. You-Xuan Zheng, Prof. Liang Zhou (Changchun Institute of Applied Chemistry), Prof. Zhao-Xu Chen and Prof. Jing-Lin Zuo are the co-corresponding authors.
The research was supported by NSFC (51773088, 21771172).