a 洛阳师范学院 化学化工学院 河南省功能导向多孔材料重点实验室 洛阳 471934;
b 电子科技大学 光电科学与工程学院 成都 610054
A Novel Yellow Thermally Activated Delayed Fluorescence emitter For Highly Efficient Organic Light-Emitting Diodes
Wang Zhiqianga, Cai Jialinb, Zhang Mingb, Zheng Caijunb, Ji Baominga
a College of Chemistry and Chemical Engineering and Henan Key Laboratory of Function-Oriented Porous Materials, Luoyang Normal University, Luoyang 471934;
b School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China(UESTC), Chengdu 610054
A novel yellow thermally activated delayed fluorescence emitter pPBPXZ was successfully synthesized using phenoxazine as electron-donor and pyrimidine as electron-acceptor by Buchwald-Hartwig and Suzuki coupling reactions. Density functional theory calculations show that pPBPXZ has highly twisted structure with the dihedrals of nearly 90° between phenoxazine and pyrimidine units, while the dihedrals between benzene ring and adjacent pyrimidine rings are almost 0°. The highest occupied molecular orbital (HOMO) is mainly confined on two phenoxazine segments, the lowest unoccupied molecular orbital (LUMO) is mainly located on the central pyrimidine and benzene segments, and there is only a slight overlap between HOMO and LUMO. Cyclic voltammetry investigation show pPBPXZ has reversible redox process, and the HOMO and LUMO energy levels were estimated to be -5.43 eV and -3.23 eV, respetively, from the onsets of oxidation and reduction curves. In diluted toluene solution, pPBPXZ exhibits the absorption band assigned to intramolecular charge-transfer transition and yellow fluorescence with a structureless emission peak at 535 nm. From the onsets of the fluorescence and phosphorescence spectra of pPBPXZ in 2Me-THF at 77 K, the lowest singlet (S1) and the lowest triplet (T1) energy levels are calculated to be 2.57 eV and 2.48 eV, respectively, and thus △EST is only 0.09 eV. The doped electroluminescence devices using pPBPXZ as guest emitter were prepared by vacuum evaporation method. These devices with doping ratios (w) of 6%, 11%, 16% and 23% show yellow emission at 552 nm-560 nm and low turn-on voltages of 3.1 V-3.3 V. The device with a doping ratio of 11% exhibits the highest maximum forward-viewing efficiencies of a maximum current efficiency of 49.9 cd/A, a maximum power efficiency of 49.0 lm/W and a maximum external quantum efficiency of 15.7% without any light out-coupling enhancement. Particularly, the efficiencies of these devices are not sensitive to the doping ratios of pPBPXZ, which would benefit the further practical application.