Organic light-emitting diode (OLED) technology is promising for ultra-high-definition (UHD) displays and other applications, but further improvements in efficiency and colour purity are desired. Compared to blue and red emissions, the requirements of green emission purity are more stringent and challengeable. Less satisfactory emission maxima, slight band broadening or the emergence of shoulders/tails in emission spectra can all lead to mixing of green with blue/red, which reduces the y coordinate value (CIEy) in the Commission Internationale de l’Eclairage (CIE) chromaticity diagram and thus harms purity of green emission.
To address this problem, recently, Prof. Xiao-Hong Zhang, Prof. Xian-Kai Chen, and Associate Prof. Kai Wang from the Institute of Functional Nano and Soft Materials of Soochow University, and Prof. Chihaya Adachi form the Center for Organic Photonics and Electronics Research (OPERA) of Kyushu University have jointly designed and synthesized an organoboron-based emitter DBTN-2, via the highly distorted fused π-conjugated molecular design concept. Via such concept, on the one hand, the relaxation energy between the geometries of the excited and ground states is significantly reduced, leading to a full width at half-maximum (FWHM) of only 20 nm; on the other hand, the different excitation characters of the singlet and triplet states (i.e., the ππ* excitation and the hybrid excitation of ππ* and πσ*, respectively) are induced, enhancing the spin-orbit couplings. The introduction of the multiple carbazole moieties gives rise to the charge-resonance-type excitation feature of the triplet states, thus resulting in a high density of the triplet states. These allow the rate of reverse intersystem crossing (kRISC) of DBTN-2 is as fast as 1.7×105 s-1. Eventually, a record-pure green OLED exploiting DBTN-2 as emitter without color filters and optimized cavity effects was thus achieved, with Commission Internationale de l’Eclairage (CIE) coordinates of (0.19, 0.74) satisfying the commercial green OLED display. Moreover, in combination with the photoluminescence quantum yield of near 100% and a strong horizontal dipole orientation in the doped film, an excellent external quantum efficiency of 35.2% with suppressed efficiency roll-off is also simultaneously obtained in the green OLED. This research not only realizes the green OLED with both the excellent device performance and the record-high color purity, but also provides the design concept for more MR-TADF emitters with high efficiencies and excellent color purity of emission. The related results have been published on Nature Photonics (DOI: 10.1038/s41566-022-01106-8).Xiao-Chun Fan, a Post-Doctor from Prof. Xiao-Hong Zhang's group, is the first author of this paper.
Figure 1. High-level coupled-cluster quantum-chemistry results of DBTN-2.
Figure 2. Photophysical properties of DBTN-2.
Figure 3. OLED performance of DBTN-2 and summary comparison.
Link to article: https://www.nature.com/articles/s41566-022-01106-8
Title: Ultrapure green organic light-emitting diodes based on highly distorted fused
π-conjugated molecular design
Editor: Guo Jia