Secrets of Luminescence in High-Efficiency Fluorescence Materials Deciphered by NTU OLED Research Team

An international research team led by Prof. Pi-Tai Chou (周必泰) from NTU Dept. of Chemistry has successfully explained the mechanism of luminescence in multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials. This has been a collaboration with Prof. Wen-Yi Hung (洪文誼) from the Dept. of Optoelectronics and Materials Technology and Center of Excellence for Ocean Engineering, National Taiwan Ocean University; and Prof. Weiguo Zhu (朱衛國) of Changzhou University. The results of their research have been published in Nature Photonics, the foremost international journal on the field of optoelectronics. The breakthrough offered by the theory is expected to provide an impetus to the accelerated development of high-efficiency fluorescent luminescent materials and to have a significant impact on the development of future display technology.

Due to their near-100% light emission efficiency and high color purity, multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials have in recent years received a deal of attention compared with conventional TADF and other metal complex phosphorescence materials. However, one phenomenon has always puzzled researchers: for some MR-TADF materials, their molecules exhibit very strong fluorescence when photoexcited, yet there is no sign of delayed fluorescence. On the other hand, doping these types of luminescent materials in a suitable host material will produce a significant delayed fluorescence. Delayed fluorescence is an important mechanism in organic electroluminescent diode (OLED); it can be obtained by capturing the triplet state upon thermal activation, thereby achieving 100% light emission quantum efficiency. MR-TAD isolates molecules and produces completely different photophysical phenomena through host-guest mixtures, which has for many years perplexed researchers across the world and hampered progress in OLED-related research.

The team led by Prof. Chou first synthesized a series of novel MR-TADF molecules, followed by systematically conducting in-depth comparative studies with classic MR-TADF fluorescent materials. Using photo- and electroluminescence excitation time-resolved spectroscopy and step-scan Fourier-transform transient absorption spectroscopy, the researchers discovered, within the organic light emission layer in mixed deposition, the presence of host–guest interactions that generated a transient charge transfer intermediate, which was capable of “bridging” the singlet and triplet states of the guest fluorescent molecules and increasing the rates of intersystem crossing and reverse intersystem crossing from the singlet to the triplet state, thereby “creating” delayed fluorescence. Fig. 1 clearly explains that the formation of the host-guest charge transfer transient bridge enables the molecules to accelerate the rates of intersystem crossing and reverse intersystem crossing, thereby generating TADF.

While ensuring that the color purity of the light remains unchanged, this new mechanism also achieves the goal of efficient utilization of the dark (or triplet) state excitons. Going forward, the mechanism will allow OLED researchers to re-examine some high-efficiency fluorescent materials with singlet and triplet states whose energy gaps are small enough to be thermally activated but lack delayed fluorescence properties. Solutions for implementing high-efficiency materials for OLED deep-blue emission are now on the horizon. Prof. Chou’s laboratory has achieved key milestones in the physics of blue and near-infrared light and in electroluminescence research. Over the past four years, researchers from his lab have published three original papers in the top journal Nature Photonics with their outstanding results. In 2020 Prof. Chou received funding from the Academic Summit Program of the Ministry of Science and Technology, and he has indeed reached the ministry’s top target in the field of optoelectronics.

Full text: The role of host–guest interactions in organic emitters employing MR-TADF, published on Sept. 28, 2021 on Nature Photonics: https://doi.org/10.1038/s41566-021-00870-3