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Department of Chemistry

Research Highlight

Unraveling the light-emitting mechanisms in highly efficient OLED materials

  • The full article entitled “The role of host-guest interactions in organic emitters employing MR-TADF” can be found at the Nature Photonics website at https://www.nature.com/articles/s41566-021-00870-3
  • Authors: Xiugang Wu, Bo-Kang Su, Deng-Gao Chen, Denghui Liu, Chi-Chi Wu, Zhi-Xuan Huang, Ta-Chun Lin, Cheng-Ham Wu, Mengbing Zhu, Elise Y. Li, Wen-Yi Hung,* Weiguo Zhu,* Pi-Tai Chou*

The research team headed by Prof. Pi-Tai Chou has resolved critical light-emitting mechanisms in multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) emitters. Such breakthrough will advance the development of highly efficient fluorescent dyes for next-generation display technologies. This work involves collaboration with Prof. Wen-Yi Hung of National Taiwan Ocean University (Department of Optoelectronic and Materials Technology) and Prof. Weiguo Zhu of Changzhou University (School of Materials Science and Engineering) and has recently been published in Nature Photonics.

Delayed fluorescence stems from harvesting the energies of triplet-states following thermal activation, hence contributes to nearly unity quantum yield and plays an important role in the development of organic light-emitting diode (OLED) materials. On this foundation, multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials exhibit both exceptional chromaticities and high quantum efficiencies, thus has attracted extensive research interests especially when compared to conventional TADF materials and phosphorescent metal complexes. However, it remains unclear why many MR-TADF molecules only exhibit TADF behavior when mixed with suitable host materials. The research team led by Prof. Chou systematically studied a series of novel MR-TADF compounds with spectroscopic approaches, including optical- and electrical-excited transient absorption and step-scan Fourier-transform transient absorption spectroscopies. A transient exciplex intermediate was found to form in host-guest systems fabricated by co-evaporation. As shown in Fig.1, this charge-transfer intermediate bridges the singlet and triplets states of the guest fluorescent molecules and accelerates both the intersystem crossing and the reverse intersystem crossing pathways, leading to the observed TADF phenomena.

To ensure the chromaticity remains unchanged while efficiently harvesting dark triplet excitons – this new performance-boosting mechanism provides potential of being generalized to other fluorescent dyes that posses thermally accessible singlet-triplet energy gap yet do not show TADF in photoluminescence measurements.

Art editor Img

Exciplex-like intermediate in host-guest systems facilitates intersystem crossing and reverse intersystem crossing between singlet and triplet states.