Carbazole/Benzimidazole-Based Bipolar Molecules as the Hosts for Phosphorescent and Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs

Zhen-Jie Gao; Tzu-Hung Yeh; Jing-Jie Xu; Chih-Chien Lee; Anuradha Chowdhury; Bo-Cheng Wang; Shun-Wei Liu; Chih-Hsin Chen

doi:10.1021/acsomega.0c00967

Carbazole/Benzimidazole-Based Bipolar Molecules as the Hosts for Phosphorescent and Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs

ACS Omega. 2020 May 3;5(18):10553-10561. doi: 10.1021/acsomega.0c00967. eCollection 2020 May 12.

Authors

Zhen-Jie Gao¹, Tzu-Hung Yeh^{2

3}, Jing-Jie Xu¹, Chih-Chien Lee³, Anuradha Chowdhury², Bo-Cheng Wang¹, Shun-Wei Liu², Chih-Hsin Chen¹

Affiliations

¹ Department of Chemistry, Tamkang University, New Taipei City 251, Taiwan.
² Department of Electronic Engineering and Organic Electronics Research Center, Ming Chi University of Technology, New Taipei City 24301, Taiwan.
³ Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan.

Abstract

A series of carbazole/benzimidazole-based molecules, namely, o-CbzBiz, m-CbzBiz, and p-CbzBiz, were readily synthesized in three steps by integrating carbazole with benzimidazole via the ortho-, meta-, and para-positions of phenyl linked to N-phenyl carbazole. These bipolar molecules exhibited a maximum UV absorption band ranging from 310 to 327 nm and a maximum emission band ranging from 380 to 400 nm. Density functional theory calculations showed that the twist angles between the donor and acceptor moieties of these molecules were from 54.9 to 67.1°. Such a twisted structure hampered the π-electron conjugation within the molecule and resulted in high-lying LUMO levels and triplet energies, which make them suitable to be applied as host materials in OLED devices. Our results showed that a maximum external quantum efficiency (EQE) of OLED reached 21.8% when p-CbzBiz was applied as the host of a green phosphorescent emitter, i.e., Ir(ppy)₂(acac). In addition, a maximum EQE of OLED reached 16.7% when o-CbzBiz with the host of a green TADF emitter, i.e., 4CzIPN. Moreover, these devices exhibited lower efficiency roll-off than the CBP-hosted device using the same emitters, which demonstrated the bipolar charge carrier property of carbazole/benzimidazole-based molecules.