Novel carbazole-based multifunctional materials with a hybridized local and charge-transfer excited state acting as deep-blue emitters and phosphorescent hosts for highly efficient organic light-emitting diodes

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Vol. 9; no. 18; pp. 5899 - 597
• Main Authors: Zhou, Haitao, Yin, Mengna, Zhao, Zhenhong, Miao, Yanqin, Jin, Xin, Huang, Jinhai, Gao, Zhixiang, Wang, Hua, Su, Jianhua, Tian, He
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 14.05.2021
• Subjects: Carbazoles; Carrier transport; Charge transfer; Electroluminescence; Emission spectra; Emitters; Glass transition temperature; Industrial applications; Multifunctional materials; Organic light emitting diodes; Oxazole; Phosphorescence; Photoluminescence; Quantum efficiency; Thermal stability
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/d1tc00494h

In this work, two carbazole- and benzo[ d ]oxazole-based novel multifunctional materials with a hybridized local and charge-transfer (HLCT) characteristic, namely, OCI and OCT , which could act as deep-blue fluorophors and phosphorescent hosts, were first designed and synthesized. Both OCI and OCT display meritorious thermal stability with a glass transition temperature ( T g ) as high as 139 and 137 °C, respectively. The two compounds also reveal excellent bipolar carrier transport ability and emit deep-blue emission with photoluminescence quantum yields (PLQY) of 34.5% and 14.0%, respectively. Used as blue fluorophors, OCI - and OCT -based non-doped devices exhibit a very low turn-on voltage ( V on ) of 3.0 V and realize deep-blue emission with peaks located at 438-453 nm, and the optimized OCI-based device achieve the maximum external quantum efficiency ( η ext,max ) reaching 3.19%. Remarkably, doped blue devices show even deeper blue emission at 424 and 428 nm and the optimized OCI-based device achieves a significantly improved η ext,max of 5.06% with CIE y < 0.04, which could rank among the deepest blue HLCT fluorophors. Additionally, green phosphorescent OLEDs hosted by OCI and OCT reveal a low V on of 2.7 V with maximum current efficiencies ( η c,max ) of 49.43 cd A −1 ( η ext,max of 14.11%) and 52.81 cd A −1 ( η ext,max of 15.57%), respectively, which are superior to that of CBP ( V on of 3.0 V, η c,max of 51.76 cd A −1 and η ext,max of 12.88%). The superb electroluminescence (EL) performance of OCI and OCT as deep-blue HLCT emitters and phosphorescent hosts demonstrates that they have great potential for industrial applications. Two novel multifunctional HLCT materials, namely OCI and OCT , were presented and they proved to be better green hosts than CBP. Acting as a deep-blue emitter, the OCI -based device achieved an EQE of 5.06% and CIE y < 0.04 with an EL peak at 424 nm.