Molecular design featuring carbazole-decorated 15 H -diindolo[2,3- b :1′,2′,3′- lm ]carbazole for improved efficiency and lifetime of thermally activated delayed fluorescence emitters

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2020-02, Vol.8 (7), p.2491-2499
• Main Authors: Konidena, Rajendra Kumar, Lee, Kyung Hyung, Lee, Jun Yeob
• Format: Article
• Language: English
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/C9TC05855A

In this report, we developed a novel electron donor named 12-(9-phenyl-9 H -carbazol-3-yl)-15 H -diindolo[2,3- b :1′,2′,3′- lm ]carbazole ( CzDICz ) by introducing a carbazole unit into the rigid 15 H -diindolo[2,3- b :1′,2′,3′- lm ]carbazole ( DICz ) donor for designing a stable green thermally activated delayed fluorescence (TADF) emitter, 5-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-(12-(9-phenyl-9 H -carbazol-3-yl)-15 H -diindolo[2,3- b :1′,2′,3′- lm ]carbazol-15-yl)benzonitrile ( CzDICzTrz ). The effect of the additional carbazole unit in the CzDICz donor on photophysical and electroluminescence properties was unveiled by comparing the CzDICzTrz with the control molecule viz. 2-(15 H -diindolo[2,3- b :1′,2′,3′- lm ]carbazol-15-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile ( DICzTrz ) featuring the DICz donor. Photophysical analysis revealed that the CzDICzTrz exhibited small singlet–triplet energy splitting, a short delayed fluorescence lifetime ( t d ), fast reverse intersystem crossing (RISC) and high photoluminescence quantum efficiency compared to DICzTrz . Therefore, the organic light emitting diode fabricated with CzDICzTrz showed superior device performance with a maximum external quantum efficiency (EQE) of 19.9% and a current efficiency of 62.3 cd A −1 compared to its congener DICzTrz , which showed a maximum EQE of 12.9% and a current efficiency of 38.6 cd A −1 . Moreover, CzDICzTrz based devices achieved 40 times extended operational lifetime up to 80% of its initial luminance ( L 0 = 1000 cd m −2 ) of 446 h compared to 11 h of DICzTrz due to its stable CzDICz donor, short t d and fast RISC. We believe that the CzDICz donor can contribute to the development of efficient TADF emitters with long operational stability and uncompromised EQE.