Molecular Design Featuring Carbazole-decorated 15H-diindolo[2,3-b:1',2',3'-lm]carbazole for Improved Efficiency and Lifetime of Thermally Activated Delayed Fluorescence Emitters
In this report, we developed a novel electron donor named 12-(9-phenyl-9H-carbazol-3-yl)-15H-diindolo[2,3-b:1',2',3'-lm]carbazole (CzDICz) by introducing carbazole unit into the rigid 15H-diindolo[2,3-b:1',2',3'-lm]carbazole (DICz) donor for designing stable green thermally activated delayed fluorescence (TADF) emitter, 5-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-(12-(9-phenyl-9H-carbazol-3-yl)-15H-diindolo[2,3-b:1',2',3'-lm]carbazol-15-yl)benzonitrile (CzDICzTrz). The effect of additional carbazole unit in CzDICz donor on photophysical and electroluminescence properties was unveiled by comparing the CzDICzTrz with the control molecule viz. 2-(15H-diindolo[2,3-b:1',2',3'-lm]carbazol-15-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (DICzTrz) featuring DICz donor. The photophysical analysis revealed that the CzDICzTrz exhibited small singlet-triplet energy splitting, short delayed fluorescence lifetime (td), 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 maximum external quantum efficiency (EQE) of 19.9% and current efficiency of 62.3 cd/A compared to its congener DICzTrz, which showed maximum EQE of 12.9% and current efficiency of 38.6 cd/A. Moreover, CzDICzTrz based device achieved 40 times extended operational lifetime up to 80% of its initial luminance (L0 = 1000 cd/m2) of 446 h compared to 11 h of DICzTrz due to its stable CzDICz donor, short td 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.