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

Abstract

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 a carbazole unit into the rigid 15H-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-9H-carbazol-3-yl)-15H-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-(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 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 (L₀ = 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.