High efficiency and long lifetime orange-red thermally activated delayed fluorescent organic light emitting diodes by donor and acceptor engineering

Abstract

Highly efficient and stable thermally activated delayed fluorescence (TADF) emitters are in urgent demand for their commercial application. However, the simultaneous achievement of high external quantum efficiency (EQE) and long lifetime in single organic molecular design is a herculean task. In particular, it is even more difficult in the long wavelength TADF emitter designs due to the compulsory molecular designing rules. Herein, we designed and synthesized two orange-red TADF emitters, namely 2-(4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazol-5-yl)phenyl)-3-phenylquinoxaline-6,7-dicarbonitrile (6,7-DCNQx-DICz) and 2-(4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazol-5-yl)phenyl)-3-phenylquinoxaline-5,8-dicarbonitrile (5,8-DCNQx-DICz) by utilizing diindolocarbazole (DICz) and 6,7/5,8 dicyano substituted quinoxaline (DCQx) as a stable donor–acceptor combination. In particular, organic light emitting diodes (OLEDs) fabricated with 6,7-DCNQx-DICz as the TADF emitter achieved a high EQE up to 23.9% and a long lifetime of 931 h with up to 90% of its initial luminescence at 1000 cd m⁻². At the same time, a comparatively low EQE of 12.5%, but a longer lifetime of 1184 h was achieved by utilizing 5,8-DCNQx-DICz as the TADF emitter. Hence, the amalgamation of the rigidified DICz unit and DCQx unit can be considered as a chemical platform in the future design of highly efficient and long lifetime orange-red TADF OLEDs.