Molecular design strategies for the highly efficient vacuum and solution-processable long wavelength thermally activated delayed fluorescence emitters

Simple device architecture-based solution-processed organic light-emitting diodes (OLEDs) are more advantageous over the multilayer vacuum-processed OLEDs for their commercial application. Highly efficient long wavelength thermally activated delayed fluorescent (TADF) emitters suitable for both vacuum and solution-processed OLEDs are rarely reported because of the stringent molecular design rules and the inherent limitation of the energy gap law. To resolve this issue, we designed and synthesized two TADF emitters namely, 5,8-bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-6,7-dicarbonitrile (2tBuTPA-QxCN) and 2,3,5,8-tetrakis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)quinoxaline-6,7-dicarbonitrile (4tBuTPA-QxCN), for their use in both vacuum and solution-processed OLEDs. High external quantum efficiency (EQE) of 22 % and emission peak at 608 nm were demonstrated for the vacuum-deposited OLED fabricated with 4tBuTPA-QxCN TADF emitter as compared to the OLED fabricated with 2tBuTPA-QxCN TADF emitter exhibiting EQE value of 21.1 % and emission peak at 605 nm. In their solution-processed OLEDs, a high EQE of 15.6 % and pure-red emission peak at 630 nm was demonstrated for the 2tBuTPA-QxCN TADF emitter as compared to the EQE value of 11.6 % and emission peak at 628 nm for the 4tBuTPA-QxCN TADF emitter.