Ultralow Roll-off Thermally Activated Delayed Fluorescent Light-Emitting Diodes Based on Furo[2,3-b]quinoxaline Emitters

Herein, a Y-type compound (67dMeOTPA-FQ) and a T-type compound (58dMeOTPA-FQ) based on furo[2,3-b]quinoxaline were synthesized. The theory calculation shows the S₁ and T₁ of both compounds own a charge-transfer feature while their T₂ states have a local excitation feature. The calculated k_RISC(T2-S1) is one to 2 orders of magnitude larger than k_RISC(T1-S1). Thus, the nonadiabatic spin-vibronic mechanism involved in the T₂ state is suggested to be responsible for the thermally activated delayed fluorescence (TADF) feature. Meanwhile, when 2-methyl-9,10-bis(naphthalen-2-yl)anthracene is selected as host, the maximum luminance of the device based on 67dMeOTPA-FQ is up to 104215 cd·m^-2, and the external quantum efficiency (EQE) keeps in the 8.2-8.0% range with the luminance changed from 55.0 cd·m^-2 to 90000 cd·m^-2, only 2.4% efficiency roll-off. As for 58dMeOTPA-FQ, a slightly lower EQE of 7.1-6.7% with the luminance range of 1-40000 cd·m^-2 was achieved for orange-red emission. Both the reverse intersystem crossing (RISC) and triplet-triplet annihilation mechanisms are supposed to concurrently contribute to the utilization of triplet excitons and suppress the notorious efficiency roll-off observed in TADF-based devices.