Novel thioxanthone host material with thermally activated delayed fluorescence for reduced efficiency roll-off of phosphorescent OLEDs

2,7-Di（9,9-dimethyl-9 H-fluoren-1-yl）-9 H-thioxanthen-9-one（DMBFTX） with thermally activated delayed fluorescence（TADF） was well designed and synthesized. The phosphorescent organic lightemitting device（PHOLED） based on this novel TADF host material displays a stable red phosphorescence region, a peak external quantum efficiency（EQE） value of 12.9% and a low EQE roll-off of 38.8% at a luminance of 10000 cd/m;, which is benefited from the reverse intersystem crossing（RISC） of TADF host and less populated triplet exitons. Notably, the red device based on the TADF host DMBFTX exhibits superior electroluminescence performance and reduced efficiency roll-off compared with the one hosted by commercially available host 1,3-bis（9-carbazolyl）benzene（mCP）, illustrating the high potential of employing the TADF host material with small energy gap to reduce efficiency roll-off in PHOLED.