Oxadiazole- and triazole-based highly-efficient thermally activated delayed fluorescence emitters for organic light-emitting diodes

We developed highly-efficient thermally activated delayed fluorescence (TADF) emitters containing 2,5-diphenyl-1,3,4-oxadiazole (OXD) or 3,4,5-triphenyl-4H-1,2,4-triazole (TAZ) electron acceptor and phenoxazine (PXZ) electron donor moieties. Oxadiazole-based compounds PXZ-OXD and 2PXZ-OXD showed green emission, while the triazole-based ones PXZ-TAZ and 2PXZ-TAZ exhibited sky-blue emission. In toluene solution, the donor–acceptor–donor-type molecules 2PXZ-OXD and 2PXZ-TAZ showed more efficient TADF and higher photoluminescence quantum yields (PLQYs) than the donor–acceptor-type molecules PXZ-OXD and PXZ-TAZ. When doped into a host material, 2PXZ-OXD displayed a high PLQY of 87%. An organic light-emitting diode using 2PXZ-OXD as an emitter exhibited an external quantum efficiency (EQE) of 14.9%, which exceeds those obtained with conventional fluorescent emitters. This high EQE results from the efficient generation of TADF in 2PXZ-OXD.