Thermally activated delayed fluorescence (TADF) emitters have attracted much interest for their great applications in organic light-emitting diodes (OLEDs), but the TADF OLEDs are limited by large efficiency roll-offs. In this study, we report two coumarin-based TADF emitters, 3-methyl-6-(10H-phenoxazin-10-yl)-1H-isochromen-1-one (PHzMCO) and 9-(10H-phenoxazin-10-yl)-6H-benzo[c]chromen-6-one (PHzBCO), with relatively high photoluminescence quantum yields (PLQYs) and extremely small singlet-triplet splittings. OLEDs using these two TADF compounds as the emitters respectively demonstrate high external quantum efficiencies of 17.8% for PHzMCO and 19.6% for PHzBCO, which are the highest among the reported coumarin-derivative-based OLEDs. More importantly, these devices based on PHzMCO and PHzBCO remained 10.3% and 12.9% at 10000 cd m, respectively, showing relatively low efficiency roll-offs at high brightness. These results reveal that the TADF emitters with high PLQYs can effectively reduce the efficiency roll-off in the devices.
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