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-benzo­c­chromen-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–2, 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.