Thermally activated delayed fluorescence (TADF) materials have continued to attract interest due to their rapid improvements in efficiency, non-toxic and potential to be manufactured at energy-saving organic light-emitting diode for full-color display and solid-state lighting. Here, The bipolar and electroluminescence (EL) characteristics of TADF host, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), were investigated. By incorporating the phosphorescent tris[2-(p-tolyl)pyridine]iridium(III) [Ir(mppy)₃] dopant into the matrix, DPDDC showed more excellent EL performance compared with the typical 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host. The turn-on voltage for phosphorescent organic light-emitting diodes (PhOLEDs) was decreased to 2.2 V from 2.5 V by exploiting the TADF host instead of CBP. Crucially, we found that the ratio of Ir(mppy)₃ in TADF host plays a significant role in EL process. Remarkable low efficiency roll-off was achieved with above 90% of the current efficiency maintained even at a brightness of 3000 cd m⁻², along with high power efficiency of 84.8 lm·W⁻¹ and the external quantum efficiency of 20.6% for DPDDC: 5 wt% Ir(mppy)₃-based device.