Highly efficient thermally activated delayed fluorescence (TADF) devices are developed by engineering mixed host materials in the emitting layer. Mixed hosts with deep highest occupied molecular orbital and high singlet energy without any exciplex formation are ideal as the host material for the TADF organic light-emitting diodes. A high external quantum efficiency of 28.6% is achieved in the green TADF organic light-emitting diodes using a mixed host of 1,3-bis(N-carbazolyl) benzene: 1,3,5-tri[(3-pyridyl)-phen-3-yl] benzene and green emitting (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile TADF emitter.