A strategy for designing highly efficient thermally activated delayed fluorescence (TADF) emitters was reported. TADF emitters with donor-acceptor-donor (D-A-D)-type structures showed highly efficient TADF because of their small singlet-triplet energy gap and large oscillator strength. An organic light-emitting diode containing a D-A-D-type TADF emitter, cis-BOX2, exhibited a high external electroluminescence quantum efficiency of 17.6%.