As one of the three primary colors, blue is significant for display and lighting applications and the development of blueemitting molecules with high quantum efficiency, and color purity for organic light-emitting diodes (OLEDs) remains a key challenge. Herein, a new series of blue thermally activated delayed fluorescence (TADF) materials featuring π-extended ladderoxaborin and thiaborin acceptors has been developed. Steadystate and time-resolved photophysical measurements revealed the advantages of the ladder-oxaborin emitter, including a tiny singlet− triplet energy splitting of 10 meV, an ideal photoluminescence quantum yield of 100%, and an ultrashort TADF lifetime of 780 ns. The TADF-OLEDs incorporating the ladder-oxaborin (deep-blue) and thiaborin (sky-blue) emitters achieved significantly high external electroluminescence quantum efficiencies of up to 20.1% and 25.9%, respectively, accompanied by suppressed efficiency roll-offs.