a AbstractEfficiency roll-off in blue organic light-emitting diodes especially at high brightness still remains a vital issue for which the excitons density-dependent mechanism of host materials takes most responsibility. Additionally, the efficiency roll-off leads to high power consumption and reduces the operating lifetime because higher driving voltage and current are required. Here, by subtly modifying the triphenylamine to oxygen-bridged quasi-planar structure, a novel thermally activated delayed fluorescence type blue host Tri-o-2PO is successfully developed. Efficiency roll-off based on Tri-o-2PO is ultralow with external quantum efficiency (EQE) just dropping by around 2% in the high luminance range from 1000 cd m(-2) to 10 000 cd m(-2). As expected, low turn-on voltage (approximate to 2.9 V) of device is also achieved, which is close to the theory limit value (approximate to 2.62 V). Super-high power efficiency (approximate to 60 lm W-1) and EQE (>22%) are also achieved when utilizing Tri-o-2PO as host. Furthermore, two-color warm-white light with CIE of (0.45, 0.43) and correlated color temperature of 2921 K is also fabricated and a champion EQE of 21% is delivered. These excellent performances prove the strategy of bridging the triphenylamine to reduce Delta E-st is validated and suggest the great potential of this novel skeleton.