Thermally Activated Delayed Fluorescence Material as Host with Novel Spiro‐Based Skeleton for High Power Efficiency and Low Roll‐Off Blue and White Phosphorescent Devices

Efficiency 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 (≈2.9 V) of device is also achieved, which is close to the theory limit value (≈2.62 V). Super‐high power efficiency (≈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 ΔE_st is validated and suggest the great potential of this novel skeleton.