Blue Thermally Activated Delayed Fluorescence‐Emitting Phosphine Oxide Hosts for Ultrasimple and Highly Efficient White Organic Light‐Emitting Diodes

Abstract Among a series of carbazole–diphenylphosphine oxide hybrids x 2ArPO , blue thermally activated delayed fluorescence (TADF)‐emitting host m 2tBCzPO with optimal electron‐withdrawing and steric effects is successfully constructed to demonstrate the superiority of blue TADF‐emitting host in adaptable exciton allocation for white light‐emitting diodes (WOLEDs), which realizes the negligible singlet–triplet splitting (Δ E ST ) of 0.02 eV, the photoluminescence quantum yield beyond 75% and the top‐rank external quantum efficiency (EQE) with the maxima of 13.8% and 22.2% from its blue and yellow devices. Its high‐efficiency blue TADF emission and excellent host characteristics are perfectly harmonized in its ultrasimple four‐layer WOLED with monodoping single‐emissive layer to support the state‐of‐the‐art performance, including EQE up to 21.3%, roll‐off within 10% at 1000 cd m −2 , and excellent spectral stability.