High-Efficiency Blue Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence from Phenoxaphosphine and Phenoxathiin Derivatives

Two novel efficient blue thermally activated delayed fluorescence (TADF) emitters, Ac-OPO and Ac-OSO, consisting of phenoxaphosphine oxide and phenoxathiin dioxide as acceptor units and dimethylacridan as a donor unit, have been developed. Maximum N_ext values of 12.3% and 20.5% were achieved in blue TADF-OLEDs by employing Ac-OPO and Ac-OSO as TADF emitters, respectively. Moreover, the Ac-OSO-based blue-emitting device exhibits a very weak efficiency roll-off with N_ext over 13% even a luminance of 1000 cd m^-2. These results demonstrate that blue TADF emitters can provide new prospects as an attractive alternative to conventional blue fluorescent and phosphorescent materials in the future.