Asymmetric Phosphorylation Integrating Host Feature and Blue Thermally Activated Delayed Fluorescence of Thiaxanthene S,S-dioxide Emitter for Efficient Heavily Doped White Organic Light-Emitting Diodes

Significant efforts have been recently devoted to develop simply structured white organic light-emitting diodes (WOLEDs) based on purely thermally activated delayed fluorescence (TADF) with a single-emissive-layer and heavy-doping configuration. Herein, through introducing a diphenylphosphine oxide group on thiaxanthene S,Sdioxide acceptor, we integrate host characteristics to blue TADF emitter CZ-MPSPO, which is beneficial to balanced exciton allocation and quenching suppression in single emissive layers of TADF WOLEDs for high performance. After co-doping with a conventional yellow TADF emitter, TADF performance of CZ-MPSPO is preserved, but external quantum efficiencies (EQE) of WOLEDs are remarkably improved through the synergistic singlet and triplet exciton harvesting by CZ-MPSPO and 4CzTPNBu. The steric hindrance of CZ-MPSPO also mitigates emission quenching by 4CzTPNBu, leading to the favourable EQE up to similar to 20 % of single-EML pure TADF WOLEDs with the doping concentration of CZ-MPSPO as high as 70 %.