Management of Delayed Fluorophor-Sensitized Exciton Harvesting for Stable and Efficient All-Fluorescent White Organic Light-Emitting Diodes

White organic light-emitting diodes (WOLEDs) using thermally activated delayed fluorescence (TADF)-based single emissive layer (SEL) have attracted enormous attention because of their simple device structure and full exciton utilization potential for high efficiency. Howeirer, WOLEDs made of an allTADF SEL usually exhibit serious efficiency roll-o1T and poor color tabilit)r due to serious exciton-annihilation and unbalanced adiative decays of different TADF emitters. Herein, a new strategyproposed to manipulate the TADF-sensitized fluorescence process by combining dual-host systems of high triplet energy th a conventional fluorescent emitter of complementary color. The multiple energy-funneling paths are modulated and shortrange Dexter energy transfer is largely suppressed due to the steric effect of peripheral tert-butyl group in the blue TADF sensitizer. The resulting all-fluorescent WOLEDs achieve an unprecedentedly high external quantum efficiency of 21.8% with balanced white emission of Commission Internationale de l'Eclairage coordinate of (0.292, 0.343), accompanied with good color stability, reduced efficiency roll-off, and prolonged operational lifetime. These findings demonstrate the validity of this strategy for lat-ecisely allocating the exciton harvesting in SEL WOLEDs.