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

White organic light-emitting diodes (WOLEDs) using thermally activated delayed fluorescence (TADF)-based single-emissive-layer have attracted enormous attention because of their simple device structure and potential of full exciton utilization for high efficiency. However, WOLEDs made of an all-TADF SEL usually exhibit serious efficiency roll-off and poor color stability due to serious exciton annihilation and unbalanced radiative decays of different TADF emitters. Herein, a new strategy is proposed to manipulate the TADF-sensitized fluorescence process by combining dual-host systems of high triplet energy with a conventional fluorescent emitter of complementary color. The multiple energy-funneling paths are modulated and short-range Dexter energy transfer is largely suppressed due to the steric effect of peripheral tert-butyl group in 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 precisely allocating the exciton harvesting in single-emissive-layer WOLEDs.