Although conventional organic fluorescent 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran) (DCJTB) molecules have been widely employed for designing high-efficiency red or white organic light-emitting diodes (OLEDs), the reasons for their high performances are still vague because the existence of hot excitons and their high-level reverse intersystem crossing (i.e., $T_{2,\mathrm{DCJTB}}\to S_{1,\mathrm{DCJTB}}$) in DCJTB has not been previously recognized. Herein, a maximum external quantum efficiency (EQE) of 14.4% is achieved by co-doping the CzDBA sensitizer and the DCJTB emitter into the CBP host. Notably, this host-sensitizer-emitter-based OLED owns an excellent property of negligible efficiency roll-off; that is, the EQEs separately exhibit 14.0% and 13.6% at $500$ and $1000\text{cd}/{\mathrm{m}}^2$ as compared to the maximum EQE of 14.4%. These significant achievements are primarily attributed to the synergistic utilization of enhanced singlet- and hot-exciton energy through multiple exciton energy transfer channels enabled by inserting the CzDBA sensitizer with the thermally activated delayed fluorescence (TADF) property and appropriate charge-transfer state energy. These efficient energy transfer channels were demonstrated through spectroscopic analyses of the film materials, fingerprint magneto-electroluminescence (MEL) studies of the control devices, and analysis of the transient electroluminescence (TEL) response. Moreover, these host-sensitizer-emitter-based devices exhibit abundant variations in optical, electrical, and magnetic properties in the presence and absence of CzDBA TADF sensitizer, which also directly verify the crucial role of an appropriate sensitizer in fully harvesting hot excitons of DCJTB emitters. Thus, our work not only deepens the physical understanding of recently discovered hot-exciton channels from typical organic fluorescent semiconductor materials but also paves a promising pathway to fabricate further high-performance DCJTB-based red or white fully fluorescent OLEDs for lighting and flat-panel display applications.

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• Carrier generation & recombination
• Charge
• Electroluminescence
• Excitons
• Magnetic coupling
• Polarons
• Device fabrication
• Organic LEDs