Red Fluorescent Organic Light-Emitting Diodes with Low-Efficiency Roll-Off

Sensitizing conventional fluorescent molecules is an effective way to improve the efficiency of fluorescent-based organic light-emitting diode (OLED) devices. The relatively long triplet exciton lifetime of thermally activated delayed fluorescence (TADF) emitters can cause serious efficiency roll-off. In this work, we use an efficient TADF emitter 2,7-TXO-PhCz to sensitize a conventional fluorescence emitter tetraphenyldibenzoperiflanthene (DBP). The energy transfer efficiency from the singlet excited state of the TADF sensitizer to the singlet excited state of the fluorescence guest can reach up to 90%. Profiting from the efficient energy transfer process, the reverse intersystem crossing can be accelerated and the delayed lifetime of the TADF sensitizer can be reduced. The TADF-sensitized fluorescent OLEDs can achieve a maximum external quantum efficiency (EQE) of 15.9% with reduced efficiency roll-off. Besides, the sensitization efficiency can achieve 77%, which indicates almost pure fluorescent emission. Our results provide an efficient approach to realize highly efficient OLEDs with low roll-off and can be helpful to overcome the limitation of TADF emitters.