Twisted Structure and Multiple Charge-Transfer Channels Endow Thermally Activated Delayed Fluorescence Devices with Small Efficiency Roll-Off and Low Concentration Dependence.

Despite the rapid development of thermally activated delayed fluorescent (TADF) materials, developing organic light-emitting diodes (OLEDs) with small efficiency roll-off remains a formidable challenge. Herein, we have designed a TADF molecule (mClSFO) based on the spiro fluorene skeleton. The highly twisted structure and multiple charge-transfer channels effectively suppress aggregation-caused quenching (ACQ) and endow mClSFO with excellent exciton dynamic properties to reduce efficiency roll-off. Fast radiative rate (k_r) and rapid reverse intersystem crossing (RISC) rate (k_RISC) of 1.6×10⁷ s^-1 and 1.07×10⁶ s^-1, respectively, are obtained in mClSFO. As a result, OLEDs based on mClSFO obtain impressive maximum external quantum efficiency (EQE_max) exceeding 20 % across a wide doping concentration range of 10-60 wt %. 30 wt % doped OLED exhibits an EQE_max of 23.1 % with a small efficiency roll-off, maintaining an EQE of 18.6 % at 1000 cd m^-2. The small efficiency roll-off and low concentration dependence observed in the TADF emitter underscore its significant potential.