Versatile Indolocarbazole‐Isomer Derivatives as Highly Emissive Emitters and Ideal Hosts for Thermally Activated Delayed Fluorescent OLEDs with Alleviated Efficiency Roll‐Off

Maintaining high efficiency at high brightness levels is an exigent challenge for real-world applications of thermally activated delayed fluorescent organic light-emitting diodes (TADF-OLEDs). Here, versatile indolocarbazole-isomer derivatives are developed as highly emissive emitters and ideal hosts for TADF-OLEDs to alleviate efficiency roll-off. It is observed that photophysical and electronic properties of these compounds can be well modulated by varying the indolocarbazole isomers. A photoluminescence quantum yield (ηPL ) approaching unity and a maximum external quantum efficiency (EQEmax ) of 25.1% are obtained for the emitter with indolo[3,2-a]carbazolyl subunit. Remarkably, record-high EQE/power efficiency of 26.2%/69.7 lm W-1 at the brightness level of 5000 cd m-2 with a voltage of only 3.74 V are also obtained using the same isomer as the host in a green TADF-OLED. It is evident that TADF hosts with high ηPL values, fast reverse intersystem crossing processes, and balanced charge transport properties may open the path toward roll-off-free TADF-OLEDs.