A Red Fluorescent Emitter with a Simultaneous Hybrid Local and Charge Transfer Excited State and Aggregation-Induced Emission for High-Efficiency, Low Efficiency Roll-Off OLEDs

Most red/deep-red fluorescent organic light-emitting diodes (OLEDs) suffer from a low exciton utilization efficiency (η_γ) and a drastic efficiency roll-off at high brightness. This work reports a new red fluorescent emitter with a D–π–A–π–D architecture, namely, 4,9-bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)naphtho[2,3-c][1,2,5]thiadiazole (NZ2AC). The new emitter shows a hybrid local and charge transfer (HLCT) excited state, which can utilize the triplet excitons by the reverse intersystem cross process via the high-lying triplet channel. A red OLED with an emission peak at 612 nm achieves a maximum external quantum efficiency (EQE) of 6.2% at a doping concentration of 8 wt% NZ2AC in a 4,4′-bis(9-carbazolyl)-2,2′-biphenyl host. Moreover, the new emitter reveals a typical aggregation-induced emission (AIE) property, and consequently, the nondoped OLEDs exhibit a deep-red emission at 663 nm with a maximum EQE of 2.8%, corresponding to a maximum exciton utilization ratio of 93%. Attributed to the simultaneous HLCT and AIE features, both the doped and nondoped devices exhibit low efficiency roll-off at high brightness, with their EQEs remaining at high values of 3.0% and 2.3% at the high luminance of 5000 cd m⁻², respectively, which are among the highest efficiencies at such high luminance for red/deep-red OLEDs.