Boosting the Efficiency of Near-Infrared Fluorescent OLEDs with an Electroluminescent Peak of Nearly 800 nm by Sensitizer-Based Cascade Energy Transfer

A sensitization-based cascade energy transfer channel is proposed to boost the electroluminescent performances of the solution-processed near-infrared organic light-emitting devices (OLEDs) featuring an electroluminescent peak of 786 nm from a new fluorescent emitter of N-4,N-4,N-9,N-9-tetra-p-tolylnaphtho[2,3-c][1,2,5]thiadiazole-4,9-diamine (NZ2mDPA) with unique aggregation-induced emission (AIE) property. The optimized device is composed of 4,4-N,N-dicarbazole-biphenyl (CBP) as the host, bis(2-phenyl-1,3-benzothiozolato-N,C-2)iridium (Ir(bt)(2)(acac)) as the sensitizer, and NZ2mDPA as the emitter, where the cascade energy transfer can occur via two steps realizing unexpected triplet-singlet energy transfer by the Forster mechanism. The first step features efficient triplet harvesting from CBP to Ir(bt)(2)(acac), and then the second step involves in resonant energy transfer from the phosphorescent sensitizer to the near-infrared AIE emitter of NZ2mDPA, which finally endows two channels of harvesting singlet and triplet excitons. The unique scheme achieves not only more efficient Forster energy transfer but also the higher utilization efficiency of triplet excitons. As a result, the near-infrared OLEDs can realize a factor of 2.7 enhancement of external quantum efficiency by employing the phosphor-sensitized AIE lumogen compared with the commonly used binary host-guest system.