Fluorescent emitters have regained intensive attention in organic lightemitting diode (OLED) community owing to the breakthrough of the device efficiency and/or new emitting mechanism. This provides a good chance to develop new near-infrared (NIR) fluorescent emitter and high-efficiency device. In this work, a D-π-A-π-D type compound with naphthothiadiazole as acceptor, namely, 4,4′-(naphtho[2,3-c][1,2,5]thiadiazole-4,9-diyl)bis(N,Ndiphenylaniline) (NZ2TPA), is designed and synthesized. The photophysical study and density functional theory analysis reveal that the emission of the compound has obvious hybridized local and charge-transfer (HLCT) state feature. In addition, the compound shows aggregation-induced emission (AIE) characteristic. Attributed to its HLCT mechanism and AIE characteristic, NZ2TPA acquires an unprecedentedly high photoluminescent quantum yield of 60% in the neat film, which is the highest among the reported organic small-molecule NIR emitters and even exceeds most phosphorescent NIR materials. The nondoped devices based on NZ2TPA exhibit excellent performance, achieving a maximum external quantum efficiency (EQE) of 3.9% with the emission peak at 696 nm and a high luminance of 6330 cd m −2 , which are among the highest in the reported nondoped NIR fluorescent OLEDs. Moreover, the device remains a high EQE of 2.8% at high brightness of 1000 cd m −2 , with very low efficiency roll-off.
To achieve high efficiencies in blue phosphorescent organic light-emitting diodes (PhOLEDs), the triplet energies (T1) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S1) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6-difluorophenylpyridinato-N,C(2'))iridium(III)] tetrakis(1-pyrazolyl)borate (FIr6; T1=2.73 eV). In both the photo- and electro-excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic-layer device, the maximum current efficiency of 37 cd A(-1) and power efficiency of 40 Lm W(-1) were achieved for the FIr6-based blue PhOLEDs.
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 9tetra-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(2phenyl-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 Förster 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 Förster 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.
Most red/deep‐red fluorescent organic light‐emitting diodes (OLEDs) suffer from a low exciton utilization efficiency (ηγ) 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.
Simple is good! Based on biphenyl molecules, two bipolar host materials with high triplet energies have been rationally designed, synthesized, and fully characterized. Deep blue phosphorescent organic light-emitting diodes, which employ the new hosts and an iridium(III) complex as triplet emitter, show a maximum current efficiency of 40 cd A(-1), a maximum power efficiency of 36 lm W(-1), and a maximum external quantum efficiency of 19.5 %.
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