Highly Efficient Simple-Structure Sky-Blue Organic Light-Emitting Diode Using a Bicarbazole/Cyanopyridine Bipolar Host

Abstract

Up to now, the most efficient blue phosphorescent organic lightemitting diode (PhOLED) was achieved with a maximum external quantum efficiency (η ext ) of 34.1% by using an exciplex cohost. It still remains a challenge to obtain such high efficiencies using a single-host matrix. In this work, a highly efficient sky-blue PhOLED is successfully fabricated using a newly developed bipolar host material, namely 5-(2-(9H-[3,9′-bicarbazol]-9-yl)phenyl)nicotinonitrile (o-PyCNBCz), which realizes a η ext of 29.4% at a practical luminance of 100 cd m −2 and a maximum η ext of 34.6% (at 23 cd m −2 ). The present device is characterized by simple configuration with a single host and single emitting layer. o-PyCNBCz also reveals high efficiency of 28.2% (94.8 cd A −1 ) when used as the host for green PhOLED. Under identical conditions, o-PyCNBCz always outperforms than its isomer 3-PyCNBCz (5-(9-phenyl-9H-[3,9′-bicarbazol]-6-yl)nicotinonitrile) in terms of more balanced charge transportation, higher photoluminescent quantum yields of over 90%, and higher horizontal orientation ratio of the emitting dipole for the host-dopant films, which finally lead to its superior performance in PhOLEDs. It is observed that all these merits of o-PyCNBCz benefit from its ortho-linking style of carbazole (p-type unit) and cyanopyridine (n-type unit) on the phenylene bridge and the resultant molecular conformation.

Self‐Host Thermally Activated Delayed Fluorescence Material with Aggregation‐Induced Emission Character: Multi‐Functional Applications in OLEDs

“…The two reference compounds were synthesized according to the literature methods. [31,32] PBCz-BP-DMAC was prepared following the procedure in Scheme 1. The intermediate 1 was synthesized by Ullmann coupling of carbazole with 3,6-dibromo-9-phenyl-9H-carbazole.…”

“…The intermediate 1 was synthesized by Ullmann coupling of carbazole with 3,6-dibromo-9-phenyl-9H-carbazole. [32] The nucleophilic addition of intermediate 1 with 4-bromobenzaldehyde generated (4-bromophenyl)(9-phenyl-9H-[3,9′-bicarbazol]-6-yl)methanol, which without further purification or characterization was then oxidized by pyridinium chlorochromate to produce the important intermediate 2. The final product PBCz-BP-DMAC was obtained via coupling reaction of compound 2 with DMAC in the presence of Pd 2 (dba) 3 catalyst.…”

“…The hole-transporting PBCz that has been used to build excellent host materials was introduced to enhance the charge transporting and balancing ability of the target emitter. [32] At the same time, the bulky nature of PBCz unit favors to generate a 3D non-planar conformation for PBCz-BP-DMAC, which is basically required to suppress the unwanted intermolecular interactions and ACQ for the efficient emitters in neat films. It was proved that the incorporation of PBCz unit endows PBCz-BP-DMAC the excellent charge transportation and self-host capability.…”

High-efficiency solution-processed green thermally activated delayed fluorescence OLEDs using a polymer-small molecule mixed host

“…Therefore, RISC can be facilitated, and mixed host systems with an exciplex generally exhibit TADF characteristics. [12][13][14] The triplet harvesting efficiency is enhanced by RISC in such a host, which reduces the tripletpolaron and TTA, thereby decreasing the efficiency roll-off at high luminance. 15,16 The mixed host with an exciplex tunes the ratio of the donor and acceptor hosts to facilitate facile charge balance and possesses a wide recombination zone.…”

A meta-linkage strategy towards high-performance hosts for efficient blue thermally activated delayed fluorescence OLEDs

“…[15][16][17] To obtain high triplet energy for a host material, the molecular conjugation must be restricted, and also, the formation of intermolecular excitation such as excimers should be avoided. 9,18,19 These requirements make the design of host materials for triplet-excited-state-involved blue emitters particularly challenging since lowering the conjugation and reducing intermolecular interactions may negatively affect the charge transport properties, which in turn results in inferior device performance.…”