Shipan Wang

New multifunctional phenanthroimidazole–phosphine oxide hybrids for high-performance red, green and blue electroluminescent devices

In this work, two novel hybrids of an electron-accepting phosphine oxide moiety attached to a phenanthroimidazole have been designed and synthesized. The PO group is used as a point of saturation between the PPI moiety and the outer phenyl groups, so the high triplet energy of PPI is preserved to act as a host for red and green phosphorescent dopants. The strong intermolecular interactions and steric effect of the diphenylphosphine oxide (DPO) moiety endows the films with high quantum yields in the deep-blue emission region. Compared to PPI, the carrier (hole- and electron-)injection/transport properties were greatly promoted by the appended DPO group according to single-carrier device measurement. Besides, the morphological and thermal stabilities were also improved. The multiple functions enable adaptation of several simplified device configurations. The undoped deep-blue fluorescent device exhibits an external quantum efficiency of 2.24% with CIE (0.16, 0.08), very close to the NTSC blue standard CIE (0.14, 0.08). High performance for green (65.4 cd A−1, 73.3 lm W−1 and 18.0%) and red (19.0 cd A−1, 21.3 lm W−1 and 13.5%) phosphorescent devices used as hosts have been achieved. The experimental and theoretical relationships between the molecular structures and the optoelectronic properties are discussed.

Deep‐Red to Near‐Infrared Thermally Activated Delayed Fluorescence in Organic Solid Films and Electroluminescent Devices

The design and synthesis of highly efficient deep red (DR) and near-infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a great challenge. A strategy was developed to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline-6,7-dicarbonitrile (QCN) were employed as electron donor (D) and acceptor (A), respectively, to synthesize a TADF compound, TPA-QCN. The TPA-QCN molecule with orange-red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light-emitting devices (OLEDs) were fabricated by regulating TPA-QCN dopant concentration in the emitting layers.

Two-Dimensional Organic Single Crystals with Scale Regulated, Phase-Switchable, Polymorphism-Dependent, and Amplified Spontaneous Emission Properties

The successful preparation of two-dimensional (2D) single crystals can promote the development of organic optoelectronic devices with excellent performance. A Schiff base compound salicylidene(4-dimethylamino)aniline with aggregation induced emission (AIE) property was employed as the building block to fabricate 2D thin single crystal plates with scales from around 50 μm to 1.5 cm. Yellow and red emissive polymorphs were concomitantly obtained during crystallization. The single-crystal-to-single-crystal (SC-to-SC) transformation from yellow polymorph to red one was demonstrated. Furthermore, both polymorphs exhibited amplified spontaneous emission (ASE) properties. Interestingly, the red polymorph displayed size-dependent ASE characteristics. The larger red polymorph showed near-infrared ASE with maximum at 706 nm, whereas the smaller one presented red ASE with maximum at 610 nm. These results suggest that the different scale single crystalline thin films with perfect optoelectronic properties may be fabricated by using the organic molecules with 2D assembly feature.

Structurally simple phenanthroimidazole-based bipolar hosts for high-performance green and red electroluminescent devices

With the aim of developing bipolar host materials, m-DPPI with two phenanthroimidazole groups meta-linked on a benzene ring and its donor–acceptor-type analogue, m-PPPI, with one of the phenanthroimidazole moieties replaced by an electron-withdrawing imidazophenanthroline group have been synthesized. The meta-linkage endows these compounds with relatively high triplet energies of ca. 2.6 eV for hosting both green and red phosphorescent emitters. The single-carrier devices indicate that both the neat and phosphor-doped films of these compounds have bipolar transporting properties, with relatively higher current densities for the films of m-DPPI. Efficient green (maximum efficiencies: 58.5 cd A−1, 67.1 lm W−1, external quantum efficiency (ηext) of 15.5%) and red (maximum efficiencies: 19.7 cd A−1, 26.9 lm W−1, ηext of 11.6%) PhOLEDs have been achieved using m-DPPI as the host. The performance of m-PPPI-based devices is also high, although a bit lower than that of m-DPPI-based devices. Moreover, all the devices exhibit low current-efficiency and ηext roll-off. The simple structures, easy syntheses and high device performance of these compounds are attractive regarding practical applications.

A diphenylamino-substituted quinacridone derivative: red fluorescence based on intramolecular charge-transfer transition

Quinacridone (QA) derivatives are long-established emitting materials for use in organic light-emitting diodes (OLEDs). However, their role in OLEDs has generally been limited to green/yellow-green emitters. Herein, we report a QA-based red emissive OLED material, NPh2-QA, constructed through the introduction of two strongly electron-donating diphenylamino groups onto the QA core of green emissive N,N′-dioctyl-substituted QA. The green-to-red change is due to an alteration of the electronic transition responsible for the population of the S1 state, i.e. from a QA-centered π → π* transition to an intramolecular charge-transfer transition. NPh2-QA can be easily synthesized and it exhibits efficient red emission, with a fluorescence quantum yield of 0.56 in toluene, as well as good thermal stability, with a decomposition temperature of 444 °C. By adopting NPh2-QA as a dopant emitter, for the first time, an efficient and bright QA-based red OLED has been realized.

One-Compound-Based Highly Efficient Deep-Red to Near-Infrared Thermally Activated Delayed Fluorescence Organic Solid Films and Electroluminescent Devices

The design and synthesis of highly efficient deep red (DR) and near-infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a great challenge. In this contribution, we developed a strategy to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline-6,7-dicarbonitrile (QCN) were respectively employed as electron donor (D) and acceptor (A) to synthesize a TADF compound, TPA-QCN. The TPA-QCN molecule with orange-red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light-emitting devices (OLEDs) were fabricated by regulating TPA-QCN dopant concentration in the emitting layers.

A dibenzo[a,c]phenazine-11,12-dicarbonitrile (DBPzDCN) acceptor based thermally activated delayed fluorescent compound for efficient near-infrared electroluminescent devices

In this paper, a near-infrared (NIR) TADF molecule with dibenzo[a,c]phenazine-11,12-dicarbonitrile (DBPzDCN) as the acceptor has been designed, synthesized and characterized. Connecting the DBPzDCN core with a diphenylamine (DPA) donor, NIR emission is successfully achieved. The relationship between the molecular structure and optoelectronic properties is discussed experimentally and theoretically. A high external quantum efficiency (EQE) of 7.68% with an emission peak at 698 nm and Commission Internationale de L’ Eclairage (CIE) coordinates of (0.68, 0.30) was achieved in a NIR organic light-emitting diode.