Shitong Zhang

Highly Efficient Near‐Infrared Organic Light‐Emitting Diode Based on a Butterfly‐Shaped Donor–Acceptor Chromophore with Strong Solid‐State Fluorescence and a Large Proportion of Radiative Excitons

The development of near-infrared (NIR) organic light-emitting diodes (OLEDs) is of growing interest. Donor-acceptor (D-A) chromophores have served as an important class of NIR materials for NIR OLED applications. However, the external quantum efficiencies (EQEs) of NIR OLEDs based on conventional D-A chromophores are typically below 1 %. Reported herein is a butterfly-shaped D-A compound, PTZ-BZP. A PTZ-BZP film displayed strong NIR fluorescence with an emission peak at 700 nm, and the corresponding quantum efficiency reached 16 %. Remarkably, the EQE of the NIR OLED based on PTZ-BZP was 1.54 %, and a low efficiency roll-off was observed, as well as a high radiative exciton ratio of 48 %, which breaks through the limit of 25 % in conventional fluorescent OLEDs. Experimental and theoretical investigations were carried out to understand the excited-state properties of PTZ-BZP.

π‐Conjugated Microporous Polymer Films: Designed Synthesis, Conducting Properties, and Photoenergy Conversions

Conjugated microporous polymers are a unique class of polymers that combine extended π-conjugation with inherent porosity. However, these polymers are synthesized through solution-phase reactions to yield insoluble and unprocessable solids, which preclude not only the evaluation of their conducting properties but also the fabrication of thin films for device implementation. Here, we report a strategy for the synthesis of thin films of π-conjugated microporous polymers by designing thiophene-based electropolymerization at the solution–electrode interface. High-quality films are prepared on a large area of various electrodes, the film thickness is controllable, and the films are used for device fabrication. These films are outstanding hole conductors and, upon incorporation of fullerenes into the pores, function as highly efficient photoactive layers for energy conversions. Our film strategy may boost the applications in photocatalysis, energy storage, and optoelectronics.

Highly efficient deep-blue OLED with an extraordinarily narrow FHWM of 35 nm and a y coordinate <0.05 based on a fully twisting donor–acceptor molecule

A highly efficient deep-blue organic light-emitting diode based on the fully twisting donor–acceptor molecule TPA–PIM exhibited a maximum quantum efficiency of 3% and a particularly narrow electroluminescence emission peaking at 420 nm with a full width at half maximum of only 35 nm and a CIE coordinate of (0.161, 0.046).

Porous Organic Polymer Films with Tunable Work Functions and Selective Hole and Electron Flows for Energy Conversions

Organic optoelectronics are promising technologies for energy conversion. However, the electrode interlayer, a key material between active layers and conducting electrodes that controls the transport of charge carriers in and out of devices, is still a chemical challenge. Herein, we report a class of porous organic polymers with tunable work function as hole- and electron-selective electrode interlayers. The network with organoborane and carbazole units exhibits extremely low work-function-selective electron flow; while upon ionic ligation and electro-oxidation, the network significantly increases the work function and turns into hole conduction. We demonstrate their outstanding functions as anode and cathode interlayers in energy-converting solar cells and light-emitting diodes.

An Efficient AIE‐Active Blue‐Emitting Molecule by Incorporating Multifunctional Groups into Tetraphenylsilane

A multifunctional AIE-active molecule, CzPySiTPE, in which carbazole (Cz) and pyridine (Py) were attached to tetraphenylsilane to facilitate carrier injection has been designed and synthesized. Tetraphenylethene (TPE) was adopted to maintain efficient blue emission. Blue electroluminescent (EL) emission of CzPySiTPE was obtained with CIE coordinates of (0.16, 0.17) and an external quantum efficiency of 1.12%.

Delayed Fluorescence in a Solution-Processable Pure Red Molecular Organic Emitter Based on Dithienylbenzothiadiazole: A Joint Optical, Electroluminescence, and Magnetoelectroluminescence Study

The discovery of triplet excitons participating in the photoluminescent processes in a growing number of pure organic emitters represents an exciting impetus for a diversity of promising opto, bio, and optoelectronic applications. In this contribution, we have studied a small-molecule dithienylbenzothiadiazole-based red-emitting dye red-1b, which shows clearly delayed fluorescence under optical and electrical excitation. The OLED device that contained red-1b as a nondoped solution-processable emitter exhibited a moderately high utilization of exciton amounting to ≈31% and slow efficiency roll-off. Magnetoelectroluminescence measurements revealed the coexistence of reverse intersystem crossing from the lowest triplet state to singlet state (RISC, E-type triplet to singlet up-conversion) and triplet-triplet annihilation (TTA, P-type triplet to singlet up-conversion). Specifically, in low current-density regime, the moderately high exciton utilization is attributed to RISC (i.e., thermally activated delayed fluorescence, TADF), whereas in high current-density regime, TTA may contribute to suppressing efficiency roll-off. Furthermore, the results showed that red-1b may represent a new kind of organic red emitters that display delayed fluorescence in a way differing from the few red emitters investigated so far.

Novel violet emitting material synthesized by stepwise chemical reactions

In this paper, we report the design and synthesis of a novel bipolar violet emitting molecule CzPySiSF. The carbazole and pyridine moieties are employed to facilitate charge injection and balance carrier transport, whereas the spirofluorene is used as the violet emitter. The three functional groups, i.e., carbazole, pyridine and spirofluorene, are connected to tetraphenylsilane in a stepwise fashion using classical coupling reactions such as Suzuki and Ullmann. The resultant bipolar molecule CzPySiSF exhibits very stable thermal properties and a uniform amorphous morphology. The introduction of spirofluorene greatly enhances the fluorescence quantum efficiency of the molecule; moreover the PL spectrum of CzPySiSF in THF is mainly located in the violet region. The EL spectrum of CzPySiSF matches well with the PL spectrum with a maximum at 408 nm. The violet OLED of CzPySiSF exhibits the maximum external quantum efficiency of 0.59%.

Efficient pyrene-imidazole derivatives for organic light-emitting diodes

Two light emitting materials, 9-phenyl-10-(4-(1,2,2-triphenylvinyl)phenyl)-9H-pyreno[4,5-d]imidazole (PyTPEI) and 9-phenyl-10-(4′-(1,2,2-triphenylvinyl)-[1,1′-biphenyl]-4-yl)-9H-pyreno[4,5-d]imidazolepyreneimidazole (PyPTPEI), containing pyrene, imidazole and tetraphenylethene units are synthesized with high yields. Both of them exhibit good thermal stability with decomposition temperatures of 458 °C and 474 °C, respectively. The fluorescent quantum efficiency in amorphous films are as high as 70% for PyTPEI and 63% for PyPTPEI, respectively. In particular, PyPTPEI shows blue-shifted emission due to the more twisted conformation and reduced intermolecular interactions as compared with PyTPEI. The maximum current efficiency and maximum brightness of a non-doped OLED device using PyTPEI as an active layer reaches 8.73 cd A−1 and 27 419 cd m−2, which is better than that of PyPTPEI (7.68 cd A−1 and 19 419 cd m−2).

Effect of cyano-substitution in distyrylbenzene derivatives on their fluorescence and electroluminescence properties

Efficient organic electroluminescent materials with both high solid-state fluorescence efficiency and high excitons usage efficiency for use in organic light-emitting diodes (OLEDs) are relatively rare. We report two isomeric-phenylethylene compounds with different positions and spatial orientation of the –CN substituent on the vinylene groups, namely α-CN-APV and β-CN-APV. The synthesis, characterization, crystal structure, optical, electrochemical, thermal, and electroluminescence (EL) properties of the two compounds are discussed in detail. The crystal structure of the β-CN-APV shows tight solid-state organization because of two, oppositely pointed, vertically aligned, hydrogen bonds between the two cyano groups and the two vinylene hydrogens (CN⋯H–CC), while the α-CN-APV showed more flexible molecular structure with absence of vinylene hydrogen intermolecular interactions in the crystal. Such tight intermolecular stacking of β-CN-APV ensures the high solid-state fluorescence quantum efficiency. Both the compounds exhibit hybrid local and charge transfer (HLCT) excited states, which facilitate the population of singlet excitons through the reverse intersystem crossing (RISC) process from the high lying triplet states. Experimental and theoretical investigation indicated that the β-CN-APV, when compared to the α-CN-APV, showed higher solid-state fluorescence quantum efficiency and also higher excitons usage efficiency, which eventually provided much higher external quantum efficiency and brightness in the corresponding EL devices.

Discrete face-to-face stacking of anthracene inducing high-efficiency excimer fluorescence in solids via a thermally activated phase transition

It is always a challenge for planar polycyclic aromatic molecules to achieve high efficiency in solids owing to their frequent encounter with aggregation-caused quenching (ACQ). An anthracene derivative with one-side meta-substituted triphenylamine (TPA) was found to show high-efficiency excimer fluorescence (ηPL = 76.8%) in G-phase (green) crystals as well as a long lifetime, in sharp contrast with that of a monomer in a doped film (ηPL = 36.6%) and that of B-phase (blue) crystals (ηPL = 8.1%). In essence, the excimer-induced enhanced emission can be ascribed to the special intermolecular stacking in the solid state, namely, discrete antiparallel dimeric stacks between anthracene moieties in G-phase crystals, which are responsible for greatly suppressed non-radiative deactivation due to a uniform emissive state preventing the formation of an energy-trapping “dark” state. Moreover, a G-phase could be obtained through a thermally-activated phase transition from B-phase crystals, corresponding to the completely synchronized change of fluorescence properties. The present results consolidate a novel strategy of designing discrete dimeric stacking of planar polycyclic aromatic molecules to achieve high-efficiency fluorescence in the solid state by an excimer-induced enhanced emission (EIEE) mechanism.