Fangzhong Shen

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.

Tight intermolecular packing through supramolecular interactions in crystals of cyano substituted oligo(para-phenylene vinylene): a key factor for aggregation-induced emission

Strong supramolecular interactions, which induced tight packing and rigid molecules in crystals of cyano substituent oligo(para-phenylene vinylene) (CN-DPDSB), are the key factor for the high luminescence efficiency of its crystals; opposite to its isolated molecules in solution which have very low luminescence efficiency.

Phenanthro[9,10-d]imidazole as a new building block for blue light emitting materials

The blue light emitting compound, BPPI, based on a phenanthro[9,10-d]imidazole group, is prepared by a facial synthetic process and exhibits excellent thermal stability, highly efficient fluorescence and balanced carrier injection. The double-layered device based on BPPI shows a higher maximum luminance and a lower turn-on voltage than the multi-layered one with an independent electron injection layer. The results suggest that the phenanthroimidazole unit is an excellent building block for tuning the carrier injection properties as well as blue emission.

Highly efficient deep-blue electroluminescence based on the triphenylamine-cored and peripheral blue emitters with segregative HOMO–LUMO characteristics

To obtain highly efficient and stable blue electroluminescence, two novel star-shaped compounds are purposefully designed and synthesized, which are composed of two functional groups, central triphenylamine (TPA) and peripheral anthracene (AN) or phenanthrene (PA) with obviously segregative electronic density distribution characteristics between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO and LUMO). As a light-emitting layer, they exhibit not only highly efficient deep-blue electroluminescence, but also good stability in organic light-emitting diodes (OLEDs) due to the combination of several advantageous properties (electrochemical, thermal and morphological stability). In particular, for triphenylamine-cored phenanthrene (TPA-PA), the external quantum efficiency of 7.23% is the best reported result for non-doped deep-blue OLEDs, and the lifetime of the device is also improved, being twice that of 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) under the same device conditions.

The Origin of the Improved Efficiency and Stability of Triphenylamine-Substituted Anthracene Derivatives for OLEDs: A Theoretical Investigation†

Herein, we describe the molecular electronic structure, optical, and charge-transport properties of anthracene derivatives computationally using density functional theory to understand the factors responsible for the improved efficiency and stability of organic light-emitting diodes (OLEDs) with triphenylamine (TPA)-substituted anthracene derivatives. The high performance of OLEDs with TPA-substituted anthracene is revealed to derive from three original features in comparison with aryl-substituted anthracene derivatives: 1) the HOMO and LUMO are localized separately on TPA and anthracene moieties, respectively, which leads to better stability of the OLEDs due to the more stable cation of TPA under a hole majority-carrier environment; 2) the more balanceable hole and electron transport together with the easier hole injection leads to a larger rate of hole-electron recombination, which corresponds to the higher electroluminescence efficiency; and 3) the increasing reorganization energy for both hole and electron transport and the higher HOMO energy level provide a stable potential well for hole trapping, and then trapped holes induce a built-in electric field to prompt the balance of charge-carrier injection.

Amplified spontaneous emission from cyano substituted oligo(p-phenylene vinylene) single crystal with very high photoluminescent efficiency

Amplified spontaneous emission (ASE) from high-quality crystal of cyano substituted oligo(p-phenylene vinylene) with very high photoluminescent efficiency is observed. The full widths at half maximum of the narrowed spectra can reach 10nm when the pumping energy is 50μJ/pulse. The threshold for ASE is about 7.9μJ/pulse corresponding to 39.5kW∕cm2, which is among the lowest values for organic crystals ever reported. The gain coefficient at the peak wavelength of ASE and loss coefficient caused by scattering are ∼33 and ∼3.9cm−1, respectively.

Separation of Electrical and Optical Energy Gaps for Constructing Bipolar Organic Wide Bandgap Materials

An electrical and optical energy gaps separation strategy is put forward for the design of organic wide bandgap semiconductors. This new principle could achieve optimization of wide bandgap (both high singlet and triplet energies) and favorable carrier injection energy levels simultaneously.

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%.

Supramolecular interaction-induced self-assembly of organic molecules into ultra-long tubular crystals with wave guiding and amplified spontaneous emission

Ultra-long tubular crystals (length up to 10 mm and diameter of 70 μm) of a small organic functional molecule, 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB), are successfully prepared through a physical vapor transport (PVT) method. On the basis of crystal structural analysis and density functional theory calculations, we show that the formation of such tubular crystals is drawn by the branched hydrogen bonds between the BCPEB molecules. High crystalline quality, highly ordered molecular orientation, and hollow-like topological structure enable the crystal to exhibit optical wave guided emission behaviors with low optical loss (3 dB mm−1) and highly polarized emission (the polarized ratio is about 20). Amplified spontaneous emission (ASE) characteristics of the tubular crystals were also studied; the net gain coefficients at the peak wavelength and the threshold are 91.7 cm−1 and 36 kW cm−2, respectively.

Synthesis and Electrochemical Properties of Peripheral Carbazole Functional Ter(9,9-spirobifluorene)s

A facile approach for synthesis of spirobifluorene trimers with peripheral carbazole functional groups by utilizing Suzuki coupling as the key reaction has been developed. These novel compounds exhibit blue emission with high quantum yields in solution and thin films, and excellent spectral stability upon photoirradiation and annealing in air. By the introduction of carbazole groups, the oxidation potentials of spirobifluorene trimers S TCPC-6 and STCPC-4 were significantly lower than that of model compound STHPH without peripheral carbazole groups, which reflect that the title compounds process higher HOMO energy level and better hole-injection ability. Highly luminescent films were obtained by electrochemical coupling between carbazole units. Pure blue-emission single-layer LEDs based on electrochemical deposition films as light emitting layers were achieved.