Hengjun Zhang

Novel copolymers incorporating dithieno[3,2-b:2′,3′-d]thiophene moieties for air-stable and high performance organic field-effect transistors

A series of semiconducting copolymers incorporating dithieno[3,2-b:2′,3′-d]thiophene moieties in the polythiophene backbone were synthesized. The resulting four copolymers are found to have large band gap (∼ 2.5 eV) and large ionization potential (−5.1 eV). Organic field-effect transistors were fabricated by spin-coating polymer solutions onto OTS-modified SiO2/Si substrates with top-contact configuration. The best resulting hole mobility afforded was 0.028 cm2V−1 s−1 with an on/off ratio of 2 × 104 and a threshold voltage of about −18.9 V in the saturation regime. Furthermore, all the devices exhibited high environmental stability against oxygen doping and relatively high ambient humidity (∼30%).

Synthesis and properties of fluorene or carbazole-based and dicyanovinyl-capped n-type organic semiconductors

Dicyanovinyl-substituted compounds as electron transport materials are rarely studied, especially oligomers. A new series of fluorene or carbazole-based and dicyanovinyl-capped oligomers consisting of benzene or thiophene segments were synthesized using the Stille coupling reaction and their physical properties were investigated. Optical spectra show that the introduction of electron-accepting groups induces an intramolecular charge transfer, resulting in a shift of the absorption onset toward longer wavelengths. Moreover, the optical spectra of their films show intermolecular interactions, leading to bathochromic shifts with respect to the spectra in solution. Cyclic voltammetry indicates that they have suitable HOMO and LUMO energy levels, and the oligomers containing thiophene segments show lower LUMO levels. The X-ray crystallography of compound FTCN shows that the compound exhibits extended π stacking of the molecules. Thermal analysis reveals that they are thermally stable and no phase transition was observed at low temperatures.

Highly efficient blue electrophosphorescent devices with a new series of host materials: polyphenylene-dendronized oxadiazole derivatives

A new series of oxadiazole derivatives TPO, PPO, MTPO, MPPO, BTPO, and BPPO have been designed and synthesized in high yields by a convenient synthetic procedure. A single crystal structure of TPO exhibited that this polyphenylene-dendronized material has a sterically crowded structure which efficiently prevents π–π stacking and endows it with good thermal stability. The photophysical properties of the materials show strong emission between 376 and 390 nm in the films. In addition, good reversible reductive waves in the cyclic voltammograms implied that these materials might have good electron transport properties. Blue electrophosphorescent devices fabricated using these materials as the host materials and iridium(III)bis[4,6-di-fluorophenyl-pyridinato-N,C2′]picolinate as the blue phosphorescent dopant emitter show very high efficiencies. A maximum luminance of 4484 cd m−2 and an external quantum efficiency of 6.20% were achieved under ambient conditions.

A non-planar pentaphenylbenzene functionalized benzo[2,1,3]thiadiazole derivative as a novel red molecular emitter for non-doped organic light-emitting diodes

A novel non-planar pentaphenylbenzene functionalized benzo[2,1,3]thiadiazole derivative (BPTBTD) has been designed and synthesized with only three steps, and applied into non-doped organic light-emitting diodes (OLEDs) as a red molecular emitter. The molecule exhibits good solubility in common organic solvents, excellent thermal stability and film-forming properties. The most important feature is that photoluminescence of BPTBTD shows excellent stablity in different solvents, at different concentrations and in different solid states. Red organic light-emitting diodes were fabricated in a facile non-doped configuration with different thicknesses of emission layers. Saturated red-emission was observed with an emission peak at 621 nm, a maximum external quantum efficiency of 1.0% and a maximum brightness of 1572 cd m−2. Especially, the properties of devices with thick emission layers are two times better than those of the devices with thin emission layers. These results indicate that the non-planar pentaphenylphenyl functional groups substantially suppressed the tendency for molecular aggregation in thin solid films, leading to a very stable photoluminescence and higher efficiency of devices with thicker layers.