Chemistry has made series progress in the regulation of crystal phases of organic semiconductors

Most organic molecules form crystals through very weak van der Waals interactions, so polycrystalline phases are a very common phenomenon in organic semiconductor materials. Crystal phases with different stacking structures have different electronic coupling effects, resulting in different charge transport behaviors. How to control the assembly and growth of high-mobility crystal phase has always been a very challenging subject in molecular electronics, involving molecular structure, crystal engineering and supramolecular self-assembly and other aspects.

Institute of Chemistry, Chinese Academy of Sciences Researchers in the organic solids laboratory used supersaturation of solution, gas phase diffusion temperature gradients, and surface nanochannels to induce thermodynamic and kinetic processes in the growth of organic semiconductor crystals. The single crystal or crystalline film exhibits very high carrier mobility. By selecting different solution concentrations to control the degree of supersaturation, a single crystal of different crystalline phases of the thiacacene derivative was prepared for the first time. The electronic coupling between β-crystal (HOMO-1) levels is significantly higher than that of α-crystals, and plays a dominant role in the charge transport performance, leading to a high mobility of 18.9 cm2 V-1 s-1 for β single crystal carriers. Different stacking structures can cause significant differences in the electronic coupling of non-degenerate (HOMO-1) energy levels, which has an important impact on charge transport, providing a new concept and train of thought for the regulation of organic semiconductor stacking structures. Adv. Mater. 2015, 27, 825).

Using the physical vapor transport method, a single crystal of α and β crystal phases of titanyl phthalocyanine was selectively obtained for the first time by controlling the temperature gradient, and a single crystal field effect transistor was constructed. The α-crystal phase has a typical two-dimensional charge transport channel with a maximum carrier mobility of 26.8 cm2 V-1 s-1, which is the highest value of the cyanide-based organic semiconductor. The β crystal phase has a three-dimensional charge transport channel, and the layer has strong electronic coupling with the layer. Its direction is perpendicular to the charge transfer direction, disturbing the charge transfer behavior, and only the migration of 0.1 cm2 V-1 s-1 is achieved. rate. This finding breaks through the traditional view that "three-dimensional charge-transporting semiconductors are superior to low-dimensional semiconductors," demonstrating that the electronic coupling between molecular layers has an important influence on charge transport (Angew. Chem. Int. Ed. 2016, 55, 5206).

Recently, the researchers found that the highly polar and nano-trough structure of PAI film, a thermal precursor of polyimide PI, can selectively induce the pentacene molecules to stand up and accumulate to form an orthorhombic phase that facilitates charge transport. It can further form high-crystalline films with large size and few grain boundaries. The mobility is as high as 30.6 cm2 V-1 s-1, which is the highest value of pentacene thin-film devices and one of the few examples of the highest mobility of organic semiconductors. . It is further found that PAA can induce the growth of crystal films of organic semiconductors such as tetracene and copper phthalocyanine with higher crystallinity, and verify the universality of nanochannel induction on the surface, providing a kind of organic semiconductor device with high performance. New ideas and methods (J. Am. Chem. Soc. 2017, 139, 2734).

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