Crystals Array via Oriented Nucleation and Growth Induced by Smectic E Mesophase of C7-T-BTBT

Long-range order crystalline thin films of organic semiconductors have attracted wide attention owing to their high charge carrier mobility. However, uncontrolled crystal nucleation and growth during the thin film drying process cause the formation of grain boundaries, thereby limiting the long-range order. Herein, we achieved the oriented nucleation and growth of organic semiconductors by off-centre spin-coating at the temperature of the smectic E(SmE) liquid crystal mesophase, and then followed by Ostwald ripening during solvent vapour annealing. The thin film of 2-(5-heptylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophne (C7-T-BTBT) blended with 40%(mass fraction) poly(methyl methacrylate)(PMMA) was prepared by off-centrespin-coating at SmE mesophase(170 °C), followed by solvent vapour annealing in chloroform for 24 h(chloroform is a good solvent for C7-T-BTBT and PMMA). The C7-T-BTBT molecules grew to rod-like crystals, which were mostly arranged parallel to each other. The crystal growth was perfect and resulted in a single crystal. The average length of the crystals was approximately 87 µm. Moreover, the highest charge carrier mobility is 1.62 cm2·V−1·s−1 as against that of the film prepared at 25 °C(0.06 cm2·V−1·s−1).