Molecularly aligned films of [1]benzothieno[3,2-b][1]benzothiophene derivatives by solution shearing: Effect of alkyl substitution on morphology and charge transporting property

The self-assembly behaviors, crystal morphologies, and charge transporting properties of π–conjugated framework can be strongly affected by the functional group/substituent it carries, as well as the way the film is prepared. In this work, thin films of mono- and di-alkylated [1]benzothieno[3,2-b][1]-benzothiophene (BTBT) derivatives were prepared on a silicon substrate by solution-shearing method. Their film morphologies and crystal packing details were examined using polarized optical microscope, grazing incident wide-angle X-ray scattering, and atomic force microscope. With 2-tridecyl[1]benzothieno[3,2-b][1]-benzothiophene (C13-BTBT), a film with well-aligned molecules packed in face-to-face π-stacking arrangement was obtained, rendering a 1D-like charge conduction channel. For 2,7-bis-tridecyl[1]benzothieno[3,2-b][1]-benzothiophene (C13-BTBT-C13), well-aligned molecules packed in a layered-herringbone arrangement were obtained. With these films as a conducting channel in transistor fabrication, high-field-effect mobility was realized for C13-BTBT-C13, with a maximum mobility (μ_h) of 0.645 cm² /Vs and low anisotropic effect, presumably owing to the 2D growth of the π–conjugated skeleton in LHB packing. This study demonstrated the advantage of 2D semiconducting channels formed from C13-BTBT-C13 by low-cost and scalable solution-shearing processes, necessary for practical use in industry-level and future flexible electronics.