SINGLE-CRYSTAL AND POLYCRYSTALLINE MORPHOLOGY OF THE THIOPHENE-BASED SEMICONDUCTOR ALPHA-HEXATHIENYL (ALPHA-6T)

We have conducted a detailed electron-microscopic investigation of the structure and morphology of alpha-hexathienyl (alpha-6T), a hexameric homologue of polythiophene that shows great promise as a thin-film-tr ansistor material. In ordinary films for device applications prepared by vacuum sublimation, alpha-6T crystals are very small (<100 nm), irr egular, and preferentially oriented parallel to the substrate. By crys tallization from the melt, very broad (tens of micrometers) and thin l amellar crystals are obtained, with their molecules perpendicular to t he lamellar surfaces. Single-crystal electron-diffraction patterns fro m a variety of zones allow resolution of the main-chain repeat at 2.38 nm, recording of over 30 orders of intramolecular reflections, discri mination among the four proposed unit cells, and determination of the preferred crystal-growth direction (which corresponds to the unique b- axis of the monoclinic cell). In addition, we have shown that the lame llae undergo splaying and warping about an axis corresponding to their growth direction, as well as very extensive fractures perpendicular t o that axis. These fractures are explained based upon poor interdigita tion of the flat extended-chain molecules between their densely packed (020) planes. By introducing such extensive internal disruptions to t he lattice, these cleavage planes are expected to have severe negative implications on single-crystal transistor characteristics.