Precursor Film Growth of 2,7-Dioctyl[1]benzothieno[3,2-<i>b</i>][1]benzothiophene (C8-BTBT) in the Smectic a Liquid Crystal Phase for High-Performance Transistor Applications

We present an approach to fabricating ultrathin organic semiconductor films via the smectic A liquid crystal precursor films of 2,7-dioctyl[1]benzothieno[3,2-<i>b</i>][1]benzothiophene (C8-BTBT) formed on Si and SiO₂/Si substrates. Using phase-shifting imaging ellipsometry, we directly visualize the lateral spread of the precursor films in discrete monolayer terraces from the reservoir region, where each layer exhibits diffusive growth behavior. Additional X-ray reflectivity measurements confirm that these monolayers adopt a standing molecular orientation. We find that the first layer grows with an apparent diffusion coefficient (<i>D</i>) of the order of 10^-9 m² s^-1 at 115 °C, whereas the second layer grows more slowly. The substrate surface condition and substrate surface treatments significantly influence <i>D</i>. Moreover, the monolayer precursor film used as a template for subsequent C8-BTBT deposition improves the carrier mobility and environmental stability of its organic field-effect transistors, which are much better than those in the devices fabricated without the precursor films. These findings highlight the potential use of smectic precursor films as templates for fabricating high-quality ultrathin organic semiconductor films and thereby achieving a higher device performance in organic electronics.