Self-assembly of PBTTT–C14 thin films in supercritical fluids

Abstract

In order to develop more atom-economical deposition methods for polymer semiconductors, we investigated physical supercritical fluid deposition (p-SFD) to form thin films of a popular bithiophene semiconducting polymer (PBTTT). We deposited thin films of PBTTT–C₁₄ on a substrate without the need for in situ chemical reactions. Depositions were performed in n-pentane at several pressures above the critical pressure of the pure liquid. The resulting films were studied with atomic force microscopy (AFM) and grazing incidence wide angle X-ray scattering (GIWAXS). At lower pressures, nanowire morphologies were observed and correlated with a more gradual decrease in the isobaric saturation solubility with respect to temperature. The isotropic distribution of crystallite textures suggests that the wires are formed in solution prior to deposition. The addition of 0.5% mol ratio of toluene also had a strong influence on thin film morphologies. Nanowires were observed at higher pressures in the presence of toluene, which we correlated with increasing saturation solubility with respect to pressure. Taken in sum, the results illustrate the profound influence of subtle changes in polymer solubility on the self-assembly process.