2D Versus 3D Crystalline Order in Thin Films of Regioregular Poly(3-hexylthiophene) Oriented by Mechanical Rubbing and Epitaxy

Highly oriented films of regioregular poly(3-hexylthiophene) (P3HT) are prepared by two methods: mechanical rubbing and directional epitaxial crystallization. The structure, nanomorphology, and optical and charge-transport properties of the oriented films are investigated by electron diffraction, high resolution transmission electron microscopy (HR-TEM), absorption spectroscopy, and transistor field-effect measurements. In rubbed films, P3HT chains align parallel to the rubbing direction and the crystalline domains orientation changes from preferential edge-on to flat-on orientation. The maximum in-plane orientation probed by absorption spectroscopy is a function of the polymer molecular weight M-w; the lower the M-w, the higher the in-plane orientation induced by rubbing. The anisotropy of field-effect mobility measured parallel and perpendicular to the rubbing shows the same trend as the absorption. The M-w-dependence of the orientation is explained in terms of chain folding and entanglement that prevent the reorientation and reorganization of the pi-stacked chains, especially when M-w >= 50 kDa. For comparison, P3HT films are oriented by directional epitaxial crystallization using a zone-melting technique. Electron diffraction and HR-TEM show that epitaxial and rubbed films differ in terms of intralamellar order within layers of pi-stacked chains. Comparison of UV-vis absorption spectra for the different samples suggests that the vibronic structure is sensitive to intralamellar disorder.