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

Abstract

AbstractHighly 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 Mw; the lower the Mw, 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 Mw‐dependence of the orientation is explained in terms of chain folding and entanglement that prevent the reorientation and reorganization of the π‐stacked chains, especially when Mw ≥ 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 π‐stacked chains. Comparison of UV‐vis absorption spectra for the different samples suggests that the vibronic structure is sensitive to intralamellar disorder.