This study presents a structural analysis of regioregular poly(3-hexylthiophene) (P3HT) based on electron diffraction from epitaxied thin films. Epitaxial orientation of the hexane fraction of P3HT was performed by slow rate directional solidification in 1,3,5-trichlorobenzene leading to highly oriented and crystalline P3HT films with different contact planes. Representative electron diffraction patterns corresponding to different zone axes were obtained by the rotation-tilt electron diffraction method. A trial-and-error method based on molecular modeling and calculation of the electron diffraction patterns for the different zone axes was used to determine the crystal structure of P3HT. The unit cell is monoclinic with space group P2 1 /c and two chains per cell (a=1.60 nm, b=0.78 nm, c=0.78 nm and γ=86.5 deg). The stacking period of successive polythiophene backbones along the b axis is 0.39 nm but short interplanar distances of 0.34 nm are observed because the conjugated polythiophene backbones are tilted to the b axis. The n-hexyl side groups crystallize in an orthogonal subcell with parameters a s =0.7 nm and b s =0.78 nm. The present structural model highlights the essential role of the linear side chain crystallization on the supra-macromolecular packing of "hairy-rod" polymers like P3HT.

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 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.