Charge Transport in Highly Face-On Poly(3-hexylthiophene) Films

We report that the pi-stacking direction in poly(3-hexylthiophene) (P3HT) films can be made to orient strongly out-of plane by uniaxially straining films in orthogonal directions, providing a valuable opportunity to evaluate charge transport in a very unusual microstructure for this material. The structure of the films was characterized using UV-visible spectroscopy, X-ray diffraction, and near-edge X-ray absorption fine structure spectroscopy, showing that unstrained films have a weakly edge-on stacking character with a large orientation distribution, whereas films strained biaxially by 100% in orthogonal directions have highly face-on stacking. In the biaxially strained films the face-on packing occurs while the P3HT long axis orientation is found to be only weakly anisotropic in-plane. Charge transport is characterized in an organic thin-film transistor (OTFT) configuration, showing that the saturated field effect mobility in the biaxially strained films is greater than that for unstrained films for channel lengths <= 10 mu m. The mobilities are found to have different channel-length dependence, attributed primarily to differences in the field-dependent charge-transport behavior, resulting in the mobility being comparable for channel lengths of 20 mu m. The results suggest that edge-on packing is not a prerequisite for relatively high-field-effect mobility in P3HT-based OTFTs.