The influence of mechanical rubbing on the field-effect mobility in polyhexylthiophene

This paper reports on improvements of the field-effect mobility in regioregular head-to-tail coupled poly(3-hexylthiophene) based transistors by mechanically induced alignment of polymer chains in the active layer. It is demonstrated that mechanical rubbing perpendicular to the source drain contacts can increase the field-effect mobility up to 800% whereas rubbing parallel to the source drain contacts results in a reduced mobility. The polymer alignment is thereby deduced from optically polarized transmission spectroscopy on polymer-coated quartz glass substrates and is shown to directly correlate with the electrical behavior of a bottom-gate field-effect transistor. The influence of layer thickness on rubbing is investigated and it is shown that annealing after mechanical rubbing at high temperature can further increase the alignment. Differences between thick drop-cast and thin spin-coated films are explained in terms of different solvent evaporation rates, allowing the material to order to a different degree. This interpretation is deduced from characteristic optical and electrical features of the differently prepared poly (3-hexylthiophene) films. (C) 2003 American Institute of Physics. [DOI: 10.1063/1.1530720].