Enhancement of field-effect mobility and stability of poly(3- hexylthiophene) field-effect transistors by conformational change

With the aim of enhancing the field-effect mobility and air stability of self-aligned regioregular poly(3-hexylthiophene), P3HT, by promoting two-dimensional molecular ordering, we transformed the chemical structure of the P3HT chain from a benzoid to a quinoid structure by doping the P3HT solution with HAuCl₄ prior to film formation. We found that, for the appropriate HAuCl₄ concentration, the P3HT nanocrystals adopt a highly ordered molecular structure with a field-effect mobility of 0.03 cm ² V^-1 s^-1, which is an improvement by a factor of more than 100. This increase in field-effect mobility is due to a significant enhancement of molecular ordering and the perpendicular orientation of the nanocrystals with respect to the insulator substrate. This resulted from the change in the P3HT chain conformation from a benzoid to a quinoid structure due to oxidation by HAuCl₄. The quinoid structure favors a linear or expanded-coil conformation, so the thiophene inter-ring bonds have increased double-bond character, which improves the molecular ordering. Furthermore, the electrical properties of a doped P3HT device had highly improved stability to air without encapsulation. These results suggest that the effect of unintentional doping by oxygen on HAuCl₄-doped P3HT film is insignificant because the p-type doping effect of HAuCl₄ is the major contributor.