We show that the electrical properties of polymer thin-film transistors can be enhanced by doping poly(3-hexylthiophene) (P3HT) with HAuCl4. Specifically, the addition of HAuCl4 causes an increase in the two-dimensional molecular ordering of P3HT and a remarkable reduction in the contact resistance at the electrode/semiconductor interface with no pre- or post-treatment process. This phenomenon is understood in terms of broadening of the transport manifold in the organic semiconductor, induced by HAuCl4, which results in a reduction in the hole-injection barrier and an enhancement of the interfacial stability at the contact between the printed electrode and the semiconductor layers.