We show that the electrical properties of polymer thin-film transistors (PTFTs) can be enhanced by controlling the solvent properties of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT/PSS) solution used as the inkjet-printed source and drain electrodes. Specifically, addition of dimethyl sulfoxide (DMSO) into the PEDOT/PSS solution increased the conductivity of the inkjet-printed PEDOT electrodes and remarkably reduced the contact resistance of the electrodes. The lower contact resistance of the DMSO-treated PEDOT electrode compared to the corresponding electrode without DMSO treatment may be due to enhanced interfacial stability at the contact between the printed PEDOT electrodes and the semiconductor layers.