Poly(3,4-ethylenedioxythiophene)—Polystyrene Sulfonate (PEDOT-PSS) is the most widely used conducting polymer as electrode material in organic (polymer) devices. However, commercially available PEDOT-PSS in our experiment has a relatively low conductivity that reduces the device performance when it is used for electrode material. The purchased PEDOT-PSS has been mixed with polar solvent dimethyl sulfoxide, which increases its conductivity from 0.07 to 30 S/cm. The enhanced conductivity has long-term stability at room temperature and short-term stability at high temperature (100$^circ$C) in air ambient. The modified PEDOT-PSS has been inkjet printed and used as source/drain (S/D) electrodes for poly(3-hexylthiophene) (P3HT) thin-film transistors (TFTs). Unmodified PEDOT-PSS and gold have also been used as S/D electrodes for comparison. The TFTs with modified PEDOT-PSS electrodes show significantly improved performance over the devices with unmodified PEDOT-PSS electrodes and are similar to the devices with gold electrodes. The difference in device performance mainly results from parasitic series resistance. In the devices with unmodified PEDOTT-PSS, high electrode series resistance has several effects on devices, e.g., restricted current growth at high negative gate voltage, reduced on/off current ratio and current output capability.