The performance of polymer:polymer solar cells that are made using blend films of poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-co- benzothiadiazole (F8BT) is improved by doping the F8BT polymer with an organosulfonic acid [4-ethylbezenesulfonic acid (EBSA)]. The EBSA doping of F8BT, to form F8BT-EBSA, is performed by means of a two-stage reaction at room temperature and 60°C with various EBSA weight ratios. The X-ray photoelectron spectroscopy measurement reveals that both sulfur and nitrogen atoms in the F8BT polymer are affected by the EBSA doping. The F8BT-EBSA films exhibit huge photoluminescence quenching, ionization potential shift toward lower energy, and greatly enhanced electron mobility. The short-circuit current density of solar cells is improved by ca. twofold (10 wt.% EBSA doping), while the open-circuit voltage increases by ca. 0.4 V. Consequently, the power conversion efficiency was improved by ca. threefold, even though the optical density of the P3HT:F8BT-EBSA blend film is reduced by 10 wt.% EBSA doping due to the nanostructure and surface morphology change.