Highly conductive poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT : PSS) attracts a strong attention as a transparent electrode material since it may replace indium tin oxide (ITO) electrodes used in many organic semiconductor devices. However, PEDOT : PSS films have been usually deposited using acidic precursors, which caused long term device degradation as well as safety issues during device fabrication processes. This paper firstly reports application of highly conductive PEDOT : PSS films deposited on polyethylene terephthalate (PET) substrates using a neutralized precursor to organic bulkhetrojunction solar cells. The sheet resistance (R_s) of PEDOT : PSS was reduced by more than two orders of magnitudes by spin coating the neutralized solution containing 5% of dimethyl sulfoxide (DMSO) and dipping the films in DMSO for 30 min. Subsequently, an approximately 55 nm-thick PEDOT : PSS layer was obtained with R_s =159 Ω/□, a conductivity of 1143 S/m, and an optical transmittance of 84%. A solar cell based on poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2, 6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3, 4-b]thiophenediyl]: [6,6]-phenyl-C₇₁-butyric acid methyl ester fabricated on the PEDOT: PSS/PET substrate exhibited a higher open circuit voltage and power conversion efficiency than did a control solar cell fabricated on an ITO-coated PET substrate. These results suggest that the highly conductive PEDOT : PSS films may contribute to realize ITO-free flexible organic solar cells.