We report an effective 2-step solvent treatment combined with a polar solvent vapor annealing process (PSVA) and a dipping process using polar solvents (DP) to enhance the electrical conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films, which is highly desirable for the fabrication of solution-processed organic photovoltaics (OPVs) on flexible substrates. The PSVA and DPtreated PEDOT:PSS films showed conductivities up to 1475 S/cm. The changes in conductivity, surface morphology, composition, and conformational PEDOT chains are investigated to elucidate the effect of the 2-step PSVA and DP treatment on PEDOT:PSS films. The highly segregated-PSS component induced by PSVA was effectively removed from the PEDOT:PSS films after successive DP treatments, eventually resulting in an enhanced conductivity of bulk films. We also employed optimized PEDOT:PSS films treated by the 2-step PSVA and DP-process as transparent anodes for indium tin oxide (ITO)-free OPVs fabricated on flexible substrates. The device performance of flexible OPVs with PSVA and DP-treated films was further improved due to the increased conductivity and decreased series resistance of devices and showed highly comparable cell-efficiency with ITO-based flexible OPVs. These results suggest that the 2-step solvent-treated PEDOT:PSS film is a promising alternative to ITO for realization of high-efficiency, flexible, and low-cost ITO-free OPVs with facile manufacturing process. (C) 2013 Elsevier B.V. All rights reserved.