A surface-interpenetrated conducting polymer, PANI:CSA on plastic substrate, especially cheap PET significantly boosts mechanical properties for outstanding performance as an ITO-free organic electrode in flexible organic solar cells and exhibits best performance of an average power conversion efficiency of 3.5 +/- 0.2% under 100 mW/cm(2) illumination, with a short-circuit current density of 8.7 mA/cm(2), an open-circuit voltage of 0.83 V. and a fill factor of 0.49 comparable to conventional organic solar cells made with ITO electrodes on glass or PET. (C) 2012 Elsevier B.V. All rights reserved.