A composite film based on carbon black nanoparticles and sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl) (CB-NPs/s-PT) is formed on a flexible titanium foil for the use as the electro-catalytic counter electrode (CE) of dye-sensitized solar cells (DSSCs). The CB-NPs provide the large amount of electro-catalytic active sites for the composite film, and the s-PT polymer serves as a conductive binder to enhance the inter-particle linkage among CB-NPs and to improve the adhesion between the composite film and the flexible substrate. The flexible CB-NPs/s-PT composite film is designed to possess good electro-catalytic ability for I-/I-3(-) redox couple by providing large active sites and rapid reduction kinetic rate constant of I-3(-). The cell with a CB-NPs/s-PT CE exhibits a good cell efficiency (eta) of 9.02 +/- 0.01% at 100 mW cm(-2), while the cell with a platinum CE shows an eta of only 8.36 +/- 0.02% under the same conditions. At weak light illuminations (20-80 mW cm(-2)), a DSSC with CB-NPs/s-PT CE still exhibits eta's of 7.20 +/- 0.04-9.08 +/- 0.02%. The low-cost CB-NPs/s-PT CE not only renders high cell efficiency to its DSSC but also shows a great potential to replace the expensive platinum; moreover it is suitable for large-scale production or for indoor applications. (C) 2015 Elsevier B.V. All rights reserved.