A flexible, slightly transparent and metal-free random network of single-walled C nanotubes (SWCNTs) on plain polyethylene terephthalate (PET) plastic substrate outperformed Pt on conductive glass and on plastic as the counter electrode (CE) of a dye solar cell employing a Co(II/III)tris(2,2'-bipyridyl) complex redox mediator in 3-methoxypropionitrile solvent. The CE charge-transfer resistance of the SWCNT film was 0.60 Ω cm2, 4.0 Ω cm2 for sputtered Pt on In Sn oxide-PET substrate and 1.7 Ω cm2 for thermally deposited Pt on F-doped Sn oxide glass, resp. The solar cell efficiencies were in the same range, thus proving that an entirely C-based SWCNT film on plastic is as good CE candidate for the Co electrolyte.