Carbon black was employed as the catalyst for triiodide reduction on fluorine-doped tin oxide glass substrates (FTO-glass) used as counter electrodes in platinum-free dye-sensitized solar cells. The fill factors were strongly dependent on the thickness of the carbon layer, and the light energy conversion efficiency also increased up to a thickness of 10 mu m. The charge-transfer resistance (R-ct) of the carbon counter electrode decreased with the thickness of the carbon layer. The R-ct for the thicker carbon layer is less than three times that for the platinized FTO-glass. The highest cell efficiency was 9.1% under 100 mW cm(-2) light intensity (1 sun AM 1.5 light, J(sc) = 16.8 mA cm(-2), V-oc = 789.8 mV, fill factor = 0.685). (c) 2006 The Electrochemical Society.