A poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et-2) counter electrode prepared by electrochemical polymerization on a fluorine-doped tin oxide (FTO) glass substrate was incorporated in a platinum-free dye-sensitized solar cell (DSSC). The surface toughness and I-/I-3(-) redox reaction behaviors based on PProDOT-Et-2, poly(3,4-propylenedioxythiophene) (PProDOT), poly(3,4-ethylenedioxythiophene) (PEDOT), and sputtered-Pt electrodes were characterized, and their performances as counter electrodes in DSSCs were compared. Cells fabricated with a PProDOT-Et-2 Counter electrode showed a higher conversion efficiency of 7.88% compared to cells fabricated with PEDOT (3.93%), PProDOT (7.08%), and sputtered-Pt (7.77%) electrodes. This enhancement was attributed to increases in the effective Surface area and good catalytic properties for 13 reduction. In terms of the film thickness effect, the fill factor was strongly dependent on the deposition charge capacity of the PProDOT-Et-2 layer, but the aggregation of PProDOT-Et-2 in thicker layers (>80 mC cm(-2)) resulted in decreases in J(SC) and the cell conversion efficiency, The charge transfer resistances (R-ct1) of the PProDOT-Et-2 counter electrodes had the lowest value of similar to 18 Omega at a deposition charge capacity of 40 mC cm(-2). These results indicate that films with high conductivity, high active surface area, and good catalytic properties for I-3(-) reduction can potentially be used as the Counter electrode in a high-performance DSSC. (C) 2008 Elsevier B.V. All rights reserved.