Poly(3,4-ethylenedioxythiophene) (PEDOT) films doped by perchlorate ions (ClO4-) have been prepared via electropolymerization with different EDOT monomer concentration and used as counter electrodes (CEs) in dye-sensitized solar cells (DSSCs). The effect of EDOT monomer concentration on the properties of obtained PEDOT CEs has been thoroughly investigated. The sponge like porous microstructure of the PEDOT films breaks gradually with the increase of EDOT concentration. Meanwhile, the conductivity of the PEDOT CEs decreases gradually due to the decreased ClO4- doping level. The catalytic mechanism is mainly comprised of the formation of weakly bonded iodine species on the undoped EDOT units, and formation and reduction of the intermediates. The undoped EDOT units in the PEDOT chain are the key catalytic reaction centers for I-/I-3(-) redox reaction. The catalytic activity of the PEDOT CEs for I-/I-3(-) redox reaction increases with the increase of EDOT concentration by virtue of the increased catalytic reaction centers in the PEDOT chain (i.e., increased undoped EDOT units due to the decreased ClO4- doping level). DSSC based on PEDOT CE polymerized with 0.05 M of EDOT and 0.1 M of LiClO4 shows good photovoltaic performance, with a power conversion efficiency of 7.42%. This result is comparable to the Pt CE based-solar cell (7.41%), suggesting that PEDOT CEs may substitute the expensive Pt CEs in the DSSCs. (C)2015 Elsevier Ltd. All rights reserved.