Conductive and catalytic multiwalled carbon nanotube (MWCNT)/poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) composite films were prepared by dispersing MWCNT in the PEDOT:PSS aqueous solution using ultrasonication without any oxidation process. The effect of the MWCNT loading in the PEDOT:PSS solution was studied in terms of the conductivity and catalytic activity. The MWCNT/PEDOT:PSS composite films showed much lower sheet resistance in the range of 730-2150 Omega/square and higher surface roughness in the range of 25-110 nm than the pure PEDOT:PSS film (980 k Omega/square and 0.82 nm) depending on the MWCNT content in the composite. The high conductivity and roughness of MWCNT/PEDOT:PSS films enhanced its performance as a catalytic counter electrode in the dye-sensitized solar cell (DSSC). The interface resistance between the MWCNT/PEDOT:PSS film and the electrolyte was evaluated with electrochemical impedance spectroscopy (EIS) and it was confirmed that as the content of MWCNT in PEDOT:PSS solution was increased, the resistance was decreased. The cell efficiency of DSSC with MWCNT/PEDOT:PSS counter electrode was highly improved as the MWCNT concentration in MWCNT/PEDOT:PSS film increases and especially, the DSSC with FTO-free spray coated MWCNT/PEDOT:PSS counter electrode showed excellent sell performance (Jsc: 10.9 mA/cm(2), F.F.: 60.6% and n: 5.4%), which is comparable with that with platinum on FTO counter electrode system (Jsc: 13.9 mA/cm(2), F.F.: 62.8% and n: 7.1%). (C) 2012 Elsevier Ltd. All rights reserved.