Flexible metal-free counter electrode for dye solar cells based on conductive polymer and carbon nanotubes

The counter electrodes (CEs) for flexible dye solar cells (DSCs) are normally prepared by sputtering platinum on indium tin oxide (ITO) plastic substrate. However both ITO and platinum are expensive materials that need to be replaced with cheaper alternatives in large scale production of low-cost DSCs. We fabricated a flexible and completely carbon-based CE for DSCs based on electropolymerized poly (3,4-ethylenedioxythiophene) (PEDOT) on single-walled carbon nanotube (SWCNT) film on a plain plastic substrate. The DSCs with such a CE had an efficiency of 4.0%, which is similar to the efficiency of the reference DSCs (3.9%) based on conventional sputtered platinum on ITO-plastic CE. The carbon-based electrode was prepared by a simple press-transfer method of SWCNTs from the collection filter used in the gas phase synthesis and by electrochemical deposition of PEDOT on it. Electrochemical impedance spectroscopy confirmed that the PEDOT–SWCNT film had the best catalytic performance among the studied CE materials, and the film was also slightly transparent. The results demonstrate a successful combination of the conductive and catalytic properties of SWCNTs and PEDOT, respectively.