The fabrication and electrochemical analysis of Pt-free counter electrodes for dye-sensitized solar cells

Abstract

In this paper, four different low-cost materials were applied as a Pt-free or Pt and TCO-free counter electrodes for dye-sensitized solar cell (DSSC). To replace conventional platinum (Pt) coated transparent conducting oxide (TCO) electrode, materials for counter electrode require high catalytic activity for I3− reduction and rapid charge transport ability. Firstly, the well-ordered inverse opal poly(3,4-ethylenedioxythiophene) (PEDOT) electrode which was prepared from PEDOT:PSS filled self-assembled PS opal template was applied as a Pt-free counter electrode for DSSC. Because of the well-ordered porous morphology of electrode, the inverse opal structured PEDOT electrode showed, in particular, improved electrocatalytic activity with high surface area and the increased quantum efficiency at specific wavelength was also observed particularly. Therefore, we could successfully apply the inverse opal structured PEDOT electrode as an effective Pt-free counter electrode for DSSCs. Furthermore, the Ni nitrides, which are known for a high electrocatalytic activity in iodide redox couple, were deposited on the ITO/PET film by sputtering deposition at low-temperature and could be also used as Pt-free flexible counter electrode for DSSC. By sputtering under nitrogen atmosphere at different temperature, a few nano-meter Ni nitride film was deposited on ITO/PET films and exhibited superior electrocatalytic activities in iodide redox electrolyte. In addition, this Ni nitride film had very thin thicknesses, about 30 nm, on the flexible ITO/PET substrate, it could be applied as a counter electrode for bifacial DSSC due to the optical transparency of Ni nitride film. On the other hand, Pt and TCO-free counter electrode was also studied to realize the cost effective counter electrode for DSSC. We found the two materials which functioned two things, high catalytic activity for triiodide reduction and high conductivity rapidly transporting the electron, in a single layer simultaneously. The nitrided Ni metal foam electrode was applied as a Pt and TCO-free counter electrode of DSSC for the first time. The nitrided Ni foam was simple synthesized by nitridation treatment of commercial Ni metal foam in ammonia atmosphere. This electrode was presented high catalytic activity on the nitrided surface and easy electron transport ability in the three-dimensional interconnected metallic structure, simultaneously. The open-pore structures in metal foam can also reduce the mass transport limitations of the electrolyte in the electrode compared to that of common metal plate. Therefore, the nitrided Ni foam using nitridation treatment under ammonia atmosphere can be a new cost-efficient counter electrode without the expensive Pt and TCO commonly used in counter electrode of DSSC. Lastly, the camphorsulfonic acid-doped polyaniline (PANI/CSA) electrode was prepared by simple spin-coating deposition flowed by low-temperature treatment was successfully used as a Pt and TCO-free counter electrode for DSSC. The PANI/CSA is a well-known conducting polymer with high conductivity and it provided excellent electrocatalytic ability because π-conjugated polymer superseded Pt as efficient electrocatalyst for iodide reduction reactions. By several optimization process of PANI/CSA electrode, it could be used as a promising Pt and TCO-free counter electrode of DSSC. Because this electrode had optical transparency and complementary color with dye, it can be used as an effective bifacial DSSC with Pt and TCO-free counter electrode.