Faradaic impedance to analyze charge recombination in photoelectrode of dye-sensitized solar cell

Charge transfer on the photoelectrode of dye sensitized solar cell (DSC) is composed of the various processes as follows: photoexcitation of electron in the dye; electron injection from the excited dye into the conduction band of oxide semiconductor; electron diffusion in the oxide semiconductor, reaction between oxidized dye and iodide ions (I−); and charge recombination. The charge recombination is one of the main factors that limit the efficiency of photoelectric conversion at photoelectrode. It is well known that 4-tert-butyl pyridine (TBP) is the useful additive in order to suppress the charge recombination. The authors fabricated the DSC with two photoelectrode materials such as titanium dioxide (TiO2) and zinc oxide (ZnO), and analyzed the charge recombination by using electrochemical impedance spectroscopy. Theoretical equation of Faradaic impedance was derived from the reaction model of photoelectorde, and the charge transfer resistance of the photoelectrode was defined from the Faradaic impedance. In addition, the influence of the charge recombination on the impedance spectrum of the photoelectrode was discussed by the comparison between simulated and experimental results.