Impedance Analysis Of Dye-Sensitized Solar Cells

With the increasingly serious energy crisis and environmental pollution, the development and utilization of new energy is becoming extremely urgent. The conversion of solar energy to electrical power is the main process for utilization of solar energy. Dye-sensitized solar cells (DSCs), with its lower cost, simpler fabrication technique and potential higher photoconversion efficiency, have attracted considerable attention in recent years and been considered a promising candidate for future solar energy conversion. Although the performance of the DSCs has been improved much and the record photoconversion efficiency of 11% has been obtained, there is still plenty of space for improvement considering the theoretical efficiency of 30% for a single bandgap junction. The theoretical studies of DSCs are far from being completed and the detailed mechanisms in this photoconversiton process are much more complex and need further research.Electrochemical impedance spectroscopy (EIS) is a commonly used electrochemistry test technology and has been proved to be a powerful tool to study the kinetics of charge transfer and recombination processes in DSCs. In this paper, EIS was used to investigate the internal resistances attributable to charge transfer and recombination processes of DSCs. An equivalent circuit was developed to describe cell's structure and the photoconversion process in DSCs. Some important parameters of the DSCs, such as charge-transfer resistance at the Pt electrode, the diffusion constant of I-3 and the effective diffusion constant of electrons in TiO2 film were determined by EIS.For the impedance spectra of DSCs which adopting P25 as photoconversion material, analytical results show that the internal resistance consisted of at least three aspects. The high-frequency region corresponds to charge transfer at Pt / electrolyte interface. Intermediate-frequency region reflects charge-transfer in films and recombination impedance, while the low-frequency region corresponds to Nernst diffusion impedance in electrolyte. Particularly, the resistance of charge transfer in TiO 2 films and recombination impedance is main factor for affecting internal resistance of DSCs. The diffusion impedance of I3- in electrolyte has little influence on the performance of cells. Moreover, the internal resistance of DSCs decreases with increasing Pt thickness and TiO2 film thickness, decreasing electrolyte thickness. Finally, a high effective diffusion coefficient and effective lifetime of electron in TiO2 were proved to be significant for improving the efficiency of cells.