Evaluation of Dye-Sensitized Solar Cells using Forward Bias Applied Impedance Spectroscopy Under Dark

Dye-sensitized solar cells (DSSCs) are promising as low cost, highly efficient solar cells. Internal resistance analysis using impedance spectra (IS) is an effective method for improving the performance of DSSCs. However, it is questionable if resistances obtained from different DSSCs by using IS are comparable under constant light illumination, because the internal resistances of DSSC might depend on current. In this study, we investigated the origin of four resistances (R1~R4) measured using IS under dark conditions, and the similarity and the difference between different three IS methods were discussed. The results revealed that forward bias applied impedance spectroscopy at constant current under dark conditions is a simple, practical method to evaluate internal resistance of DSSCs. In conclusion, development of high efficiency DSSCs will require low resistances of the ohmic (R1), redox reactions on the counter electrode (R2, 100~10k Hz) and I-/I3- diffusion (R4, 2~100 Hz) and high resistance of back charge transfer from TiO2 to electrolyte (R3, 100m~2 Hz).