Anodic TiO2 Nanotube Arrays for Dye-Sensitized Solar Cells Characterized by Electrochemical Impedance Spectroscopy

This paper reports on the microstructure of anodic titanium oxide (TiO₂) and its use in a dye-sensitized solar cell (DSSC) device. When voltages of 60 V were applied to titanium foil for 2 hr under 0.25 wt% NH₄F+ 2 vol% H₂O+C₂H₄(OH)₂, TiO₂ with a nanotube structure was formed. The film, which had a large surface area, was used as an electron transport film in the DSSC. The DSSC device had a short-circuit current density (J_sc) of 12.52 mA cm⁻², a fill factor (FF) of 0.65, an open-voltage (V_oc) of 0.77 V, and a photocurrent efficiency of 6.3% under 100% AM 1.5 light. The internal impedance values under 100%, 64%, 11%, and 0% (dark) AM 1.5 light intensities were measured and simulated using the electrical impedance spectroscopy (EIS) technique. The impedance characteristics of the DSSC device were simulated using inductors, resistors, and capacitors. The Ti/TiO₂, TiO₂/Electrolyte, electrolyte, and electrolyte/(Pt/ITO) interfaces were simulated using an RC parallel circuit, and the bulk materials, such as the Ti, ITO and conducting wire, were simulated using a series of resistors and inductors. The impedance of the bulk materials was simulated using L₀+R₀+R_b, the impedance of the working electrode was simulated using (C₁//R₁)//(R_a+(C₂//R₂), the electrolyte was simulated using C₃//R₃, and the counter electrode was simulated using C₄//R₄.