Impedance Investigation of Dye-Sensitized Solar Cells Employing Outer-Sphere Redox Shuttles

Electrochemical impedance spectroscopy was used to investigate dye-sensitized solar cells (DSSCs) employing outer-sphere redox shuttles. A three-electrode cell was used to overcome limitations associated with a high charge transfer resistance at the counter electrode, accurately compare alumina coated and bare photoanodes, and compare DSSCs with different solution potentials. The effect of an alumina coating, which has been shown to improve the performance of such DSSCs, was determined by comparisons of the chemical capacitance, charge transport and charge recombination. Contrary to previous reports and our expectations, no evidence of recombination from surface states was observed at any potential range for either coated or uncoated electrodes, further confirmed with open circuit voltage decay measurements. Instead, even a single atomic layer deposition cycle of alumina acts as a tunneling barrier layer, which produces the decrease in recombination rate of electrons from the TiO2 to the oxidized redox shuttle. Impedance measurement results allowed calculation of charge transport times and lifetimes for DSSCs employing a series of cobalt bipyridyl redox shuttles. Comparisons of these transport times and lifetimes produce estimates of the effective charge diffusion lengths. The relative diffusion lengths are in excellent agreement with the photovoltaic response of the DSSCs studied; however, the magnitude appears to be a slight overestimate.