The influence of cations on charge accumulation in dye-sensitized solar cells

The relation between the VOC, light intensity, φ, and accumulated charge, Q, was studied for dye-sensitized solar cells (DSCs) contg. different counterions for the iodide/triiodide redox couple. At higher light intensities, VOC scaled in the order Cs+ > K+ > Na+ > Li+, which was caused in part by shifts in the conduction band edge. The relation between VOC and Q was fitted to an exponential trap model. Inclusion of a capacitive term, C, improved the fit significantly. The detd. values of C are large, up to 75 μF/cm2, and dependent on the cation. The largest fraction of C can be ascribed to the TiO2 bulk or the TiO2/dye/electrolyte interface. The interpretation of the trap distribution broadening parameter, β, is dependent on the fitting model. Using the model including the linear CVOC term, β was independent of cation and could be viewed as a TiO2 material parameter, while in the model excluding CVOC, β was dependent of cation. Voltage decay expts. were performed to study the cationic influence on recombination. Electron lifetimes were calcd. from the voltage decay curves and the DSC contg. Li+ yielded the shortest lifetime followed by the DSCs contg. Na+, K+ and Cs+. Voltage decay curves include the effect of the TiO2 conduction band shifts in the comparison of electron lifetimes with different cations. It is accordingly suggested that electron lifetimes be calcd. from charge decay curves. From such a comparison, the DSC contg. Li+ yielded the shortest lifetime whereas the DSCs contg. Na+, K+ or Cs+ showed approx. identical lifetimes.