Electric Fields Control TiO2(e(-)) + I3(-) → Charge Recombination in Dye-Sensitized Solar Cells

The electric fields generated by excited-state electron injection into anatase TiO2 nanocrystallites are screened by cations present in the external electrolyte. With some assumptions, a newly discovered electroabsorption signature enables quantification of the electric field strength experienced by surface-anchored dye molecules. Here, it was found that the fields increased in the order Na(+) < Li(+) < Mg(2+) < Ca(2+), with magnitudes of 1.1 MV/cm for Na(+) and 2.2 MV/cm for Ca(2+), values that were insensitive to whether the anion was iodide or perchlorate. The magnitude of the field was directly related to average TiO2(e(-)) + I3(-) → charge recombination rate constants abstracted from time-resolved kinetic data. Extrapolation to zero field provided an estimate of recombination dynamics when diffusion alone controlled I3(-) mass transport, k = 300 s(-1). The decreased rate constants measured after excited-state injection were attributed to migration of I3(-) away from the TiO2. Cation transference coefficients were tabulated that ranged from t = 0.97 for Ca(2+) to 0.40 for Na(+) and represented the ability of the unscreened electric field to block the TiO2(e(-)) + I3(-) → charge recombination reaction. This data provides the first compelling evidence that the anionic nature of I3(-) inhibits unwanted charge recombination in dye-sensitized solar cells.