Effect of electrolyte constituents on the motion of ionic species and recombination kinetics in dye-sensitized solar cells

The dynamic motion of ions in electrolyte solutions and its effect on recombination was investigated by the heterodyne transient grating method in addition to transient absorption and transient photocurrent methods in dye sensitized solar cells. Realignment of ionic species at the electrode/electrolyte interface was observed after the electron injection in TiO2 on the order of μs. The process was affected by the total quantity of ionic species as well as cation species in the electrolyte. The recombination processes of the electrons were also affected by the constituents; the probability of the electron–electrolyte recombination decreased with decrease in I2 concentration; the dominant recombination process changed from the electron–electrolyte to the electron–dye recombination by decreasing I− concentration. It is concluded that sufficient I− is necessary for the suppression of the electron–dye recombination and that sufficient I2 is necessary for an efficient redox cycle, while low concentration of I3− ions at the electrolyte/TiO2 interface is preferable to suppress the electron–electrolyte recombination. The effect of the cation size in an electrolyte solution on the charge dynamics was also investigated, and it was revealed that the steric hindrance of cations changed the penetration of ionic species into the nanoporous dye/TiO2 electrode, causing a change in the electrostatic properties at the interface. The cation dependence indicated that the presence of large-sized cations suppressed the electron–electrolyte recombination by disturbing the approach of I3− paired with the cations.