Theory of the impedance of charge transfer via surface states in dye-sensitized solar cells

The dynamics of trapping coupled with charge transfer is analysed in the frequency domain for a model of the dye-sensitized solar cell. We solve the steady state occupations of the surface state and the resulting recombination currents, based on the assumption of detailed balance. We determine all the parameters that control the frequency response in Impedance Spectroscopy as a function of the steady state, and the characteristic shape of the impedance spectra. At large charge transfer rate the low frequency capacitance of the surface states is considerably reduced with respect to the thermodynamically defined equilibrium capacitance. We also show that the usual kinetics of recombination does not give a negative capacitance contribution.