Determining the conductivities of the two charge transport phases in solid-state dye-sensitized solar cells by impedance spectroscopy

To apply impedance spectroscopy (IS) for studying charge transport in the mesoporous film of solid-state dye-sensitized solar cells (ss-DSCs), it is necessary to determine the relative conductivities of the electron and hole transporting phases, given the equivalent positions of the distributed electron and hole transport resistances (r_t and r_h) in the equivalent circuit. Here, in-plane transistor-like devices employing dye-sensitized TiO₂ and the spiro-OMeTAD hole conductor were fabricated and characterized with IS. By design, r_t and r_h are no longer in equivalent positions in these devices, thus providing a means to determine their values independently. Fitting and simulation results of transistor-like devices combined with cross checks against results obtained for ss-DSCs with regular solar cell geometry clearly show that r_t is significantly larger than r_h under all conditions studied. With r_t and r_h confidently assigned, charge transport and transfer in ss-DSCs are discussed and effective carrier diffusion lengths are calculated. The results show that charge collection is limited by an inadequate electron diffusion length in ss-DSCs, implying the necessity to enhance electron transport or retard recombination in order to improve the performance of ss-DSCs. We expect that the experimental methodology proposed here will find important use in other ss-DSCs.