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

Abstract

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 (rt and rh) 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, rt and rh 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 rt is significantly larger than rh under all conditions studied. With rt and rh 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. © 2013 American Chemical Society.