Impedance spectroscopy study of dye-sensitized solar cells with undoped spiro-OMeTAD as hole conductor

The photoelectrical properties of a solid state dye-sensitized (DSSC) solar cell with 2,2′,7,7′- tetrakis(N,N-di-p-methoxyphenilamine)-9,9′-spirobifluorene (Spiro-OMeTAD) as transparent hole conductor have been studied in comparison with liquid electrolyte DSSC. Measurements of photopotential decay, transient spectroscopy, and impedance spectroscopy show that the lower electron lifetime determines the lower efficiency of the solid state DSSC with respect to the liquid electrolyte cell. Due to the high transport resistance of pure spiro-OMeTAD the cell response is transport limited at the more positive potentials studied, but recombination dominates at the potentials near the maximum efficiency peak and more negative potentials. Low frequency capacities presented by the solid state cells are much smaller than those of respective liquid cell versions.