Charge Transport and Back Reaction in Solid-State Dye-Sensitized Solar Cells:  A Study Using Intensity-Modulated Photovoltage and Photocurrent Spectroscopy

Solid-state dye-sensitized solar cells employing spiro-MeOTAD 2,2'7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene as a hole transport phase were studied by intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) over a wide range of illumination intensity. The IMPS and IMVS responses provide information about charge transport and electron−hole recombination, respectively. For the range of light intensities investigated, the dynamic photocurrent response appears to be limited by the transport of electrons in the nanocrystalline TiO2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO2 was found to be 4.4 × 10-4 cm. This value was almost independent of light intensity as a consequence of the fact that the electron diffusion coefficient and the rate constant for electron−hole recombination both increase in the same way with light intensity (proportional to I00.64).