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

Abstract

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 TiO 2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO 2 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 I 0 0.64 ).