Characterization of the Pore Filling of Solid State Dye Sensitized Solar Cells with Photoinduced Absorption Spectroscopy

Near steady-state photoinduced absorption (PIA) and UV-Vis absorption spectroscopy are used to characterize the pore filling of spiro-MeOTAD (2, 2',7, 7'-tetrakis-(N, N-di-p-methoxyphenylamine) 9, 9'-spirobifluorene) into the nanoparticulate TiO2 electrode of a solid-state dye-sensitized solar cell (ssDSC). The volumetric ratio of filled to unfilled pore volumes, as well as the optical signature of interacting chemical species, that is, the hole-transfer yield (HTY), are investigated. PIA spectroscopy is used to measure the HTY, relative to the amount of spiro-MeOTAD present, without needing to determine the extinction coefficients of the dye and spiro-MeOTAD cation species. The Beer-Lambert law is used to relate the relative PIA signal to the penetration length of the hole-conductor in the TiO2 film. For the sample thickness range of 1.4-5 mu m investigated here, the optimum characteristic penetration length is determined to be 3.1 + 0.46 mu m, which is compared to 1.4 mu m for the 200 mg mL(-1) concentration of spiro-MeOTAD conventionally used. Therefore, doubling the effective penetration of spiro-MeOTAD is necessary to functionalize all the dye molecules in a ssDSC.