Influence of Electrical Potential Distribution, Charge Transport, and Recombination on the Photopotential and Photocurrent Conversion Efficiency of Dye-Sensitized Nanocrystalline TiO2 Solar Cells: A Study by Electrical Impedance and Optical Modulation Techniques

The role of electrical potential, charge transport, and recombination in determining the photopotential and photocurrent conversion efficiency (IPCE) of dye-sensitized nanocrystalline solar cells was studied. Electrostatic arguments and electrical impedance spectroscopy (EIS) are used to obtain information on the electrical and electrochemical potential distribution in the cell. It is shown that on the macroscopic level, no significant electrical potential drop exists within the porous TiO₂ when it contacts the electrolyte and that the electrical potential drop at the transparent conducting oxide substrate (TCO)/TiO₂ interface occurs over a narrow region, one or two layers of TiO₂. Analyses of EIS and other data indicate that both the photopotential of the cell and the decrease of the electrical potential drop across the TCO/TiO₂ interface are caused by the buildup of photoinjected electrons in the TiO₂ film. The time constants for the recombination and collection of the photoinjected electrons are measured by EIS and intensity-modulated photocurrent spectroscopy (IMPS). As the applied bias is varied from short-circuit to open-circuit conditions at 1 sun light intensity, recombination becomes faster, the collection of electrons becomes slower, and the IPCE decreases. The decrease of IPCE correlates directly with the decline of the charge-collection efficiency η_cc, which is obtained from the time constants for the recombination and collection of the photoinjected electrons. Significantly, at open circuit, η_cc is only 45% of its short-circuit value, indicating that the dye-sensitized nanocrystalline TiO₂ solar cell behaves as a nonideal photodiode.