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The performance enhancement of dye-sensitized solar cells (DSCs) in lithium-free and lithium-containing electrolytes under visible light-soaking was examined by impedance spectroscopy and photovoltage transient decay measurements. The improvement was found to arise from the formation of electronic transport levels close to the conduction band, resulting most likely from photoinduced proton intercalation in the TiO2 nanoparticles. These shallow trapping states accelerate the charge carrier transport within the nanocrystalline films without deteriorating the open circuit photovoltage. Subjecting the cells to forward bias in the dark produces a similar effect, whereas the introduction of lithium ions in the electrolyte suppresses the phenomena due to prevailing lithium ion intercalation. The redistribution of localized states in the band gap of TiO2 and the resulting conduction band edge movement appears to play a significant role in the performance of the DSC.
Wang et al. (2008) studied this question.