Lithium Ion Effect on Electron Injection from a Photoexcited Coumarin Derivative into a TiO2 Nanocrystalline Film Investigated by Visible-to-IR Ultrafast Spectroscopy

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Abstract
The dynamics of ultrafast electron injection from a coumarin derivative (NKX-2311), which is an efficient photosensitizer for dye-sensitized solar cells, into the conduction band of TiO2 nanocrystalline films have been investigated by means of femtosecond transient absorption spectroscopy in a wide wavelength range from 600 nm to 10 μm. In the absence of Li+ ions, electron injection into the TiO2 conduction band occurred in about 300 fs. In the presence of Li+ ions, however, electron injection occurred within ∼100 fs, and the oxidized dye generated was found to interact with nearby Li+ ions. Possible positions of Li+ ion attachment to the dye molecule were examined by means of semiempirical molecular orbital calculations. The electron injection efficiency was found to increase by a factor of 1.37 in the presence of Li+ ions. The effects of Li+ ions on the energy of the TiO2 conduction band and the electronic interaction between the dye molecule and Li+ ions are discussed, and the major cause for the acceleration of electron injection was suggested to be a conduction-band shift of TiO2.

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