Lithium ion effect on electron injection from a photoexcited coumarin derivative into a TiO2 nanocrystalline film investigated by visible-to-IR ultrafast spectroscopy

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 it wide wavelength range from 600 nm to 10 mu m. 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 similar to 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.