Dynamics of charge recombination at surface observed by highly sensitive transient absorption spectrometer

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Charge recombination at interface is an interesting process and it has been applied to solar energy conversion devices, such as photocatalysts and dye-sensitized solar cells. Upon photoexcitation charge separation occurs and then cations and anions are produced. These are non-emitive species and therefore dynamics of these species cannot be probed by fluorescence spectroscopy. Transient absorption technique is a powerful tool for this target. However, in reality it is quite difficult to probe these phenomena due to low sensitivity. Thus, we proposed to develop highly sensitive transient absorption spectrometer.Firstly, we developed highly sensitive transient absorption spectrometer. Using the spectrometer, we can observed very small absorbance change (<10^<-6>). This is enough sensitivity to observe transient absorption in mono-layer film.We studied charge separation and recombination dynamics of dye-sensitized semiconductor films, organic dyes adsorbed on oxide semiconductors (TiO_2,ZnO, SnO_2 etc.). We found that quantum yield of the charge separation is very sensitive to the exciting light intensity and it becomes constant at lower intensity. We also estimated absolute quantum yield of the charge separation. We also studied primary process of photoexcitation of photocatalyst, such as TiO_2 films. Upon photoexcitation electrons and holes are produced. Using our spectrometer reactivity of these species can be studied.