Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye-Semiconductor (TiO2

The charge injection dynamics of dye sensitization from a surface-bound dye (coumarin 343 (C343)) to the conduction band (CB) of the TiO2 is reported here for the first time. Ultrafast fluorescence dynamics demonstrate that the charge injection from the C343 dye to the CB of the TiO2 occurs on a time scale of ca. 200 fs. The charge injection efficiency is attributed to strong electronic coupling between the dye and TiO2 energy levels. The results yield a rate of injection of 5 × 1012 s-1. Photoinduced electron transfer between dyes and colloidal semiconductors plays a vital role in silver halide photography, electrophotography, and more recently in solar energy cells. Sensitizing semiconducting oxides such as TiO2,1,2 SnO2,3 and ZnO24 has proven an effective way to achieve electron injection for solar cells.5 The photostability of TiO2, combined with its ability to effectively and quickly separate charge, makes it a potentially attractive material for solar energy cells. However, since the bandgap lies in the ultraviolet, to make TiO2 colloids sensitive to visible (sun)light, sensitizing dyes must be adde