| The electron transfer dynamics at the semiconductor-liquid interface determines the efficiency of solar cells based on dye-sensitized semiconductor electrodes. These cells are promising devices for clean, efficient, and renewable energy sources. The objectives of this study are: (i) To determine the relevance of the different photophysical processes that compete with the forward and back electron transfer reactions for excited dye molecules adsorbed at the surface of semiconductor nanoparticles; and (ii) to ascertain the importance of external conditions that affect electron transfer reactions at interfaces, such as solvent, reaction driving force, etc. In order to achieve these goals, transient absorption experiments were performed with ca. 200 fs time resolution. The results of these experiments show that the surface of the semiconductor plays a fundamental role in controlling the electron transfer dynamics. Specifically, a strong electrostatic interaction between the surface of SnO2 and cresyl violet induces dimerization of the dye molecules with a consequent increase in the rate of internal conversion, in detriment to charge injection into the semiconductor. In addition, the effect of the solvent on the surface of the nanoparticles dramatically changes the kinetics of the system. For example, the addition of small amounts of water to an ethanolic suspension of TiO2 particles increases the rate of back electron transfer for 9-anthracenecarboxylic acid adsorbed at the semiconductor surface. Furthermore, the electron transfer dynamics depend on both the crystalline structure of the semiconductor and the chemical structure of the dye. Specifically, the 3 isomers of anthracenecarboxylic acid present faster forward and back electron transfer times for anatase TiO2 particles compared to amorphous TiO2 particles. This difference is attributed to a difference in the electronic coupling between the dye and the semiconductor. On the other hand, the rate constants for the different isomers are controlled by the energetics of the system. Finally, the size of the nanoparticles does not have any effect on the electron transfer dynamics, for sizes on the 4 to 40 nm diameter range. The insight into surface electron transfer gained from these studies will aid the design of solar cells based on dye sensitization. |
