Mechanism of Molecular Control of Recombination Dynamics in Dye-Sensitized Nanocrystalline Semiconductor Films

Abstract

Recent experiments have shown that it is possible to switch between electron transport limited and interfacial electron transfer (IET) limited recombination dynamics of photoinjected electrons and the oxidized dye molecules anchored to the TiO2 under the same spectroscopic conditions, by appropriate design of the dye. The kinetics not only is slower in the IET limited case but also single exponential, in contrast to the highly dispersive transport limited recombination kinetics. We suggest that exponential kinetics could be a result of the Coulomb trap effect. Once an electron reaches the cation, electrostatic interaction keeps it near the cation until recombination takes place. Otherwise, dispersive transport would not allow for the exponential kinetics to develop. An analytical expression for the cation survival probability is derived that shows very good agreement with available experimental data. Physically, the Coulomb trap effect is justifiable if the relative dielectric constant of the surrounding ...