Preferential Direction of Electron Transfers at a Dye–Metal Oxide Interface with an Insulating Fluorinated Self-Assembled Monolayer and MgO

Slowing nondesirable electron transfer reactions at metal oxide–dye interfaces is important for many technologies. In particular, after an interfacial photoinduced charge separation event at a metal oxide–dye interface, it is critically important to limit the rate of electron transfer reactions back to sensitizers and to limit electron transfer reactions between the electrolyte and the metal oxide. Ruthenium-based dyes at metal oxide interfaces are widely used in many fields; however, these dyes often have poor surface insulation resulting in fast recombination kinetics with transition metal-based redox shuttles (RSs) in an electrolyte. This work explores two semiconductor surface modification strategies designed to minimize recombination events of electrons in TiO2 with oxidized RSs using a fluorinated siloxane insulator (PFTS) and a metal oxide insulator (MgO) with a well-known Ru dye, B11. Additionally, the influence of these treatments on the rate and duration of photoinduced interfacial charge separation at the TiO2–dye interface was examined. The TiO2-dye-RS systems were studied via dye-sensitized solar cell current–voltage curve, incident photon-to-current conversion efficiency, small modulated photovoltage transient, time-correlated single-photon counting, and transient absorption spectroscopy measurements. MgO was found to decrease the rate of the electron transfer reaction from the metal oxide to the electrolyte, decrease the rate of the electron transfer reaction to the oxidized dye from TiO2, increase the electron transfer reaction rate from a reduced RS to an oxidized dye, and decrease the electron injection rate from the photoexcited dye to TiO2. Interestingly, 1H,1H,2H,2H-perfluorooctyltrimethoxysilane (PFTS) was found to desirably improve these rates relative to MgO or untreated TiO2. A model based on electrostatic interactions is presented to explain the exceptional behavior of PFTS with density functional theory computational analysis of PFTS supporting this model.