Charge Rectification at Molecular Nanocrystalline TiO2 Interfaces: Overlap Optimization To Promote Vectorial Electron Transfer

Three pyridinium molecules were synthesized and were anchored to the mesoporous TiO2 thin films that are commonly used in dye-sensitized solar cells. The first reduction potential of the pyridiniums spanned a 660 mV range and occurred commensurate with or after the direct reduction of TiO2. The interfacial redox chemistry was non-Nernstian and was modeled by the inclusion of ideality factors. The kinetics for TiO2 and pyridinium reduction were quantified spectroscopically after a potential step. The reduction rate of TiO2 and of the pyridiniums were within experimental error the same consistent with a band-filling or "cup of wine" model. In contrast, oxidation of the reduced pyridiniums was dependent on the pyridinium formal reduction potential: Those with the most positive formal reduction potential required the most time. The behavior is understood based on the overlap of the pyridinium redox states with the acceptor states in TiO2 and provides a means for optimization of vectorial electron transfer at these interfaces.