Heterogeneous electron transfer from dye-sensitized nanocrystalline TiO2 to [Co(bpy)3]3+: insights gained from impedance spectroscopy.

Dye-sensitized solar cells (DSCs) employing the Co(bpy)3 redox mediator have recently attained efficiencies in excess of 12%, increasing the attractiveness of DSCs as an alternative to conventional photovoltaics. Heterogeneous electron transfer from dye-sensitized nanocrystalline TiO2 to Co(bpy)3 ions in solution, a process known as recombination in the context of DSC operation, is an important loss mechanism in these solar cells. Here, we employ impedance spectroscopy over a range of temperatures to characterize electron storage, transport, and recombination in efficient DSCs based on the Co(bpy)3 redox mediator, with either the amphiphillic ruthenium sensitizer Z907 or the state-of-the-art organic sensitizer Y123. The temperature dependence of the electron-transport resistance indicates that transport occurs via states at energies lower than commonly assumed for the TiO2 conduction band edge. We show that a non-exponential dependence of capacitance, transport resistance, and recombination resistance on photovoltage can be interpreted as evidence for partial unpinning of the TiO2 energy levels. We also find that the nature of the sensitizing dye determines the predominant recombination route: via the conduction band for Y123 and via band gap states for Z907, which is the main reason for the superior performance of Y123. The different mechanisms appear to arise from changes in electronic coupling between TiO2 donor states and Co(bpy)3 acceptor states, as opposed to changes in the density of TiO2 states or their energetic matching with the acceptor-state distribution. These findings have implications for modeling heterogeneous electron transfer at dye-sensitized semiconductor-solution interfaces in general and for the optimization of DSCs.