Influence of Ionic Liquid on Recombination and Regeneration Kinetics in Dye-Sensitized Solar Cells

Nonvolatile electrolyte solutions are necessary for dye-sensitized solar cells (DSCs) with good long-term stability. Such electrolytes usually contain room-temperature ionic liquids (RTILs) and consequently possess higher viscosity and ionic strength than the volatile electrolytes used in current champion cells. In this study, we systematically investigated the effect of an RTIL additive on the performance of DSCs employing either a classical Ru-complex dye or a recently developed organic D-A-pi-A dye, in combination with either I-/I-3(-) or [Co(bpy)(3)](2+/3+) as redox mediator. Using impedance spectroscopy and transient absorption measurements under various background illumination intensities, recombination and regeneration kinetics were examined. Recombination is accelerated in the I-/I-3(-) devices upon addition of RTIL, regardless of the dye used, but it is retarded in the [Co(bpy)(3)](2+/3+) devices. Addition of RTIL slowed regeneration in I-/I-3(-) devices for both sensitizers, marginally accelerated it for [Co(bpy)(3)](2+/3+) with the Ru-complex dye, and did not significantly affect it for [Co(bpy)(3)](2+/3+) with the D-A-pi-A dye. We show that these findings cannot be explained by diffusion limitations caused by increased solution viscosity or by a shift in the TiO2 conduction band relative to the electrolyte redox level. These findings should be optimization of RTIL-based DSCs.