Impedance Spectroscopy with Microscopic Reference Electrodes as a Technique to Study Electrode Kinetics in Dye-Sensitized Solar Cells

Abstract

Measurements with microscopic reference electrodes in electrochemical impedance spectroscopy (µRE-EIS) were used to study electrode kinetics of dye-sensitized solar cells (DSSCs). A detailed impedance analysis of DSSCs is essential to separately optimize different rate-determining steps in these cells. Two-electrode measurements as typically used, however, often do not allow a separate analysis of charge transfer reactions at different electrodes that occur with similar time constants. Such problems, e.g., arise in DSSCs with water-based electrolytes using 2,2,6,6-tretramethylpiperidine-1-oxyl (TEMPO) as a redox mediator. The use of a quasi-reference electrode offers the possibility to separately record impedance spectra of the photoanode and the counter electrode of a DSSC under operating conditions in one experiment. With this method, electron transfer kinetics could be studied for both electrodes individually and contributions of charge carrier recombination at the photoanode could be studied separately from the reduction of the redox mediator at the counter electrode.In DSSCs, the sensitized photoanode and the counter electrode are commonly separated by no more than 60 µm, depending on the thickness of the sealing foil. Into such an arrangement, a thin platinum wire was introduced as a third electrode, serving as a microscopic quasi-reference electrode (µRE). The architecture was adjusted stepwise and the influence of the adjustments was studied to ensure that important cell properties such as solar cell parameters and diffusion resistance remained comparable to cells in a typical two-electrode architecture.Subsequently, three-electrode experiments were carried out, using the photoanode as the working electrode (WE), a counter electrode (CE), and the µRE. For µRE-EIS experiments, the potential was modulated between CE and WE and the overall cell impedance Z 0 was determined. Simultaneously, the impedance between µRE and WE was recorded (Z 1) and the impedance between µRE and CE (Z 2) was calculated from Z 0 and Z 1. Thus, one experiment yielded spectra for the full cell and both respective half-cells WE vs. µRE and CE vs. µRE.Here, we investigated DSSCs based on TiO2 photoanodes sensitized with the organic dye Y123, aqueous electrolytes with the organic redox mediators TEMPO+/0 or OH-TEMPO+/0, and poly(3,4-ethylenedioxythiphene) (PEDOT) counter electrodes. The impedance spectra revealed that there are contributions in the same frequency range in both half-cell spectra, suggesting that both charge transfer processes from the electrodes to the oxidized redox species, i.e., recombination at the photoanode and the reduction of the redox mediators at the counter electrode, respectively, occur on similar time scales. These contributions superimpose in a typical two-electrode measurement and cannot be separated. µRE-EIS, however, proved to be a powerful tool to simultaneously study the interactions of novel redox mediators with photoanodes and counter electrode materials in detail. Figure 1