Simultaneous Measurement of Charge Transfer Resistances at Photoanodes and Counter Electrodes in Aqueous Dye-Sensitized Solar Cells
The replacement of potentially dangerous and technically problematic volatile organic solvents by water in dye-sensitized solar cells (DSSCs) is desirable in order to enhance outdoor applicability, and sustainability. Since the liquid electrolyte solution interacts with all components of the cell, all interfacial processes have to be studied thoroughly. Electrochemical impedance spectroscopy (EIS) is a common and powerful characterization technique to investigate charge transfer processes in electrochemical cells. However, it lacks spatial resolution in the typically used and technically relevant two-electrode configuration and, thus, processes at the same time scale but at different electrodes superimpose in the recorded impedance spectra. By incorporating an additional microscopic quasi reference electrode (µRE) into the DSSC architecture, the contributions of the photoanode (Z1) and the counter electrode (Z2) half-cell, to the full cell impedance could be recorded separately but simultaneously under operating conditions. In the case of aqueous DSSCs with organic nitroxide radicals as redox mediators, significantly different behavior for structurally similar molecules (TEMPO and OH-TEMPO) was observed. By µRE-EIS, it was found that poor charge transfer kinetics at the counter electrode limited the overall cell performance for OH-TEMPO electrolytes, which are otherwise very promising because of a high solubility and, hence, potentially low diffusion resistance.
