Electrochemical Impedance Analysis for Corrosion on Current Collecting Electrodes in Dye-sensitised Solar Cells

Dye-sensitized solar cell (DSC) is paid attention to next generation’s energy device because of various advantages such as simple manufacturing process, low cost and low environmental loading. Recently, the modularization technology has been developed for improvement of the DSC performance. For example, Toyoda et al. fabricated large scale DSC modules with glass substrate, and indicated that the modules have the long term stability for a half year. Itagaki et al. reported that a plastic DSC module maintained its high conversion efficiency under the accelerated condition of 55 oC and 95 % relative humidity. Typical plastic DSC module is composed of three parts shown as follows: (1) Photoelectrode (anode) which consists of a porous TiO2 film with absorbed dye and the fluorine-dope SnO2 (FTO) or indium tin oxide (ITO) coated a plastic substrate. (2) Organic electrolyte solution containing redox couples such as iodide ions (I) / triiodide (I3). (3) Pt or carbon counter electrode (cathode) formed on the plastic substrate. In the modulation of the DSC cell, current collecting electrodes are normally used for reducing the resistance of the substrate. A silver wiring is often printed as the collecting electrode on the substrate. The silver wiring is covered by a polymer sealing film to present the silver wiring from corroding. But, a corrosion of the silver wiring under the polymer film often arises with deterioration of the polymer sealing film. The corrosion including the electrochemical migration (ECM), induces a trouble of the DSC. The authors previously investigated oxide film formation and ECM of a silver wiring electrode using electrochemical impedance spectroscopy (EIS). The initial growth and growth process of ECM oxide film formation of a silver wiring electrodes printed on a polyimide substrate were detected by EIS. In the present study, we apply the EIS to analysis of corrosion of current collecting electrodes in a DSC module fabricated by screen-printing. The DSC module with two silver current collecting electrodes was fabricated by screen printing (Fig. 1). All measurements were performed using a potentio/galvanostat with FRA analyzer (HZ-5000, Hokuto Denko) under the simulated solar radiation of 100 mWcm produced by solar simulator (PEC-L11, Peccel Technologies). Electrochemical impedance was measured in the frequency range from 100 mHz to 100 kHz. The dc potential of the electrode was kept at open circuit potential. Figure 2 shows typical impedance response measured just after the start of solar radiation. The three capacitive loops were observed in the Nyquist plot. The loop in the high frequency range was dependent on the cathodic reaction. The loop in the middle frequency range was dependent on the anodic reaction. The diffusion impedance in the low frequency range was dependent on the diffusion of triiodide in the electrolyte solution. The impedance was increased markedly after two hour from the start of insolation (Fig. 3). No change in the loop depending on the cathodic reaction was observed in high frequency range. On the other hand, the capacitive loops in middle frequency range increased drastically. The dendrite growth was observed on the cathode. From these results, it suggests that increase of the impedance is mainly due to the corrosion of the current collecting electrode. In the present study, we discuss the impedance spectra by performing impedance simulations with assuming equivalent circuits.