Electrochemical impedance spectroscopy: a powerful tool to unveil the charge transport/recombination processes in aqueous dye-sensitized solar cells

Long term stability of Dye-Sensitized Solar Cells is one of the main issues preventing the large-scale commercialization of this class of device. Indeed, high-efficiency DSSCs are prepared mainly with liquid electrolytes based on organic solvents, such as acetonitrile (ACN) and methoxypropionitrile (MPN). These are often oil derivatives characterized by toxicity and flammability; additionally, they possess high vapor pressure and volatility, which often straightforwardly lead to electrolyte leakage and decrease of photovoltaic performances over time. For this reason, the researchers’ efforts move towards alternatives solvents to obtain efficient, safe and low-cost devices. Above all, DSSCs with water-based electrolytes are amongst the best solutions providing reduced costs, non-flammability, better stability and environmental compatibility, all at once. In the last years, scientific literature on this topic has significantly increased and, recently, devices have been reported that achieved 5.5% photoconversion efficiency with a efficiency 100% aqueous electrolyte. Electrochemical Impedance Spectroscopy (EIS) has been proven to be a very powerful tool to investigate the charge transport/recombination phenomena occurring in classical device (i.e., organic-based). Yet, just few studies are reported in which EIS is used to investigate aqueous systems. As a matter of fact, the fastening of the recombination processes occurring at the electrode/electrolyte surface and the slackening of the charge transfer reactions at the counter-electrode could worsen the interpolation of experimental data. Herein, we report the electrochemical impedance spectroscopy analyses of both liquid and polymeric aqueous-based electrolytes to unveil the charge transport and recombination processes. The latter are compared to those of reference devices assembled with standard organic-based electrolyte. Results obtained confirm that the diffusion processes throughout the electrolyte are slower in aqueous environment with respect to the organic counterpart. Very interestingly, the resistance of the charge recombination reactions at the electrode/electrolyte interface is sensibly higher when an aqueous electrolyte (both liquid and polymeric) is employed. Unfortunately, this evidence does not lead to a better photoconversion efficiency because of the simultaneous worsening of the charge injection (transport) processes into (throughout) the TiO2 electrode. Nevertheless, the results discussed in this work could be considered as a starting point to further enhance the overall efficiency of aqueous-based Dye-Sensitized Solar Cells and demonstrate that EIS might be a very powerful tool in this respect.