Impedance Spectroscopy: a General Introduction and Application to Dye-sensitized Solar Cells

Impedance Spectroscopy (IS) has become a major tool for investigating the properties and quality of dye-sensitized solar cell (DSC) devices. This chapter provides an introduction of IS interpretation methods focusing on the analysis of DSC impedance data. It also presents a scope of the main results obtained so far. IS gives access to fundamental mechanisms of operation of solar cells, for which reason we discuss our views of basic photovoltaic principles required to realize the interpretation of the experimental results. The chapter summarizes some 10 years of experience of the authors with regard to modeling, measurement and interpretation of IS applied in DSC. A good way to start this subject is a brief recollection of how it evolved over the fi rst years. The original " standard " confi guration of a DSC [12.1] that emerged in the early 1990s is formed by a large internal area constituted of a nanostructured TiO 2 semiconductor, connected to a transparent conducting oxide (TCO) and coated with photoactive dye molecules. It is furthermore in contact with a redox I I − 3 − / electrolyte that is in turn connected to a Pt-catalyzed counterelectrode (CE). The DSC was initially developed to be a photoelectrochemical solar cell. Electrochemical Impedance Spectroscopy (EIS) is a traditional method, central to electrochemical science and technology. Electrochemistry usually investigates interfacial charge transfer between a solid conductor (the working electrode, WE) and an electrolyte. This is done with a voltage applied between the WE and CE, with the assistance of a reference electrode (RE), rendering it possible to identify the voltage drop at the interface between the WE and the electrolyte. In addition, the electrolyte often contains a salt that provides a large conductivity in the liquid phase and removes limitations by drift transport in an electrical fi eld. Electrochemistry is thus mostly concerned with interfacial charge transfer events, possibly governed by diffusion of reactants or products. It is with EIS