Phenanthroline-based Copper complexes as redox mediators in DSSCs

The extraordinary energy dependence from fossil fuels has been forcing humanity to consider also other, and possibly renewable, sources. Among these the sunlight is probably the optimal task being ubiquitous, abundant, and practically inexhaustible. Since 90’s of the last century dye-sensitized solar cells, DSSCs, have been studied as light-to-electricity-conversion devices alternative to the already mature silicon-based photovoltaic technology. Especially in the last years a lot of efforts have been made in the optimization of electrolytes for DSSCs, searching for efficient redox mediators that can overcome some intrinsic drawbacks of the most common I3-/I- redox couple. Among the possible classes of electron shuttles our research group has bet on first-row transition metal complexes, focusing on copper. In this contribution we will focus on substituted 1,10-phenanthroline-based copper complexes which revealed to be effective redox mediators characterized by i) a simple and high yielding synthesis, ii) a high chemical and electrochemical reversibility, iii) and an easy-tunable oxidation half-wave potential E1/2(Cu2+|Cu+) obtainable through modification of the diimine chelating scaffold. A quite complete characterization of the aforementioned N^N chelated copper complexes will be presented, resulting from a multi-technique approach which combined electrochemical and photoelectrochemical techniques (including electrochemical impedance spectroscopy) with time-resolved absorption spectroscopy. We will start with the electrochemical rationalization of their thermodynamic and kinetic features as a function not only of the ligand structure (and of the related complex geometry) but also of solvent and counteranion nature. After that we will show the results obtained employing copper complexes as redox mediators in DSSCs, screening different kinds of photoanodes and cathodes, different TiO2 sensitizers (both Ru-based and fully-organic dyes), and various electrolytes changing the absolute and relative content of oxidized and reduced form of the mediators. Up to now the best combination of cell components gave rise to a power conversion efficiency higher than 5% recorded under 100 mW cm-2 AM 1.5G illumination.