Copper complexes as electron shuttles in DSSCs

One of the challenges that humanity will soon have to face is the problem of the increasing energy demand. Depletion and environmental impact of fossil fuels necessitate abandoning them in favor of alternative, possibly cleaner and renewable, energy 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. Researches started in the last decade have identified in first-row transition metal complexes promising redox mediators alternative to the most common I3-/I- redox couple. In this contribution we will focus on substituted bis(1,10-phenanthroline) 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, and iii) an oxidation half-wave potential E1/2(Cu2+|Cu+) and an electron transfer rate easy-tunable through modification of the diimine chelating scaffold. Starting from the electrochemical rationalization of their thermodynamic and kinetic features as a function of the ligand structure (and of the related complex geometry), solvent and counteranion nature, and electrode material we tested some complexes in liquid-type DSSCs. Variations of the dye, of the counter electrode material and of the electrolyte formulation were performed reaching a 4.5% efficiency under 100 mW/cm2 AM1.5 G illumination with the homolepctic [Cu(2-mesityl-4,7-dimethyl-phenanthroline)2]+/2+ redox couple.