BIS-phenanthroline copper complexes in iodine-free electrolytes for DSSCs

Dye-sensitized solar cells, DSSCs, are photoelectrochemical devices well contextualized within the global commitment for the progressive increase of the percentage of electric energy produced by renewable resources. This technology should free us from all problems and risks related to fossil fuels exploitation, in favor of the ubiquitous and practically inexhaustible sunlight. Since the milestone paper of Graetzel and O’Regan, dye engineering has been for many years the unique task of scientists to improve the photon-to-current conversion efficiency, PCE, of cells. Only in the last decade the crucial role played by redox mediators has emerged, attracting attention mainly on tris(diimine) cobalt complexes capable of raising PCEs up to 14%. In the frame of iodine-free electrolytes, our group has proposed novel homoleptic 1,10-phenanthroline copper complexes able to significantly exceed the unique effective literature benchmark proposed to date, based on neocuproine ligands. In this contribution we will present our results dealt with the study of how ligand substituents affect the electrochemical and optical features of this class of complexes so as to reduce light harvesting competition with dye and to overcome kinetic dichotomies active in DSSCs. These “structure vs activity maps” have been then exploited to choose a selected ensemble of compounds to be tested in DSSCs as electron shuttles and to rationalize their photoelectrochemical performances. Particular attention was also dedicated to select a cathode material able to minimize the overpotential for the mediator regeneration reaction. As a result we have proposed a convenient Cu-based redox couple based on bulky 2-mesityl-4,7-dimethyl-1,10-phenanthrolines that, in combination with two completely different dyes (i.e. a Ru-based and an organic one), has more than doubled the PCE of cells filled with the benchmark neocuproine-based shuttles and has reached values even slightly higher than a equimolar I–/I3– control electrolyte.