Copper Complexes as Electron Mediators in DSSCs

The fossil fuels depletion puts the challenge to humanity to replace such energy sources with renewable and more ethic ones. The Sun could be considered a practically inexhaustible energy source on Hearth whose light could be converted into electric energy exploiting the photovoltaic effect exploited for decades by the well-known solar panels based on mono- or polycrystalline silicon cells. Starting from the pioneering study of Graetzel and co-workers, in the last years a lot of efforts have been made by different scientific groups all other the world to try to make competitive cells that employ the “indirect” excitation of cheaper wide-band gap semiconductors (e.g. titanium dioxide) by means of sensitizer molecules able to adsorb light in the visible region of the electromagnetic spectrum. Leaving out the inorganic semiconductor the two main components are object of an intense study: the dye and the mediator redox couple. This work is focused on copper complexes with 1,10-phenanthroline ligands as possible alternative mediators to the common I3-/I- redox couple which however suffers of some drawbacks (e.g. volatility and corrosive nature of iodine species). A rational study on the modulation of the electrochemical behavior of these copper complexes was performed in order to obtain useful information for their clever design; different substituents (varying their electronic and steric properties) placed in different positions on the phenanthroline scaffold were considered. In order to obtain a full characterization, electrolytes containing the copper complexes were tested in dye-sensitized solar cells combining photoelectrochemical studies with fast pulsed laser experiments. The effect of iron polypyridine co-mediators was also considered. As an example we cite the significant performances obtained with the bulky asymmetric substituted bis-(2-mesitil-1,10-phenanthroline) copper hexafluorophosphate complex (see Figure). This mediator allowed us to increase the fill factor (> 0.60) and to double both the short circuit current and the overall conversion efficiency of the cell taking as reference the symmetric neocuproine copper complex already proposed in literature (which is able to reach h = 7.0% in combination with C218 dye]).