Bis(1,10-phenanthroline) copper complexes with tailored molecular architecture: From electrochemical features to application as redox mediators in dye-sensitized solar cells

In the last few years, copper coordination compounds turned out to be effective competitors of cobalt complexes as redox mediators in the formulation of iodine-free electrolytes for dye-sensitized solar cells (DSSCs). However, the lack of a clear correlation between electrochemical signatures of copper complexes (i.e. half-wave potential and heterogeneous electron transfer rate) and photoelectrochemical performance of solar devices makes difficult the optimization of their coordination sphere. Therefore, to partially fill this gap and to elucidate the intrinsic correlation between the molecular architecture of these complexes and their electrochemical features, we prepared four Cu+/2+ redox couples in which the copper center is coordinated by two 1,10-phenanthrolines bearing various substituents in position 2. These complexes were well characterized, from both electrochemical and spectroscopic point of view, and tested as electron shuttles in lab-scale photoelectrochemical cells sensitized with two efficient π-extended benzothiadiazole dyes. It appeared that 2-aryl-1,10-phenanthrolines effectively combine suitable optical and electrochemical properties. While a fast electron transfer kinetics generally positively affects the dye regeneration process, an optimal balance between dye regeneration efficiency, mass transport and heterogeneous electron transfer at both the counter electrode and at the TiO2 interface, must be achieved in order to optimize DSSC performance. Within our series, the top performer was [Cu(2-tolyl-1,10-phenanthroline)2]+/2+ which achieved a relative 20% and 15% improvement in power conversion efficiency (under 100 mW s−1 simulated AM 1.5G illumination) with respect to control cells filled with [Co(bpy)3]2+/3+ (bpy = 2,2′-bipyridine) and I−/I3− electrolytes, respectively.