First application of the HETPHEN concept to new heteroleptic bis(diimine) copper(i) complexes as sensitizers in dye sensitized solar cells

We report here the synthesis and full chemical and physical characterizations of the first stable heteroleptic copper(I)-bis(diimine) complexes designed for implementation in dye sensitized solar cells (DSC). Thanks to the HETPHEN concept, pure and stable heteroleptic copper(I) complexes were isolated. Anchorage of the sensitizers was provided by 2,2'-biquinoline-4,4'-dicarboxylic acid (dcbqH2), while sterically challenged ligands 2,9-dimesityl-1,10-phenanthroline (L0) and N-hexyl-2,9-dimesityl-1,10-phenanthroline-[a:b]imidazo-(4'-dianisylaminophenyl) (L1) were used to complete the copper(I) coordination sphere. The resulting heteroleptic complexes C1 and C2 exhibit a broad MLCT transition spreading over a wide wavelength domain, especially when adsorbed onto nanoparticulate TiO2 photoanodes, providing a rather comprehensive visible light collection. The corresponding DSC were evaluated under AM 1.5 simulated solar light and rather weak performances were obtained owing to small J(sc) and V(oc). This is due to a combination of low extinction coefficient and poor driving forces for the various interfacial processes. However, significant improvements in the performances were monitored upon ageing in the dark, likely due to beneficial reorganization of the dye monolayers. The possibility to isolate stable asymmetric systems paves the way for structurally assisted photo-induced charge injection from the chemisorbed copper(i) based sensitizers into the conduction band of TiO2, through charge vectorialization.