Synthesis and Characterization of Diethylphosphonate and Carboxylate–appended Iridium Complexes for the Application on Dye-Sensitized Solar Cells

In this work, cyclometalated iridium(III) complexes were synthesized, presenting a general formula [Ir(ppy)2(N N)]PF6, where ppy=2-phenylpyridine or 2-(2,4-difluorophenyl)pyridine and N N=4,4′-bis(diethylphosphonate)-2,2′-bipyridine or 4,4′-bis(carboxy)-2,2′-bipyridine. The complexes were characterized by 1H-NMR, 13C-NMR 31P-NMR and UV-Vis spectroscopy. The assembled Dye Sensitized Solar Cells were characterized by current versus potential curves and by photon-to-current conversion efficiency measurements. The complexes present intense absorption bands in the UV region (250–350 nm), which are assigned to symmetry allowed singlet 1LC ππ* transitions involving both phenylpyridyl and bipyridyl ligands. At longer wavelengths (350-450 nm) the weaker absorption bands are related to transitions with mixed metal-to-ligand and ligand-to-ligand charge transfer character. Dye desorption measurements revealed that the phosphonate group provide a more stable anchoring than the carboxylate. The most efficient devices were obtained by using iridium complexes with carboxylate anchoring groups.