Improved performance of dye-sensitized solar cells by tuning the properties of ruthenium complexes containing conjugated bipyridine ligands

Three heteroleptic ruthenium complexes cis-[Ru(H2dcbpy)(L)(NCS)2], where H2dcbpy is 4,40-dicarboxy-2,20-bipyridine and L is 4-(4-(N,N-di-(p-anisyl)amino) styryl)- 40-methyl-2,20-bipyridine (Dye-1), 4-(4-(N,N-di-(p-hexyloxyphenyl) amino) styryl)-40-methyl-2,20-bipyridine (Dye-2) or 4-(5-(N,N-di-(p- hexyloxyphenyl)- amino)-thiophene-2-yl-ethenyl)-40-methyl-2,20-bipyridine (Dye-3) have been synthesized and characterized. The influence of differently conjugated bipyridine ligands on these complexes was studied using UV-Vis spectroscopy and cyclic voltammetry. These heteroleptic complexes show appreciably broad absorption ranges and quite high extinction coefficients. These new dyes were used as photosensitizers in nanocrystalline TiO2 dye-sensitized solar cells. It was found that the difference in light-harvesting property between Dye-1, Dye-2 and Dye-3 is associated mainly with molar extinction coefficients and alignment of the HOMO-LUMO energy levels. The power conversion efficiencies of solar cells based on Dye-1 and Dye-2 are 4.21% and 4.41%, while Dye-3 delivered a lower efficiency of 2.88% under the same device fabrication and measurement conditions. © 2010 Vietnam Academy of Science & Technology.