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

Hong Minh Nguyen, Duc Nghia Nguyen and Nakjoong Kim

Abstract

Three heteroleptic ruthenium complexes cis-[Ru(H₂dcbpy)(L)(NCS)₂], where H₂dcbpy is 4,4'-dicarboxy-2,2'-bipyridine and L is 4-(4-(N,N-di-(p-anisyl)amino)styryl)-4'-methyl-2,2'-bipyridine (Dye-1), 4-(4-(N,N-di-(p-hexyloxyphenyl)amino)styryl)-4'-methyl-2,2'-bipyridine (Dye-2) or 4-(5-(N,N-di-(p-hexyloxyphenyl)-amino)-thiophene-2-yl-ethenyl)-4'-methyl-2,2'-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