Ruthenium dyes with heteroleptic tridentate 2,6-bis(benzimidazol-2-yl)-pyridine for dye-sensitized solar cells: Enhancement in performance through structural modifications

Three heteroleptic pyridine/benzimidazole-based ruthenium sensitizers, [Ru(L1)(dcbpyH2)NCS](SCN) (Y1, L1 = 2,6-bis(1-benzylbenzimidazol-2-yl)-pyridine, dcbpyH2 = 4,4′-dicarboxylic acid-2,2′-bipyridine), [Ru(L2)(dcbpyH2)NCS](SCN) (Y2, L2 = 2,6-bis(1-ethylbenzimidazol-2-yl)-pyridine), and [Ru(L3)(dcbpyH2)NCS](SCN) (Y3, L3 = 2,6-bis(1-hexylbenzimidazol-2-yl)-pyridine) are synthesized and applied to dye-sensitized solar cells (DSSCs). The power-conversion efficiency of dye Y3 is higher than those of dyes Y1 and Y2, due to the modification of the auxochromic ligand with a hexyl moiety. The origins of the performance diversity in these devices have been studied with respect to power-conversion efficiency by UV–Vis absorption and emission spectra, cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy (IMPS), intensity modulated photovoltage spectroscopy (IMVS), and density functional theory (DFT) studies. The better solar-cell performance of Y3 compared to Y1 and Y2 was believed to derive from the enhanced electron lifetimes.