Near‐IR Sensitization of Dye‐Sensitized Solar Cells Using Thiocyanate‐Free Cyclometalated Ruthenium(II) Complexes Having a Pyridylquinoline Ligand

Three thiocyanate-free cyclometalated ruthenium(II) complexes, [Ru(dcpq)(2)(CN)]Cl [where dcpq is 2-(4-carboxypyridin-2-yl)quinoline-4-carboxylic acid and (CN)-N-perpendicular to is a bidentate cyclometalating ligand] (compounds FT22, FT23, and FT61), were designed and synthesized as near-IR (NIR) sensitizers for dye-sensitized solar cells (DSSCs) to investigate the effect of the ligand structure on the photovoltaic performance. Density functional theory calculations of these complexes show that the highest occupied molecular orbital is mainly localized on the metal center with the (CN)-N-perpendicular to ligand, and the lowest unoccupied molecular orbital is mainly localized on the anchoring dcpq ligand. These complexes exhibit a broad absorption band and superior light-harvesting properties in the NIR region. FT22-, FT23-, and FT61-based DSSCs exhibit efficiencies of 3.81, 3.43, and 5.53%, respectively. The modification of the (CN)-N-perpendicular to ligand improves the photovoltaic performance of this class of cyclometalated ruthenium(II) complexes. The DSSC sensitized with FT61 exhibited a 25% incident photon-to-current conversion efficiency at 900 nm.