A computational study on Ru complexes with bidentate carboxylate ligands: Insights into the photocurrents of dye-sensitized solar cells

• We explored the origin of the Jsc variation in DSSCs with N^O ligands in Ru dyes.
• Higher Jsc can be obtained for N^O that involve pyridine or quinoline.
• The relative disposition of N^O to the central pyridyl group must be trans.

Factors associated with the short-circuit photocurrent densities (Jsc) of dye-sensitized solar cells (DSSCs) were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). The goal was to explore the origin of the significant variations in the Jsc values of tricarboxyterpyridine-Ru(II) complex dyes containing bidentate carboxylate ligands (N^O). These variations in Jsc were observed despite the efficient driving force for dye regeneration (ΔG2) that was estimated by the difference between the redox potential of the dye and the iodine/iodide electrolyte. The change in the orientation of the coordinated 2-pyridinecarboxylate ligand of FT27 (cis) weakened its light-harvesting ability at wavelengths longer than 470 nm relative to that of FT28 (trans). This change also reduced its ability to inject an electron into the TiO2 conduction band, while it accelerated the recombination of the injected electrons in the TiO2 with the oxidized FT27, leading to the lowest Jsc among the five tested complexes. The 4-imidazolecarboxylate ligand of FT36 promoted intermolecular interactions with I2 and enhanced the recombination of the injected electrons in TiO2 with the I2 acceptor, leading to poor charge collection and the second-lowest Jsc among the five complexes. Conversely, FT12, which contains a 2-quinolinecarboxylate ligand, exhibited the highest Jsc due to its superior light-harvesting, injection, and collection abilities. The computational results strongly indicate that the value of ΔG2 along with other factors such as the light-harvesting, and electron-injection/collection abilities of the dye sensitizer affect the photocurrents of DSSCs.