6-(2-Quinolinyl)-2,2′-bipyridine ruthenium complexes for near-infrared sensitization in dye-sensitized solar cells

Abstract

In an effort to obtain a dye-sensitized solar cell (DSC) with a broader absorption band, four ruthenium (Ru) complexes based on 6-(2-quinolinyl)-2,2′-bipyridine ligands with a large π-conjugated system were synthesized. We investigated the relationship between the conversion efficiency of DSC and the structure of quinoline-based Ru complex as well as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) potentials. As compared with black dye, all of these complexes showed red-shifted absorption edges. The results of electrochemical analyses and theoretical calculations indicated that the shift of the absorption bands comes from the electron-withdrawing or electron-delocalization effect of the quinoline ring of the ligands. In addition, the number and position of carboxyl groups significantly affected the distribution of HOMO and LUMO thereby shifting their potentials. Comparison of the performance of the DSCs using different complexes revealed that the electron injection efficiency was also significantly affected by the position of the carboxyl groups. In this study, the Ru complex with the ligand composed of both 4-carboxypyridine and 4-carboxyquinoline efficiently sensitized near-IR photons as well as visible photons.