A New Route to Enhance the Light-Harvesting Capability of Ruthenium Complexes for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) have been explored for more than a decade for realistic photovoltaic applications owing to their high conversion efficiency and low cost. Molecular engineering of the sensitizers to achieve high photovoltaic performance and long-term device stability is one of the critical strategies. Since the first high-efficiency ruthenium-based sensitizer, cis-di(thiocyanato)-bis-(2,2′-bipyridyl)-4,4′-dicarboxylate ruthenium(II) (N3), reported by Grätzel and coworkers in 1993, various structural modifications have been performed to improve the molar extinction coefficient of the sensitizers. It was found that elongating the conjugation length of the anchoring or ancillary ligand is the best route, although it may come up against the problem of solubility, which is not only a critical point for dye preparation, purification, and identification but also one of the crucial factors for the photovoltaic performance of DSCs. Nevertheless, it is essential to enhance the light-harvesting capacity and at the same time maintain the desirable solubility of dyes to be used in DSCs. Here we report the synthesis and performance of a new well-designed ruthenium complex, SJW-E1 (cis-di(thiocyanato)-4,4′-di(octylethylenedioxythienyl)-2,2′-bipyridine4,4′-dicarboxylate-2,2′-bipyridine ruthenium(II)), which showed high light-harvesting capacity and good solubility in organic solvents. Another new ruthenium complex, denoted CYC-B3 (cis-di(thiocyanato)-4,4′di(octylthienyl)-2,2′-bipyridine-4,4′-dicarboxylate-2,2′-bipyridine ruthenium(II)), was also prepared not only to explore the effect of thiophene moieties but also for comparison with SJW-E1 to investigate the impact of the ethylenedioxy groups on the physicochemical properties and performance of the dye molecules. The structures of SJW-E1 and CYC-B3, which incorporate a-octyl-ethylene-dioxythiophene (O-EDOT) and octyl-thiophene-substituted bipyridine, respectively, as an ancillary ligand, are shown in Figure 1. The synthetic details and structure characterizations are provided in the Supporting