A new ruthenium polypyridyl dye, TG6, whose performance in dye-sensitized solar cells is surprisingly close to that of N719, the ‘dye to beat’ for 17 years

Abstract

A new ruthenium polypyridine sensitizer for dye-sensitized solar cells (DSSCs) is proposed containing a hexasulfanyl–styryl-modified bipyridyl group as an ancillary ligand. The advantages of this dye are the much larger absorption coefficient and the small shift of the absorption envelope to the red. We compare this new dye, TG6 (cis-bis(thiocyanato)(2,2′-bipyridyl-4,4′-dicarboxylato){4,4′-bis[2-(4-hexylsulfanylphenyl)vinyl]-2,2′-bipyridine}ruthenium(II) mono(tetrabutylammonium) salt), to the current best-performing dye, N719 (cis-bis(thiocyanato)bis(2,2′-bipyridine-4,4′-dicarboxylato)ruthenium(II) bis(tetrabutylammonium) salt). We have applied a suite of evaluation tools including: varying the electrolyte, varying the TiO₂ film thickness, charge density and recombination rate constant measurements, fluorescence lifetime and magnitude, and transient absorption techniques. The combined results indicate that TG6, as presently constructed, can surpass the performance of N719 under some conditions, but is likely to need some modification before surpassing cells designed to give record energy efficiency using N719. The higher absorption coefficient may be relevant to mass-produced DSSCs on metal where thinner TiO₂ films are advantageous. The main disadvantage is the slight catalysis of the electron/electrolyte recombination, which is possibly due to the extended π-system. A factor that requires further optimization involves the complex interaction of the slightly lower LUMO position, the composition of the electrolyte, the band edge position of the TiO₂, and the electron injection rate. We show why the maximum output from TG6 cells will not occur using the exact electrolytes used to maximize N719 cells.