Effect of different auxiliary ligands and anchoring ligands on neutral thiocyanate-free ruthenium(II) dyes bearing tetrazole chromophores for dye-sensitized solar cells

Abstract

Four new bistridentate Ru(II) complexes having a combination of ligands, 4,4′-dicarboxy-2,2′:6,2″-terpyridine or its 2-hexylthiophene-substituted derivative as the anchoring ligand on one hand and one of tetrazolylpyridine-based ligands having a NˆNˆN coordination pattern or C−ˆNˆN coordination pattern as the auxiliary ligand on the other are reported as sensitizers for dye-sensitized solar cells (DSSCs), along with their spectroscopic, electrochemical, and theoretical characterizations. For the anchoring ligand, the introduction of 2-hexylthiophene unit leads to narrower spectral response and lower molar extinction coefficients and a smaller driving force for dye regeneration. For the ancillary ligand, the cyclometalating RuC bond induces a red shift in absorption compared with a RuN bond and thus affords a photocurrent generation at wavelengths of up to 950 nm. Further, the overall efficiencies of DSSCs are higher with the NˆNˆN complexes than with the C−ˆNˆN complexes, mainly due to higher open-circuit voltages (Voc). Overall, the DSSC based on the bistetrazolate dye without the hexylthiophene unit for the anchoring ligand and having the NˆNˆN coordination pattern for the auxiliary ligand exhibited the highest efficiency of 5.9% when employing 0.5 M t-butylpyridine in the electrolyte.