Design and characterization of heteroleptic ruthenium complexes containing benzimidazole ligands for dye-sensitized solar cells: The effect of fluorine substituents on photovoltaic performance

We designed heteroleptic ruthenium complexes (RD12-RD15) containing fluoro-substituted benzimidazole ligands for dye-sensitized solar cells (DSSCs). These dyes were synthesized according to a typical one-pot procedure with the corresponding ancillary ligands produced in two simple steps; they were prepared into DSSC devices according to the same conditions of fabrication. The eventual devices show a systematic trend of increasing V_OC and decreasing J_SC with fluorine atoms of increasing number substituted on the ligand. The charge-extraction results show that upward shifts of the TiO ₂ potential occurred when the fluoro-substituted dyes were sensitized on TiO₂ with a systematic trend of shift N719 > RD15 (with 5 F) > RD12 (with 2 F) >RD5 (no F); the intensity-modulated photovoltage spectra indicate that those fluoro substituents retard charge recombination with the electron lifetimes (τ_R) in the order RD15 > RD12 > RD5 > N719, consistent with the variation of V_OC for the systems.