Ruthenium complexes of 2-(2′-pyridyl)benzimidazole as photosensitizers for dye-sensitized solar cells

Abstract

N-Alkylated carboxylic acid derivatives of 2-(2′-pyridyl)benzimidazole (pbimH) with different chain-lengths (pbim(CH)_nCO₂H where n = 1–3) and their ruthenium complexes [Ru(bpy)₂(pbim)](PF₆)₂ have been synthesized and characterized. 2D COSY and NOESY NMR spectroscopy were used to aid the assignment of the pbim NMR spectrum. The effect of chain-length on the cyclic voltammetry (CV) was studied and the voltammetry of the parent pbimH complex was re-investigated. The ability of the carboxylic acid groups to bind to TiO₂ coated electrodes was confirmed by the observation of a symmetrical, surface-confined Ru^III/II wave, while the specular reflectance IR revealed a band at 1620 cm⁻¹ due to the bound carboxylate (COO⋯Ti) group. The efficiencies of solar cells using these sensitizers were rather low, due to the distance between the sensitizer and the surface and the inefficient coupling of the charge-separated excited state to the surface. A fall in the cell open-circuit voltage with chain length reflected this distance effect. Time-resolved luminescence spectroscopy indicated that rapid electron injection into the TiO₂ conduction band was occurring (<30 ns), but this is not fast enough to compete effectively with alternative excited state processes.