Bis-tridentate Ru(ii) sensitizers with a spatially encumbered 2,6-dipyrazolylpyridine ancillary ligand for dye-sensitized solar cells

Abstract

Bis-tridentate Ru(II) sensitizers with a 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine anchor (i.e. tctpy) and a 2,6-dipyrazolyl pyridine ancillary ligand with either 5-dodecylthien-2-yl or t-butyl substituents at the central pyridyl unit and four distinctive perfluoroalkyl fragments (e.g. CF₃, C₃F₇, C₅F₁₁ and C₇F₁₅) at the terminal pyrazolyl sites were designed, synthesized and applied as sensitizers for the fabrication of dye-sensitized solar cells. All these sensitizers exhibited suitable optical properties and electrochemical characteristics. In addition, despite the TF-tBu series of sensitizers with t-butyl substituents showing a lowered absorption extinction coefficient vs. their 5-dodecylthien-2-yl substituted counterparts (i.e. TF-2′ series) in solution, their smaller molecular size allowed a larger dye loading on TiO₂ photoanodes, which offsets the inferior optical response and makes them the better DSC sensitizers. After appropriate selection of C₃F₇ substituents, the sensitizer coded TF-tBu_C₃F₇ showed the highest overall efficiencies of J_SC = 18.47 mA cm⁻², V_OC = 767 mV, FF = 0.71 and PCE = 10.05% under simulated one sun irradiation, due to the fine balance between dye loading and reduced charge recombination. The corresponding enlarged solar cell module with an active area of 11.2 cm² also showed the best PCE of 7.55% under one sun irradiation with PCE reaching 12.70% under T5 lighting at 2400 lux.