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

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 JSC = 18.47 mA cm−2, VOC = 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.