Heteroleptic Ruthenium Sensitizers with Hydrophobic Fused-Thiophenes for Use in Efficient Dye-Sensitized Solar Cells

Two fused-thiophene-containing ruthenium sensitizers, [Ru(Hdcbpy)(ottip)(NCS)₂]^-·0.5[H]⁺·0.5[N(C₄H₉)₄]⁺ [Z1, H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridine, ottip = 2-(5-octylthieno[3,2-b]thiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline] and [Ru(Hdcbpy)(odttip)(NCS)₂]^-·0.5[H]⁺·0.5[N(C₄H₉)₄]⁺ {Z2, odttip = 2-[6-octyldithieno(3,2-b:2′,3′)thiophen-2-yl]-1H-imidazo[4,5-f][1,10]phenanthroline}, were designed for use in dye-sensitized solar cell (DSSC) applications. The fused-thiophene-based Z1 and Z2 have stronger light-harvesting capabilities compared with a non-fused-thiophene analog, [Ru(Hdcbpy)(otip)(NCS)₂]^-·0.5[H]⁺·0.5[N(C₄H₉)₄]⁺ [JF-2t, otip = 2-(5-octylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline], and a standard N3 dye. The power-conversion efficiency of the Z1- and Z2-sensitized solar cells were superior (8.70 % and 8.15 %) to JF-2t- and N3-sensitized solar cells (7.85 and 7.40 %, respectively) under standard AM1.5G full sunlight irradiation. The photovoltaic performance of Z1 was better than Z2, primarily owing to the fact that Z1-based cells have a higher level of dye loading and less charge recombination, as seen from the electrochemical impedance spectroscopy data. This study not only demonstrates the excellent power-conversion efficiency of new fused-thiophene ruthenium sensitizers, but also points to the promise that fused-thiophene antennas might be a useful molecule in the engineering of ruthenium dyes for DSSCs. Optimization of the power-conversion efficiency in ruthenium sensitizers containing fused-thiophene antennas was achieved. This result clearly confirms that fused-thiophenes can be used as a superior antenna in the design of ruthenium sensitizers for greater dye-sensitized solar cells (DSSC) performance.