Development of New Heteroleptic Ruthenium Photosensitizers for Use in Efficient Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have recently emerged as efficient green energy sources that can operate even under conditions of limited sunlight or indoor lighting, and, hence, can be installed at a wide variety of locations for various applications. Five new ruthenium photosensitizers, Ru(Hdcbpy)(otip)(NCS)2]-•0.5[H]+• 0.5[N(C4H9)4]+ (JF-2t, H2dcbpy = 4,4''-dicarboxy-2,2''-bipyridine, otip = 2-(5-octyl-thiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline), [Ru(Hdcbpy)(ottip)(NCS)2]-•0.5[H]+•0.5[N(C4H9)4]+ (Z1, ottip = 2-(5-octyl- thieno[3,2-b]thiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline), [Ru(Hdcbpy)(odttip)(NCS)2]-•0.5[H]+•0.5[N(C4H9)4]+ (Z2, odttip = 2-(6-octyl- dithieno[3,2-b:2′,3′]thiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline), [Ru(H2dcbpy)(bhttp)(NCS)2]-•0.6[H]+•0.4[N(C4H9)4]+ (Z3, bhttp = 4,7-bis(2- hexylthiothiophen-5-yl)-1,10-phenanthroline, and [Ru(H2dcbpy)(bhtbtp)(NCS)2]-•0.67[H]+•0.33[N(C4H9)4]+ (Z4, bhtbtp = 4,7-bis(5-hexylthio-2,2′-bithiophen-5′-yl)-1,10-phenanthroline, for use in DSSCs applications were synthesized. The compounds were photophysically and electrochemically characterized and density functional theory investigations were carried out. The fused-thiophene-based Z1 and Z2 had stronger light-harvesting capabilities compared to the nonfused-thiophene analogue, JF-2t, 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 the 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 due to the fact that the level of dye loading of the Z1-based cells was higher and charge recombination was less, as evidenced by EIS data. This study not only demonstrates the excellent power-conversion efficiency of such fused-thiophene-incorporated ruthenium sensitizers but also points to the promise that fused-thiophene antennas might be useful molecules for use in the engineering of ruthenium dyes for DSSCs. Moreover, two new heteroleptic phenanthroline-based ruthenium dyes, Z3 and Z4, incorporating two light-harvesting oligothiophene antennas with electron-rich hexylthio-terminal chains were designed and synthesized. In the calculated electronic spectra, Z3 exhibited a red-shift and the MLCT transition for efficient electron injection was lower than Z4, in which longer oligothiothiophene antennas had been incorporated. This outcome can be attributed to the noncoplanar link between a phenanthroline base and the oligothiophene antennas in the computed molecular structure. Therefore, Z3-based cells can be expected to have superior power-conversion efficiency than Z4-based cells.