Ruthenium sensitizers with 2,2′-bipyrimidine or a 5,5′-disubstituted 2,2′-bipyrimidine ligand: Synthesis, photo- and electrochemical properties, and application to dye-sensitized solar cells

Novel ruthenium sensitizers with 2,2′-bipyrimidine (TUS-26) or a 2,2′-bipyrimidine derivative bearing two 5-hexyl-2,2′-bithiophene units at the 5,5′-positions (TUS-27) have been synthesized for application in dye-sensitized solar cells. TUS-27 shows a strong absorption at around 460 nm, mainly attributed to a π-π* transition in the 5,5′-disubstituted 2,2′-bipyrimidine ligand, and an onset wavelength of the absorption spectrum that is redshifted by about 50 nm relative to those of structurally analogous sensitizers (cis-[Ru(dcbpy)(bpy)(NCS)₂] and N719). The energy level of the LUMO of TUS-27 is relatively close to that of the conduction band of TiO₂. Dye-sensitized solar cells containing TUS-26 and TUS-27 showed much poorer performances than those containing cis-[Ru(dcbpy)(bpy)(NCS)₂] and N719, even though an electrolyte containing no 4-tert-butylpyridine was used to adjust the energy of the conduction band of TiO₂. The results of DFT calculations indicated that both unfavorable populations of LUMO and LUMO+1, and the lower energy level of LUMO+1, contribute to the much poorer solar cell performances of the TUS sensitizers. In addition, photovoltaic measurements also suggested that enhancement of backward electron transfer from the conduction band of TiO ₂ to the one-electron-oxidized form of the dye or to I ₃^- in the electrolyte is another reason for the inferior performances of the TUS sensitizers. Two novel ruthenium sensitizers with a 2,2′-bipyrimidine (TUS-26) or a 2,2′-bipyrimidine derivative bearing two 5-hexyl-2,2′-bithiophene units at the 5,5′-positions (TUS-27) have been synthesized. The performances of these sensitizers in dye-sensitized solar cells have been investigated.