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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
Author(s) -
Ozawa Hironobu,
Kawaguchi Hiroki,
Okuyama Yu,
Arakawa Hironori
Publication year - 2013
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201300345
Subject(s) - homo/lumo , dye sensitized solar cell , chemistry , ruthenium , photochemistry , solar cell , band gap , absorption spectroscopy , electrochemistry , electrolyte , molecule , organic chemistry , optoelectronics , materials science , electrode , optics , physics , catalysis
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 TiO 2 . 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 TiO 2 . 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.