Highly efficient solid-state dye-sensitized TiO2 solar cells via control of retardation of recombination using novel donor-antenna dyes

Two series of heteroleptic tris(bipyridyl)Ru(II) and bis(bipyridyl)(NCS)2Ru(II) complexes have been synthesized and characterized. This is a part of a new concept of covalent linkage of donor-antenna groups, e.g., triphenylamine or N,N′-bis(phenyl)-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) to Ru(II) dye center. For the covalent attachment of donor units, a multi-step synthesis was carried out starting from 4,4′-dimethyl-2,2′-bipyridine followed by chlorination and Wittig reaction with donor aldehydes. This was followed either by a metallation reaction using bis(4,4′-dicarboxy-2,2′-bipyridyl)Ru(II)dichloride ((bpy(COOH)2Ru(II)2Cl2 2H2O) as precursor to get tris(bipyridyl) dyes or by a one pot synthesis starting from dichloro(p-cymene)Ru(II) dimer resulting in bis(bipyridyl)(NCS)2 dyes. The complexes (bpy(COOH)2)2(bpyMe2)Ru(II) 2PF6 and (bpy(COOH)2)(bpyMe2)(NCS)2Ru(II) without donor-antenna groups were also prepared to study and compare the properties. The influence of donor-antenna groups in these complexes was studied using UV–Vis spectroscopy and cyclic voltammetry. The heteroleptic complexes carrying donor groups show appreciably broad absorption ranges and extraordinarily high extinction coefficients. These high extinction coefficients are explained as due to the extended delocalization of π-electrons in the donor-antenna ligands. The HOMO/LUMO energy values obtained from cyclic voltammetry support the multi-step charge transfer cascade possible in these donor-antenna dyes. Examples of solid-state dye-sensitized solar cell utilizing these novel donor-antenna dyes revealed spectacular performances of power conversion efficiencies of up to 3.4%, for the dye carrying a TPD donor group as measured under AM 1.5 spectral conditions. This is attributed to highly efficient light harvesting of these novel dyes and the improved charge transfer dynamics at TiO2–dye and dye–hole conductor interfaces.