The need of reliable and economically viable alternative and renewable energy sources has boosted research on new materials for unconventional photovoltaics (Moore, J. S. J. Am. Chem. Soc., 2008, 130, 12201, JACS Select special issue; Jacoby, M. Chem. Eng. News 2007, 85, 16). In the field of molecular-based photovoltaic devices, dye-sensitized solar cells (DSCs) have reached the best efficiencies, approaching those exhibited by conventional systems such as amorphous silicon solar modules. In addition, they show the best potential and promise for high-conversion low-cost devices (O'Regan, B.; Graetzel, M. Nature 1991, 353, 737; Graetzel, M. Nature 2001, 414, 338).
In DSCs semiconductor (for example, TiO2) nanoparticles are coated with light-harvesting sensitizer dyes and are typically surrounded by a charge transporting medium, for example a liquid-phase electrolyte or an organic electron or hole transporting material. The dye-sensitizer captures photons and an electron/hole pair is generated and transferred at the interface with the inorganic semiconductor and, for example, the redox electrolyte. The photosensitizer dye is the first device interface to the external input and thus represents one of the strategic components of these cells. Ru(II)-polypyridyl dyes (Polo, A. S.; Itokazu, M. K.; Murakami Iha, N. Y. Coord. Chem. Rev. 2004, 248, 1343) yielding power conversion efficiencies of ca. 9-11% under standard AM 1.5 conditions, have best performed so far (Nazeeruddin, M. K.; DeAngelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Graetzel, M. J. Am. Chem. Soc. 2005, 127, 16835). The most representative dye of this series is the homoleptic complex bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II), carrying 4 (N3) (Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Graetzel, M. J. Amer. Chem. Soc. 1993, 115, 6382) or 2 (N719) (Nazeeruddin, M. K.; DeAngelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Graetzel, M. J. Am. Chem. Soc. 2005, 127, 16835) protonated carboxylic anchoring groups to TiO2.
The present inventors believe that one of the disadvantages of the currently existing dyes is the mismatch between the dye absorption band, limited to the UV-Vis region, and the solar emission, which spans to the lower energy portion of the spectrum. Therefore, it would be advantageous to design new metal complex sensitizers having a panchromatic response with improved molar extinction coefficients.
The inventors are also aware that polypyridines other than bi-pyridines have been only rarely investigated, although a series of ruthenium(II) sensitizers derived from carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) (black dye) have exceeded 10% efficiencies (Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. J. Am. Chem. Soc. 2001, 123, 1613).
Abbotto, A.; Barolo, C.; Bellotto, L.; Angelis, F. D.; Gratzel, M.; Manfredi, N.; Marinzi, C.; Fantacci, S.; Yum, J.-H.; Nazeeruddin, M. K. Chem. Commun. 2008, 42, 5318 have used a heteroaromatic donor 3,4-ethylenedioxythiophene ring as a substituent in a bpy ancillary ligand; such ligand was used in a Ru(II) sensitizer named Ru-EDOT, which was endowed with high photocurrent and overall conversion efficiency.
The present inventors observed, however, that heteroleptic sensitizers with anchoring sites localized on the same bipyridine ligand give rise to a decrease of the open circuit potential because of the different sensitizer adsorption mode onto TiO2 compared to homoleptic sensitizers (De Angelis, F.; Fantacci, S.; Selloni, A.; Graetzel, M.; Nazeeruddin, M. K. Nano Lett. 2007, 7, 3189).
The inventors as well as other researchers have verified this behavior for a rather large series of heteroleptic Ru(II)-dyes; as a consequence, despite delivering increased photocurrents compared to the prototypical N3/N719 dyes, heteroleptic Ru(II)-dyes have so far not provided increased performances when employed in DSCs employing a liquid I-/I3-electrolyte.
The present invention addresses the problems set out above. More specifically, the present invention addresses the problem of providing new sensitizing dyes, which preferably absorb light in the lower energy portion of the spectrum.
It is in particular an objective of the invention to overcome the disadvantages encountered with heteroleptic dyes currently known. For example, it is an objective to provide the advantages of a heteroleptic dye such as the enhanced optical properties and photocurrents while at the same time retain a high open circuit potential or to even increase the open circuit potential.
It is thus an objective of the invention to provide sensitizing dyes that increase the overall power conversion efficiency of dye-sensitized solar cells, if compared to dyes currently available.