High Molar Extinction Coefficient Heteroleptic Ruthenium Complexes for Thin Film Dye-Sensitized Solar Cells Daibin Kuang, Seigo Ito, Bernard Wenger, Cedric Klein, Jacques-E Moser, Robin Humphry-Baker, Shaik M. Zakeeruddin,* and Michael Gra ¨ tzel* Contribution from the Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fe ´ de ´ rale de Lausanne, 1015 Lausanne, Switzerland Received December 16, 2005; E-mail: shaik.zakeer@epfl.ch; michael.graetzel@epfl.ch Abstract: Two novel heteroleptic sensitizers, Ru((4,4-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(p-hexyloxy- styryl)-2,2-bipyridine)(NCS)2 and Ru((4,4-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(p-methoxystyryl)-2,2′- bipyridine) (NCS)2, coded as K-19 and K-73, respectively, have been synthesized and characterized by 1 H NMR, FTIR, UV-vis absorption, and emission spectroscopy and excited-state lifetime and spectroelec- trochemical measurements. The introduction of the alkoxystyryl group extends the conjugation of the bipyridine donor ligand increasing markedly their molar extinction coefficient and solar light harvesting capacity. The dynamics of photoinduced charge separation following electronic excitation of the K-19 dye was scrutinized by time-resolved laser spectroscopy. The electron transfer from K-19 to the conduction band of TiO 2 is completed within 20 fs while charge recombination has a half-life time of 800 μs. The high extinction coefficients of these sensitizers enable realization of a new generation of a thin film dye sensitized solar cell (DSC) yielding high conversion efficiency at full sunlight even with viscous electrolytes based on ionic liquids or nonvolatile solvents. An unprecedented yield of over 9% was obtained under standard reporting conditions (simulated global air mass 1.5 sunlight at 1000 W/m 2 intensity) when the K-73 sensitizer was combined with a nonvolatile “robust” electrolyte. The K-19 dye gave a conversion yield of 7.1% when used in conjunction with the binary ionic liquid electrolyte. These devices exhibit excellent stability under light soaking at 60 °C. The effect of the mesoscopic TiO 2 film thickness on photovoltaic performance has been analyzed by electrochemical impedance spectroscopy (EIS). Introduction Following its discovery in 1991, 1 research on the dye- sensitized solar cell (DSC) has progressed remarkably, rendering it a credible chemical alternative to solid-state silicon based devices. 2-5 The high efficiency of the DSC arises from the collective effect of numerous well-tuned physical-chemical properties, the key issue being the panchromatic sensitization of large band-gap mesoscopic semiconductor electrodes. Ru- thenium polypyridyl complexes such as the cis-Ru(SCN) 2 L 2 (L ) 2.2′-bipyridyl-4,4′-dicarboxylate (N3) or the black dye Ru- (SCN) 3 L (L ) 4,4′,4′′-tricarboxy 2,2′:6′,2′′-terpyridine) 6 showed the best performance as sensitizers so far due to their advanta- geous spectral properties and high stability. While more than 11% conversion efficiency has been reached with the N3 dye, a film thickness of over 15 microns and a volatile redox electrolyte were required to achieve this performance. The long- term containment at elevated temperatures of the volatile solvent mixture employed still remains a major challenge. 7 This dilemma is currently being addressed by the development of novel sensitizers with an increased optical cross section allowing thinner TiO 2 films and nonvolatile electrolytes to be employed. A successful approach has been to replace one of the 2.2′-bipyridyl-4,4′-dicarboxylate groups in the N3 dye by a styryl-subsituted bipyridine. 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K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi C. A.; Gra ¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (b) Nazeeruddin, M. K.; Kay, A.; RodicioI,; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra ¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (7) Gra ¨tzel, M. Inorg. Chem., 2005, 44 (20), 6841. Published on Web 03/07/2006 4146 9 J. AM. CHEM. SOC. 2006, 128, 4146-4154 10.1021/ja058540p CCC: $33.50 © 2006 American Chemical Society