A new ion-coordinating ruthenium sensitizer for mesoscopic dye-sensitized solar cells Daibin Kuang, Cedric Klein, Henry J. Snaith 1 , Robin Humphry-Baker, Shaik M. Zakeeruddin * , Michael Gra ¨tzel * Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fe ´de ´ rale de Lausanne, 1015 Lausanne, Switzerland Received 19 April 2007; accepted 15 May 2007 Available online 31 May 2007 Dedicated to Professor Michael Gra ¨tzel Abstract A new ion-coordinating ruthenium polypyridyl sensitizer, NaRu(4-carboxylic acid-4 0 -carboxylate)(4,4 0 -bis[(triethyleneglycolmethyl- ether) heptylether]-2,2 0 -bipyridine)(NCS) 2 (coded as K68), has been synthesized and characterized by 1 H NMR, FTIR, UV–Vis absorp- tion and emission spectroscopy. A power conversion efficiency of 6.6% was obtained for dye-sensitized solar cells (DSCs) based on the K68 dye and a newly developed binary ionic liquid electrolyte containing 1-propyl-3-methyl-imidazolium iodide (PMII) and 1-ethyl-3- methyl-imidazolium tetracyanoborate (EMIB(CN) 4 ). For a non-volatile organic solvent based electrolyte, a photovoltaic power conver- sion efficiency of 7.7% was obtained under simulated full sun light and exhibited a good thermal stability during the accelerated test under 80 °C in the dark. Solid-state DSCs incorporating K68 also perform remarkably well, out-performing our previously best ruthe- nium complexes employed in this type of DSC. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Dye-sensitized solar cell; Solid state DSC; Ruthenium (II) sensitizer; Ionic liquid; TiO 2 1. Introduction In view of their potential technological implication in photovoltaic energy, relentless efforts are underway on the development of dye-sensitized solar cells (DSCs) [1]. In this area maximum effort has been focused on the devel- opment of sensitizers [2–6], and optimization of electrolyte and anode materials [7–10,1]. The most efficient photosen- sitizers employed so far in DSC are ruthenium(II) polypyr- idyl complexes that yielded more than 11% sunlight to electrical power conversion efficiencies [5,11,12]. Attention has been paid to a number of details in the development of sensitizers in order to improve the photoelectric conversion efficiency and stability [2–6]. Recently, an ion-coordinating sensitizer (K51) contain- ing triethylene oxide methyl ether (TEOME) at the 4,4 0 position of a 2,2-bipyridine ligand has been reported, which showed higher efficiency compared with a non ion-coordi- nating analogy (Z907) [13]. Further, a high molar extinc- tion coefficient ion-coordinating dye (K60) was developed by extending the pi system of the peripheral ligand, which showed high efficiency and excellent stability when used in combination with a non-volatile electrolyte [4]. DSCs based on non ion-coordinating dyes (Z907) and ion-coordinating dyes (K51) showed remarkable differ- ences in photovoltaic performance and long-term stability tests [13]. K51 dye, having an ethyleneoxide chain as an 0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.05.031 * Corresponding authors. Tel.: +41 21 6936124; fax: +41 21 6934111 (S.M. Zakeeruddin); tel.: +41 21 6933112; fax: +41 21 6936100 (M. Gra ¨tzel). E-mail addresses: shaik.zakeer@epfl.ch (S.M. Zakeeruddin), michael. graetzel@epfl.ch (M. Gra ¨tzel). 1 Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK. www.elsevier.com/locate/ica Available online at www.sciencedirect.com Inorganica Chimica Acta 361 (2008) 699–706