Journal of Photochemistry and Photobiology A: Chemistry 164 (2004) 87–92 High performance dye-sensitized solar cells using ionic liquids as their electrolytes Ryuji Kawano a , Hiroshi Matsui b,1 , Chizuru Matsuyama a , Akihiro Sato a , Md. Abu Bin Hasan Susan a , Nobuo Tanabe b,1 , Masayoshi Watanabe a,∗ a Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan b Material Technology Laboratory, Fujikura Ltd., 1-5-1 Kiba, Koto-ku, Tokyo 135-8521, Japan Received 24 September 2003; received in revised form 19 December 2003; accepted 25 December 2003 Abstract Room temperature ionic liquids have been used as electrolytes to investigate the performance and the characteristics in dye-sensitized solar cells (DSSCs). The ionic liquids used are 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (EMImTFSI), 1-ethyl-3-methy- limidazolium tetrafluoroborate (EMImBF 4 ), 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF 6 ), 1-ethyl-3-methylimidazolium dicyanamide (EMImDCA), and 1-butylpyridinium bis(trifluoromethane sulfonyl)imide (BPTFSI), in which 1-ethyl-3-methylimidazolium iodide (EMImI) and I 2 are dissolved as a redox couple. The structure and the property greatly influence the DSSC performances. Es- pecially, the photocurrents are affected by the ionic conductivity (viscosity) and charge transport by the exchange reaction between the iodide/tri-iodide redox couple. EMImDCA is specific in terms of enhancement of the open-circuit voltages. The photo-energy conversion efficiency of DSSCs with EMImDCA under 100 mW cm -2 can be optimized up to 5.5%, when [I - ] + [I 3 - ] = 2 M and [I - ]:[I 2 ] = 10:1 with the addition of 4-t-butylpyridine and LiI. © 2004 Elsevier B.V. All rights reserved. Keywords: Dye-sensitized solar cell (DSSC); Ionic liquid; Photo-voltaic cell; Photo-energy conversion; Dicyanamide 1. Introduction Ionic liquids are room temperature molten salts that entirely consist of cations and anions. The ionic liquids have afforded and are expected to afford potentials for the use in diverse fields, such as non-volatile green solvents for syntheses [1] environmentally benign separation pro- cesses [2] and new electrolytes in electrochemical systems [3,4] based on their unique physico-chemical properties. There have been several attempts to use ionic liquids for electrochemical devices by utilizing their characteristics, such as non-volatility, non-flammability, high ionic con- ductivity, and gel-forming property with polymers [5]. The use of ionic liquids for dye-sensitized solar cells (DSSCs) [6–10] is one of the fascinating applications of ionic liq- uids, because DSSCs present an important alternative to current solar technology. The solar cell is constituted by nano-crystalline TiO 2 , a molecular dye and an electrolyte containing a redox couple. The function of such devices is ∗ Corresponding author. Tel.: +81-45-339-3955; fax: +81-45-339-3955. E-mail address: mwatanab@ynu.ac.jp (M. Watanabe). 1 Tel.: +81-3-5606-1067; fax: +81-3-5606-1511. based on the injection of an electron from a photo-excited state of the sensitizer dye into conduction band of the nano-crystalline semiconductor. These cells usually employ a liquid electrolyte to reduce the dye cation, generated by the injection of the photo-excited electron [11,12]. Typi- cally, an iodide/tri-iodide (I - /I 3 - ) redox couple dissolved in organic solvents has been used. However, use of the volatile solvents causes deterioration of the cell by evaporation of the solvent over the prolonged use. Ionic liquids with their characteristic properties seem to be the most appropriate solvents to solve such problems, whereas large viscosity of the ionic liquids is a serious problem for the development of such devices using these promising solvents. The viscos- ity of typical ionic liquids is about 100 times larger than that of acetonitrile, and also 30 times larger than that of water at room temperature [13,14]. Photocurrents in such systems are affected by the series resistances of the elec- trolytes, which are usually in proportion to the viscosity. Grätzel and coworkers reported [6] the use of ionic liquids, consisting of 1-hexyl-3-methylimidazoliun iodide (HMImI) and I 2 , as an electrolyte of DSSCs and revealed the high short-circuit currents (J sc ), in spite of the extremely high vis- cosity. It was implicated that the Grotthuss-like mechanism 1010-6030/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2003.12.019