DOI: 10.1002/cssc.201301015 Ionic Liquid with a Dual-Redox Couple for Efficient Dye- Sensitized Solar Cells Te-Chun Chu, [a] Ryan Yeh-Yung Lin, [a, b] Chuan-Pei Lee, [a] Chih-Yu Hsu, [b] Pei-Chieh Shih, [a] Ray Lin, [a] Sie-Rong Li, [b] Shin-Sheng Sun,* [b] Jiann T. Lin,* [b] R. Vittal,* [a] and Kuo- Chuan Ho* [a, c] Introduction Dye-sensitized solar cells (DSSCs) have been fabricated from low-cost materials and simple fabrication process and they ex- hibit high energy conversion efficiencies; these advantages make them attractive candidates for the next generation of solar energy conversion technologies. [1] A typical DSSC consists of three adjacent thin layers: a dye-adsorbed mesoporous TiO 2 layer supported on a transparent conducting glass, an electro- lyte, which contains iodide and triiodide (I  /I 3  ) as the redox couple, and a counter electrode. For large-scale commercial DSSC modules, the corrosion properties of the I  /I 3  redox couple for Ag and Cu metals make the scale-up of DSSCs more difficult and could also affect the stability of the DSSC mod- ules. [2] The difference between the redox potential of I  /I 3  (0.35 V vs. the normal hydrogen electrode (NHE)) [3] and that of a usual sensitizer necessitates an excessive driving force of approximately 600 mV for dye regeneration. This excessive driving force leads to energy loss in DSSCs and thereby to a low efficiency of the cell. It is known that the open-circuit voltage (V OC ) of DSSCs is the energy difference between the redox potential of the redox couple and the Fermi level of the semiconductor material. The less positive redox potential of I  / I 3  is a bane to the power conversion efficiency of its DSSCs. Hence there is considerable research to find alternative redox couples for DSSCs. Among the alternatives for the I  /I 3  redox couple, a Co complex has been identified to be one of the best. Grätzel et al. have used a redox electrolyte based on cobalt(II/III) tris- (bipyridyl) in conjunction with a custom-synthesized zinc por- phyrin dye as the sensitizer (YD2-o-C8) to obtain a power con- version efficiency of 12.3 % for the DSSC, [4] which is even higher than that obtained hitherto with an I  /I 3  system. The redox potentials of the Co complex redox couples can be al- tered by modification of the ligand. This trait has attracted many researchers, and a series of Co complex redox shuttles have been investigated. [5] Ferrocene/ferrocenium (Fc/Fc + ) is an- other metal complex redox couple. [6] The redox potential of this couple is more positive (0.62 V vs. NHE) than that of I  /I 3  (0.38 V vs. NHE), which is tantamount to a lower difference in the redox potentials of the dye and the electrolyte and leads to a lower energy loss for the dye regeneration. In 2011, a power conversion efficiency of 7.5 % was achieved for DSSCs that used the ferrocene redox couple and the metal-free or- ganic sensitizer Carbz-PAHTDTT [3-(5-{6-[5-(2-{4-[bis-(4-carba- zol-9-yl-phenyl)-amino]-phenyl}-vinyl)-3-hexyl-thien-2-yl]-dithie- A new type of ionic liquid that contains a nitroxide radical (N OC) and iodide as two redox couples, JC-IL, has been suc- cessfully synthesized for high-performance dye-sensitized solar cells (DSSCs). Both of the redox couples exhibit distinct redox potentials and attractive electrochemical characteristics. The UV/Vis absorption spectra of JC-IL shows a low-intensity peak compared to the strong absorption of I 2 in the wavelength region of 350–500 nm. The high open-circuit voltage of DSSCs with JC-IL is over 850 mV, which is approximately 150 mV higher than that of the DSSCs with a standard iodide electro- lyte. The dramatic increase in the standard heterogeneous electron-transfer rate constant leads to an increase in the short-circuit current for JC-IL compared to that of 2,2,6,6-tetra- methylpiperidin-N-oxyl (TEMPO). DSSCs with the JC-IL electro- lyte show promising cell efficiencies if coupled with dyes CR147 (8.12 %) or D149 (6.76 %). The efficiencies of the DSSCs based on the JC-IL electrolyte are higher than those of DSSCs based on either TEMPO electrolyte or standard iodide electro- lyte alone. [a] T.-C. Chu, R.Y.-Y. Lin, Dr. C.-P. Lee, P.-C. Shih, R. Lin, Dr. R. Vittal, Prof. K.-C. Ho Department of Chemical Engineering National Taiwan University Taipei 10617 (Taiwan) E-mail : javittal1@yahoo.com kcho@ntu.edu.tw [b] R. Y.-Y. Lin, Dr. C.-Y. Hsu, Dr. S.-R. Li, Prof. S.-S. Sun, Prof. J.T. Lin Institute of Chemistry Academia Sinica Nankang 11529, Taipei (Taiwan) E-mail : sssun@gate.sinica.edu.tw jtlin@gate.cinica.edu.tw [c] Prof. K.-C. Ho Institute of Polymer Science and Engineering National Taiwan University Taipei 10617 (Taiwan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301015.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 146 – 153 146 CHEMSUSCHEM FULL PAPERS