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Eutectic Melts Improve Stability of Electrolytes Used in DyeSensitized Solar Cells Dye-sensitized solar cells (DSCs) are rapidly emerging as a low-cost alternative to traditional silicon photovoltaics. Based on a photoelectrochemical process that is analogous to photosynthesis, DSCs have intrinsically lower fabrication costs and potentially very high efficiencies. However, one key barrier to their wider commercial adoption and large-scale use is the fact that they incorporate volatile, plasticpermeating solvents. Now a group led by the original inventor of the DSC, M. Grätzel of the Swiss Federal Institute of Technology in Lausanne, and P. Wang of the Chinese Academy of Sciences in Changchun have introduced the use of a eutectic melt to produce solvent-free DSCs with efficiencies that approach the highest reported to date. They reported their findings in the August issue of Nature Materials (DOI: 10.1038/nmat2224; p. 626). The use of solvents in electrolytes has been understood as an obstacle to largescale adoption of DSCs for some time, and a number of researchers have pursued alternatives such as solvent-free roomtemperature ionic liquids. To date, the highest-efficiency solvent-free DSCs have been based on 1-propyl-3-methyl imi dazolium iodide (PMII), which has a lower viscosity than other iodide melts, enabling high fill factors and no mass-transport limitations on photocurrent. The Lausanne/ Changchun group observed that three other salts that are solid at room temperature, 1-ethyl-3-methylimidazolium iodide (EMII); 1,3-dimethylimidazolium iodide (DMII); and 1-allyl-3-methylimidazolium iodide (AMII), are more conductive than PMII above their melting points. This inspired the researchers to mix EMII, DMII, and AMII at a molar ratio of 1:1:1, resulting in a eutectic-based melt with melting point below 0ºC and a conductivity of 1.68 mS cm-1, almost three times higher than that of PMII. The group then fabricated a DSC device using this eutectic-based melt (with routine additives guanidinium thiocyanate and N-butylbenzoimidazole), as well as

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one based on a melt of EMII, DMII, and 1-ethyl-3-methylimidazolium tetracyanoborate (EMITCB). The first device yielded an overall power conversion efficiency in full sunlight of 7.1%, significantly better than corresponding devices based on a PMII electrolyte. The second device displayed overall efficiency of 8.2%, the highest reported to date for a DSC with a solvent-free electrolyte, and retained 93% of this conversion efficiency after 1000 hours of light soaking with full solar intensity at 60ºC, confirming that it is highly stable. These results, and particularly the introduction of the use of eutectic melts to produce electrolytes, offer important clues for further improving the stability and efficiency of DSCs. The