(472c) Inexpensive Room Temperature Ionic Liquids for Low Volatility Electrolytes of Dye-Sensitized Solar Cells

Electrolytes are one of the four key components of dye-sensitized solar cells (DSSC), in addition to photoanodes, counter electrodes, and dyes. The solvent evaporation issue of the electrolyte however poses a great challenge to the practical application of DSSCs. Consequently, the development of electrolytes with low volatilities has drawn a great deal of research attention in recent years. Imidazolium salts, a class of room temperature ionic liquids (RTIL), possess extremely low volatilities and excellent ionic conductivities, and are thus considered a promising alternative to the commonly used acetonitrile (AN) as the solvent in electrolyte preparations. Imidazolium salts however are expensive. In this work, a series of inexpensive, easy preparation, low volatility, excellent ionic conductivity RTILs is developed to serve as the solvents in electrolytes for DSSCs. This series of RTILs is prepared from heated mixtures of quaternary ammonium salts and hydrogen bond donors. One such RTIL series is composed of heated mixtures of ethylene glycol and acetylcholine iodide (termed EG-ACI). The RTIL is prepared by simply mixing its two constituents at a suitable molar ratio and heated at 50 oC till a clear solution is obtained. The corresponding electrolyte is prepared by dissolving LiI, I2, and N-methylbenzimidazole (NMBI) in the EG-ACI. For an electrolyte composition of 0.8M LiI, 0.1M I2, and 0.5M NMBI (based on an EG-ACI of 9:1), the ionic conductivity is measured to be 5.47 mS cm^-1 and the diffusion coefficient of the tri-iodide ion to be 3.98x10^-7 cm^2/s, both considered appropriate for DSSC applications. When used in DSSCs, constructed from P25 powders and D149 dye (a metal-free dye), the electrolyte gave a Jsc of 8.57 mA/cm^2, a Voc of 0.645, a fill factor of 0.61, and an efficiency of 3.36% which is 82% of that achieved by using the AN based electrolyte (efficiency of 4.1%) under the same measurement condition. This series of RTILs is thus a promising alternative to imidazole-based solvents in electrolyte preparations of DSSCs for long term outdoor applications.