High performance dye-sensitized solar cells using ionic liquids as their electrolytes

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-methylimidazolium tetrafluoroborate (EMImBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6), 1-ethyl-3-methylimidazolium dicyanamide (EMImDCA), and 1-butylpyridinium bis(trifluoromethane sulfonyl)imide (BPTFSI), in which 1-ethyl-3-methylimidazolium iodide (EMImI) and I2 are dissolved as a redox couple. The structure and the property greatly influence the DSSC performances. Especially, 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−] + [I3−] = 2 M and [I−]:[I2]=10:1 with the addition of 4-t-butylpyridine and LiI.