Binary room-temperature ionic liquids based electrolytes solidified with SiO2 nanoparticles for dye-sensitized solar cells

In this study, binary ionic liquids (bi-IL) of imidazolium salts containing cations with different carbon side chain lengths (C = 2, 4, 6, 8) and anions such as iodide (I−), tetrafluoroborate (BF4−), hexafluorophosphate (PF6−) and trifluoromethansulfonate (SO3CF3−) were used as electrolytes in dye-sensitized solar cells (DSSCs). On increasing the side chain length of imidazolinium salts, the diffusion coefficients of I3− and the cell conversion efficiencies decreased; however, the electron lifetimes in TiO2 electrode increased. As for different anions, the cell which contains 1-butyl-3-methyl imidazolium trifluoromethansulfonate (BMISO3CF3) electrolyte has better performance than those containing BMIBF4 and BMIPF6. From the impedance measurement, the cell containing BMISO3CF3 electrolyte has a small charge transfer resistance (Rct2) at the TiO2/dye/electrolyte interface. Moreover, the characteristic frequency peak for TiO2 in the cell based on BMISO3CF3 is less than that of BMIBF4 and BMIPF6, indicating the cell with bi-IL electrolyte based on BMISO3CF3 has higher electron lifetime in TiO2 electrode. Finally, the solid-state composite was introduced to form solid-state electrolytes for highly efficient DSSCs with a conversion efficiency of 4.83% under illumination of 100 mW cm−2. The long-term stability of DSSCs with a solidified bi-IL electrolyte containing SiO2 nanoparticles, which is superior to that of a bi-IL electrolyte alone, was also presented.