Size Effects of Imidazolium Cations Bearing Cyanoethyl Group on Performance of Dye-Sensitized Solar Cells

•Imidazolium ionic liquids are utilized in electrolytes in dye-sensitized solar cells.•Cations in the ionic liquids are differently sized while bearing cyanoethyl group.•Bulky cations exhibit slow I3− diffusion and electron recombination.•But, unexpectedly, bulky cations suppress downward shift of TiO2 conduction band.•This interesting result is closely associated with function of cyanoethyl group.