Size effects of imidazolium cations bearing cyanoethyl group on performance of dye-sensitized solar cells

Imidazoliums are commonly used ionic liquid cations in electrolyte in dye-sensitized solar cells (DSSCs). Here, the size effects of imidazolium cations bearing the cyanoethyl group on DSSC performance have been systematically investigated by introducing the different alkyl chains (C1–C12), by which size control is allowed in a range of 9–20 Å. The large cations slow down electron recombination and ion diffusion, but, unexpectedly, they suppress downward shift of TiO ₂ conduction band significantly affecting on electron injection process. While all the cations used in this work demonstrate a better power conversion efficiency than that for the common imidazolium analogue, the interplay among the key processes results in the best performance with the medium-sized cation C4 (11 Å). This work paves a way for design of efficient electrolyte cations for photovoltaic devices.