Ionic liquid crystals: Synthesis, structure and applications to I2-free solid-state dye-sensitized solar cells

A novel type of ionic liquid crystal (ILC) was synthesized and used as a solid electrolyte in I2-free solidstate dye-sensitized solar cells (ssDSSCs). In particular, the properties of two ILCs, 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium iodide (EBII) with a single aliphatic C=C bond and 1-[(4-ethenylphenyl)methyl]-3-vinyl-imidazolium iodide (EVII) with two aliphatic C=C bonds, were evaluated. The structures and morphologies of the ILCs were characterized using Fourier transform infrared spectroscopy (FTIR) and polarized optical microscopy (POM). Ultraviolet (UV)-visible spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analyses revealed that EBII exhibited weaker π-π stacking interactions, longer d-spacing, and a lower melting temperature. The energy conversion efficiency of I2-free ssDSSC with EBII (4.7% at 100 mW/cm2) was higher than with EVII (3.8%) due to facile charge transport and lower electron recombination in the former, as supported by electrochemical impedance spectroscopy (EIS).