Electrospun membranes of imidazole-grafted PVDF-HFP polymeric ionic liquids for highly efficient quasi-solid-state dye-sensitized solar cells

Three novel polymeric ionic liquids (PILs), denoted as PFII-F, PFII-E, and PFII-S, are successfully synthesized by grafting different molar ratios (one-fourth, one eighth, and one sixteenth, respectively) of 1-butylimidazolium iodide onto poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). These PFII PILs are fabricated as polymer membranes via a simple electrospinning technique, which are used as the electrolyte for quasi-solid-state (QSS) dye-sensitized solar cells (DSSCs). The PFII membranes have multiple functions including: (1) encapsulation of the liquid electrolyte with good charge transfer and ionic conductivity properties, (2) chelation of Li through the lone pair electrons on their fluoride atoms, and (3) filling the dye-uncovered TiO surface with their imidazolium segment. Function (1) endows DSSCs with prominent long-term stability, while functions (2) and (3) suppress the dark current. The best QSS-DSSC with the PFII-F membrane shows a larger open-circuit voltage (V), comparable short-circuit current density (J), better power conversion efficiency (η) of 9.26%, and superior long-term stability (up to 97% of its initial η) over 1500 h compared to the cell with standard liquid electrolyte (8.63%).