A comparative study of gel polymer electrolytes based on PVDF-HFP and liquid electrolytes, containing imidazolinium ionic liquids of different carbon chain lengths in DSSCs

The photoelectrochemical characteristics of titanium dioxide (TiO₂)-based dye-sensitized solar cells (DSSCs) containing gel polymer electrolyte (GPE) and organic liquid electrolyte (OLE) were studied in detail. GPE was prepared by adding poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) to imidazolinium ionic liquids (IILs) of the type, 1-methyl-3-alkyl imidazolinium iodides (alkyl is C_nH_2n+1, where n=3-10) in methoxy propionitrile (MPN) and the OLE contained the above molten salt in MPN. The IILs were synthesized in the laboratory and characterized by ¹H nuclear magnetic resonance spectroscopy (NMR). The conductivities (σ) of both GPE and OLE decrease with increase in chain length (n) of the alkyl group of IILs; however, the effect is more drastic in the former case. The performance of the DSSCs containing OLE increases with the increase in alkyl chain length of IIL from C3 to C7, whereas, there is a linear decrease in the efficiency of the DSSCs incorporated with GPE containing IIL of alkyl chain length from C3 to C10. The change in short circuit current density (J_SC) determines the cell efficiency as the V_OC of the DSSCs remains almost the same with increase of alkyl chain length of IILs for both the electrolytes. The change in J_SC values and the consistency of the V_OC of the DSSCs for both the electrolytes may be explained on the basis of increase in viscosity of IILs from C3 to C10 and the dominating role of the 4-tertiary butyl pyridine (TBP), respectively, on the phenomenon of charge recombination.