Highly stable dye-sensitized solar cells with quasi-solid-state electrolyte based on Flemion

A polymer gel electrolyte containing Flemion® (a perfluorocarboxylated polymer) as polymer matrix, lithium iodide, iodine, 1,2 dimethyl-3-propylimidazolium iodide (DMPII) and 4-tert-butylpyridine (TBP) was developed for quasi-solid-state dye-sensitized solar cells (DSSCs). The Flemion-based electrolyte was gelled by heating the Flemion solution to evaporate the ethanol solvent. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy suggested three regions in the Flemion-based gel electrolyte: the fluorocarbon polymer frame, the ion cluster, and their interface. The open-circuit voltage, the short-circuit current density, and the fill factor were markedly enhanced by adding TBP and DMPII, respectively, to the Flemion-based gel electrolyte. When irradiated with light at 100 mW cm−2, the DSSC containing the Flemion-based gel electrolyte with 0.6 M DMPII and 0.5 M TBP exhibited an energy conversion efficiency (η) of 4.1%, while η of the acetonitrile-based DSSC was 5.1%. According to electrochemical impedance spectra, gelation greatly increased the charge transfer resistance at the TiO2/electrolyte interface in DSSCs containing the Flemion-based electrolyte without DMPII. However, this effect was negated by adding 0.6 M DMPII to the electrolyte. The η of Flemion-gel-based DSSCs stored for up to 4392 h decreased by less than 10%. In contrast, the η of Flemion-liquid-based and acetonitrile-based DSSCs decreased by 95% during 480 h storage, and by 93% during 115 h storage, respectively. This result demonstrates that Flemion-based gel electrolytes solve the leakage problem that degrades the performance of DSSCs.