Amplifying charge-transfer characteristics of graphene for triiodide reduction in dye-sensitized solar cells

The fabrication and functionalization of large-area graphene and its electrocatalytic properties for iodine reduction in a dye-sensitized solar cell are reported. The graphene film, grown by thermal chemical vapor deposition, contains three to five layers of monolayer graphene, as confirmed by Raman spectroscopy and high-resolution transmission electron microscopy. Further, the graphene film is treated with CF₄ reactive-ion plasma and fluorine ions are successfully doped into graphene as confirmed by X-ray photoelectron spectroscopy and UV-photoemission spectroscopy. The fluorinated graphene shows no structural deformations compared to the pristine graphene except an increase in surface roughness. Electrochemical characterization reveals that the catalytic activity of graphene for iodine reduction increases with increasing plasma treatment time, which is attributed to an increase in catalytic sites. Further, the fluorinated graphene is characterized in use as a counter-electrode in a full dye-sensitized solar cell and shows ca. 2.56% photon to electron conversion efficiency with ca. 11 mA cm^-2 current density. The shift in work function in F^- doped graphene is attributed to the shift in graphene redox potential which results in graphene's electrocatalytic-activity enhancement.