Flexible TCO-free counter electrode for dye-sensitized solar cells using graphene nanosheets from a Ti–Ti(III) acid solution

Abstract

A rapid and simple route to synthesize highly conductive graphene-based nanosheets for use as a flexible counter electrode in dye-sensitized solar cells is presented. The flexible counter electrode is free of transparent conductive oxide layer, i.e., TCO-free. A clean graphene with high quality is obtained by the chemical reduction of graphene oxide (GO) using titanium metallic powders in a hydrochloric acid solution. The Ti+3 ions that dissociated from metallic Ti particles in a hydrochloric acid solution result in a clean graphene material with no formation of TiO2 nanoparticles, which are always present on graphene when only Ti+3 ions are used for the reduction, i.e., an anatase TiO2 nanoparticle by-product will be always left on the graphene product when not using metallic Ti particles. The chemical reaction mechanisms for these differences are revealed in this report. The reduced materials are characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, thermo-gravimetric analysis, Fourier transform infrared spectrometry, UV–vis spectroscopy and X-ray photoelectron spectroscopy. The four-point probe method is also employed to characterize the surface conductivity of the graphene films. This high quality graphene film exhibits comparable or better performance than those obtained using conventional sputtered Pt counter electrode when used as a flexible counter electrode of dye-sensitized solar cells.