Reduced graphene oxide films fabricated by gel coating and their application as platinum-free counter electrodes of highly efficient iodide/triiodide dye-sensitized solar cells

Abstract

This paper reports the preparation of reduced graphene oxide (rGO) films through the gel formation of rGO with liquid polyethylene glycol (PEG) of low molecular weight and the application of the rGO films as the platinum (Pt)-free counter electrode of highly efficient iodide/triiodide dye-sensitized solar cells (DSCs). rGO prepared through the reduction of graphene oxide can form physical gels with PEG by mechanical grinding. The rGO sheets form 3-dimensional solid networks in the gels. The rGO–PEG gels are processed into rGO films on substrates with good adhesion by doctor blade and subsequent removal of PEG through heating. The rGO films can effectively catalyze the electrochemical reduction of triiodide and are used as the counter electrode of iodide/triiodide DSCs. The optimal photovoltaic efficiency of DSCs with the rGO counter electrode is 7.19% under AM1.5G illumination (100 mW cm⁻²), which is only slightly lower than that (7.76%) of control DSCs with Pt prepared by pyrolysis as the counter electrode. This is the highest photovoltaic efficiency observed on the DSCs with iodide/triiodide as the redox couple and graphene as the counter electrode. The rGO films exhibit better stability in catalyzing the electrochemical reduction of triiodide and give rise to better aging stability of DSCs than Pt prepared by pyrolysis. The photovoltaic efficiency of DSCs is strongly dependent on the thickness of the rGO film, and the optimal rGO film thickness is 15 μm. The photovoltaic performance of DSCs is also affected by the method to prepare the rGO–PEG gels. The photovoltaic efficiency is only 6.15% or lower when the rGO–PEG gels are prepared by ultrasonication rather than mechanical grinding. This is attributed to the different morphologies of the rGO films achieved by different methods.