Synthesis of copper sulfide/reduced graphene oxide nanocomposites for use as the counter electrodes of high-performance CdS-sensitized solar cells

Copper sulfide (CuxS)/reduced graphene oxide (RGO) nanocomposites were prepared by a one-pot hydrothermal method with various contents of GO in the initial precursor. The nanocomposites were first blended with a polyvinylidene fluoride binder, then coated onto SnO2−xFx substrates, which were used as the counter electrodes (CEs) of quantum dot solar cells (QDSCs) using a CdS-sensitized TiO2 as a photoanode. The microstructure and performance of the CEs were characterized by FE-SEM, XRD, Raman spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Results show that the CuxS/RGO CEs are superior to the conventional Cu2S/brass CE. The stoichiometry and morphology of the CuxS nanocrystals are significantly influenced by the initial GO content in the precursor. A CuxS/RGO nanocomposite with more active sites for effective Sx2− ion reduction in a polysulfide electrolyte (S2−/Sx2−) is optimally obtained at a medium GO content in the precursor. The QDSC assembled with the optimized CuxS/RGO CE exhibits a reproducible high and stable power conversion efficiency of 2.36% under an illumination intensity of 100 mW/cm2, which is higher than the value (1.57%) of the cell with the Cu2S/brass CE. The improved performance is attributed to the synergistic effect between the CuxS nanocrystals and conductive RGO in the CuxS/RGO CE, where RGO acts as both a co-catalyst to accelerate the polysulfide reduction and a conductivity promoter to decrease the series resistance of the CE.