PbS nanoparticles were in situ deposited on graphene sheets by a successive ionic liquid adsorption and reaction method to prepare a graphene/PbS composite counter electrode for CdS/CdSe quantum dot sensitized solar cells (QDSCs). Under 1 sun illumination, the cells with graphene/PbS counter electrodes (CEs) show a maximum energy conversion efficiency of 2.63%, which is remarkably higher than that of those employing PbS (1.28%) or graphene (0.23%) CEs. Electrochemical impedance spectroscopy analysis shows that graphene/PbS composite counter electrodes have lower charge-transfer resistance at the interface of the CE and the polysulfide redox electrolyte, compared to those cells with PbS and graphene counter electrodes. For the first reported CdS/CdSe-sensitized cells with the G/PbS CE, a cell performance with 2.63% efficiency (V oc = 559 mV, J sc = 11 mA/cm2, FF = 0.42) was obtained, which is comparable with the photovoltaic properties of the cells with optimized conventional Cu2S CEs (η = 3.01%, V oc = 564 mV, J sc = 11.6 mA/cm2, FF = 0.46). Our results indicate the potential application of graphene/noble metal sulfide composite electrodes in high-efficiency QDSCs.

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