Quantum dot-sensitized solar cells (QDSSCs), which are cost-effective, facilely prepared and environmentally friendly, are regarded as a promising third-generation photovoltaic technology, although the conversion efficiency of QDSSCs is still far lower than the theoretical efficiency (44%). Furthermore, the traditional Cu₂S/brass counter electrode (CE) is susceptible to continuous corrosion from the polysulfide electrolyte, reducing the stability and electrocatalytic activity of the CE, thereby affecting cell performance. To enhance the stability of the Cu₂S CE and make full use of its excellent catalytic properties, we used eco-friendly polyoxometalates (K₆SiW₁₁O₃₉Co(II) (H₂O), SiW₁₁Co) and porous sponge-like CuO as precursors to prepare a novel SiW₁₁Co-doped high-efficiency Cu₂S/SiW₁₁Co CE on FTO by adopting the method of in situ sulfidation in the electrolyte. Electrochemical analysis revealed that the novel Cu₂S/2%-SiW₁₁Co (Cu₂S-2) CE exhibits extremely high stability and electrocatalytic performance compared with other electrodes. Meanwhile, adding an appropriate amount of SiW₁₁Co can inhibit charge recombination and improve cell performance. The CdS/CdSe co-sensitized QDSSCs assembled with the novel Cu₂S-2 CE obtained a power conversion efficiency (PCE) of 5.94%, which was 20% more efficient than QDSSCs based on Cu₂S/brass CE (4.96%). The method reported here puts forth a new direction for the preparation of efficient Cu₂S counter electrodes.