New insight into copper sulfide electrocatalysts for quantum dot-sensitized solar cells: composition-dependent electrocatalytic activity and stability.

Despite recent significant strides in understanding various processes in quantum dot-sensitized solar cells (QDSSCs), little is known about the intrinsic electrocatalytic properties of copper sulfides that are the most commonly employed electrocatalysts for the counter electrode of QDSSCs. Given that the physical properties of copper sulfides are governed by their stoichiometry, the electrocatalytic activity of copper sulfides toward polysulfide reduction may also be dictated by their compositions. Using a new, simple approach to prepare robust copper sulfide films based on chemical bath deposition (CBD), we were able to delicately control the compositions of copper sulfides, which allowed us to perform a systematic investigation to gain new insight into copper sulfide-based electrocatalysts. The electrocatalytic activity is indeed dependent on the compositions of copper sulfides: Cu-deficient films (CuS and Cu1.12S) are superior to Cu-rich films (Cu1.75S and Cu1.8S) in their electrocatalytic activity. In addition, the stability of the Cu-deficient electrocatalysts is substantially better than that of the Cu-rich counterparts.