Phase transformations of novel CuxS nanostructures as highly efficient counter electrodes for stable and reproducible quantum dot-sensitized solar cells

Abstract

An ideal counter electrode (CE) that is cost-effective and has high electrocatalytic activity, high performance stability, and a simple fabrication process is essential for quantum dot-sensitized solar cells (QDSSCs). We report a facile one-step chemical bath deposition method for the growth of an L-cysteine-dependent Cu_xS thin film on a fluorine-doped tin oxide substrate, which was used as a highly efficient CE for QDSSCs. SEM images revealed that the surface morphology of Cu_xS changed from nanoplatelets to nanosheets to nanospheres to nanoflowers based on the concentration of L-cysteine. Also, the concentration of L-cysteine in the formation of Cu_xS thin films is expected to play an important role in determining the phase of Cu_xS thin films (Cu₂S, Cu_1.75S, Cu_1.12S and CuS). QDSSCs based on Cu_1.12S nanosphere CEs possess a high power conversion efficiency of 5.88%, which is much higher than those of QDSSCs based on Cu₂S nanoplatelets (5.10%), Cu_1.75S nanosheets (5.32%), CuS nanoflowers (4.88%) and Pt (1.36%) CEs. Electrochemical measurements revealed that a Cu_1.12S nanosphere CE has better electrocatalytic activity, which increases the rate of reduction of polysulfide in comparison to those of Cu₂S nanoplatelets, Cu_1.75S nanosheets, CuS nanoflowers and Pt CEs. A QDSSC also displayed superior stability in an operational state for over 20 h, which is a major challenge for Cu_xS CEs.