Copper selenide (Cu3Se2 and Cu2−xSe) thin films: electrochemical deposition and electrocatalytic application in quantum dot-sensitized solar cells

In this work, high crystallinity copper selenide thin films directly deposited onto conducting substrates were obtained through a potentiostatic electrodeposition approach. The as-deposited copper selenides involve annealing induced phase transformation from tetragonal Cu3Se2 to cubic Cu2−xSe. The annealing also leads to a remarkable morphology change from dendritic nanosheets to connected networks and separated particle shapes for the annealed (A-Cu2−xSe) and selenized (S-Cu2−xSe) samples, respectively. The copper selenide thin films were demonstrated to serve as efficient counter electrodes (CEs) in quantum dot-sensitized solar cells (QDSCs) for electrocatalyzing polysulfide electrolyte regeneration. The CdS/CdSe QDSCs constructed with copper selenide CEs deliver considerable power conversion efficiencies (PCEs), especially an optimal value of 3.89% for the A-Cu2−xSe CE-based device. The enhanced photovoltaic performance benefits from the connected network microstructure of A-Cu2−xSe films which afford a large number of reaction sites and efficient charge transport pathways. The Tafel polarization characterization further indicates that, in contrast to the commonly used Cu2S and Pt CEs, the non-stoichiometric Cu2−xSe CE exhibits better electrochemical catalytic activity. This work highlights the great potential of electrodeposition for fabricating promising copper selenide CEs for high performance QDSCs.