Highly Electrocatalytic Cu₂ZnSn(S_1–xSe_x)₄ Counter Electrodes for Quantum-Dot-Sensitized Solar Cells

Traditional Pt counter electrode in quantum-dot-sensitized solar cells suffers from a low electrocatalytic activity and instability due to irreversible surface adsorption of sulfur species incurred while regenerating polysulfide (S(n)(2-)/S(2-)) electrolytes. To overcome such constraints, chemically synthesized Cu(2)ZnSn(S(1-x)Se(x))(4) nanocrystals were evaluated as an alternative to Pt. The resulting chalcogenides exhibited remarkable electrocatalytic activities for reduction of polysulfide (S(n)(2-)) to sulfide (S(2-)), which were dictated by the ratios of S/Se. In this study, a quantum dot sensitized solar cell constructed with Cu(2)ZnSn(S(0.5)Se(0.5))(4) as a counter electrode showed the highest energy conversion efficiency of 3.01%, which was even higher than that using Pt (1.24%). The compositional variations in between Cu(2)ZnSnS(4) (x = 0) and Cu(2)ZnSnSe(4) (x = 1) revealed that the solar cell performances were closely related to a difference in electrocatalytic activities for polysulfide reduction governed by the S/Se ratios.