Cost-effective and morphology controllable PVP based highly efficient CuS counter electrodes for high-efficiency quantum dot-sensitized solar cells

Currently, copper sulfide (CuS) is the most commonly used counter electrode (CE) in high-efficiency quantum dot-sensitized solar cells (QDSSCs) because of its superior electrocatalytic activity in the presence of polysulfide electrolyte. For the first time, CuS thin films were prepared by a facile chemical bath deposition method with different concentrations of polyvinylpyrrolidone (PVP) and directly used as CEs in QDSSCs without any further post treatment. The quantum dot photoanode with the optimized 0.25 mM PVP-based CuS CE exhibits higher short circuit current density (J(sc)), open circuit voltage (V-oc), fill factor (FF), and power conversion efficiency (PCE) of 17.57 mA cm(-2), 0.578 V, 0.514, and 5.22%, respectively, which are much higher values than those of a bare CuS CE (Jsc: 12.36 mA cm(-2); Voc: 0.591 V; FF: 0.436; PCE: 3.18%) and Pt CE (Jsc: 11.25 mA cm(-2); Voc: 0.464 V; FF: 0.296; PCE: 1.54%) under one-sun illumination (AM 1.5 G, 100 mW cm(-2)). Moreover, the 0.25 mM PVP- based CuS CE produces a charge-transfer resistance of only 4.39 Omega with the aqueous polysulfide electrolyte commonly applied in QDSSCs. This value is several orders of magnitude lower than that of a typical Pt electrode (69.75 Omega) and bare CuS electrode (9.27 Omega). This enhancement is mainly attributed to the improved morphology of the 0.25 mM CuS CE with high catalytic activity, which plays a main role in the reduction processes of the oxidized polysulfide electrolyte, as well as the increased sulfur atomic percentage with Cu vacancies. Cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel polarization were performed to study the underlying reasons behind the efficient CE performance.