Electrochemical deposition of molybdenum sulfide thin films on conductive plastic substrates as platinum-free flexible counter electrodes for dye-sensitized solar cells

In this study, pulsed electrochemical deposition (pulsed ECD) was used to deposit molybdenum sulfide (MoS_x) thin films on indium tin oxide/polyethylene naphthalate (ITO/PEN) substrates as flexible counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The surface morphologies and elemental distributions of the prepared MoS_x thin films were examined using field-emission scanning electron microscope (FE-SEM) equipped with energy-dispersive X-ray spectroscopy. The chemical states and crystallinities of the prepared MoS_x thin films were examined by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. The optical transmission (T (%)) properties of the prepared MoS_x samples were determined by ultraviolet–visible spectrophotometry. Cyclic voltammetry (CV) and Tafel-polarization measurements were performed to analyze the electrochemical properties and catalytic activities of the thin films for redox reactions. The FE-SEM results showed that the MoS_x thin films were deposited uniformly on the ITO/PEN flexible substrates via the pulsed ECD method. The CV and Tafel-polarization curve measurements demonstrated that the deposited MoS_x thin films exhibited excellent performances for the reduction of triiodide ions. The photoelectric conversion efficiency (PCE) of the DSSC produced with the pulsed ECD MoS_x thin-film CE was examined by a solar simulator. In combination with a dye-sensitized TiO₂ working electrode and an iodine-based electrolyte, the DSSC with the MoS_x flexible CE showed a PCE of 4.39% under an illumination of AM 1.5 (100 mW cm⁻²). Thus, we report that the MoS_x thin films are active catalysts for triiodide reduction. The MoS_x thin films are prepared at room temperature and atmospheric pressure and in a simple and rapid manner. This is an important practical contribution to the production of flexible low-cost thin-film CEs based on plastic substrates. The MoS_x thin films produced by pulsed ECD are good candidates for catalysts in flexible DSSCs. - Highlights: • We used indium tin oxide/polyethylene naphthalate (ITO/PEN) as a substrate. • Molybdenum sulfide (MoS_x) thin films were deposited on ITO/PEN. • We used electrochemical deposition to deposit MoS_x thin films on ITO/PEN. • The MoS_x/ITO/PEN was used as a flexible counter electrode (CE) for DSSCs. • The DSSC with MoS_x/ITO/PEN CE showed a photoelectric conversion efficiency of 4.39%