MoSe₂ nanosheet/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) composite film as a Pt-free counter electrode for dye-sensitized solar cells

A composite film of molybdenum diselenide nanosheets and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (MoSe₂ NS/PEDOT:PSS) was prepared for the counter electrode (CE) of a dye-sensitized solar cell (DSSC), via a low-cost drop-coating method. The two-dimensional (2D) nanosheets of MoSe₂ (MoSe₂ NS) were aimed to provide the composite film not only a large amount of electrocatalytic active sites but also orientational pathways for electron transfer, from the conducting substrate of the CE to the interface at the CE electrolyte. The PEDOT:PSS acts as the conductive matrix for the composite film, and also enables multiple interfacial electron transfer pathways between the MoSe₂ NS and the substrate. Owing to the combination of the advantages of both MoSe₂ NS and PEDOT:PSS, the DSSC with the composite film of MoSe₂ NS/PEDOT:PSS on its CE exhibited a power conversion efficiency (η) of 7.58 ± 0.05%, which is comparable to that of the cell with a Pt CE (7.81 ± 0.03%). The counter electrode films were characterized by scanning electron microscopy. Various electrochemical analyses, including those with cyclic voltammetry, rotating disk electrode, Tafel polarization curves, and electrochemical impedance spectroscopy, were performed intending to quantify the electrocatalytic ability of the counter electrode films; these techniques were able to precisely distinguish the functions of MoSe₂ NS and PEDOT:PSS in the composite films; the former worked as an electrocatalyst and the latter as a conductive binder. Incident photon-to-current conversion efficiency (IPCE) was used to substantiate the photovoltaic parameters. When the MoSe₂ NS/PEDOT:PSS composite film was coated on a flexible titanium (Ti) foil, the pertinent DSSC showed even a higher η of 8.51 ± 0.05%, as compared to a lower η of 8.21 ± 0.02% using the Pt-coated Ti foil CE. The results indicate the promising potential of MoSe₂ NS/PEDOT:PSS composite film to replace the expensive Pt.