Transparent Cobalt Selenide/Graphene Counter Electrode for Efficient Dye-Sensitized Solar Cells with Co2+/3+-Based Redox Couple.

In this study, we demonstrate a facile, one-pot, and low-temperature (∼85 °C) chemical bath method for the preparation of a composite of cobalt selenide/graphene (Co0.85Se/Gr) as the electrocatalyst for the counter electrode (CE) of dye-sensitized solar cells (DSSCs) with a cobalt-based electrolyte. The Co0.85Se/Gr composite film was envisaged to have the advantages of both components, that is, the high electrochemical surface area of Co0.85Se and the straight paths for electron transfer from Gr. The DSSCs with Co0.85Se/Gr exhibited a power conversion efficiency (η) of 11.26%. According to the results of the rotating disk electrode, the film of Co0.85Se/Gr showed a high electrocatalytic surface area (Ae) and an extremely large intrinsic heterogeneous rate constant (k0). Furthermore, the composite film of Co0.85Se/Gr exhibits a high transparency in the wavelength region of 400-800 nm (>82%), which implied that the corresponding electrode shall be a potential CE in rear-side illuminated DSSCs. The photovoltaic parameters of the DSSCs with Pt, Co0.85Se, Gr, and Co0.85Se/Gr were obtained for rear-side illumination and additionally for front- and rear-side illuminations (AM 1.5, 100 mW/cm2) using different electrolytes. As the cobalt-based electrolyte of [Co(bpy)3]2+/3+ exhibited a low light absorption and low overpotential for dye regeneration, a rear-side illuminated DSSC with a cobalt-based electrolyte showed the highest efficiency of 9.43 ± 0.02%, which is greater than that of the DSSC with an I-/I3--based electrolyte (η = 7.63 ± 0.04%).