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

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 (Co_0.85Se/Gr) as the electrocatalyst for the counter electrode (CE) of dye-sensitized solar cells (DSSCs) with a cobalt-based electrolyte. The Co_0.85Se/Gr composite film was envisaged to have the advantages of both components, that is, the high electrochemical surface area of Co_0.85Se and the straight paths for electron transfer from Gr. The DSSCs with Co_0.85Se/Gr exhibited a power conversion efficiency (η) of 11.26%. According to the results of the rotating disk electrode, the film of Co_0.85Se/Gr showed a high electrocatalytic surface area (A_e) and an extremely large intrinsic heterogeneous rate constant (k₀). Furthermore, the composite film of Co_0.85Se/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, Co_0.85Se, Gr, and Co_0.85Se/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%).