In situ fabrication of Co_0.85Se and Ni_0.85Se hierarchical thin films as high-performance counter electrode for dye-sensitized solar cells

Cobalt selenide (Co_0.85Se) and nickel selenide (Ni_0.85Se) hierarchical thin films assembled using by nanosheets and nanoparticles, respectively, were fabricated in situ on fluorine-doped tin oxide substrates in a ligand-free environment to prepare counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) consisting of a N719-sensitized nanocrystalline TiO₂ film. The Co_0.85Se- and Ni₀.₈₅Se-based CEs were rapidly synthesized via a microwave-assisted one-step method at 170 °C within a short reaction time of 10 min. The morphology and structure of the Co_0.85Se and Ni_0.85Se hierarchical thin films were characterized by means of scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results of electrochemical impedance spectroscopy, cyclic voltammetry, and Tafel polarization measurements showed that both CEs exhibited good electrocatalytic activity and I⁻/I₃⁻ redox reactions and much lower charge-transfer resistance. A power conversion efficiency (PCE) of 8.58% for the Co_0.85Se CE-based solar cell was obtained, which is comparable to that of cells using Pt CEs (8.57%); 7.09% PCE was obtained for the Ni_0.85Se-based CE. The results suggest that in situ-prepared Co_0.85Se-based CEs could be a promising alternative to Pt-based CEs for DSSCs.