Size-controllable synthesis of NiCoSe2 microspheres as a counter electrode for dye-sensitized solar cells

NiCoSe2 microspheres have been successfully synthesized by a facile one-step hydrothermal method at different hydrothermal temperatures. The prepared samples are divided according to their reaction temperatures (90, 120, 150 and 180 degrees C) and named NiCoSe2-90, NiCoSe2-120, NiCoSe2-150 and NiCoSe2-180, respectively. The diameters of the NiCoSe2 microspheres strongly depend on the different hydrothermal temperatures. When the temperature is increased to 150 degrees C, the size of the resultant NiCoSe2 microspheres changes from 200 to 800 nm, and the interior of NiCoSe2-150 possesses a flocculent structure. However, NiCoSe2-180 displays a cauliflower-like aggregated structure. The prepared NiCoSe2 alloys are used as high-performance Pt-free counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). Cyclic voltammogram measurement indicates that NiCoSe2-150 CE has larger current density than Pt CE. Electrochemical impedance spectroscopy shows that NiCoSe2-150 CE has a low charge-transfer resistance of 1.8 Omega cm(2). Due to their unique morphologies and well-defined interior and exterior structures, DSSCs based on NiCoSe2-120 and NiCoSe2-150 CEs achieve high power conversion efficiencies of 8.48% and 8.76%, respectively, which are higher than that of the solar cell based on Pt CE (8.31%).