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

NiCoSe 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 °C) and named NiCoSe-90, NiCoSe-120, NiCoSe-150 and NiCoSe-180, respectively. The diameters of the NiCoSe microspheres strongly depend on the different hydrothermal temperatures. When the temperature is increased to 150 °C, the size of the resultant NiCoSe microspheres changes from 200 to 800 nm, and the interior of NiCoSe-150 possesses a flocculent structure. However, NiCoSe-180 displays a cauliflower-like aggregated structure. The prepared NiCoSe alloys are used as high-performance Pt-free counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). Cyclic voltammogram measurement indicates that NiCoSe-150 CE has larger current density than Pt CE. Electrochemical impedance spectroscopy shows that NiCoSe-150 CE has a low charge-transfer resistance of 1.8 Ω cm. Due to their unique morphologies and well-defined interior and exterior structures, DSSCs based on NiCoSe-120 and NiCoSe-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%).