Hydrothermal synthesis of CoMoO4/Co9S8 hybrid nanotubes based on counter electrodes for highly efficient dye-sensitized solar cells

Abstract

CoMoO₄/Co₉S₈ hybrid nanotubes were fabricated by a simple two-step hydrothermal method, which was similar to that for preparing Co₉S₈ nanotubes. Then the CoMoO₄/Co₉S₈ nanotubes were coated onto fluorine-doped tin oxide glass to fabricate a counter electrode (CE) by spin-casting. Field emission scanning electron microscopy images show that the introduction of CoMoO₄ to Co₉S₈ makes the surface of the CoMoO₄/Co₉S₈ nanotubes rougher. Cyclic voltammetry shows that the electrocatalytic activity of the CoMoO₄/Co₉S₈ CE is similar to that of a Pt CE when the (NH₄)₂MoO₄ content was 60 wt%. Meanwhile, the electrochemical impedance spectroscopy and Tafel measurements demonstrated that the CoMoO₄/Co₉S₈ CE had smaller values of R_s and R_ct than a Pt CE. The dye-sensitized solar cells assembled with a CoMoO₄/Co₉S₈ CE achieved excellent values of open-circuit voltage (0.743 V), short-circuit current density (17.276 mA cm⁻²), fill factor (0.670) and a wonderful power conversion efficiency (8.60%), which is higher than that of DSSCs with a Co₉S₈ CE (7.69%) or a Pt CE (8.13%) under the light intensity of 100 mW cm⁻² (AM 1.5 G).