Synthesis of nitrogen-doped carbon dot/tin disulfide nanosheet composite electro-catalysts for dye-sensitized solar cells

Nitrogen-doped carbon dots (N-CDs) and vertically-grown tin disulfide (SnS₂) nanosheets are synthesized via hydrothermal method and chemical vapor deposition technique, respectively. The SnS₂nanosheets are directly fabricated on flexible carbon cloth (CC), and then their basal planes are decorated with N-CDs. The as-prepared composite electrodes are used as the counter electrode for the application in dye-sensitized solar cells (DSSCs). The characterizations of N-CDs and SnS₂nanosheets are studied by high resolution transmission electron microscopy, scanning electron microscopic, energy dispersive x-ray spectrometer, Raman spectrometer and x-ray photoelectron spectroscopy etc. Moreover, the cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and photocurrent-density voltage are utilized to understand the electro-catalytic performance of N-CDs/SnS₂/CC composite counter electrode. The N-CDs/SnS₂/CC composite electrode shows higher cathodic reduction current density and lower charge transfer resistance in CV and EIS measurements, respectively, as compared to those of the electrodes with N-CDs or SnS₂alone. Meanwhile, the DSSC using N-CDs/SnS₂/CC exhibits cell efficiency (<i>η</i>) of 7.68%, which is higher than those of cells having SnS₂/CC (<i>η</i>= 7.54%) and N-CDs/CC (<i>η</i>= 5.66%) counter electrodes, respectively; it also reaches 94% cell efficiency of the cell using Pt/CC counter electrode (<i>η</i>= 8.15%). The design concept of the modification of the basal planes by defect-rich carbon dots (i.e. N-CDs) and highly-exposed edge sites (i.e. vertically-grown SnS₂nanosheets) makes promising route to enhance the performance of two-dimensional electro-catalysts.