The Application Of CuInS₂/Graphene Composites In Dye-sensitized Solar Cells

CuInS2,a typical Ⅰ-Ⅱ-Ⅵ2 type semiconductor, possesses the properties of semiconductor. Generally. CulnSi has three crystal type:chalcopyrile, zinc-blende and unknown. The semiconductor type can alter through controlling the ratio of (Cu+ln)/S. The band gap of CuInS2 is about 1.5eV, close to the optimum band gap of dye-sensitized solar cells (1.45 eV). CuInSi is a direct band gap semiconductor, which reduces the diffusion of minority carriers. The above statements indicate that CuInS2 have excellent electrocatalytic activity, which can be used as the counter electrode material for dye-sensitized solar cells. Graphene is a two dimension hexagonal honeycomb carbon material with superlarge specific area, excellent electron mobility (15000 mA·m-1·K-1) and mechanical strength, which have the potential application in solar cells, lithium-ions cells, super capacitor, desalination et al. Therefore, the purpose of our research is to combine graphene and CuInS2 through a two-step hydrothermal approach so that the composite can possesses excellent electrocatalytic activity from CuInS2 and conductivity from graphene, which enhances the catalytic activity of counter electrode and the cell performance for the substitution of noble metal Pt.This work aims at synthesis of CuInS2 and graphene composites through a two-step solvothermal approach with the assistance of L-Ascorbic acid. The main procedures are as follows:We successfully synthesize CuInS2/RGO nanocomposites via a two-step hydrothermal method with CuCl, InCl3·4H2O, graphene oxide as the reagent, anhydrous ethanol as solvent and the L-Ascorbic acid as reductant. And the crystallinity. morphology and the reduction degree of GO were characterized via X-ray powder diffraction (XRD), Raman scattering spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM). The results indicated that nanocomposites has good crystallinity and GO are well reduced to graphene. The morphology of nanocomposites is that CuInS2 nanoparticles with the average diameter of ca.200 nm evenly disperse on graphene sheets. The intercalation of CuInS2 nanoparticles breaks up the stack of graphene sheets, which enlarge the specific area to benefit to the transport of electron. Afterwards, the counter electrodes (CEs) were prepared by nanocomposites, and the electrocatalytic activity was characterized via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel polarization. CV results indicate that the CEs have higher oxidation and reduction peaks, which demonstrates that the nanocomposites can well catalyze the reduction of I3-. EIS indicates that nanocomposites CEs have a less series resistance (Rs), charge-tranfer resistance (Rct) and Nernst diffusion impedance. Tafel polarization curve indicates that nanocomposites CEs have a larger exchange current density (Jo) and limiting diffusion current density (Jiim). The result indicates that nanocomposites CEs have excellent electrocatalytic activity, which is close to that of Pt. The CE was assembled to a DSSC and test under the simulating light source via a Keithley 2410 digital source meter. The current density-photo voltage (J-V) curve gives the power conversion efficiency (PCE) of 6.96%. As a comparison, we also test PCE of RGO, CuInS2, Pt CE. The PCE of nanocomposites CE is larger than any component of nanocomposites, indicating that nanocomposites have high conductivity of graphene and electrocatalytic activity of CuInS2, and the synergistic catalytic effect enhances PCE.We successfully synthesize graphene-wrapped CuInS2 ocomposites via a two-step hydrothermal method with CuCl, InCl3·4H2O, graphene oxide ethanol solution as the reagent, anhydrous ethanol as solvent and the L-Ascorbic acid as reductant. The characterization results indicated that nanocomposites has good crystallinity and GO are well reduced to graphene. However, the size, morphology and the number of CuInS2 microspheres graphene wrapped are not uniform. Afterwards, the current density-photovoltage (J-V) curve gives the power conversion efficiency (PCE) of 6.4%, indicating that nanocomposites have high conductivity of graphene and electrocatalytic activity of CuInS2,and the synergistic catalytic effect enhances PCE.In summary, CulnS2 and graphene composites have a big potential application in DSSCs. In the future research, we can enhance the PCE through optimizing the fabrication technology.