Synthesis, Characterization And Photovoltaic Applications Of Cu₂ZnSnS₄（CZTS）

Copper-zinc-tin-chalcogenide CZTS materials have considerably attracted scientists attention asabsorbing thin film of solar cells. The CZTS is a semiconductor whose the compounds areabundant in the earth’s crust, with extremely low toxicity, an absorption coefficient greater than104cm-1, and a direct band gap about1.5eV, making it more ecological friendly and alsoeconomic. These characteristics make it more suitable to have exciting results, demonstrating thepromise of low-cost application in thin film for solar cells and photovoltaic devices. One of themost important problems in the preparation of CZTS materials is the identification of optimalconditions leading to the formation of single-phase CZTS with the remarkable properties andcharacterization of these different second phases. In this thesis, using solutions-based techniques,we have successfully identified the optimal conditions for CZTS materials phase formation,under the poly-condensation of precursors, with the remarkable optical and electrical properties,leading to their application in photoelectrochemical and solar cells. The details are as follows:In order to analyze the elements concentration effects on the properties of nanoparticles,CZTS nanoparticles with Kesterite structure were successfully prepared by the easyhydrothermal method. The concentrations of sulfur and copper were varied and the properties ofCZTS nanoparticles observed. The copper-rich (2.8:1:1:6) gave better results with remarkable crystallinity. The absorption spectra of UV-Vis-NIR show that CZTS nanoparticles have strongabsorption due to their superior optical properties. X-Ray Diffraction (XRD), Ramanspectroscopy, Scanning electron microscope (SEM), Energy Dispersive X-ray analysis (EDAX),Transmission electron microscopy (TEM), current-voltage(I-V), Mott-Schottky characterizationsclearly confirm the structure, composition, morphology, elemental distribution andphotoelectrochemical properties of CZTS nanoparticles obtained.The extremely important problem in the high yield production of thin films is to find theappropriate inks and preparation techniques by using non-toxic solvents, or non-pollutingelements. In order to overcome this requirement, the CZTS thin films were successfully preparedby sol-gel technique, using the less toxic solvents such as ethylene-glycol and dichloromethane,under thermal treatment from450to600oC and N2flow for2hours. To study the influence ofstoichiometric ratios from the precursors on the samples, the concentration ratio of sulfur andmetal precursors (S/Metal) was varied from1.5to3. The stoichiometric ratio S/Metal2.5presented remarkable properties than others, based on XRD, Raman and SEM characterization.Thereafter, the deposited CZTS, CZTSe and CZTSSe thin films were used inphotoelectrochemical cell (PEC) for a comparative study of Photocurrent and Mott-Schottkyplots measurements.To enhance light harvesting and holes collection efficiency, the CZTS thin films wereprepared by solution-based synthesis approach leading to the manufacture of the tandemquantum dot sensitized solar cells (QDSSCs) with3.15%of efficiency. The results demonstratethat CZTS materials with the band gap of1.56eV is active to the visible and near infraredradiation, and may also be beneficial for holes collection as well as light harvesting in thetandem QDSSCs. Here, we showed a new approach, beneficial for the utilization of CZTS materials in QD based tandem cells towards efficient and low cost photovoltaics. We have alsodeveloped an efficient sol-gel technique for the production of thin films, based on low cost andless toxic alternative. This technique may be considered as an economic and environmentallyfriendly method. By applying with a lot more investigations, it may provide enormous benefits,time/energy saving, high efficiency, versatility and better reproducibility, leading to an importantstep in the industrial large scale production of high quality nanomaterials thin films.