| The majority of the world’s energy consumption is derived from fossil fuels. However, due to the environmental issues associated with burning fossil fuels and the diminishing availability of these resources, it is necessary for mankind to develop an alternative source of energy that is more sustainable in the long-term. Solar energy resources are vast:more energy hits the earth in one hour than humankind consumes in entire year. Solar cells offer the potential to change the landscape of how we produce and use energy. However, solar cells technologies have only been used to a limited degree in energy production thus far because of high costs. If developed into a mature technology, they present the opportunity of significantly reduce solar energy costs through earth abundant materials, efficient installation, and roll-to-roll production. Ⅰ2-Ⅱ-Ⅳ-Ⅵ4semiconductors, including Cu2ZnSnS4(CZTS), Cu2ZnSnSe4(CZTSe) and the sulfo-selenide Cu2ZnSn(S,Se)4(CZTSSe), have attracted increased attention for the production of low cost thin film solar cells since they mainly consist of earth abundant or readily available elements in recent years. Owing to the processing complexity associated with the vacuum-based fabrication of CZTS(e) thin film photovoltaic cells, non-vacuum solution-processing can overcome these issues due to the inherent processing advantages over conventional vacuum-based processes.The focus of this study is to demonstrate a viable route of fabricating highly efficient thin-film solar cells by ways of nanoparticles based solution-processing. This dissertation is organized in the order of annals describing the synthesis of CZTS nanoparticles, and the means to improve the CZTSSe solar cells efficiency from zero to about7.5%.Firstly, hot-injection method was used for synthesizing a uniformed and composition controllable CZTS nanoparticle. The effects of the precursor concentrations, the kind of precursors, the metal precursor ratio on the morphology and composition of obtained CZTS nanoparticles were systematically investigated. A narrow size distribution, uniformed, and pure Kesterite CZTS nanoparticle can be achieved using the thiourea as the sulfur source, and the Cu/Zn+Sn ratio and Zn/Sn ratio of the fabricated CZTS nanoparticle can be ranged from0.8to1and1.0to1.3, respectively. Thiourea plays a key role in the formation of nanocrystals with uniform shape and narrow size distribution because thiourea will gradually release sulfur into the solution which triggers reactionwithmetal ion and eventually leads to the formation of CZTS. Time-dependent results showed that CZTS nanoparticle with uniform shape was form initially, and CZTS nanopartilces grew in one direction because of the thiourea’s symmetrical structure. UV-vis absorption spectra of the CZTS nanoparticles revealed a strong absorption in the visible light region with a direct band gap of1.52eV which was optimal for solar cell materials.Secondly, spin-coating and drop-cast methods were investigated to obtain the CZTS thin films. The effect of the various concentrated ink and different spin speeds on the thickness of the thin films were investigated using the spin-coating method. A thickness of1μm CZTS thin films was obtained by4times spin-coating using the200mg/ml ink. The influence of the different selenium atmosphere and annealed temperatures on the structure, morphological and optical properties of CZTS thin film were investigated in the sealed quartz tube. It was concluded that an evolution of the initial formation of Cu2-xSe at the surface, and a subsequent extinction completely was performed during the formation of CZTSSe thin films. Sn loss also be observed during the annealing process under Se atmosphere, which has a negative effect on CZTSSe thin film solar cells, only a efficiency of3.8%thin films solar cells was achieved; While adding Sn powders during annealing process could avoid Sn loss and an improved efficiency thin film solar cell with7.50%was obtained at last. Mott-Schottky measurement showed that the higher efficiency solar cells had a lower carrier concentration which was an advantageous case for high efficiency solar cells.Finally, a new earth-abundant element material Cu2MgSnS4had been synthesized by hot-injection methods. X-ray diffraction, Raman spectrum and transmission electron microscopy elucidated as-prepared nanoparticles are Kesterite Cu2MgSnS4without binary compounds coexisting. Scanning transmission electron microscopy energy-dispersive spectroscopy element mapping shows the copper, magnesium, tin, and sulfur elements distributed uniformly in the nanoparticles. The result of the Uv-vis absorption spectra revealed the Cu2MgSnS4nanoparticles have an optical band gap of1.63eV, which is suitable for application in the field of photovoltaic. |
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