| In recent years, the harsh environment, global climate anomalies, environmental pollution and haze are associated with a large consumption of fossil fuels. Solar energy as a sustainable development and green energy has been given considerable attention and great development in recent decades. But the PV industry is still faced with the enormous challenge of competition between conversion efficiency and the cost of photovoltaic devices. Compared to the silicon-based photovoltaic devices, thin film solar cell having a high utilization rate of raw materials, simple manufacturing process and excellent theoretical conversion efficiency features. In traditional copper indium gallium selenide (CIGS) compound semiconductors, indium reserves are scarce and most of the elements are used in the flat panel industry, which limit the development of the CIGS industrialization process. Through replacing Indium of the CIGS by zinc and tin, the Cu2ZnSnSe4 (CZTSe) thim films is developed and expected to reduce the cost of thin film photovoltaic devices from raw material selection.Although CZTSe thin film solar cells made more prominent in recent results, but the device conversion efficiency is limited by the problem of the CZTSe thin films, such as, the component is difficult to precisely control, the exist of secondary phases and intrinsic defects in the CZTSe films. In this study, magnetron sputtering method is used for CZTSe thn films preparation which is widely used in the preparation of a thin-film semiconductor industry. Preparation and performance improvements of the CZTSe absorber layer are systematic and deeply studied. The thesis will mainly include the following aspects:The effect of the Cu/Zn/Sn presurcor stack order by magnetron sputtering on the structure, morphology, composition and electrical properties of CZTSe thin films is systematic studied. The Cu/Zn+Sn/Mo(bottom) order of the precursor leads to better uniform morphology, optical band gap, the carrier concentration,and the hole mobility,and the Cu:(Zn+Sn), Zn:Sn ratio can be controlled within 0.8 to 1.0 and 1.1 to 1.3 range, of the CZTSe films. The solar cell device based on the stack order obtained 2.12% conversion efficiency.For the problem of the secondary phases generated easily during the CZTSe selenization prcess, the selenization reaction mechanism is studied. By changing the seleniazion temperature, the formation temperature zone of the secondary phases such as SnSe, SnSe2, Cu2-xSe, Cu2Se and Cu2SnSe3 are confirmed. To inhibit the formation of secondary phases, the rapid thermal processs (RTP) is explorated and optimized which led to the phase and crystalline quality of the CZTSe films improved significantly. X-ray diffraction pattern showed no secondary phase peaks and narrow FWHM of the CZTSe films, and the conversion efficiency of the device has improved significantly, reaching 3.43%.In order to further improve CZTSe film crystalline quality, the active role of the Sodium (Na) in CZTSe polycrystalline thin film growth process is studied. By depositing appropriate amount of NaF between Mo and precursor, the CZTSe films shows better crystalline quality, the grain size increased from about 500nm to over 1 u m. The electrical properties improved at the same time and obtained a solar cell device with 4.33% conversion efficiency.The high resistance layer of MoSe2 generated between the CZTSe films and Mo layer during the selenization process, which lead to the high series resistance and low fill factor and conversion efficiency of the CZTSe device.The Cu2ZnSnS4(CZTS) nanoparticles layer is deposited on the surface of the Mo by spin coating method which reduce the spread of Se element during the selenium annealing process, so that to preventing MoSe2formation and the decreasing the series resistance effectively. The conversion efficiency of CZTSe solar cell devices is improved to 5.05%.Quaternary compounds CZTSe has a small formation region in the phase diagram which leads to more deminding preparation condition. The ternary compounds Cu2SnSe3 (CTSe) thin films which is prepared by magnetron sputtering and RTP method is preliminary studied. High quality CTSe thin films is fabricated and provide the basic experiment for the further development of a ternary compound based solar cells. |
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