Preparation and microstructural characteristics of solar-grade multicrystalline silicon by directional solidification in
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Preparation and microstructural characteristics of solar-grade multicrystalline silicon by directional solidification in an axial magnetic field Xiao-Hui Chen1,a)
, Senlin Rao2, Fayun Zhang2,b)
1
School of Mechanical and Electrical Engineering, Xinyu University, Xinyu 338004, People’s Republic of China; and Key Laboratory of University in Jiangxi for Silicon Materials, Xinyu 338004, People’s Republic of China 2 School of New Energy Science and Engineering, Xinyu University, Xinyu 338004, People’s Republic of China; and Key Laboratory of University in Jiangxi for Silicon Materials, Xinyu 338004, People’s Republic of China a) Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] Received: 4 August 2019; accepted: 6 November 2019
Solar-grade multicrystalline silicon ingots as raw material for solar cells were obtained from upgraded metallurgical silicon by directional solidification in an axial magnetic field. The influence of preparation technology on the microstructural characteristics of silicon ingots was investigated. Governing equations were used to simulate the silicon fluid flow and thermal fields during directional solidification. The results show that appropriately increasing melt temperature and/or decreasing pulling-down rate can be conductive to the growth of a coarse columnar grain. Meanwhile, the axial magnetic field promotes the formation of low-energy P 3 twin boundaries and reduces the dislocations and impurities, where the total concentration of major metal impurities is with a mean of 0.459 ppmw in the range of 1/9 to 8/9 of height along the growth direction. It is shown from the simulation results that suppressing silicon melt flow in both radial and azimuthal directions and reducing the growth rate in the edge regions contribute to the formation of a flat solid–liquid interface, which is more consistent with the experimental results. Moreover, the formation mechanism of the twins and removal mechanism of the impurities were discussed.
Introduction In recent years, the demand for solar-grade silicon (SoG-Si) material as raw material for solar cells has drastically increased due to the rapid development of photovoltaic industry [1, 2]. Upgraded metallurgical grade silicon (UMG-Si) attracts more and more attention because it can be directly used for preparing solar-grade multicrystalline silicon, leads to significantly reduced production cost of silicon ingots for photovoltaic application [3, 4]. Directional solidification, which is one of the metallurgical routes, has become a primary method to fabricate solar-grade multicrystalline silicon because of its low cost and environment-friendly [5, 6, 7]. Nevertheless, the microstructures of silicon material, such as dislocations, highenergy grain boundaries, and impurities, greatly influence the electrical properties of the solar cells. How to control the microstructural characteristics of silicon ingot up to solar
ª Materials Research Society 2019
grade, thus, is the main problems to be solved when preparing mult
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