使用晶種直接固化法生長大尺寸太陽能多晶矽之熱流場與雜質傳輸數值分析;Numerical Study of Thermal Flow Field and Impurity Transport during the Growth of Large Size Multicrystalline Silicon Ingots by the Seeded Directional Solidification Process

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Title:	使用晶種直接固化法生長大尺寸太陽能多晶矽之熱流場與雜質傳輸數值分析;Numerical Study of Thermal Flow Field and Impurity Transport during the Growth of Large Size Multicrystalline Silicon Ingots by the Seeded Directional Solidification Process
Authors:	廖思涵;Liao,Szu-Han
Contributors:	機械工程學系
Keywords:	多晶矽晶錠;晶種直接固化法;數值模擬;Multi-crystalline silicon ingot;Seeded Direction Solidification;Numerical Simulation
Date:	2015-08-27
Issue Date:	2015-09-23 15:30:28 (UTC+8)
Publisher:	國立中央大學
Abstract:	發展低生產成本及高品質多晶矽晶錠，對太陽能電池持續朝向市電平價的目標邁進，具關鍵重要地位。在多晶矽晶錠生長過程中，常見高濃度的碳與氧雜質，會在晶錠中形成差排導致太陽能電池轉換效率降低，因此，控制雜質濃度是影響晶錠品質之關鍵。本研究中以有限體積法（FVM）進行數值模擬，使用晶種直接固化法模擬生長大尺寸多晶矽晶錠過程之熱流場與氧、碳雜質濃度場，將模擬結果與實際生長數據進行比較，數據由SAS公司測量晶錠內雜質濃度分佈。探討不同的熱場設計、調整氬氣流速與石墨上蓋板的設計對於氧與碳雜質濃度之影響。由模擬結果顯示，熔湯的對流主要由浮力造成，對流形態隨生長過程固化的程度而改變，固液界面形狀大部分呈現凹向熔湯，使晶粒朝向中心軸生長。對於固液界面形狀的影響，透過調整坩堝支柱的間距與排列方式，以及加入保溫塊於爐體中，兩者藉以增加堝壁的軸向溫度梯度，使晶粒在堝壁生長的結晶速率降低且在中央區微凸，改善固液界面形狀，加熱器的輸出功率降低能達到節能熱場的效果。在晶錠生長過程中，流動型態對於雜質傳輸具重要影響。氬氣流速增加，能將更多從自由表面氣化的氧化矽氣體帶往爐體外，當氬氣流率增加為52.5slpm時，熔湯中有較低的雜質濃度。在固定氬氣流速下，調整不同的石墨上蓋板長度，藉此影響氣體區與熔湯的流動型態。當石墨上蓋板長度為四分之一且放置於坩堝上方時，熔湯中的流動型態在高固化分率時明顯改變，能使更多氧雜質從自由液面氣化再由氬氣帶往爐體外，對於降低雜質濃度有明顯效果。關鍵字：多晶矽晶錠；晶種直接固化法；數值模擬 ;The multi-crystalline silicon (mc-Si) solar cells have the highest market share in the photovoltaics (PV) market. To accomplish the goal of grid parity, the production cost of silicon solar cell must be reduced further and the efficiency has to be improved. These are strongly dependent on the wafer production and quality. Nowadays the growth of large size mc-Si ingots with the high quality became the main development direction of wafer production. However the dislocation caused by the high concentration of oxygen and carbon in the mc-Si wafer can reduce the efficiency of solar cell. Therefore, the control of these impurities needs to be paid attention during growth process. In the present study, the thermal flow field, the concentration of oxygen and carbon in seeded directional solidification system (DSS) are numerically investigated by CGSim (Crystal Growth Simulator) program of STR Inc. based on Finite Volume Method (FVM). The distributions of impurity concentration in the grown ingot measured by the SAS Company are compared with the computational results. In addition, the effects of the hot zone and the graphite cover design on impurity content are also discussed. The simulation results show that the melt convection is induced by buoyancy force and the flow pattern in the melt changes during the growth process. Most of the c-m interface is concave to the melt. Therefore the grain tends to grow toward the center axis. The flatter c-m interface shape can be gotten by changing the position of crucible support and arrangement method. Moreover, the vertical temperature gradient at the crucible wall increases as the insulation block is added in the furnace. The c-m interface becomes more convex at the central section and its slope at the crystal wall section is reduced. That to reduce the heating consumption to obtain the favorite interface shape and enhance the energy saving. In the growth process, the flow pattern is a major impact affected the transport of impurity. Increasing the argon gas flow rate can bring more evaporated silicon monoxide above the free surface outwards the furnace. It is found that the impurity concentration in silicon melt gets lower when argon flow rate is 52.2slpm. The effect of different lengths of graphite cover on impurity transport was also investigated at a fixed argon flow rate. The graphite cover affects the gas and melt flow pattern in the chamber, which affect the transport of carbon monoxide and silicon monoxide in the gas as well as oxygen and carbon in the melt. The melt flow pattern was significantly changed at the higher solidification fraction when the graphite cover length is a quarter. This results in the larger amount of impurity evaporated out of the free melt surface. Keywords: Multi-crystalline silicon ingot; Seeded Direction Solidification;
Appears in Collections:	[Graduate Institute of Mechanical Engineering] Electronic Thesis & Dissertation

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