| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 103 === 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;
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