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Reparationand Properties of Large Grains of Solar Grade Multicrystalline Silicon by Cold Crucible Directional Solidification

Author: ShenBinHua
Tutor: ChenRuiRun
School: Harbin Institute of Technology
Course: Materials Processing Engineering
Keywords: multicrystalline silicon Directional solidification Cold crucible Crystal growth
CLC: O78
Type: Master's thesis
Year: 2012
Downloads: 52
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Abstract


Grain boundaries in polycrystalline silicon, in particular by the metal elementpollution, which will greatly reduce the performance of polycrystalline silicon, soincreasing the grain size of the ingot is very meaningful. This paper uses numericalsimulation software to study the rate and distribution of the flow field in the crucible,combined with the simulation results using the insulation method and seeded to preparelarge grain polysilicon ingots.In this paper, the mechanism of crystal growth wasanalyzed and its performance was tested to study the influence of the grain size on theirelectrical properties.The simulation results show that the flow rate in the crucible becomes larger andlarger with the current and the frequency increasing.When the frequencies is up to50kHz, its velocity changed little with the frequency increasing. The flow of Sillicon inthe molten pool appears the swirling circular shape and the upper and lower layers ofthe molten pool shows two sets of circulation. The velocity is small generally in thebottom of the melt, but there is a small areas with high rate. The flow rate in this areawill cause a greater impact on the growth of crystal.The polysilicon ingots with big grain was prepared,when heat preservation timewas controlled,The ingots was analyzed,and it showed that the grain become biggerwith the extension of the holding time.The average grain size of the ingot can beachieved5mm,when the holding time reach35min.The microstructure of the ingots wasobserved with scanning electron microscopy (SEM),there is a large number of whitegranules with the size of2μm when the holding time reach5min.It contains Si and Fewith the analysis of EDS. The number of white granules Significantly reduced with theextension of the holding time and it disappeared when the holding time reach35min.High melting point impurities of the silicon feedstoctk was melted with the extension ofthe holding time.It reduce the heterogeneous nucleation of particles and conducive thegrowth of grains.Finally, large-grain polycrystalline silicon ingot was prepared by seed crystalmethod.The seed was easy to be melted,because it must use Graphite to help melting inthe starting. Thermal insulation measures need to be taken to protect the seed.Theelectrical properties of the ingots was tested,it showed that the resistivity become larger and the lifetime of minority carrier become higher with the growth of grain.

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CLC: > Mathematical sciences and chemical > Crystallography > Crystal growth
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