定向凝固法多晶硅杂质控制数值模拟概述

A Review of Numerical Simulation for Impurity Control in the Directional Solidification Process of Multicrystalline Silicon

在传统能源日渐消耗及可再生能源开发利用日趋受到重视的形势下,太阳能光伏发电逐渐成为最具潜力的可再生能源技术之一。多晶硅凭借高效率、低成本的优势成为最主要的光伏材料,其铸锭的品质和成本将直接影响太阳能电池的成本和光电转换效率。定向凝固法是制备多晶硅铸锭的重要方法,该方法晶硅生长过程中存在很多问题,包括熔体流动、杂质传输、固液界面的形状和结构以及缺陷。定向凝固过程中引入的有害杂质严重影响晶硅的机械和电学性能,是限制多晶硅光电转换效率的关键因素。长晶过程中定向凝固炉处于高温环境中,内部的传热传质极其复杂,不具备单一的线性关系和可推断性,且难于进行实验测量,因而数值模拟是研究定向凝固过程中传热传质现象的重要方式。降低长晶过程中杂质的含量可从两方面入手:(1)杂质的来源——原材料本身所携带的杂质和长晶过程中生成的杂质;(2)杂质的输运——找到杂质在熔体和氩气中的输运规律,并利用该规律控制杂质的分凝与输运。近年来,从控制杂质产生和输运的角度考虑,国内外对降低多晶硅中有害杂质的研究主要采用以下手段:(1)控制杂质产生,包括减缓坩埚与挡板之间的化学反应、优化顶部坩埚盖板、增设碳化硅涂层等;(2)优化氩气流动,例如使用导流系统、调整炉膛压力和氩气流量;(3)优化熔体对流形式,包括控制熔体流动方式及分凝、调整加热器功率大小及其排布、采用可旋转坩埚和调整石墨碳毡位置等。为进一步降低定向凝固法多晶硅铸锭成本,坩埚尺寸和投料量不断增大,熔体对流、杂质输运和界面形状也更加难于控制,外加磁场则成为控制熔体对流的一种强有力工具,并能进一步控制杂质输运。目前利用外加磁场控制定向凝固法多晶硅杂质的研究仍刚刚起步,具有很大的研究价值,其中电磁场(EMF)和行波磁场(TMF)在控制搅拌熔体对流方面具有巨大潜力,逐渐被用于多晶硅长晶过程。本文在深入分析杂质来源和输运机理的基础上,综述了国内外对多晶硅定向凝固过程中有害杂质的产生、分布、输运以及排出等问题的研究现状,总结了数值模拟中氩气导流系统、加热器以及外加磁场等因素对杂质的影响。

The continuous consumption of traditional energy sources is exaggerating the prominence of renewable energy production and utilization,and solar photovoltaic power generation has gradually been regarded as one of the most promising renewable energy techniques.Multicrystalline silicon,owing to high efficiency and low cost,has now become the most important photovoltaic material,and quality and cost of its ingot will directly affect the cost of solar cells and the efficiency of photoelectric conversion.Directional solidification（DS）method is an important way to obtain multicrystalline silicon ingot,but suffers many problems in the growth of crystalline silicon,including melt flow,impurities transfer,shape and structure of solid-liquid（S-L）interface,and defects.Harmful impurities introduced during the directional solidification process will seriously affect the ingot＇s mechanical and electrical properties,and thus are the key factor limiting the photoelectric conversion efficiency of multicrystalline silicon.The heat and mass transfer in the DS furnace under high temperatures during the crystal growth process is extremely complicated and shows no single linear relationship,no inferentiality,and difficulty for experimental measurements.So the numerical simulation is considered as an important way to study the heat and mass transfer in the DS process.There have been two useful aspects for the reduction of impurities：I.the source of impurities—impurities in the raw materials and impurities generated during the crystal growth process;II.transport of impurities—finding the transport rule of impurities in melts and argon,and using this rule to control the segregation and transport of impurities.In recent years,from the perspective of controlling the impurity generation and transport,most of the research works which intend to reduce the harmful impurities in multicrystalline silicon involve the following methods：I.controlling the source of impurities,including inhibiting the chemical reaction between crucible and baffle,optimizing top crucible cover plate,and introducing silicon carbide coating,etc.;II.improving argon flow by adopting argon guidance system,regulating furnace pressure and argon flow rate;III.optimizing melt convection,including controlling melt flow and segregation,adjusting the power and arrangement of heaters,adopting rotatable crucible,adjusting the position of graphite carbon felt and so on.The intention to cut the cost of DS multicrystalline silicon ingot gives rise to increasingly bigger crucible and larger feeding amount,which make the melt convection,impurity transport and interface shape more difficult to control.The external magnetic field has been proved to be powerful in controlling melt convection and further controlling impurity transport,and the relevant research is still in its infancy.As the electromagnetic field（EMF）and traveling magnetic field（TMF）display great potential in controlling the stirring melt convection,they have gradually found application in the crystal growth process.On the basis of an analysis over impurity sources and transport mechanism,this review provides a comprehensive description of the generation,distribution,transport and removal of harmful impurities in the directional solidification process,as well as a summary of the effects of argon guidance system,heaters and magnetic field on impurities with respect to numerical simulation optimization.