Numerical Study Of Heat And Mass Transfer Under Magnetic Field In Directional Solidification Method Multicrystalline Silicon

Multicrystalline silicon solar cells play an important role in the PV market.Directional solidification system(DSS)is the main method for growing solar grade multicrystalline silicon at present.In the process of multicrystalline silicon growth,there are still many problems,such as melt flow,transfer of impurities,shape and structure of solid-liquid(S-L)interface,and defects,which directly affect the cost of solar cells and the efficiency of photoelectric conversion.The external magnetic field can control the melt flow and impurity transport accurately,which has great research value.In this paper,the effect of magnetic field on heat and mass transfer in the growth of multicrystalline silicon by DSS method is mainly studied and optimized.The specific research contents and conclusions are as follows:(1)The research status of controlling melt flow in a DSS method for multicrystalline silicon solar cells is reviewed.The different types and principles of magnetic field applied in crystal growth are introduced in detail.The application of magnetic field in crystalline silicon growth in recent years is also reviewed.(2)A two-dimensional axisymmetric numerical model of global heat and mass transfer for G5 ingot furnace is established.Taking into account the nonaxisymmetric structure of some parts in the ingot furnace,three-dimensional 1/8axisymmetric numerical simulation is carried out for a G6 ingot furnace.Based on the heat and mass transfer equation and the electromagnetic field theory and equation,the heat and mass transfer numerical model and electromagnetic field coupling model are established.(3)Temperature field and flow field in melt during DS process without magnetic field are studied numerically.The influence of argon velocity and marongoni convection on heat transfer in melt is analyzed.It is found that the higher the argon flow rate,the higher the flow velocity near the melt free surface,the larger the vortex on the radial side,and the smaller the difference between the upper and lower vortex;marangoni convection effects on the flow near melt free surface are obvious,but the flow pattern in the melt is still dominated by buoyancy convection.The heat transferlaw of melt in different crystal phases is studied.It is founded that at full melting and early crystal growth stage,the excessive flow velocity near S-L interface may lead to interfacial instability and more impurities entering the melt.In the middle and later stage of crystal growth,the melt flow velocity is too small,which is not conducive to the uniform transmission of impurities.(4)Aiming at the problems of excessive melt flow rate at melting and initial stage of crystal growth,the temperature field,flow field and S-L interface in the melt are studied by adding different intensities of axial magnetic field.The study shows that in two-dimensional simulation,the melt flow velocity is inhibited obviously after the axial magnetic field strength increases to 0.05 T,and two vortexes become a vortex,the heat transfer at the bottom of crucible decreases and the S-L interface slightly rises.In three-dimensional simulation,when the axial magnetic field strength is greater than 0.05 T,the flow velocity is obviously suppressed.The flow pattern changed greatly,and the horizontal velocity is restrained,and then bifurcates into several branches flowing to all directions.The convective vortex evolves vertically from the horizontal direction without magnetic field.(5)In view of the fact that stirring the melt in the process of multicrystalline silicon growth can rapidly remove impurity from the S-L interface and homogenate the doped impurities,the effect of different alternating magnetic field directions,regions,frequencies and time periods on the Lorenz force and heat transfer in melt are studied.It is found that when alternating magnetic field is excited in Y direction,Lorenz force is perpendicular to the side wall of crucible,and efficiency of mixing is the highest.the skin depth is bigger of 50 Hz alternating magnetic field than that of500 Hz,and affect more melt region,which is helpful to stir the melt better.The flow rate in a specific area of the melt can be continuously strengthened with time increases.