Study On Directional Growth Of Polysilicon By Multi-physical Fields

Solar energy is abundant in the earth and clean to the environment as a renewable clean energy.Therefore,people can enter the era of energy conservation and emission reduction.In the field of solar cells,polysilicon solar cell has been widely used because of its low cost and simple production process.The growth process of polysilicon is a very slow and complicated process,directional solidification technology is one of the most critical factors to determine the performance of polycrystalline silicon solar cells.COMSOL Multiphysics is used to simulate the transient process of polysilicon growth as a finite element simulation software.The effects of crucible with thickness of 15 mm,20mm and 25 mm on melting and crystallization of silicon were simulated and analyzed.When the thickness of the crucible is 15 mm,the solid-liquid interface presents a "W" shape;When the crucible thickness is 20 mm,the solid-liquid interface is concave;however,these conditions have a bad influence on the growth of the ingot.The solid-liquid interface is flat when the crucible thickness is 25 mm,which is beneficial to the growth of the crystal.The effect of axial magnetic field on directional solidification of polysilicon is simulated,the effects of different current intensities on the convection and the Lorenz force of the melt were investigated and verified simulation results by experiments.The axial magnetic field has obvious inhibition on the melt on the surface of silicon,with the increase of magnetic field intensity,the melt flow rate decreases gradually,and also the maximum Lorenz force increases.In the experiment,when the axial magnetic field which intensity is 100 mT was added,the solid-liquid interface of silicon ingot becomes smooth from concave,which can improve the quality of polysilicon.The cooling rate has an effect on the solid-liquid interface,the minority lifetime,resistivity and the crystal structure during the directional solidification of the metallurgical polycrystalline silicon.When the cooling rate decreased from 8K/h to 4K/h,the curvature radius of the solid-liquid interface was increased,and then the minority lifetime increased from 0.23?s to 1.36?s obviousily.However,when the cooling rate decreased to 2K/h,the minority lifetime decreased to 0.21?s.When the cooling rate is too low,the silicon crystal grows from crystal plane(100)to the crystal plane(111).