Numerical Simulation Of Interface Morphology Evolution Of Directionally Solidified Multicrystaline Silicon Based On LBM-CA Coupling Model

The morphology of directionally solidified polysilicon interface directly controls the microstructure of crystal,which in turn affects crystal quality,and ultimately determines the photoelectric conversion efficiency of polycrystalline silicon solar cell.The evolution of interface morphology is complex and lacks effective experimental observation methods.It is difficult to study its formation and evolution mechanism,while numerical simulation technology of crystal growth has advantages in this respect.This is because of the superiority of numerical simulation of crystal growth.On the one hand,the growth process is simulated according to physical model and numerical model,which can provide information on the formation and development of microstructure during whole process of crystal growth.On the other hand,it can be predicted by simulation.The growth of crystal is compared with experimental results to verify the formation and evolution mechanism of interface.At the same time,it can provide the necessary reference data for determining optimal growth process parameters of crystal,so as to reduce the related research cost and improve efficiency.In this paper,the lattice boltzmann-cellular automaton(LBM-CA)coupling method is used to simulate competitive growth,facet growth and grain boundary concave growth of silicon.Among them,LBM is used to calculate coupling of flow field,temperature field and impurity concentration field,and growth process of crystal is calculated by CA method.The main research results are as follows:(1)The competitive growth process of crystal grains was studied.Simulation calculations were carried out for different nucleation modes,bottom cooling rates and composite heating conditions.Calculation results show that grain competition mainly exists in two stages,namely temperature-dominated competition stage and anisotropy-dominated competition stage.Controlling nucleation sites and densities by inducing nucleation facilitates the acquisition of high quality polysilicon.The smaller the bottom cooling rate,the more competitive the horizontal direction of the crystal.Composite heating is beneficial to suppress sidewall nucleation,but excessive heating power causes bottom nucleation grains to favor lateral wall growth.(2)The process of facet growth of grains was studied.High anisotropic equations were added to the LBM-CA coupling model to realize anisotropy of the model.Simulation results of different interface energy anisotropy strength and dynamic anisotropy strength are compared with theoretical analysis and experimental results,which are highly consistent.The necessity of interface disturbance and negative temperature gradient during growth of facet is verified.At the same time,it is found that initial overcooling of the melt will cause the growth interface to be undercooled,which will destroy stability of facet growth.(3)The problem of grain boundary concave angle of crystal grains was studied.Numerical simulation for origin of the grain boundary concave angle was carried out.Simulation results of different grain boundary energies were compared with theoretical analysis and experimental results.Accuracy of the model was verified,and grain boundary concave corners under different grain boundary energies and cooling rate conditions were simulated.Distribution of temperature,law of fluid flow and phenomenon of impurity accumulation show that influencing factors of the three are mainly the area of the concave corner of grain boundary.