Study On Microstrucyure Evolution Of Directionally Solidified Polysilicon Based On 3D CA-LBM Coupling Model

At present,photovoltaic solar cells have become the leading part of new energy industry,and directional solidification(ingot)polysilicon is still the mainstream material of photovoltaic cells.The growth process of directionally solidified polysilicon crystal is studied by numerical simulation.The growth behavior and the formation mechanism of micro defects can be explored,which is beneficial to the improvement of crystal growth quality and photoelectric conversion efficiency.The traditional numerical simulation technology of crystal growth generally uses the finite element method and other methods,and it is two-dimensional simulation,which can not fully reflect the crystal growth information.It has important research and application value to establish macro micro coupling model and simulate its three-dimensional crystal growth process.In this paper,a three-dimensional cellular automata lattice Boltzmann(CA-LBM)coupling model is built to simulate the three-dimensional small plane interface growth process,three-dimensional small plane dendrite growth process,three-dimensional grain competition,three-dimensional small plane columnar crystal to equiaxed crystal transformation process.Among them,the lattice Boltzmann method is used to calculate the three-dimensional multi physical field,and the cellular automata method is used to simulate the nucleation,growth and capture process of the crystal.(1)In the three-dimensional CA-LBM model,two factors,i.e.interface energy anisotropy and dynamic anisotropy,are considered synthetically,and a new interface energy anisotropy function is selected to describe the growth process of highly anisotropic small plane interface.The calculated results can show the three-dimensional small plane interface growth characteristics of silicon crystal.The three-dimensional temperature field in the growth process was simulated,and the negative temperature gradient at the front of the growth interface and the phenomenon of small plane annexation caused by solidification latent heat release were analyzed.(2)A three-dimensional CA-LBM model with higher interface energy anisotropy and dynamic anisotropy is established.The three-dimensional small plane growth process of single Si equiaxed crystal was simulated.The calculation results show that the single equiaxed silicon dendrite has obvious small plane characteristics,which shows the high anisotropy completely different from the non-small plane dendrite.Under the condition of high anisotropy,some typical high anisotropy features such as orientation loss and interface of small plane dendrites appear.When the undercooling is large,the growth rate of small plane dendrites increases,and the small plane interface is sharper.The different anisotropic strength will lead to the change of the small plane interface morphology of dendrites.When the anisotropy is small,the interface is smoother,and when the anisotropy is strong,the interface is sharper.(3)A three-dimensional model for global simulation of polysilicon directional solidification process is established.The results show that there will be competition among grains with different orientations during directional solidification,the volume of grains with competitive advantage will increase,and the grains with weak position will disappear.As the interface of small plane changes to equiaxed crystal,the temperature of adjacent dendrites will increase due to the release of latent heat of solidification,and the growth speed and shape of dendrite arm will be affected.The growth rate of the dendrite is slow and the dendrite arm is relatively short.The three-dimensional flow field and growth morphology were simulated and analyzed.