Molecular Dynamics Simulation Study On Silicon Solidification And Structure Evolution

Multicrystalline silicon solar cells have become the mainstream products of photovoltaic market due to the high conversion efficiency and low cost.The preparation of multicrystalline silicon ingots is a process of solidification and growth from silicon melt,which determines the quality of the ingots and further affects the conversion efficiency of solar cells.Consequently,providing a good deal of insight into the solidification process of silicon is quite important.However,it is difficult to grasp the evolution laws of microstructure in the solidification process because of the limitation of the experimental conditions,which is unfavorable for further improving quality of multicrystalline silicon ingots.This work employed the molecular dynamics simulation based on the SW potential that used to describe the interaction between silicon atoms to construct the two-dimensional homogeneous silicon melt model and bicrystal silicon growth model with more than one hundred thousand atoms.The homogeneous nucleation and growth laws of silicon melt at different temperatures,and the competition between nucleation and crystal growth during the growth of bicrystal silicon have been investigated.The results are as follows:?1?In the homogeneous nucleation stage of silicon melt,two different nucleation modes were exhibited:spontaneous nucleation at high temperatures and sporadic nucleation at low temperatures.Occasionally,local heterogeneous nucleation occurred during homogeneous nucleation.The nucleation rate reached the maximum at the critical temperature of⁰.65T_m.In the crystal growth stage,the nuclei growth exponent is the maximum at⁰.65T_m,and the grain number showed three different changing patterns with time at different temperatures.During the long-time grain coarsening stage,the log-normal distribution function could suitably describe the grain size distribution.The grain coarsening exponent approximately lower one order of magnitude than the nuclei growth exponent.?2?The crystal defect analyses in homogeneous nucleation and growth of multicrystalline silicon showed that the dislocation density is about 10⁵ cm^-2.Most dislocations were generated at grain boundary,and some self-growth dislocations could be formed in the internal of grain,which could slip and even be absorbed by the grain boundary.More than 90%of the grains contained twin boundaries,and the percentage of twin atom exceeded 40%.CSL grain boundary is about 30%in the multicrystalline silicon grown at 0.71T_m.?3?It shows the competitive relationship between nucleation and crystal growth in bicrystal silicon.At high temperatures,the growth of seed crystal predominated.At medium temperatures,the seed crystal growth was accompanied with the heterogeneous nucleation.At low temperatures,nucleation came to dominate.Heterogeneous nucleation and homogeneous nucleation exist in the solidification.?4?After the solidification of bicrystal silicon with different orientations,at the same temperatures,the percentages of twin boundary in?110?-?100?and?112?-?100?bicrystal are lowest,and the dislocation density of?111?-?100?bicrystal is the lowest.