Study On Microstructure Simulation Of Multi-crystalline Silicon Ingot By Cellular Automata Method

Recently,Multi-crystalline silicon?mc-Si?had been widely used in solar photovoltaic industry as the semiconductor material with excellent properties.It is a formation of polycrystalline elemental silicon with high-purity and the preparation method commonly used directional solidification technology.The microstructure morphology and detects in multi-crystalline silicon ingot has much influence on photoelectric conversion efficiency of photovoltaic cells.In this paper,the process of multi-crystalline silicon ingot with microstructure were analyzed through the method of Cellular Automaton during crystal growth.The microstructure morphology and formation process of multi-crystalline silicon ingot were simulated and the simulation results are verified with experimental results.In order to get perfect crystal quality,the formation mechanism of grain boundary detects was analyzed.Firstly,establishment of microstructure numerical calculation model.A numerical model was set up with reasonable conditions according to the principles of thermodynamics and kinetics of crystal growth and the numerical model was deconstructed and corrected via cellular automata and finite difference method.The C language was used in this study and the whole process can be divided into four parts:definition module,macrocosmic temperature field calculation module,microstructure simulation module and image processing module.The contact of temperature field calculation-Nucleation-Growth-transition and capture has been built through the coupling of macro and micro grid in time and space scale.Secondly,simulation of nucleation and growth process for multi-crystalline silicon ingot.We simulated the temperature field,crystal-melt interface shape,columnar grain formation process and doping process during crystal growth via calculation module and conclusions were obtained as follows:on the one hand,the number of nucleus formed at the bottom boundary,then a part of the columnar crystal grains grown and continue to melt in advance.On the other hand,the temperature gradient in horizontal direction was lower and the crystal-melt interface may keep straight with heat transfer coefficient of 500W?m^-2?K^-1.Besides,the growth velocity of columnar zone decreases with increase of crystal length.The crystal-melt interface is likely concave to melt and impurities were enriched in grain boundaries obviously after doped in simulation.Thirdly,simulation of silicon grains competition process.Only a part of crystal grains might be grown up to columnar grains and got into silicon melt stably through the competition,consolidation and elimination among silicon grains due to the grain boundary energy was different among silicon grains.What is more,the grain number decreases with the length of multy-crystalline silicon ingot increases and the zigzag-faceted interface that produced by many tiny grains decreases with competition and elimination mechanism among silicon grains,then the smooth crystal-melt interface was formed.The velocity of crystal growth and convexity of crystal-melt interface increased at start and decreased later during mc-Si ingot growth.It is beneficial to obtain large silicon grain size and suitable temperature field with heat transfer coefficient of 1000W?m^-2?K^-1at bottom boundary through comparison of different heat transfer coefficient at the bottom boundary.