Numerical Study On Thermal Stress And Dislocation In The Growth Process For Multi-crystalline Ingot

multi-crystalline silicon photovoltaic cells are currently mainstream products of solar cells,but a large number of internal dislocations and grain boundaries have seriously affected their electrical performance and become major obstacles to improvingcellefficiency.Duringthesolidificationandannealingof multi-crystalline silicon,the crystal temperature varies with space and time,and thermal stress is caused due to the difference in the cooling rate between different parts of the crystal.Under the action of thermal stress,the stress concentration inside the crystal and the defect site will produce dislocations.Dislocations and grain boundaries will provide precipitation centers for various impurities.They promote each other and become a powerful carrier recombination center.Reduces the performance of solar cells.The dissertation summarizes the basic principles of multi-crystalline silicon dislocation formation and research status at home and abroad.Then,from the beginning of the solidification process to the end of the cooling process,multi-crystalline silicon dislocations are subjected to transient numerical simulations to study the temperature field and thermal stress of multi-crystalline silicon ingots at different growth stages.The relationship between dislocation density.The main work of this paper is as follows:（1）The temperature,thermal stress and dislocation density of multi-crystalline silicon at different growth stages are studied through transient numerical simulation of the solidification and cooling process.The simulation results show that dislocations move and increase with thermal stress in the growth and cooling process,and the temperature gradient in the crystal is the key factor to the dislocation density.It is found that the high dislocation densities appear at the top,center and outer edge of the silicon ingot.The dislocation density of the ingot surface decreases from the center to the outer edge which is due to the enrichment of impurities at the front of the crystal-melt interface.The maximum dislocation density is about 2.4×10⁴ cm^-2,which occurs at the top of the central axis of the silicon ingot.The maximum local dislocation density is about 2.2×10⁴ cm^-2,which occurs at the bottom of the ingot.（2）The effect of the change in lift rate on the growth and thermal stress and dislocation density of the silicon ingot during solidification and later cooling of the silicon ingot was investigated.The simulation results show that when the lifting rate is 0.2 mm/min,the dislocation density in the upper right corner of the silicon ingot is the minimum,but the thermal stress is the maximum at three different lifting rates;the upper right corner dislocation of the silicon ingot is at the lifting rate of 0.4mm/min.The highest density,but the smallest thermal stress,the dislocation density difference of 3.4×10² cm^-2,thermal stress difference of 1.6×10⁵ pa,and the final residual stress of the three are not much difference.（3）The effect of the high temperature annealing process on the thermal stress and dislocation density of the silicon ingot after the insulation cage is closed.The distribution and characteristics of the thermal stress and dislocation density in the later stage of the silicon ingot were studied.The simulation results show that:although the thermal stress of the annealed silicon ingot with annealing is lower than that of the silicon ingot without annealing,the dislocation density does not decrease but it increases a lot.This is because a certain degree of dislocation proliferation can release the thermal stress in the silicon ingot.Therefore,high-temperature annealing of silicon ingots can effectively reduce the thermal stress of the silicon ingot before the furnace is released,but at the same time it will increase the dislocation density in the silicon ingot.These dislocations will affect the photoelectric conversion efficiency of solar cells.It is necessary to consider the influence of dislocation density on the silicon ingot while annealing and find a balance between thermal stress and dislocation density.