Research On Red Zone Suppression And Process Optimization Of Low Poly Minority Lifetime Of Polycrystalline Silicon Ingots

Multicrystalline silicon solar cells occupy the main share of photovoltaic industry because of its high photoelectric conversion efficiency and low cost.The preparation of multicrystalline silicon ingot by directional solidification method is an important link in the fabrication process of multicrystalline silicon solar cells.There is a certain length of low life time area in the bottom,top and side walls of silicon ingot named red zone,which can't meet the requirements of the preparation of solar cells.It is also reduced the utilization ratio of silicon ingot and improve the production cost of multicrystalline silicon solar cells.This paper researched the influence of impurity distribution and occurrence form,stress distribution and crucible type on the red area at the edge of polycrystalline silicon ingot by numerical simulation and experimental analysis.The ways to reduce the length of the red zone of silicon ingot were discussed from the aspects of improving the purity of silicon raw material and crucible,surface modification of crucible and optimizing the process parameters.The specific contents include the following parts.1.The process of N diffusion,which was produced by the decomposition of Si3N4 on the crucible surface,in the silicon melt was analyzed during the high temperature directional solidification process.The distribution of Fe in the red zone effect by N diffusion on the at the bottom of the ingot was also studied.The results can be described:the occurrence of Fe impurities in the red zone of the ingot is mainly FeSi2,When Fe is sufficient,(3-FeSi2 is formed,and when Fe is insufficient,the metastable a-FeSi2 was formed.After annealing process,accelerated cooling method was used to suppress the decomposition of the supersaturated a-Fe(Si)of solid solution,which made the length of the red side of the ingot side wall reduced 5 mm averagely,and minority-carrier lifetime of silicon ingots outside the red zone increased by 2?s averagely,its distribution was more uniform.2.The thermal stress values and distribution of silicon ingots were simulated under the two conditions of pilot test and industrialization experiment.The result was carried out by specific experiment,high press in bottom and top of silicon ingot was the main reason why minority carrier lifetime decrease and red area formed.The maximum stress values of the ingot by two experimental conditions are at the top edges,the stress value of the ingot prepared by the pilot condition was up to 410.7MPa,the length of red area in top of the silicon ingot was about 5mm,it can reach 10mm in the edges area.At the bottom,the length of red area was 20mm.The stress on the top edge of the ingot prepared at the industrial conditions was up to 70.63 Mpa,when silicon ingots were cut,the length of red area at the top was 30.23mm,at bottom is 48.37mm.There were many flocculent defects in silicon ingots and the sizes were small,and flocculent defects dispersed in the silicon were uniform,this was another main reason why red area formed.3.The effects of different surface morphology of side wall and the bottom of the ingot in crucible and the effect of the surface modification of the crucible on the red zone length were studied.The micro-field silicon ingots prepared by four different morphologies at the bottom of the crucible were shown:when the bottom of the crucible was concave and flat,the overall lifetime of the ingot was the best,with a maximum of 2.68?s and 2.33?s.Crucible surface modification,can reduce the length of the red ingot from 40mm to about 26mm.Polysilicon ingot ingots prepared in two kinds of crucibles under grouting and pouring in industrial conditions after cutting,B6 small ingot of the overall minority carrier of the highest life up to 6.9?s,the width of the red zone of the ingot B region prepared in the grouting crucible was 16.6 mm and the width of the red zone of the ingot B region prepared in the grouting crucible was 13.4 mm.The use of injection molding crucible can increase the utilization of ingot by about 2%.In the case of the same amount of investment,At the bottom of the crucible is enlarged by 6 cm,the ingot height will drop by 3.6 cm.The stress distribution in the ingot was basically same as that without expansion,but the radial temperature gradient of polysilicon ingots increased slightly.The utilization of Silicon ingot would be reduced by 0.44%,resulting in production waste.4.In order to reduce the length of red area,experimental study was carried out from two aspects:reducing impurity pollution and optimizing directional solidification process.Research indicates that silicon material should be pre-sourized prior to charging to improve the purity of the silicon raw material,under the industrial conditions,the length of the red zone after the ingot can be reduced by 7 mm,the maximum lifetime of the minority carriers in the red zone increased from 4?s to 6?s.When the crucible was subjected to a prepared pickling treatment,the length of the red zone of the ingot B in ingot can be reduced from 20.8 mm to 14.9 mm.The average lifetime of silicon ingots prepared by directional solidification optimization was increased to more than 2.99 ?s.Minority-carrier lifetime was not high or low,strip distribution phenomenon.Seed crystal semi-melting process was used to make minority-carrier in small A1 ingot at the middle of the ingot basically wasn't at red zone.The lifetime distribution of the minority carriers in the silicon wafer was relatively uniform.The average life expectancy of the whole minority carrier of the silicon ingot was increased from 4?s to 8?s.When doing the pilot test,chillednucleation directional solidification method was used to reduce the length of the red zone at the bottom of the ingot from 56.7 mm to 42.5 mm.Minority carrier lifetime in conventional method to produce Multicrystalline silicon ingot from bottom to the top were less than 2?s of the region accounted for about 56%of the total height.Multicrystalline silicon ingots produced by Chilled nucleation directional solidification method,Minority carrier lifetime were less than 2?s of the region accounted for about 40%of the total height,the utilization of silicon ingot increased by 16%.