Study On The Preparation Of High-quality Cast Single Crystalline Silicon For Industrialization

Photovoltaic solar energy is one of the most important types of clean and renewable energy.Crystalline silicon is commonly used in the production of photovoltaic industry.Crystalline silicon accounts for more than 95% of photovoltaic market,which is mainly produced using Czochralski method(Cz-Si)and cast method.Cast single-crystalline silicon has received more attention on account of its cost-effective compared with Czochralski silicon(CZ-Si)and higher efficiency compared with high-performance multicrystalline silicon.In this thesis,several typical problems that hindered the industrial production of high-quality cast monocrystalline silicon were studied.The techniques of polycrystalline barrier at the edge and dislocation control of cast single-crystalline silicon ingot and red zone reduction at the bottom were studied,which effectively improved the quality of cast single-crystalline silicon and mass applications based on these results had been achived.The performance of cast single-crystalline silicon was characterized by the use of minority carrier lifetime tester,PL,EL,ICP-MS and other test methods,which effectively improved the quality of cast single-crystalline silicon and formed a large-scale application.The main research results are as follows:(1)By laying seed crystal strips around the seed crystal at the crucible edge to form multiple large-angle grain boundaries,three cases without seed crystal strips,with seed crystal strips and with graphite soft felt as insulation material between the crucible and the heater were investigated to study the effects of seed crystal strips and graphite soft felt insulation material on blocking of edge polycrystals and ingot quality.These results indicated that compared with the silicon ingot without seed crystal strips,the proportion of whole-monocrystalline silicon wafers was increased by 14%,and the dislocation ratio of the edge silicon block was reduced by 5.3%.The average photoelectric conversion efficiency of the solar cell was increased by 0.35%,which indicated that the multiple large-angle grain boundaries could effectively block the intrusion of edge polycrystals;After introducing the graphite soft felt thermal insulation material under the condition of seed crystal strips,compared with the silicon ingot without thermal insulation layer,the proportion of whole-monocrystalline silicon wafers was increased by further 8%,and the dislocation ratio of the edge silicon block was reduced by further 5.7%.The average photoelectric conversion efficiency of the solar cells were increased by further 0.29%,which indicated that the graphite soft felt insulation layer could effectively protect the edge seed crystals from being melted through and reduce the generation of polycrystals on the crucible wall.(2)Two preparation schemes with and without impurity barrier layers were investigated by laying high-purity quartz sheets between the crucible and the seed crystal as an impurity barrier layer to study the effect of the impurity barrier layer on the red zone at the bottom of cast single-crystalline silicon.It was found that the high-purity quartz sheets were used as the impurity barrier layer,which could effectively prevent the metal impurities in the crucible body into cast single-crystalline silicon ingot.These results suggested that the bottom red zone of the silicon ingot was reduced from 65 mm to 42 mm.In other words,the height ratio of the red zone at the bottom was reduced by 35% and the effective length was increased by 7%.In addition,the average minority carrier lifetime of the silicon blocks were0.68 us higher than those of the traditional process,increased more than 14%,which indicated that the impurity barrier layer of the high-purity quartz sheets could effectively prevent the metal impurities in the crucible body from diffusing into cast single-crystalline silicon ingot.