A Study Of Growth Process And Quality Of Cast Monocrystalline Silicon And High-Performance Multicrystalline Silicon

Directional solidification growth is one of the main preparation methods for photovoltaic crystalline silicon because of its advantages of low cost and low feestock requirements.The cast monocrystalline silicon and high-performance multicrystalline silicon in the industry are currently both fabricated by the directional solidification method.They are both seed-assisted growth technologies,but there is a difference between single crystal and multicrystalline of seed type.With the development of the PV market and technology,higher requirements for the quality of crystalline silicon have been put forward.In this paper,cast monocrystalline silicon and high-performance multicrystalline silicon were prepared using a directional solidification furnace,and the effects of the seed type and process conditions on the crystal quality during the preparation of cast monocrystalline silicon were investigated,focusing on the structural characteristics and generation mechanism of shadow defects,as well as the effects of the seed type and heat dissipation method on the crystal quality during the preparation of high-performance multicrystalline silicon.In addition,the effects of thermal field structure and crystallization rate on the growth and quality of crystalline silicon by directional solidification method were simulated by CGsim software.The main conclusions are as follows.(1)The study on the seed paving method of cast monocrystalline silicon found that the dislocation sources at the"-"grain boundaries formed by the melting between seeds can be effectively controlled when only 40°symmetric large-angle grain boundaries are used for splicing between seeds,but the dislocation sources at the"+"grain boundaries in the middle of four seeds cannot be suppressed.The dislocation sources at the"-"grain boundaries between the four seeds can be effectively controlled,but the dislocation sources at the"+"grain boundaries between the four seeds cannot be suppressed.We have improved and proposed various methods for splicing seeds with asymmetric large-angle grain boundaries,and successfully suppressed the dislocation sources at the"-"grain boundaries between any two seeds and at the"+"grain boundaries between any four seeds,which are less likely to induce twinning and significantly improve the crystal quality of cast monocrystalline silicon and the absolute value of the corresponding solar cell conversion efficiency increased by 1.23%.(2)The study of shadow defects in cast monocrystalline silicon ingots revealed that the shadows are mainly of dispersed silicon carbide grains.They induced dislocations,subgrain boundaries and some alien grains with long strips shape which(221)planes or(313)planes perpendicular to the growth direction.These grains formed Σ9 grain boundaries between them and Σ3 grain boundaries between them and the substrate.The shadows and the defects induced by shadows significantly reduced the crystal quality of the cast monocrystalline silicon.We found that the essence of shadow generation is the enrichment of carbon impurities at the solid-liquid interface due to composition subcooling,and obtained the process conditions to prevent shadow formation as growth rate(V)≤1.1 cm/h and the ratio of liquid phase temperature gradient and growth rate G_L/V>0.235.(3)According to the investigation on the seed types of high-performance multicrystalline silicon,it is found that the semi-cylindrical dense polysilicon rod is better than the granular polysilicon.There is no cavity at the bottom of high-performance multicrystalline silicon ingot prepared by semi-cylindrical dense primary polysilicon rod seed crystal,the grain size is uniform and the dislocation density is low.In addition,the minority carrier lifetime is more uniform,and the average minority carrier lifetime is about 0.32μs higher than that of multicrystalline silicon ingot fabricated by granular polysilicon,and the length of the red zone at the bottom of silicon ingot is 6 mm shorter.(4)The research on the heat dissipation mode in the preparation of high-performance multicrystalline silicon shows that increasing the heat dissipation in the nucleation stage can improve the undercooling and obtain smaller grains and more large-angle grain boundaries andΣgrain boundaries,and release some growth stress at the grain boundaries to achieve the purpose of inhibiting dislocation generation;Adjusting the heat dissipation at the bottom ingot bottom to obtain a slightly convex solid-liquid interface can reduce the growth stress in the grain and reduce the probability of dislocation generation.The lateral uniformity of resistivity of silicon ingot is improved,and the absolute value of conversion efficiency of corresponding solar cells is improved by 0.09%.(5)The thermal field structure and growth rate of directionally solidified silicon ingots are studied by CGsim finite element simulation.It is found that additional blocks beside the graphite heat exchange block in the directionally solidified furnace can effectively reduce the heat dissipation rate,and the heater power is reduced by about 6kW at the same growth rate.By simulating three growth rates of 10mm/h,12.5mm/h and 15mm/h,it is found that the ratio of liquid temperature gradient to growth rate decreases with the increase of growth rate,which is prone to constitutional supercooling phenomenon.When there is no additional insulation block,the shape of solid-liquid interface is concave as a whole,and the carbon impurities are concentrated near the solid-liquid interface in the center of silicon ingot.The solid-liquid interface is W-shaped when the additional block is added,and the enrichment of carbon impurities in the center of the silicon ingot is reduced,which can effectively reduce the probability of shadow in silicon ingot grown by directional solidification.