A Study Of Seed-assisted Growth Of High Performance Cast Crystalline Silicon

Seed-assisted growth of cast crystalline silicon is an important growth technology in the photovoltaic industry.Both cast multicrystalline silicon(mc-Si)and cast monocrystalline silicon(mono-Si)can be grown from different types of seeds.Cast mc-Si has the advantages of high capacity,low cost and simple process.However,compared with CZ mono-Si,high defect density in cast mc-Si reduces its photovoltaic performance and still needs further research and improvement.On the other hand,cast mono-Si has become a research hotspot in recent years with the popularity of a series of high-efficiency solar cell technologies for mono-Si.Cast mono-Si combines the high efficiency of mono-Si solar cells and the low cost of mc-Si,which is a very promising method for crystalline silicon fabrication.Nevertheless,cast mono-Si ingots have problems such as uneven distribution of crystal defects,low mono-Si area,and high seed cost.In recent years,researchers have conducted a lot of research around these problems and proposed improvement solutions,but the industrial application of cast mono-Si is a systematic project,and new problems arise during the application process that need further research and improvement.In the present thesis,some of the challenges in the growth process of cast crystalline silicon are systematically investigated and improvement methods are proposed.Low-cost high-performance mc-Si ingots were successfully grown by using silicon-based composite nucleation layers.By studying the mechanism of defect generation at the seed joints,a method to reduce seeding defects of casting mono-Si is proposed.The mono-Si area and defect distribution of cast mono-Si ingot were significantly improved by inducing grain boundary barrier zone.The preparation cost of cast mono-Si ingot was significantly reduced by using composite barrier coatings and seed recycling.The main findings of this thesis are as follows.(1)The preparation method of silicon-based composite nucleation layer was studied.The nucleation rate can be significantly improved by using silicon powder and silicon particles with lower nucleation energy and increasing their surface roughness.The prepared high performance mc-Si ingots have fine and uniform grains,low dislocation density,and a small range of low minority lifetime zone(called red zone in the industry)at ingot bottom,the yield of the ingots is improved by more than6% compared with conventional high performance mc-Si ingots.The average solar cell efficiency of the ingots is comparable to that of conventional high performance mc-Si ingots,and the low interstitial oxygen content also controls the light induced degradation(LID)rate of solar cells below 1%.(2)The effect of different widths of seed junctions on the seeding defects of cast mono-Si ingots was investigated.Narrow seed junctions would generate large thermo-mechanical stresses in the corner regions of seeds due to volume expansion at high temperatures,and serious dislocation sources are formed at the corner joints of the seeds and the intersection of grain boundaries after seeding.When the seed junctions are increased to a suitable width,the seeding defects in the corner region of the seeds are significantly reduced.Numerical simulation results also show that the shear stress in the corner region of the seeds decreases significantly with increasing the width of seed junctions.(3)The effect of silicon melt infiltrated from the ingot bottom on the quality of the ingots was investigated.The infiltrated silicon melt not only increases the length of the red zone at the bottom of the ingot to varying degrees,but also causes serious dislocation sources at the bottom and junctions of mono-Si seeds and then causes the formation of seeding defects.However,polysilicon seeds,with their large number of pores and high density of random grain boundaries,can effectively release the thermomechanical stresses generated during the solidification of infiltrated silicon melt and do not cause seeding defects.(4)The effect of grain boundary barrier zone on the crystal quality of cast mono-Si was investigated.The formation of grain boundary barrier zone by inducing large-angle random grain boundaries could effectively block the intrusion of mc-Si at the edge of ingot,and the mono-Si area of the ingot could exceed 95%,reaching the leading level at home and abroad.The grain boundaries in the grain boundary barrier zone are significantly deformed and generate a large number of dislocations under the extrusion of mc-Si,while the multilayer grain boundaries in the barrier zone could block the dislocations from entering the mono-Si region,thus improving the defect distribution in the mono-Si region at the edge of ingot.(5)The effect of convex growth interface on the crystal quality of cast mono-Si ingots was investigated.Although the convex growth interface further increases the mono-Si area in the edge region of ingot,the bending of the growth interface causes distortion and deformation of the grain boundaries in the edge region of ingot,and the bent interface and distorted grain boundaries lead to the generation of a large number of dislocation sources.In addition,the bent growth interface increases the propargation rate of dislocation,resulting in a significant increase in the number of subgrain grains and the angle of small-angle grain boundaries in the top region of ingot.(6)The reduction of the diffusion distance of impurities in crucibles by non-oxide composite barrier coatings was investigated.Silicon nitride covering layer could protect the high-purity silicon powder layer from contamination by impurity atmospheres in the casting furnace,while the high-density grain boundaries in the high-purity silicon powder layer strongly absorb the impurities diffused from the crucible.The use of the composite barrier coating reduces the red zone length at the bottom and side of cast mono-Si region by 14 mm and 18 mm,respectively,while also improving the photovoltaic performance of the silicon wafers in the bottom region of the ingot.(7)The recycling process of seeds and the effect on the quality of cast mono-Si ingots were studied.Different concentrations of metal impurities in recycled seed crystals increase the length of the red zone at the bottom of ingots to varying degrees.Most of the dislocations in the recycled seeds are effectively blocked by the phase boundaries formed at the seeding interface,and the defect distribution in the ingots grown with recycled seeds is comparable to that of the ingots grown with new seeds.The recycling and utilization of seeds could reduce the cost share of seeds in cast mono-Si ingots from 23% to 8%.