The Formation Mechanism Of Horizontal Silicon Ribbon Growth And Prototype Device Design

Silicon ribbon growth technology has distinct advantages in reducing the manufacturing cost of crystal silicon solar cells due to their promise of avoiding the wafering of ingots and surface damage caused by the wire cutting process in the traditional production process of the silicon wafer.During the horizontal ribbon growth(HRG)process,the horizontal configuration extends the solid-liquid interface and allows the latent heat of crystallization to be dissipated over a far greater area than in vertical growth methods.Thus,much higher growth rates can be realized.However,the HRG method requires a high level of control over the solidification process,and the current HRG growth model has not given a clear understanding of the growth process.Therefore,the purpose of this study is to solve the solidification formation,heat and mass transfer,transport of diluted species,and efficient heat transfer mechanism during horizontal silicon ribbon growth by establishing multi-physical field coupling models which included heat transfer,fluid flow,segregation,and moving mesh during HRG process.The horizontal solidification forming mechanism of the silicon ribbon,the complex convection mechanism inside the melt,and the surface heat exchange and stress distribution characteristics of jet impingement on the silicon sheet were systematically studied.Finally,the prototype device of the horizontal silicon ribbon growth equipment was designed.The main works and innovation results of this dissertation can be summarized as follows:1.A multi-field coupled finite element model of the HRG solidification growth process was established based on hydrodynamics,thermodynamics,and surface mechanics.The effect of forced convection induced by the horizontal motion of the silicon ribbon on thermal transfer and the influence of the surface thermal flux and pull velocity on the forming thickness of the silicon ribbon were analyzed.The convective diffusion mechanism of oxygen impurity concentration coupled to heat flow was illustrated,revealing the solid-liquid-gas three-phase contact interface change laws under the multi-physical field coupling.It was found that the model predicts the growth and diffusion behavior of solidification,where 2D nucleation occurs at the triple-phase contact line,first forming steps and then propagating down along the solidification interface.The thickness of the silicon ribbon forming in a steady-state was affected by the horizontal pull velocity and the jet heat exchange flux.The faster the pull rate forms,the thinner the silicon ribbon.Stable growth conditions were disrupted when the pull rate was above the critical value.The latent heat released during solidification was removed through the vertical direction,one of the main conditions for rapid growth during the HRG process.The distribution of oxygen impurity content at the solidification interface was mainly affected by the internal convection of the melt,and its oxygen concentration is high in the areas of sizeable thermocapillary convection intensity.2.A complex flow model inside the melt during HRG was constructed,studying the thermocapillary-buoyancy effect under temperature gradient and the influence of the moving wall on the silicon melt.The effects of the dimensionless parameters M_a,R_a,and B_i on heat transfer and the velocity and temperature distribution at the solid-liquid interface were systematically investigated.The effect of the crucible size aspect ratio A on the molten silicon heat transfer was discussed,the free surface temperature and velocity oscillatory properties were analyzed,and the interface flow mechanism was clarified.The results indicated that in the case of small M_a(M_a(<100),the heat transfer inside the silicon liquid was carried out in heat conduction,and the pull rate size plays a decisive role in the convection inside the melt.An increase in B_i number could reduce the degree of nonlinearity of the free surface temperature,thereby weakening the thermocapillary effect.In the case of large heat exchange flux(more extensive B_i),the silicon ribbon obtained a high production rate,while the thermocapillary effect reduced the temperature.When M_a increased to a critical value,temperature and velocity oscillations were generated by the thermocapillary effect at the free surface,with a larger M_a giving rise to earlier temperature oscillation times and larger amplitude of the temperature oscillations.The oscillatory properties of the temperature were almost unaffected by the temperature for B_i and V_g.Appropriate reduction of the crucible size aspect ratio A value can effectively weaken the thermocapillary action,which can better maintain the stability of the temperature field at the solid-liquid interface.3.The characteristics of inert gas argon on silicon wafer surface heat exchange during HRG growth were systematically studied,the influence of the jet parameters on the heat exchange was explored,and the distribution law of silicon wafer heat stress under jet heat exchange conditions was revealed.It was found that reducing jet spacing H/w_j,increasing inlet Reynolds number Re,reducing jet inlet fluid temperature T_in can enhance the heat exchange flux at the triple-phase contact line and improve the production efficiency of the HRG process.The number of Nu on the impact surface showed a Gaussian distribution shape,with the highest heat exchange flux in the stagnation point area,and the heat change in the stagnation area away from the stationary point was gradually reduced.The ratio of the heat exchange and the stagnation point area at the triple-phase contact point is 96%.In addition,thermal stress was generated inside the silicon wafer,while shock stress was formed on the surface in the use of jet shock heat exchange flux.Under significant heat exchange flux parameters,the silicon ribbon has a relatively low internal stress while obtaining a high growth process.4.Based on the above model analysis,the prototype device of the HRG process was designed and constructed from the heat field structure,jet cooling device,lift mechanism,and related control system of the horizontal silicon ribbon growth prototype.The heat field structure of the prototype consists mainly of a crucible,heater,jet device,temperature measurement and feedback system,and heat insulation material.For the phenomenon of incomplete melting and surface oxidation on the upper surface of the silicon melt,the horizontal lift silicon ribbon prototype device was optimized,and the increasing diversion device was proposed.