Numerical And Experimental Study Of Thermal Field And Dislocation Mechanism For Large-size Cast-mono Silicon Furnace

Mass production of crystalline silicon solar cells which is high-efficiency and low-cost is a precondition for photovoltaic grid parity.The cast-mono silicon is studied for improving the single crystal area and reducing the defects such as dislocations.Firstly,the heat and mass transfer numerical models of G6 small directional solidification furnace and new large G7 furnace are established by CGSim numerical simulation software.Simulation-based and experiment-assisted methods are used to compare and analyze the advantages and disadvantages of the thermal field of G6 furnace and G7 furnace in the cast-mono silicon growth.Secondly,the advantages and disadvantages of the thermal field of G6 furnace and G7 furnace in crystal growth of cast-mono silicon are compared and analyzed by numerical simulation and experiment.Then the bottom grille is designed according to the problems in G7 furnace.And the feasibility of the design scheme is verified by simulation and experiment.Finally,using the theory of grain boundary engineering,three groups of experimental studies were carried out on the mechanism of seed dislocation in G7 furnace.The main research topics are as follows:(1)Based on the previous publications and author’s own practice,the principle of cast-mono silicon methods,the research status and a number of important issues in Si crystal growth are introduced.Emphasis has been placed on the driving forces of the heat and mass transfer mechanism,the dislocation generation mechanism,solid/liquid interface,and the impact factors of minority-carrier lifetime.Thus the detailed background is provided for understanding the solar grade silicon growth.(2)The numerical modeling of the small-scale G6 furnace and G7 furnace are established.And the thermal field and flow field under 10%,50% and 90%crystallization of crystal growth in the two tape of furnace are compared and analyzed.The results show that the G6 furnace has a “W” solid-liquid interface.The side warpage is easy to occur in the early of crystal growth,which is not conducive to the maintenance of single crystal area.And the side heat insulation is not easy to improve.For G7 furnace,the convexity of the solid-liquid interface height difference becomes larger in the early of crystal growth because of the size upgrade and the bottom center position of the heat dissipation.Uneven heat loss under the support block is not only not conducive to maintaining the crystal growth direction,but also generates thermal stress,which leads to dislocation generation in the crystal growth.(3)According to the problems in the G7 furnace thermal field,three grille design schemes are proposed—interlayer grille,interlayer grille perforation,and pyramid grille.The thermal field and flow field of 10%,50% and 90% crystalline silicon are simulated for the three schemes.The results show that the three grille schemes can improve the heat dissipation uniformity of the support block and optimize the solid-liquid interface at the initial stage of crystal growth.In the middle and later of crystal growth,the side is hotter due to less heat loss,making the solid-liquid interface convex.The thickness of the central interlayer in the pyramid grille scheme is the thickest,and the thickness gradually decreases,leaving enough space on the side.Heat loss and solid-liquid interface is more uniform.Comprehensive experiments show that the pyramid grid scheme is more conducive to maintaining the uniformity of the solid-liquid interface,increasing the area of cast-mono silicon,reducing the generation of thermal stress,and growing higher quality cast-mono silicon ingots.(4)Through designing the experiment of cast-mono silicon,the influence of dislocation is studied from three aspects: different surface treatment of seed,seed crystal with different crystal orientation and functional grain boundary of seed crystal.Research shows that dislocations generated from the surface of cast-mono silicon produced by using surface-treated seed crystal will be greatly reduced.Using <100>orientation seed crystal,dislocations diffuse along a direction with an angle of 35.3degrees to the growth direction.For <110> orientation seed crystal,dislocations are not to extend laterally,but defect zones will be formed,which will affect battery performance.The small angle grain boundary between seed crystals cannot reduce dislocation due to its instability of grain boundary.Large-angle coincidence lattice grain boundaries or large-angle random grain boundaries between seed crystals can absorb initial dislocations and block dislocations,reduce dislocation density and improve solar cell efficiency.