Study On Growth Mechanism For Solar-Grade Casting Quasi-Single Silicon Prepared By Induction Heating

As an important renewable energy, photovoltaic is a significant basic energy in the first half of 21st century. About 90% of solar modules are based on crystalline silicon. The conversion efficiency of solar cells fabricated by traditional Czochralski silicon is higher because of less impurities and defects, but its production cost is high. While the production cost of cast multi-crystalline silicon is lower than that of Czochralski silicon, but there are large number of grain boundaries, dislocations and impurities. These factors have a significant influence on the properties of cast multi-crystalline silicon solar cells. quasi-single silicon combines the advantages of conversion Czochralski silicon and cast multi-crystalline silicon. The production cost of quasi-single silicon is lower than that of Czochralski silicon and conversion efficiency of solar cells is higher than that of cast multi-crystalline silicon. quasi-single silicon has a significant influence on the photovoltaic industry.Towards low cost high efficiency solar grade crystalline silicon materials, In this paper, the commercial CG-Sim software has been employed to analyze the heater configuration and the convective behavior of silicon melt in the vacuum induction melting furnace used for preparing solar-grade casting quasi-single crystal silicon and the influence of pulling rates on the solid-liquid interface, oxygen concentration and V/Gn. In the following are the results of this thesis.(1) We can got same melt convection by induction heating and resistance heating. The temperature is more uniformity and temperature gradient is more lower in induction heating. This is advantage for crystal growth.(2) The electromagnetic force is one of the driving forces to convection. Meanwhile, both strength and distribution of the electromagnetic force are influenced strongly by the ratio of induction coil height to melt depth (defined to k). As "k" value is 1.2, melt convection evaluate from two vortexes to one dominating vortex, which is benefit to the impurities evaporation. At the same time, for the inductive frequency within the range from 3000 Hz to 5000 Hz, the flow strength is decreased which could increase the thickness of boundary layer between silicon melt and silica crucible, resulting in the deceasing of oxygen concentration in silicon melt.(3) The curvature of solid-liquid interface was increased with increasing the pulling rates, which increased nucleation frequency of the grains in the ingot edgy area. On the other hand, the melt temperature decreased, resulting in oxygen concentration decreased with increasing the pulling rates. At the same time, the V/Gn value will be more than a critical value when pulling rate is above 10 mm/h. Lastly, the optimized pulling rates for preparing the quasi-single silicon is in the range of 10 mm/h to 15 mm/h.