Removal Of Oxygen, Carbon And Their Compounds From Silicon Through Electron Beam Melting

With the increasing energy crisis and environmental complications, all the countries around the world have begun to focus on the renewable energy. Solar energy has caught more and more attention for its environmental protection, high reserves, safety and potential economy. Photovoltaic technology is the main form of solar energy consumption. The casting multicrystalline silicon is the most widely used material for solar cell. One of the important ways to promote the development of photovoltaic industry sustainability is to explore silicon preparation with low cost and low energy consumption.High purity silicon is used as the raw material for casting. However, silicon melt and vapor in high temperature inevitably react with quartz crucible, graphite heater and graphite susceptor, so that a lot of oxygen and carbon can be incorporated into silicon by diffusion and convection. After solidification, these impurities enrich at the top, side or bottom of silicon ingot in the form of solution or precipitation. It deteriorates the electrical performance of material, so these contaminated parts, more than 30% of the ingot, cannot be used for preparation of solar cell. Traditionally, the area with high oxygen was used to mix with high purity silicon to make new ingot. However, oxygen was not removed, leading to the degradation of the photoelectric conversion efficiency. The area with high carbon was reused after refining by primary directional solidification. However, this method is with low efficiency and high cost. Therefore, there is an urgent need to develop the recycling method with high efficiency and low cost.In this work, oxygen, carbon and their compounds were removed successfully through electron beam melting (EBM), so that those areas (contaminated with oxygen and carbon) of ingot were recycled and reused. Oxygen can be removed from molten silicon by evaporation under high temperature and high vacuum condition. The removal efficiency and mass transfer coefficient were also obtained with specific experiment parameters. Meanwhile, the silicon after EBM treatment was mixed with high purity silicon to obtain a casting ingot. The performance of the ingot was evaluated and compared with an ingot by traditional method. During EBM process, there is great temperature gradient existing in silicon melt. Carbon precipitates and deposits at low temperature area. So carbon contamination region can be separated after complete silicon solidification. The separation mechanism of SiC from silicon was investigated. The conclusions are as follows:(1) The oxygen content in silicon decreased rapidly at constant power with the increase in melting time. It decreased from 6.177 ppmw to 0.0571 ppmw at 15 kW after 600 s, which is a 99.08% reduction of the oxygen. The mass transfer coefficient of oxygen was determined to be 1.51 × 10-5 kg/s, indicating the evaporation rate of oxygen larger than that of phosphorus, aluminum and calcium. During the melting process, the loss of silicon was very small, with an evaporation rate of 1.10 × 10-5 kg/s. The loss of silicon could be controlled up to 1.7% during the oxygen removal process to a desirable value, indicating EBM as an effective method to remove oxygen from silicon and decrease the loss of silicon.(2) On industrial scale EBM equipment, the oxygen content was reduced from 10 ppmw to< 0.0517 ppmw at 500 kW for 30 min, with removal efficiency up to 99.429%. However, at bottom oxygen content was 0.12 ppmw, which is considered to be affected by the combined action of evaporation on gas-liquid interface and segregation on solid-liquid interface. Compared with the traditional method, the homogeneity of life time of recycled silicon ingot was much better than cycled silicon, although the average values were almost the same. The efficiencies of recycled solar cell and cycled solar cell were both measured to be 17.55% initially. But after 4 h the efficiency of the recycled silicon solar cell is decreased to 17.05% and the efficiency of cycled silicon solar cell was decreased to 15.775%.(3) After EBM, carbon enriches in the form of SiC at the bottom of the ingot but not in the center. The numerical simulation results show that there is great temperature gradient existing in the melt during EBM, so the melt near copper crucible shows low temperature and bad fluidity. Carbon in silicon melt flows with the melt, precipitated and gathered in this area so that it is separated. This technology is applied on industrial scale EBM equipment. The results show that a majority of SiC was deposited in the bottom of the refining crucible and the carbon contaminations were not found in the most of the area of the solidified ingot in crucible.