Multi-crystalline Silicon Ingot Casting By Directional Solidification Using Medium-frequency Induction Melting

The increasing demand of the energy resources had great influence on the environment and the climate in the whole world. The solar cells which can resolve the two problems of the environment and the resources have wide prospects as a renewable and non-pollution energy source. Nowadays, the silicon solar cells have been widely used because of its mature production technology. The multi-crystalline silicon solar cells hold a large market share due to its low production cost. The casting of the multi-crystalline silicon ingot is the largest energy consumption process during the multi-crystalline silicon solar cells production. So the improvement of technology of the multi-crystalline silicon ingot casting by directional solidification is an effective way to decrease the cost of the solar cells. The grain boundaries,the dislocation,the defect and the impurities in the multi-crystalline silicon ingot affect the energy conversion efficiency of the solar cells, so the research of casting technique of the multi-crystalline silicon ingot has important significance.Multi-crystalline silicon ingot casting by directional solidification using the medium-frequency induction melting furnace had been studied. The temperature distribution in the induction heating zone has been investigated by the temperature measured in the silicon melt. The structure of the silicon ingots casting by power reduction method and crucible pull-down method had been analyzed by the theories of the directional solidification and the electromagnetic induction melting. The results was shown that the crystalline structure of the silicon ingot casting by the crucible pull-down method was better than the one casting by the power reduction method. The longitudinal section of the ingot had been shown that the columnar crystal grains were grown from the bottom of the ingot to the top of the ingot.The concentration of iron,aluminum,copper and phosphorous in the silicon ingot along the longitudinal direction has been measured by ICP-Mass. The results was shown that the concentration of iron,aluminum,copper and phosphorous in the silicon ingot decreased significantly after directional solidification. The removal efficiency of iron,aluminum,copper was above 98% and the removal efficiency phosphorous was about 61%. The actual removal efficiency of iron and copper was closed to the theoretical value calculated by Scheil's equation. However, the theoretical removal efficiency of aluminum and phosphorous was higher than the actual value. The theoretical removal efficiency of aluminum and phosphorous by considering evaporation mechanism was more correspond to the actual value.The multi-crystalline silicon ingots casting by directional solidification had been prepared by using crucible pull-down method. The velocities of the crucible pulling down were 0.23mm/min and 0.12mm/min respectively. The concentrations of the impurities were higher in the silicon ingot with the faster pulling rate by the ICP measurement. The concentration of iron affected the crystal structure of the silicon ingot. The columnar crystal grain growth broken up at the position of the concentration of iron sharply increased. The result was shown that the proportion was increased by decreasing the crucible pulling rate. The conduction type was P-type at the bottom of the silicon ingot. Oppositely, the conduction type was N-type at the top of the silicon ingot. The resistivity and the minority carrier lifetime were low at the top of the ingot and the bottom of the ingot. However, the resistivity and the minority carrier lifetime reached to the maximum at the polarity transition position. The electrical characters in the multi-crystalline silicon ingot were related to the concentration of aluminum,boron and phosphorous. The concentration of aluminum and boron which can generate the hole carriers was higher than the concentration phosphorous which can generate the electronic carriers in p-type region. Oppositely, the concentration of aluminum and boron was lower than the concentration phosphorous in n-type region...