Investigation of the Red Zone of Multicrystalline Silicon Ingots for Solar Cells
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This thesis is dealing with the “Red Zone” of multicrystalline silicon (mc-Si) ingots, a region which exhibits low performances and which is usually removed before slicing the wafers for solar cells fabrication. It is situated in the bottom, top and sides of the ingots. The aim of this research work is firstly to build a numerical model of this zone based on the defects which are believed to be the cause of its poor performance, and secondly to develop a novel defect engineering technique able to improve the performance of the Red Zone material. The distribution of iron in mc-Si ingot was simulated taking into account the initial contamination of the feedstock, the solid diffusion of iron from the crucible during the crystallization, its segregation to the melt and the back diffusion after the end of the solidification process. A good correlation between the depth of diffused iron and the depth of the Red Zone was revealed. This result confirms conclusions reported in the literature indicating that iron is the principal defect limiting the performance of the Red Zone material. A model simulating the lifetime distribution in mc-Si ingot was successfully developed. The model was based on the combined effect of iron and dislocations on the lifetime of the material. A good correlation between the measured lifetime and the calculated one was found. Some poor fit regions were observed and tentative reasons were proposed. This model could predict the width of the Red Zone as function of some crystallization parameters and the purity of the used crucible and coating. Internal gettering is a defect engineering technique widely used in microelectronics CZ-Si; it uses oxygen precipitates as sinks to getter metal impurities in the bulk of wafers. The impact of internal gettering on the performance of the bottom Red Zone was investigated through several approaches. Significant reduction of the bottom Red Zone of mc-Si ingot was revealed, the thickness of the Red Zone was reduced from about 2 cm to just few millimeters after gettering.