| With the depletion of fossil energy, the development and utilization of new energy has been becoming a hotspot of the whole world. The research and utilization of photoenergy is attracting more and more attention for its many advantages, such as huge reserves, easy accessibility, environmental friendliness, etc. The annual consumption of silicon is very large as the main raw material of the photovoltaic industry. Usually, the silicon used in solar cell can be classified into monocrystalline silicon and multicrystalline silicon. Compared with monocrystalline silicon, multicrystalline silicon occupy most of the photovoltaic market for its high quality and low cost. A new metallurgical method of directional solidification purification process for solar-grade multicrystalline silicon production has attracted many attentions for its low cost, simple process, less investment, and environment pollution, etc., and great achievements and progresses have been made. However, the crucibles used in this metallurgical process usually contain many different impurities, the interaction of these impurities with the grain boundaries dislocations and defects of the multicrystalline silicon would decrease quality of the material, and finally the conversion efficiency of the solar cell. So the reduction of the pollution from the impurities is one of the keys to high-quality solar-grade multicrystalline silicon.In order to improve the performance of the multicrystalline silicon material prepared by directional solidification purification, the surface modified graphite crubcibles by high-purity Si3N4were used in the progress. The affect of the surface modification on the many properties was investigated at different condensation rates, such as the morphology, density of defects, density of dislocation, boundary type, grain size and electronic properties, with the characterization means of optical microscopy, x-Ray diffraction, SEM, secondary ion mass spectroscopy (SIMS) and glow discharge mass spectroscopy (GDMS). And the affects of the purity of the raw materials and the types of Si3N4were also studied at the optimal condensation rate. The main results as follows:The surface modification can significantly improve the silicon's performance.. The grains all grew normal to the bottom plate of the crucible. When the condensation rate was 20μm/s, the best performances of multicrystalline silicon ingots were obtained. The grain size from the central part of the ingots slice grew up from633.2μm to921μm, and the trend of the growth orientation was significantly enhanced; the density of point defects on the slice surface reduced to7.8×103/cm2, and the dislocation density shown a "V" shaped distribution with the lowest value of1～30×103/cm2. The grain boundaries belonged to the large-angle type, and the main type was of∑3in the R and CSL. The∑3(111) type accounted for about67.4%, and∑3(211) type accounted for about12.2%; preferential growth surfaces of the crystal were (111),(311),(422), and(533). Other non-preferential growth orientation has been perfectly eliminated. The average minority carrier lifetime increased from the0.81μs to1.89μs; the maximum resistivity increased from110mΩ·cm to227mQ-cm.In addition, the affects of the purity degree of raw materials and the types of Si3N4at the condensation rate of20μm/s was studied as the control experiment. The results showed that:the electrical properties from the raw material No.1with the highest purity degree got the best, the highest average minority carrier lifetime approached1.89μs, and the resistivity approached227mΩ·cm. It thus came to a conclusion that the level of impurities in the raw material was the most important factor on the electrical properties; the electrical properties of the ingots all increased with the modification of Si3N4to graphite crucibles. |
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