| Silicon is the most important material for PV today. While multicrystalline silicon (mc-Si) can be used to fabricate solar cells with higher performances and low fabrication costs. The metallurgical method as a new way to produce solar grade silicon (SoG-Si), has made great progress. Compared with conventional Siemens and casting method, metallurgical method has relative low cost, the advantage of accommodate raw materials of low purity silicon, and can be directly used to purify metallurgical grade silicon to SoG-Si. However, mc-Si produced by metallurgical method, among which impurities, crystal defects and disadvantage growth orientations reduce the efficiency of the solar cells. Thus, it is important to study how to control the concentration of impurities, structure defects (dislocation and grain boundaries) and growth orientations of mc-Si.In this paper, optical microscopy (Zeiss Axioimager Al), x-Ray diffraction (D/max 3B), electron backscatter diffraction measurements (EBSD, EDAX-TSL) fitted on SEM (Philips XL30), secondary ion mass spectroscopy (SIMS) and glow discharge mass spectroscopy (GDMS) were performed for various prepared wafers sliced from different ingots. We studied the effect of different solidification rates on dislocation density changes, crystal growth orientations, grain boundary (GB) types, grain sizes, macroscopic morphology of crystal growth for vertical cross section, the concentration of carbon. oxygen and partial metal impurities. Meanwhile, we also studied the effect of conventional phosphorus gettering on the performance of wafers, the experiment included different gettering temperature, holding time was 2h. The following progresses have been achieved according to our experiment.With crucible withdrawal rate increasing, the dislocation density of crystalline silicon is increased. Among all silicon ingots that we prepared, when the growth rate is 10μm/s. it has the minimum dislocation density. While the growth rate is 20μm/s, the concentration of some important transition metal impurities is minimal in middle part of silicon ingot. The dislocation density of top with withdrawal rate 10μm/s is larger than 20μm/s. Preferential growth orientation of crystal faces changes from (111) to (331), then transformed to (111) again. When the withdraw rate is 30μm/s, the percentage of CSL boundaries is maximum compared with other withdrawal conditions, while the percentage of CSL boundaries is minimum for 20μm/s. The number ofΣ3 boundary increased with the solidification rate increasing. The grain size of middle part of ingot is maximal at 20pμm/s, average grain size is 547.5μm. While grain size of middle part of ingot is minimal at 40μm/s, average grain size is 345μm. Through corroding vertical cross section of all ingots, we observed crystal growth orientations parallel to crucible axial direction well, also got better quality of crystals for the withdrawal rate condition is 20μm/s.In addition, phosphorus gettering study was performed for samples sliced from ingot prepared by induction furnace. We find it's the most efficient for iron removing. With gettering temperature increasing, the resistivity and minority carrier lifetime of samples increase. Different gettering temperature can cause the change of GBs, common feature is the number of general high angle GB (R) tends to reduce, while the number of coherent GB (CSL) increase. At 800℃, the number of CSL GB is maximum. Through metallographic observed we found the dislocation density of sample surface reduce dramatically. Among all the withdrawal rate conditions, especially at 800℃and 900℃, there are many twins appear on the crystal surface, indicating thermal stress of inside crystal get release greatly, meanwhile, improving the quality of crystal. |
