Research On Metallurgical Purifying Method Of Solar Grade Multicrystalline Silicon

Clean and renewable energy is the only choice when the 3E(Energy,Economy and Environment) problem is considered as the main problem of human continuous development. Solar energy will soon be in great demand since it is inexhaustible and much cleaner than any other conventional energy resources.With more and more national photovoltaic(PV) engineering starting,the multicrystalline silicon(mc-Si) demand will be increased greatly and the PV industries will develope rapidly.Solar grade silicon(SoG-Si) is not only the basic raw materials of PV industry,but also is the mainly raw materials of semiconductor industry. Nowadays,the SoG-Si mainly prepared by chemical method which has the disadvantages of high cost,heavy pollution and the key techniques are monopolized by few overseas enterprises,which results in short supply and speeds an upsurge of researching on preparation SoG-Si by new routes.In these methods,great attentions have being paid to the metallurgical method because of its low costs,non-pollution,etc.Based on the carefully comparative research of different metallurgical grade silicon (MG-Si) purifying routes,we put forward and experience an effective metallurgical process of purifying MG-Si to SoG-Si.The mc-Si ingot used for solar cells are prepared by 5 steps including acid leaching,vacuum refining,first directional solidification(DS),electron beam (EB) refining and second directional solidification.In addition,a series of mc-Si purification equipments are designed and manufactured.The results of acid leaching experiment indicate that the removal effect of impurities in silicon is different when leached by different acid.The HF acid leaching process has the best effect of removing Al and Fe,the removal effect of Ca,Ti,Cu and Zn is not significant when leach by HF or HCl.The optimized parameter is as follows:0.1-0.5 mm of silicon particles, 60℃of leaching temperature,HF with 4 mol/l leach for 24 h.Furthermore,it is found that the acoustic streaming and cavitation can remove the metallic impurities in grain boundary slit more thoroughly and the purifying effects are improved when ultrasonic field was imposed on acid leaching process.It can be found in vacuum refining experiment that the main parameters are 10^-2 Pa of vacuum and 30～40 min of refining time.Based on experimental data,a equation are summarized for optimizing technology plan.Hence,the yield of silicon can be calculated according to the concentration of impurity P of pre-and post vacuum process. Y_m =(C_fin / C_ini)⁽1/((?)-1)-0.966(1-C_fin / C_ini)A multi-region temperature-control DS equipment for preparation of multicrystalline silion is designed and fabricated.During DS process,the effect of refining magnetic field, temperature gradient and solidified times on purifying of silicon are studied by calculation and experiments.The experimental results indicate that segregation of impurities in silicon ingot is enhanced by refining magnetic field which is induced by self-designed refining unit. When refining temperature of molten silicon is 1550℃,drawing velocity is 1×10^-5～5×10^-5 m/s and cooling water flow rate is 300～800 L/h,the impurities can be removed effectively by more than twice solidification process.In EB experiment,the influence of working mode,refining temperature,refining time,EB power and feed rate on EB refining effectiveness are studied in customized EB refining furnace.The relations of molten silicon temperature,EB power and refining time are calculated which resolves the problem of temperature measurement in EB furnace.The experimental results show that the impurities in silicon can be removed effectively under the condition of 2550℃of molten pool and 1 h of EB refining time.According to the above technology route,one mc-Si,ingot with the diameter of 100 mm and the length of 170 mm is prepared.The direction of the most grains is parallel to the axial of silicon ingot and the size of grains is approximately 5 mm.There are a few stomata on the bottom of silicon ingot and there are no obvious defects inside the silicon ingot.Inductively coupled plasma atomic emission spectrometry analysis results indicate that the concentration of total impurities in silicon ingot decreased from above 6000 ppmw to less than 30 ppmw, the concentration of impurities can reach 5N in middle-lower part and meet the demands of preparing solar cells.Through further technique optimizing,the purifying effect can be improved continually and the mc-Si ingot with whole purity of 5N can be prepared.