SILICON PRODUCTION, REFINING & RECYCLING FOR PHOTOVOLTAIC CELLS

Refining of metallurgical grade silicon is one of the most challenging scientific topics for metallurgists and material scientists. Slag refining is one of the promising refining methods which could remove impurities like phosphorus and boron from the silicon matrix. Published data are available for high purity silicon only whereas slag refining of metallurgical grade silicon is feebly studied. In the present work metallurgical grade silicon has been treated with the slag of Na2O-CaO-SiO2 system. After cooling down the microstructure of silicon has been studied in comparison with the untreated silicon. The composition of the slag has been varied with concentrations of Na2O from 10 to 30 wt% at a fixed ratio CaO/SiO2=1.21 to ensure a high basicity of the slag. Metallurgical grade silicon contains about 2 wt% of impurities, with major impurity elements Fe, Al, Ti and Ca. They form many small inclusions, which are randomly distributed throughout the matrix. B and P are dissolved in the silicon matrix, which is the main difficulty for their refining. The main impurity phase in inclusions is FeSi2. After slag treatment no more small inclusions have been observed and impurities have been found enriched in clear grain boundaries, whereas along with FeSi2 a large amount of CaSi2 has been detected exceeding its initial content. Thermodynamic analysis of possible reactions shows the probability for CaSi2 formation only at high temperatures in an argon atmosphere from CaO and silicon. This conclusion should be taken into account for study of the slag refining process of metallurgical grade silicon. Additionally, in the authors’ previous work on alloying, the impurity phase FeSi2Ti has been found capable to absorb up to 0.4 wt% of phosphorus, and this observation has been confirmed in slag treatment experiments. This fact indicates a concurrence between possible processes: Oxidation of phosphorus by a slag and dissolution of phosphorus in the Fe-Si-Ti intermetallic phases. It could point to a different mechanism of phosphorus refining in comparison to boron refining.

Low-cost metallurgical refining of metallurgical grade silicon is one of the most developing topics for fast growing PV industry. Acid leaching is a suggested method for removal of most metallic impurities, but not efficient for removal of B and P as most harmful impurities in solar wafers. Despite the thirty years of research in leaching treatment of silicon, there are different conditions claimed as best, but no correlation with microstructure is published. Little attention is paid also to the microstructure of the metallurgical grade silicon. In the present work, the microstructure of silicon (98 wt% purity) is studied in relation to its leaching behavior. For a better leaching efficiency, silicon of different microstructure has been obtained by alloying with 3-10 wt% of calcium using different cooling rates. It has been shown that the major impurity and its location determine the preferred leaching agent. Source metallurgical grade silicon contains 2000-3000 ppmw of Fe and several hundreds ppmw of Al, Ti and Ca as major impurities, randomly distributed throughout the matrix. For efficient leaching it requires very fine grinding and a HF solution as a preferable leaching agent. Calcium is a major impurity after alloying with calcium and it promotes agglomeration of impurities in inclusions. Starting from 5 wt% Ca well-pronounced grain boundaries with most impurities could be observed. Leaching of these structures could be performed using a HCl solution only; However in this case formation of a side-product FeSi2 takes place. Better removal of phosphorus and boron has been detected in silicon after alloying with calcium. Different cooling conditions result in different microstructure of silicon. The leaching efficiency is connected with the applied cooling rate taking into account the diffusivity of impurity elements in silicon. In this still continuing work 82% of phosphorus and 100% of Fe and Ti have been removed. Thus, study of the microstructure of silicon prior to its refining is a key to better leaching efficiency.

In the metallurgical process (MP-SOG) of purifying metallurgical grade silicon (MG-Si) to Solar grade silicon (SoG-Si), B, different from Fe, Al etc. Has larger segregation coefficient as 0.8, which formed a consciousness that B almost completely dissolved inside Si matrix as the only existing form in MG-Si. The directional solidification, by which most of the impurities can be removed from molten Si, has no removal effect on B. As the crucial impurity in purification, slag refining, plasma refining and alloy melting method were used to remove B from Si in MP-SOG. The crushing classification of lump MG-Si showed that B had the enrichment behavior inside the finer particles as those of metal impurities. The largest relative contents (β) of B in three different MG-Si materials were 1.798, 1.758, 1.647 within the size range of less than 38.5μm, respectively. It was found that B contents in different sizes of MG-Si showed an eminent decrease after its being leached by aqua regia or HF. However, the boron distributions in the same sizes of the recrushed Si after being remelted and sequential acid leaching had no marked difference. The phenomenon was probably related with slag refining of the MG-Si industrial processes, which made part of boron change to oxide and remain on MG-Si grain boundary. After crush the segregated B easily exposed and removed by acid leaching revealed that its present form was changed by slag refining. B segregation in MG-Si did not agree with the concept of B dissolving completely with Si. In addition, the discovery of B exposure was important to improve the slag refining in SoG-Si preparation by metallurgical method or MG-Si production.

Metallurgical grade silicon is the starting material for production of silicon for crystalline silicon solar cells. This silicon has to be purified before it can be used for solar cells, and the dominating purification process today is the Siemens process, which is an expensive and energy demanding process. The silicon produced through this process has a purity of 9N, while the required purity level for crystalline solar cells is 6N. One promising method for reaching this purity level with less cost and less use of energy is to refine metallurgical silicon through a pyro-metallurgical approach. One of the challenges with this approach is to bring down the amount of boron in silicon to the required level. The present study is focusing on removal of boron by slag treatment of molten silicon, which is one of the most promising methods. Boron is absorbed from the silicon and oxidized before it is transferred into the slag in this method. The distribution of boron between slag and silicon determines the efficiency of the process, and thus the cost. The distribution of boron between molten silicon and calcium-silicate based slags was determined at 1823 K. The slags had a CaO/SiO2 ratio between 0.56 and 1.16 and contained up to 40% Al2O3 and 5% MgO.

Porous silicon thin layers are of great interest in many multidisciplinary fields such as optoelectronics, photovoltaics, chemical and biological sensors, in-vivo biomedical applications and electrochemical batteries. We report several new electrochemical methods of silicon surface texturing in molten salt by porous silicon. Well-defined silicon substrates with thin SiO2 layers thermally grown on p-type silicon were used as precursors. Rich and versatile surface architectures were obtained composed of micro-nano pores, spheres and nano-fibre structures. The samples exhibited specifically high surface to volume ratio, which showed great ability to absorb visible light. The proposed method offer new opportunities in silicon surface technologies and applications, for instance, in preparation the electrodes for rechargeable lithium batteries, the anti-reflection coatings in solar cells or the platforms for biological and chemical sensors.

Studied on electrorefining metallurgical grade silicon to prepare solar grade silicon(SOG-Si) with electrochemical method in molten KCl and NaF mixture salt. Molten KCl and NaF mixture salt as electrolyte, the metallurgical grade silicon as anode and a metal Ni crucible for collecting silicon powder as cathode, electrorefining experiments were performed at 800℃ and 2.0V for 8h and 24h under dry argon atmosphere, respectively. The results showed that the metallurgical grade silicon can be anodically dissolved and cathodically deposited from electrolytes based on molten KCl and NaF mixture salt. Most of impurities like B and P were reduced significantly. The collection efficiency of electrorefining silicon reached 90%. The electrochemical method could be applied to electrorefining solar grade silicon, which had a great advantage in cheap cost, short technical process, protecting environment and saving energy.

Filtration with foam filters is reported as a successful way to remove inclusions from top-cut silicon scrap. Top-cuts in the multicrystalline silicon ingots have a high content of solid inclusions which are SiC and Si3N4 particles. For this reason they are lost as scrap. However, on removing the solid inclusions by a simple filtration process with ceramic foam filters we can recycle silicon as PV feedstock. 10, 20 and 30 ppi (pores per linear inch) SiC filters are used in our experiments. Top-cut silicon scrap before and after filtration is analyzed by light microscopy, and particle size distribution versus number % is reported. Filtration efficiency is determined by counting inclusions before and after filtration and by measuring their surface area with light microscopy. A high filtration efficiency of more than 99% for 30 ppi SiC filters is achieved. The inclusions remaining are mainly SiC particles smaller than 10µm . Possibly these are primarily secondary inclusions. The filtration efficiency increases with decreasing filter pore size. The main actor that plays a role in deep bed filtration seems to be interception. Various models are considered to estimate removal efficiency of foam filters by this mechanism. Here we propose a new named “the branch” model. This gives a high filtration efficiency, and agrees best with the experimental results.

Recently, solar energy has received much attention as an important subsection of green energy. To overcome the issue of cost, production of SoG-Si using a metallurgical refining process has been considered. It is important to understand the thermodynamic behavior of boron in slag for optimized refinement of SoG-Si. In this study, the thermodynamic behavior of boron in the CaO-SiO2-Al2O3 slag system was investigated at 1773 K for various oxygen potential and slag composition. In addition, evaporation of boron in the reduction process was observed during slag refining by addition of H2 gas. Experimental results showed that the stability of boron in slag depends on both the O2−(basicity) content of the slag and the solubility of Ca in silicon, and the dissolution mechanism of boron into slag was derived. The effective removal of boron for SoG-Si during refining processes was evaluated quantitatively.

This study presents a simple method to reduce the contents of phosphorous and boron from silicon melt using vacuum and steam plasma refining. Recently, solar energy has been attracted to substitute the fossil fuel since it is clean and inexhaustible. Conventionally, Solar grade silicon (SoG-Si) has been generated by Siemens or fluidized bed reactor (FBR) method leading to higher initial cost and environmental problems. Thus, a metallurgical refining process to produce SoG-Si has been considered. Among the various impurities, however, it is more difficult to remove phosphorous and boron than metallic impurities which mainly disturb what the purity of silicon reaches to solar grade (≥6N). In this work, we investigate the refining process to get rid of phosphorous and boron. Phosphorous is evaporated in a vacuum because of its higher vapor pressure and boron is eliminated as more volatile compounds such as BxHzOy, B(OH)x and BxOy by steam plasma. In the single vacuum process, the contents of phosphorous decreased up to 69% (from 4.2ppm to 1.3ppm) and in the single steam plasma process, boron decreased up to 93% (from 2.9ppm to 0.2ppm). This work is ongoing, we expect that the silicon purity would be increased by combining of two processes, steam plasma and vacuum refining.

As one of the most likely special technologies for Solar-grade Silicon (SoG-Si), metallurgical process has two serious problems, high energy-consuming and quality instability, which blocked its prevalence. The ultimate reason of metallurgical process dilemma is the difficult removal of boron and phosphorous. Easy oxidation of B and high volatility of P are generally used as the removing fundamental in metallurgical process. Due to its low concentration, the speed of interphase mass transfer and impetus of reaction were ultra-low, which was the ultimate reason of removal difficulty of B and P. Novel removal method of B and P is the key for the further development of metallurgical process. In this paper, the recrystallizing purification of silicon in metal liquid, called metal-liquating method, was proposed as one new method for the removal of impurities. With the replace of purification system, the segregation coefficients of B and P were improved, and the interphase reallocating was strengthened. With the replace of purification means, the virtual surface of mass transfer was increased, and the mass transfer of B and P was strengthened. Since the nature of problems was completely changed by the method, the rapid and low-temperature removal of B and P could be achieved. The selection criteria of metal-liquating system were summarized and tin was selected as the preferred medium. The total segregation coefficient of metal-liquating purification was proposed and deduced, which can directly indicate the removal rate. The value of boron in Sn-Si system decreased with two magnitudes from 0.8, evidencing the feasibility of tin as liquating medium. With the change of operating temperature from 1200 to 600 oC and the recrystallizing time of 2 hours, the removal efficiency of B reached more than 85%, and the removal efficiency of phosphorous reached 60%. For MG-Si with B and P of 10ppmw respectively, two times of metal-liquating could reduce them down to less than 0.1ppmw. The generation of insoluble compound between impurity and silicon was the main way of impurity removal. Silicon-melting under low-temperature provided the possibility for its formation. The compound attached to Si crystal and removed by acid-leaching, which become the main outputting way of impurities for the metal-liquating system. The liquating medium tin could be reused directly.

Direct carbothermic reduction of ultra-pure quartz and carbon materials in a submerged arc furnace is one of the routes to produce solar grade silicon for the PV industry. This process, called Solsilc, was developed by Fesil Sunergy and SINTEF as an alternative to the standard Siemens process for poly-silicon, with the aim to lower the total cost of the solar grade silicon. However, a limitation of the process is the residual carbon originating from the carbon materials. The carbon content, about 700 mass ppm, is in the form of SiC particles with size between 1 - 50 µm. Settling of the SiC particles in molten silicon is investigated and reported here. Two experiments were carried out where 15 and 50 kg of Solsilc silicon were melted in two different induction furnaces. Liquid silicon samples were taken at the top and the bottom of crucible and the extracted silicon samples were analyzed for the total carbon content by employing a Leco carbon analyzer. The results show that initially the carbon content in the silicon melt is high reaching up to 1350 mass ppm at 1550°C. As the temperature is lowered with time so does the carbon content in the melt. At 1480°C the carbon level ranges from 100 – 170 mass ppm. The results are compared with the theoretical models on settling of SiC particles and dissolution of carbon in silicon.

Since solid silicon has retrograde solubilities of major impurities, it is effective to remove impurities during the refining process at low temperature. For example, we have clarified that the low-temperature partial solidification of silicon from Si-Al solvent is extremely effective to reduce impurity concentrations. In order to develop the optimal refining process on solid silicon, thermodynamic properties of silicon solid solutions are of importance. In the present paper, thermodynamic analysis was carried out on the reported solid solubilities of silicon as well as segregation coefficients of impurities between solid / liquid silicon at its melting point. The excess Gibbs energies of silicon solid solutions were determined for 15 binary systems. Based on determined thermodynamic data, the refinability during the low-temperature partial solidification of silicon will be demonstrated.

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