Production Of Solar Grade Silicon Research Articles

Structure and Properties of Metallurgical-grade Silicon

The main raw material for the production of solar-grade silicon used in the production of photovoltaic energy converters for solar cells is metallurgical silicon. When choosing any technology for the production of solar-grade silicon, special attention should be given to improving the quality of the initial MG-Si. The aim of this work was to study the structure and properties of metallurgical silicon. Samples of MG-Si were prepared in laboratory conditions. Samples, obtained by plasma-arc melting in the furnace with graphite electrodes, contained admissible impurity limits. The structure and physico-mechanical properties of MG-Si with different content of impurities were studied in a wide range. The electrical resistivity index depended on the presence of grain boundaries and the level of impurity elements content, as well as the presence of carbides and silicides. At the maximum grain size of 105.58 μm, the maximum value of the electrical resistivity of 2.65 Ohm⋅cm was observed. The size and shape of the grains also had an effect on mechanical properties. Selection of the optimal composition of all components, as well as the conditions of the melting, allowed to achieve a defect-free structure. Such MG-Si samples can be used for various practical applications.

Thermochemical Aspects of Boron and Phosphorus Distribution Between Silicon and BaO-SiO2 and CaO-BaO-SiO2 lags

In the production of solar grade silicon by metallurgical route the distribution of B and P between slags and liquid silicon is the most important key issue. The equilibrium and thermochemistry of reactions between liquid silicon and BaO-SiO2 slags and up to 10% BaO-containing CaO-BaO-SiO2 slags is studied through experimental work and using thermodynamic calculations. It is shown that the distribution coefficient of B (LB) is higher for the CaO-BaO-SiO2 slags than that for BaO-SiO2 slags and it is not significantly affected by temperature and composition changes of the slags. In contrast, the distribution coefficient of P (LP) is higher for BaO-SiO2 slags than that for the CaO-BaO-SiO2 slags, and it is higher at lower temperatures. The chemical activities of the dilute solutions of Ba in liquid silicon, and the dilute solutions of B2O3, P2O5 and BaO in the slags are calculated. Moreover, the reaction mechanisms for B, P, Ba and Ca transport between liquid silicon and the slags are explained.

Dephosphorization of Levitated Silicon-Iron Droplets for Production of Solar-Grade Silicon

The treatment of relatively inexpensive silicon-iron alloys is a potential refining route in order to generate solar-grade silicon. Phosphorus is one of the more difficult impurity elements to remove by conventional processing. In this study, electromagnetic levitation was used to investigate phosphorus behavior in silicon-iron alloy droplets exposed to H2-Ar gas mixtures under various experimental conditions including, refining time, temperature (1723 K to 1993 K), gas flow rate, iron content, and initial phosphorus concentration in the alloy. Thermodynamic modeling of the dephosphorization reaction permitted prediction of the various gaseous products and indicated that diatomic phosphorus is the dominant species formed.

The mechanism of boron removal from silicon alloy by electric field using slag treatment

Abstract Stubborn nonmetallic impurity boron is the most difficult to be removed by the metallurgical route for solar grade silicon (SoG-Si) production. To break through the restriction of separation of trace impurity, a novel approach by applying an electrical potential between the slag and the solvent phases to enhance silicon alloy refining using slag was developed. The effect of the external potential and refining time on boron removal has been investigated. For silicon, Al-Si, or Sn-Si system, it can be found that the removal efficiency of B substantially increased with the augment of the external potential. When the external potential was increased to 6 V, the partition ratio of boron increased to 5.7, which was about ten times than that without the external potential applied. Under the constant external potentials, the boron content also decreased with increasing refining time in the Al-Si system, and three stages of average removal velocity of boron were presented within 2 h. In addition, the synergistic removal mechanism of boron with electric field was investigated for the first time. For the electrode reaction, B is oxidated to the oxidizing products, then the oxide crosses the boundary layer rapdily and finally is absorbed by slag owing to the transfer of O2− at the anode, at the same time Si is produced at the cathode, which provides theoretical support for this new purification process.

Process design and intensification for the production of solar grade silicon

Solar grade silicon (SiSG) is typically used in photovoltaic applications, and it is commercially manufactured via the Siemens process. Despite the fact that current levels of demand are satisfied, there may be a shortage of SiSG in the near future. To improve the low yield of the Siemens process, two alternative types of SiSG production processes have been developed and analyzed using a stochastic optimization scheme within ASPEN PLUS. The first one is an intensified Fluidized Bed Reactor (FBR) process using a reactive distillation column. The second one is a hybrid process combining both the Siemens and the conventional FBR processes. In addition to supplying future demand, these processes are intended to reduce the use of raw materials. The results show the great value of optimizing the processes, since it achieves savings in the TAC of 53.28%, 67.65% and 62.58% (Siemens, Intensified FBR and Hybrid process), respectively. Siemens process shows the lowest TAC ($0.50 M/y), but this does not mean that it is the process with the highest potential, since it has the lowest silicon production rate, 0.47 kt/y. The Intensified FBR Union Carbide Process turns out to be the most expensive of the three ($2.57 M/y), with a large production of Si(SG), 1.49 kt/y. However, it is the hybrid process which shows the larger yield by far, with a production of 1.89 kt/y of Si(SG) and TAC of $1.95 M/y, showing the highest profit from sales, $40.47 M/y. However, from the environmental point of view, the Siemens process shows the lowest environmental impact based on the eco-indicator 99, while the Hybrid process is the second best.

The Use of Silicon Wafer Barriers in the Electrochemical Reduction of Solid Silica to Form Silicon in Molten Salts

Nowadays, silicon is the most critical element in solar cells and/or solar chips. Silicon having 98 to 99% Si as being metallurgical grade, requires further refinement/purification processes such as zone refining [1,2] and/or Siemens process [3] to upgrade it for solar applications. A promising method, based on straightforward electrochemical reduction of oxides by FFC Cambridge Process [4], was adopted to form silicon from porous SiO2 pellets in molten CaCl2 and CaCl2-NaCl salt mixture [5]. It was reported that silicon powder contaminated by iron and nickel emanated from stainless steel cathode, consequently disqualified the product from solar applications. SiO2 pellets sintered at 1300oC for 4 hours, were placed in between pure silicon wafer plates to defeat the contamination problem. Encouraging results indicated a reliable alternative method of direct solar grade silicon production for expanding solar energy field.

Material requirements and availability for multi-terawatt deployment of photovoltaics

This study investigates growth rates and material flows required to reach and sustain multi-terawatt installed capacity of photovoltaics (PV). The dynamics of material flows over time are captured, taking account for the life expectancy of PV technology. Requirements of solar grade silicon and silver for crystalline silicon (c-Si) technology, as well as indium, gallium, selenium, tellurium, and cadmium for currently commercial thin film (TF) technology are explored, accounting for different technology choices and potential improvements in material intensities. Future availability of these materials from primary resources, as well as secondary resources from end-of-life recycling, is also analyzed. Rapid deployment of c-Si technologies would require a major expansion of solar grade silicon production, and significant quantities of silver. Availability of materials such as indium and tellurium could become problematic for major implementation of TF technology, unless production can be scaled up significantly, or material intensities radically decreased. Availability of secondary resources from end-of-life recycling have little impact on material availability during the growth phase, but could be important for sustaining a low-carbon energy system over longer time perspectives. Material availability could cause problems for rapid PV growth, but does not necessarily limit total PV deployment, especially if material intensities are decreased.

Binary Phase Diagrams and Thermodynamic Properties of Silicon and Essential Doping Elements (Al, As, B, Bi, Ga, In, N, P, Sb and Tl).

Fabrication of solar and electronic silicon wafers involves direct contact between solid, liquid and gas phases at near equilibrium conditions. Understanding of the phase diagrams and thermochemical properties of the Si-dopant binary systems is essential for providing processing conditions and for understanding the phase formation and transformation. In this work, ten Si-based binary phase diagrams, including Si with group IIIA elements (Al, B, Ga, In and Tl) and with group VA elements (As, Bi, N, P and Sb), have been reviewed. Each of these systems has been critically discussed on both aspects of phase diagram and thermodynamic properties. The available experimental data and thermodynamic parameters in the literature have been summarized and assessed thoroughly to provide consistent understanding of each system. Some systems were re-calculated to obtain a combination of the best evaluated phase diagram and a set of optimized thermodynamic parameters. As doping levels of solar and electronic silicon are of high technological importance, diffusion data has been presented to serve as a useful reference on the properties, behavior and quantities of metal impurities in silicon. This paper is meant to bridge the theoretical understanding of phase diagrams with the research and development of solar-grade silicon production, relying on the available information in the literature and our own analysis.

Influence of Growth Velocity on the Separation of Primary Silicon in Solidified Al-Si Hypereutectic Alloy Driven by a Pulsed Electric Current

Investigating the separation of the primary silicon phase in Al-Si hypereutectic alloys is of high importance for the production of solar grade silicon. The present paper focuses on the effect of growth velocity on the electric current pulse (ECP)-induced separation of primary silicon in a directionally solidified Al-20.5 wt % Si hypereutectic alloy. Experimental results show that lower growth velocity promotes the enrichment tendency of primary silicon at the bottom region of the sample. The maximum measured area percentage of segregated primary silicon in the sample solidified at the growth velocity of 4 μm/s is as high as 82.6%, whereas the corresponding value is only 59% in the sample solidified at the growth velocity of 24 μm/s. This is attributed to the fact that the stronger forced flow is generated to promote the precipitation of primary silicon accompanied by a higher concentration of electric current in the mushy zone under the application of a slower growth velocity.

Optimization of the Raw Material Input Molar Ratio on the Carbothermal Production of Solar-Grade Silicon

Optimization of the Raw Material Input Molar Ratio on the Carbothermal Production of Solar-Grade Silicon

From Metallurgical-Grade to Solar-Grade Silicon: An Overview

The non-renewable nature of fossil fuels as an energy source means its future availability is a cause for concern. The world’s energy demand is ever increasing and there is a growing interest in finding alternative renewable, environmentally benign and cheap energy sources like solar energy. This has resulted in the shortage of silicon feedstock for the photovoltaic industry. This is mainly due to the non-availability of a dedicated solar silicon production and the growing demand for silicon feedstock. There has been tremendous research in a quest to develop methods for the production of solar-grade silicon in a cheap and environmentally friendly way. The metallurgical and chemical routes for the production of solar-grade silicon from metallurgical-grade silicon have evolved. The chemical methods are the most researched ones and they are mostly preferred than the metallurgical ones since the former are capable of producing silicon of higher purity. This review discusses some of the available methods so far for the production of solar-grade silicon using metallurgical-grade silicon as a starting material.

Electrochemical Reduction of Solid Silica to Form Silicon in Molten Salts

Nowadays, Silicon is the most critical element in solar cells and/or solar chips. Silicon having 98 to 99% Si as being metallurgical grade, requires further refinement/purification processes such as zone refining [1,2] and/or Siemens process [3] to upgrade it for solar applications. A promising method, based on straightforward electrochemical reduction of oxides by FFC Cambridge Process [4], was adopted to form silicon from porous SiO2 pellets in molten CaCl2 and CaCl2-NaCl salt mixture [5]. It was reported that silicon powder contaminated by iron and nickel emanated from stainless steel cathode, consequently disqualified the product from solar applications. SiO2 pellets sintered at 1300oC for 4 hours, were placed in between pure silicon wafer plates to defeat the contamination problem. Encouraging results indicated a reliable alternative method of direct solar grade silicon production for expanding solar energy field.[1] P.R. Mei, S.P.Moreira, E.Cardoso, A.D.S.Cortes, F.C.Marques, “Purification of metallurgical silicon by horizontal zone melting”, Sol. Energ. Mat. Sol., 98 (2012), 233-239. [2] P. Siffert, E.F. Krimmel, eds. Silicon: Evolution and future of a technology. Berlin,Springer (2004). [3] Siemens & Halske: BRD Patents 1.102.117 and 1.140.1594 filed 1954, issued 1956 [4] G.Z. Chen, D. J. Fray, T. W. Farthing, “Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride”, Nature, 407 (2000), 361-364. [5] E. Ergül, İ. Karakaya, M. Erdoğan, “Electrochemical decomposition of SiO2 pellets to form silicon in molten salts” Journal of Alloys and Compounds, 509 (2011), 899-903.

High Purity Composite Briquette for Direct UMG-Si Production in Arc Furnaces

In metallurgical grade Si (MG-Si), the coal (B) and charcoal (P) contents are on average above 30 ppm as the carbon reduction materials used in the arc furnace are either rich in B or in P. A decrease of both impurities by a factor of 3 using purer raw materials would allow for the direct production of the upgraded metallurgical grade (UMG).This would significantly improve the efficiency of the resulting photovoltaic (PV) cells made with the refined solar grade silicon (SoG-Si) or massively decrease the costs of Si purification by shortening the number of steps needed for reaching B and P contents below 1 ppm requested for the SoG-Si used for the PV cells. A composite C/SiO2 briquette fulfilling the purity targets for the direct production of UMG-Si in the arc furnace was developed. The composite contains several carbon materials with different levels of reactivities and quartz sand. The raw materials aspects, the paste and briquette preparation, as well as the final carbonization step are discussed. The finished briquettes are free of volatiles and are mechanically and thermally very stable, thus, ensuring stable arc furnace charges with minimum losses of dust and SiO gas. Semi-industrial trials including the downstream purification steps for the production of SoG-Si by a metallurgical low-cost route are contemplated.

Numerical modelling of silicon melt purification in induction directional solidification system

Solar grade silicon production is an energy intensive and harmful to the environment process. Yet 40% of this valuable product material is lost into sawdust (kerf loss) during wafering. The kerf waste from Fixed Abrasive Sawing of PV silicon wafers is pelletized and then remelted in an induction furnace. The furnace has a square cross-section quartz crucible, surrounded by graphite susceptors and heated by an induction coil that enables directional solidification of the new ingot. Top and bottom 'pancake' coils provide additional temperature control. Once melted, silicon becomes electrically conductive and subject to stirring by induction. To recycle the silicon, particulate impurities (due to the sawing, condensed silicon oxides or carbides) need to be removed. Flow control and the electromagnetic Leenov-Kolin force are used to expel particulates, through a novel dual frequency induction scheme. Three-dimensional, multi-physics numerical modelling captures the electromagnetic, fluid-flow and heat-transfer effects in this process. The presented results show it is possible to retain the impurity particles on the sides of the solidified ingot where they can be sliced off and removed.

Life cycle assessment of grid-connected power generation from metallurgical route multi-crystalline silicon photovoltaic system in China

A life cycle assessment (LCA) has been performed for the grid-connected electricity generation from a metallurgical route multi-crystalline silicon (multi-Si) photovoltaic (PV) system in China. The energy payback time (EPBT), environmental impacts and total environmental impact indexes were calculated. Based on the contribution analysis, it can find out the most critical stage (or process) and the greatest environmental impact. Meanwhile, the main cause of them were traced respectively. The sensitivity analysis reveals that aluminum consumption makes the most obvious influence. However, PV station retirement affects the result significantly by recycling the important materials. The total environmental impact indexes of solar grade silicon (SoG-Si) production via metallurgical route were compared with purification via modified Siemens process. Results show that the metallurgical route is with obvious advantage. Among the renewable energy, photovoltaics with the minimal environmental impact is only 3.33% of coal-fired power generation. The environmental impact of current work is about 56–66% of other nations’ PV results. In order to promote the green and sustainable development of the Chinese PV industry, there is usually a brief comment or relevant suggestion for improvement after each analysis and comparison in this research.

Optical and spectroscopic characterizations of Algerian silica raw material to predict high quality solar-grade silicon

We assess the potential use as raw material for photovoltaics of Algerian silica samples from the quartz veins of the Tirek deposit and quartz sandstones of the Ain Barda deposit. With 97–98% purity, they all require enrichment before their industrial utilization. Acid leaching and gravimetric separation are used to remove the impurities at the grain boundaries and within the crystal lattice. We obtain course, middle, and fine products. The acid leaching process and the gravimetric separation increase the content of SiO2 up to 99.68%; the residue concentration of iron, alumina and zirconium is decreased to 70, 72 and 58 ppm respectively. These values are in agreement with requirements for silica as raw material destined for solar-grade silicon production.

A New Electrolytic Production Process of Silicon Using Liquid Zn Alloy Cathode in Molten Salt

1. Introduction In order to replace the present manufacturing process of solar-grade silicon (SOG-Si), the Siemens process, the establishment of low-cost SOG–Si production process is required. As a new approach, we demonstrated that solid SiO2can be directly reduced to solid Si by electrolysis in molten salts at 1123 K [1]. SiO2 + 4 e− (through conductor) = Si + 2 O2−(1) We also proposed that the electrolytic reduction of purified SiO2 is a potential method to realize the low-cost production of SOG-Si [2]. In recent years, we have been studying the electrochemical reduction of powdery SiO2 to establish a continuous process [3]. However, washing treatment using water or acid is necessary to recover the produced powdery Si from molten CaCl2 and the residual SiO2. To effectively recover the product, we recently proposed an electrolytic process utilizing a liquid Si–Zn alloy cathode as shown in Fig. 1 [4, 5]. In this process, solid SiO2is reduced to form a Si–Zn liquid alloy on the cathode, and then metallic Si is recovered by lowering the temperature to decrease the solubility of Si in the alloy. The Zn-rich liquid alloy after the separation step is recycled for the cathode. The produced Si is further purified to SOG-Si by directional solidification process. In this study, we investigated (i) the electrolytic reduction of SiO2 granules on a liquid Zn cathode in molten CaCl2 and (ii) the impurity segregation during the precipitation of Si from liquid Si-Zn alloy. 2. Experimental High-purity CaCl2 was charged in an alumina crucible and dried under vacuum at 773 K for 24 hours. All the electrochemical experiments were carried out in a dry Ar atmosphere at 1123 K. Electrochemical behavior was investigated by cyclic voltammetry and potentiostatic electrolysis using a liquid Zn electrode. The counter electrode was a carbon rod. An Ag+/Ag electrode was used as a reference electrode. 3. Results and discussion The potentiostatic electrolysis was conducted using a liquid Zn electrode with SiO2 granules (diameter ca. 0.1 mm) at 0.9 V vs. Ca2+/Ca for 1 h. By cross-sectional SEM measurement, several particles were observed in the Zn sample after the electrolysis. EDX analysis confirmed that the SiO2 powder had been reduced to Si. According to the Si–Zn phase diagram, solubility of Si in liquid Si–Zn alloy is 6 at% at 1123 K and that at room temperature is very low. Thus, it is concluded that SiO2 was reduced to form liquid Si–Zn alloy and then Si particles were precipitated during the cooling process. In the presentation, the mechanism of SiO2reduction and the impurity segregation behavior during the cooling process will be discussed. Acknowledgement This study was partly supported by the Core Research for Evolutionary Science and Technology (CREST) of the Japan Science and Technology Agency (JST), and Grant-in-Aid for Scientific Research (A) from Japan Society for the Promotion of Science (JSPS). Reference [1] T. Nohira, K. Yasuda, and Y. Ito, Nat. Mater., 2, 397 (2003). [2] K. Yasuda, T. Nohira, R. Hagiwara, and Y. H. Ogata., Electrochim. Acta, 53, 106 (2007). [3] T. Toba, K. Yasuda, T. Nohira, X. Yang, R. Hagiwara, K. Ichitsubo, K. Masuda, and T. Homma, Electrochemistry, 81, 559 (2013). [4] T. Shimao, X. Yang, K. Yasuda, T. Nohira, R. Hagiwara, K. Ichitsubo, K. Masuda, and T. Homma, Technical Digest for The 6th World Conference on Photovoltaic Energy Conversion, 4WePo.7.52, (2014). [5] K. Yasuda, Molten Salts (Yoyuen Oyobi Koon Kagaku), 58, 20 (2015). Figure 1

Production of Solar-Grade Silicon by the SiF4 and Mg Reaction

Because crystalline Si substrate still accounts for a significant portion of the total cost of solar cells, substantial efforts have been invested in the development of new processes that can significantly lower the production cost of solar-grade silicon. At SRI International, we developed a low-cost process of producing solar-grade silicon based upon the SiF 4 -Na reaction. This process has now been scaled up to pilot-plant level, and solar cells obtained using such silicon feedstock reached > 18% efficiencies. Due to continuing increases in demand and production capacity and the simultaneous drop in price of Mg metal, we decided to explore its potential as a reducing agent instead of Na metal. Experimental conditions for obtaining a fast and complete metallothermic reaction between SiF4 and Mg are identified and aspects of the reaction mechanism are discussed. The reaction involves both a heterogeneous liquid-gas phase reaction and a homogeneous gas-gas phase reaction. The separation of the products, Si and MgF 2 , is described. When pure Mg was used, Si obtained after melt separation had the potential to be suitable for solar cell applications.

Segregation behavior of iron in metallurgical grade silicon during Si[sbnd]Cu solvent refining

Impurity removal from metallurgical grade silicon (MG-Si) is an important issue for solar-grade silicon production, and solvent refining is used as an effective method to purify silicon. To evaluate the purification ability of solvent refining with SiCu melt at 1343–1603 K, the effective segregation ratio of the main impurity element Fe between solid Si and SiCu melt was experimentally and theoretically determined. The results demonstrated that the effective segregation ratio of Fe increased with the increasing temperature at the range of 1343–1603 K. The corresponding removal mechanism of Fe was investigated in detail. It was found that the effective segregation ratio of Fe calculated by Scheil equation presented the same tendency with experimental results, and the theoretical calculation was more consistent with experimental results when back diffusion was considered. Moreover, Fe was found to form SiFeMn and FeMnTi phases into SiCu alloy phase from MG-Si with the presence of Cu. It can be concluded that removal mechanism of Fe was not only a segregation process, but also a recombining process of impurity phase transformation.

In Search of an Alternative Solar Grade Silicon Production By Electrochemical Reduction of SiO2

Silicon plays an important role in fabrication of solar cells and solar chips. Metallurgical grade silicon (98 to 99 % Si) demands further purification such as zone refining [1, 2], metallothermic reduction [3] and Siemens process [4] to convert it to solar grade. A promising method, based on direct electrochemical reduction of oxides by FFC Cambridge Process [5], was adopted to form silicon from porous SiO2 pellets in molten CaCl2 and CaCl2–NaCl salt mixture [6]. The contamination of silicon powder by iron and nickel originating from stainless steel cathode was reported to disqualify the product from solar applications. SiO2 pellets, sintered at 1300oC for 4 hours, were sandwiched between pure silicon wafer plates to minimize contamination of silicon. The promising results indicated a potential alternative method of direct solar grade silicon production for expanding solar energy industry. [1] P.R. Mei, S.P.Moreira, E.Cardoso, A.D.S.Cortes, F.C.Marques, “Purification of metallurgical silicon by horizontal zone melting”, Sol. Energ. Mat. Sol., 98 (2012), 233-239. [2] P. Siffert, E.F. Krimmel, eds. Silicon: Evolution and future of a technology. Berlin,Springer (2004). [3] K. Yasuda, T.H. Okabe, “Solar-grade silicon production by metallothermic reduction”, JOM,62 (12) (2010), 94-101. [4] Siemens & Halske: BRD Patents 1.102.117 and 1.140.1594 filed 1954, issued 1956 [5] G.Z. Chen, D. J. Fray, T. W. Farthing, “Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride”, Nature, 407 (2000), 361-364. [6] E. Ergül, İ. Karakaya, M. Erdoğan, “Electrochemical decomposition of SiO2 pellets to form silicon in molten salts” Journal of Alloys and Compounds, 509 (2011), 899-903.