Production Of Solar Grade Silicon Research Articles

Zero-Direct-Carbon-Emission Production of Si Epitaxy Enabled by Solid Oxide Membrane (SOM) Electrolysis

State-of-the-art Solar-grade silicon production is energy intensive and has a negative impact on the environment. Due to the rapid growth of the Si-based photovoltaic (PV) industry, it is necessary to develop a greener technology for silicon production. Solid oxide membrane (SOM) electrolysis is a proven versatile green technology that can be developed to economically produce many important metal or metal compounds from their oxides. This work will discuss the application of SOM electrolysis to produce solar-grade silicon epitaxially from silica in a single-step, which results in net zero-carbon emission. The high-temperature SOM electrolysis cell employs stable molten oxide-fluoride bath with silicon wafer cathode and stabilized zirconia membrane-based novel anodes. The cell design and process parameters are selected to enable high-efficiency silicon deposition on the Si-wafer cathode and mitigate undesired side-reactions. The SOM process is electrochemically characterized with detailed microstructural analysis of the Si deposit on the Si wafer cathode.

Toward sustainability for upcycling SoG-Si scrap by an immersion rotational segregation purification process

Recycling solar grade silicon (SoG-Si) scrap is critical for the sustainability of solar energy and fulfilling to close material loops in circular economics. A major challenge is to control trace impurities efficiently in short time for upgrading scrap to a level of SoG-Si. Here, a new immersion rotational separation purification (IRSP) process is developed to enhance trace impurities removal and the crystal growth from remelted metal by driving the rotating shear flow to improve the efficiency of solute and heat transfer. In a departure from conventional practice that forces extremely low solidification rate limited by the diffusion layer depth at approximately equilibrium solidification interface, here IRSP achieves the deep removal of impurities and the increase of solidification rate by the non-equilibrium rotating solidification interface. In the non-equilibrium solidification process caused by mold rotation, the rotating shear flow not only reduces the diffusion layer depth to improve impurity removal efficiency, but also increases the temperature gradient of the melt at the solidification front that is conducive to the increase of crystal growth rate. The new method is applicable to molten Si and the production of SoG-Si (99.9998%) has been demonstrated, which is able to improve the quality of recycled SoG-Si requiring a low energy input. It is expected that this process helps close high purity Si material loops, reduce resource consumption and the carbon footprint of the photovoltaic industry.

Optimization and sensitivity analysis of a multi-product solar grade silicon refinery: Considering environmental and economic metrics

Today, one of the main lines of solar energy development is the creation of environmentally friendly, waste-free and inexpensive solar grade silicon production technologies. Presently, the main solar grade silicon production technologies are based on the reduction of silicon hydrogen chloride compounds (trichlorosilane, tetrachlorosilane and silane). This paper will examine the results of the optimization of a multi-product silicon refinery, where the objective functions are Profit and Eco-indicator99. Seven scenarios are considered for the work. The best scenario (S4) showed a tendency to maintain a balance between Profit (98.66 M$/y) and the environmental indicator Eco99 (6.04 MP/y). Similarly, a sensitivity analysis was carried out by varying the market prices of each of the products obtained (solar grade silicon, TEOS at different purities, and chlorosilanes) with an increase of 10% and a decrease of 10%, resulting in multiple production scenarios depending on what the market dictates. By increasing the cost of solar grade silicon by 10%, the multi-product refinery achieves the highest profit (151.84 [M$/y] with an environmental impact of 9.8 [MP/y]), and by decreasing the cost of the same silicon by 10%, the lowest environmental impact is achieved (36 [M$/y] with an environmental impact of 3 [MP/y]).

Towards sustainability assessment through a flexibility index as the condition number

Green chemistry and green engineering, projected in the concept of sustainability, provide inherently safe processes. These safety aspects can be determined indirectly through process controllability and flexibility. Although several indexes have been proposed to measure these parameters, the flexibility index and controllability measured by the condition number are notable. However, these two indicators are usually measured individually, and information inherent to synergies is lost due to the link between profitability and process safety. In the present work we aim to evaluate a quantitative parameter (from the condition number), to simultaneously measure flexibility and controllability of processes to generate a simple measure of sustainability to processes. To demonstrate the relationship of controllability and flexibility, through the condition number, three sequences of solar grade silicon production were explored. In addition, the advantages of using the proposed indicator were demonstrated through a pair of binary and multicomponent reactive distillation sequences. The results clearly showed that the condition number measuring controllability at low frequencies can be related to flexibility. And that this is potentially an indicator that can simultaneously measure controllability and flexibility, and with its use in process design, sustainable processes will be ensured.

The effect of Ni on Fe and Al impurities by MIVM model for the silicon production

Many impurities in industrial silicon significantly affect the production of solar-grade silicon. Based on a large amount of actual production data, the effect of Ni content on the Fe and Al contents in an actual production process was evaluated using statistical methods and MIVM models in this study. The results obtained by statistical analysis (linear regression) based on the big data of an actual production are consistent with the results of Si–P–Fe and Si–P–Al calculated using the MIVM model. The presence of Ni is beneficial to the removal of Fe and Al in Si. When the content of Ni in Si is high, pickling and refining removal of Fe and Al will increase. The effect of impurities in Si–Ni–Fe and Si–Ni–Al ternary systems and Si–Fe–Al–Ni quaternary system calculated using the MIVM model is consistent. In the actual production process, the remaining low-concentration impurities have a slight effect on the results. The nickel content can be adjusted based on the MIVM calculation results. The power consumption of industrial silicon production can be increased by first removing the Fe and Al and then removing the Ni, which is helpful for preparing solar-grade polysilicon using the metallurgical method.

Electropurification of Reusable CaCl2-Based Molten Salt for Silicon Electrorefining

Electrorefining in molten salt is a promising solar-grade silicon production process due to its low-cost, energy-saving and eco-friendly characteristics. After the silicon electrorefining process, the impurities in the metallurgical-grade silicon are induced into the salt. However, the salt has to be ultrapure and the impurity concentration in molten salt must be strictly controlled to produce high-purity silicon. This work electropurified the eutectic CaCl2-CaF2 molten salt after the silicon electrorefining process. The cyclic voltammetry is used to identify the impurity level of the salt. The single-potential and multi-potential electropurification process have been conducted and compared. The multi-potential electropurification process is proven to be more efficient in impurity removal. After the multi-potential electropurification process, the salt can be regenerated to its original impurity level, indicating a reusable high-purity salt for solar-grade silicon electrorefining.

In situ high temperature spectroscopic study of liquid inclusions and hydroxyl groups in high purity natural quartz

High purity natural based quartz is used as raw material for production of crucibles for solar-grade silicon production. The crucible properties are strongly related to presence of water and hydroxyl groups in the raw material. In this work, in situ temperature-dependent diffuse reflectance infrared (DRIFT) and Raman spectroscopy in the spectral range of the OH stretching modes, 3000–4000 cm−1, were used to investigate water inclusions and hydroxyl groups at temperatures up to 800 °C. DRIFT shows that a substantial amount of liquid water was removed at 700 °C whereas new silanol groups characterised by stretching modes at about 3650 cm−1 were formed above 200 °C. A surface OH mode was also identified by multivariante curve resolution (MCR) of Raman maps. These surface OH groups were interpreted as the sign of formation of a hydrated layer at the surface of inclusions. It was not possible to detect water signals in other types of structural defect like cracks. Images were acquired in an in situ cell showing different stages of the rupture of an inclusion with a diameter of ≈8 μm between 400 and 500 °C. The in situ Raman spectrum of the inclusion water showed the typical liquid water bands at room temperature with a maximum moving to higher wavenumbers with increasing temperature. At 400 °C, the single peak at 3645 cm−1 indicated a situation close to the critical point, triggering rupture. The rupture of such large inclusions was thus identified as the dominating channel for the removal of liquid water inclusions. A small inclusion with a diameter ≈4 μm, however, was stable even at 800 °C. These small inclusions contain supercritical water, as evidenced by the sharp Raman peak at 3600 cm−1. Via the peak position, the internal pressure in this stable inclusion was estimated to be 90 MPa. Stability may be facilitated by the formation of a hydrated interfacial layer; the different reactivities of the inclusion fluid at different temperatures leads to different interfacial structures at different temperatures. The presented evidence suggests that inclusions with diameter in the range of few μm are the main source of remaining water and hydroxyl groups in thermally treated high purity quartz sand.

Effect of Si Content on the Morphology Evolution of the Si Primary Dendrites in Al-Si Alloy Solvent Refining Process

Solvent refining with Al-Si alloy is a promising purification method for the production of solar-grade silicon (SoG-Si) feedstock owing to the advantages of low production cost and high impurity removal efficiency. In this process, larger refined Si primary dendrites should be easily collected after acid leaching, which is favorable to recovery, thereby reducing the production cost. Hence, the growth behavior of the precipitated Si crystal must be investigated systematically. In the present work, the morphology evolution of solidified Al-Si alloys with a wide range of Si content (30 ~ 70 wt%) was analyzed. The typical plate-like Si primary dendrites grown following the twin plane re-entrance edge (TPRE) mechanism formed in all alloy compositions. As increasing the Si content from 30 wt% to 50 wt%, the Si primary dendrites underwent a coarsening process attributed to the preferred growth along with < 211 > and < 111 > directions, leading to an increase in the experimental recovery rate. However, the preferred growth along < 211 > direction was inhibited when the Si content is higher than 55 wt%. Moreover, the broken effect originating from grain collision and thermal stress on the Si primary dendrites was enhanced by further increasing the Si content, resulting in a decrease in the experimental recovery rate. Therefore, the optimum composition is determined as Al-50 ~ 55 wt% Si for solvent refining solution, based on the cost reduction consideration.

Effects of La2O3 Addition into CaO-SiO2 Slag: Structural Evolution and Impurity Separation from Si-Sn Alloy

Boron (B) and phosphorus (P) are the most problematic impurities to be removed in the production of solar-grade silicon by the metallurgical process. In this work, the distribution of B and P between CaO-(La2O3)-SiO2 slags and Si-10 mass pct Sn melt was experimentally studied. B distribution coefficient increased from 2.93 in binary CaO-SiO2 slag to 3.33 and 3.65 with 2 and 10 mass pct La2O3 additions, respectively. In the followed acid-leaching experiments, the slag-treated Si-Sn alloys exhibited higher B and P removal than that of the initial alloy without slag treatment. Molecular dynamics simulations were performed to study the effect of La2O3 addition on the slag structural and transport properties. A novel oxygen classification method was proposed to distinguish the different structural roles of La and Ca in the CaO-La2O3-SiO2 system. It was found that La3+ prefers to stay in the depolymerized region, mostly connects with 6-7 non-bridging oxygen, and requires a weak charge compensation with Ca2+. Possible silicothermic reduction was evaluated to discuss the slag chemistry and the mass transfer between slag and metal phase. A thermodynamic model was derived to theoretically study the alloying effect on impurity distribution in slag refining where positive interaction coefficient and high alloying concentration were found most beneficial to improve the impurity removal.

Planning of intensified production of solar grade silicon to yield solar panels involving behavior of population

The need to use clean sources for producing electricity has been a great topic of discussion in recent years. Many countries have begun to take advantage of solar energy through the production of solar panels for electric power generation. In addition, knowing people's preferences allows predicting their behavior to propose better planning at macroscopic level. This work presents a mathematical programming model to address the conduct of people. The objective is to know their inclinations using the matching law, which involves the actions of users through different scenarios considering economic incentives and punishments. It includes a strategic planning of the production and distribution of solar grade silicon that is used for the construction of solar panels to meet the demands of electricity in residential sector of Mexico as a case study. Process intensification was used to enhance different ways to obtain solar grade silicon such as an Intensified Fluidized Bed Reactor and a hybrid, which is a combination of Siemens and conventional FBR processes. Also, the Siemens process was considered the most common process to produce silicon. Results show that the difference between the analyzed scenarios lies in the behavior of people while profit maintains constant.

Vacuum refining of silicon at ultra-high temperatures

In the production of solar grade silicon (SoG-Si), Phosphorus (P) removal from Si is a challenge as it cannot be eliminated effectively by the final key directional solidification step in the value chain of the silicon solar cell technologies. The present research investigates the application of ultra-high temperatures (UHTs) up to 1900 °C for the P removal from Si in the vacuum-induction refining (VIR) process. Kinetic parameters such as mass transfer coefficient and activation energy for P removal from Si melt by vacuum evaporation are determined. It is shown that the P removal kinetics is significantly accelerated with increasing temperature, and about 1800 °C is a critical temperature in which the process rate is doubled. The silicon loss of the process to reach SoG-Si quality is formulated, and it is shown that it is lower at ultra-high temperatures, while it is insignificantly increased with the temperature rise in each temperature regime, and is independent of the melt geometry. The results from UHTs experiments showed complete phosphorus removal from silicon melts with even as high as P concentrations as 92.71 ppmw in short durations. It is shown that the application of UHTs in VIR process reduces the power consumption to reach SoG-Si.

Simultaneous structural and operating optimization of process flowsheets combining process simulators and metaheuristic techniques: The case of solar-grade silicon process

This paper presents a new optimization approach for the simultaneous structural optimization of process flowsheets with the operating conditions through combining process simulators with metaheuristic techniques. The proposed approach allows optimization of a superstructure in process simulators and reduce the computation time. A superstructure for different configurations for producing solar-grade silicon is considered, which includes three different configurations for solar-grade silicon production (Siemens Process, Intensified FBR Union Carbide Process, and Hybrid Process). The operating conditions with major impact in the performance of each of the proposed configuration were considered as decision variables. The improved multi-objective differential evolution (I-MODE) algorithm was selected as search method from others metaheuristic techniques because its efficiency to solve multi-objective problems in a short central process unit (CPU) time. The optimization algorithm consists in linking the process simulator software Aspen PlusTM with the metaheuristic technique. The results offered attractive options for the considered objective functions in the addressed case study.

Inherent occupational health hazards in the production of solar grade silicon

Solar energy has become one of the most developed renewable energy sources in recent years. As with any energy source or product, there are health risks associated with the manufacturing of solar cells. And even though the photovoltaic industry uses far lesser amounts of toxic and flammable substances than many other industries, the use of hazardous chemicals can present occupational and environmental hazards. One of the most important aspects in the selection of new processes lies in the protection of workers’ health. Health risks can be reduced if a process is chosen properly and in preliminary phases. Since we have found that it is necessary to carry out an evaluation of the health risks to workers in the production of polycrystalline silicon for the manufacturing of photovoltaic cells, in this work we will use the Process Route Healthiness Index to quantify the health risk that each silicon production process represents (the higher the index, the higher the hazards). The polycrystalline silicon production processes evaluated with the healthiness index are: Siemens Process, Intensified Fluidized Bed Reactor Union Carbide Process, and Hybrid Process. Our results show that the Siemens Process is the healthiest process, but with the Process Route Healthiness Index values are closer to the Hybrid Process. Apart from this, a guide to the assessment of inherent occupational health hazards in SiSG production processes was also developed, which provides results alike those to the PHRI methodology.

Separation and Recovery of Refined Si from Al-Si Melt by Modified Czochralski Method.

Separation of refined silicon from Al–Si melt is still a puzzle for the solvent refining process, resulting in considerable waste of acid and silicon powder. A novel modified Czochralski method within the Al–Si alloy is proposed. After the modified Czochralski process, a large amount of refined Si particles was enriched around the seed crystalline Si and separated from the Al–Si melt. As for the Al–28%Si with the pulling rate of 0.001 mm/min, the recovery of refined Si in the pulled-up alloy (PUA) sample is 21.5%, an improvement of 22% compared with the theoretical value, which is much larger 1.99 times than that in the remained alloy (RA) sample. The content of impurities in the PUA is much less than that in the RA sample, which indicates that the modified Czochralski method is effective to improve the removal fraction of impurities. The apparent segregation coefficients of boron (B) and phosphorus (P) in the PUA and RA samples were evaluated. These results demonstrate that the modified Czochralski method for the alloy system is an effective way to enrich and separate refined silicon from the Al–Si melt, which provide a potential and clean production of solar grade silicon (SoG-Si) for the future industrial application.

Boron removal for solar-grade silicon production by metallurgical route:A review

Solar energy has received considerable attention over the past few decades, due to its importance as a green and renewable energy. Low-cost solar-grade silicon production is critical for the widespread use of solar cells. Conventional routes (e.g., modified Siemens process: chlorosilane and hot filament) have still dominated the production of solar-grade silicon. The metallurgical route offers benefits in the productivity and cost, but efficient removal of boron is one of the most daunting challenges in front of us. This paper reviews thermodynamic and kinetic properties (solubility, diffusivity, diffusion coefficients, mass transfer rate, and activity coefficient) of boron and recent research topics (slag treatment, solvent refining, gas injection, plasma treatment, and acid leaching) for boron removal.

Approaches to the development of environmentally friendly and resource-saving technology for solargrade silicon production

ABSTRACTCurrently, the main material for the production of solar cells is still silicon. More than 70% of the global production of solar cells are silicon based. For solar-grade silicon production the technologies based on the reduction of silicon from organosilicon compounds are mainly used. These technologies are energy-consuming, highly explosive and unsustainable.The present paper studies the technology of purification of metallurgical-grade silicon by vacuum-thermal and plasma-chemical treatment of silicon melt under electromagnetic stirring using numerical simulation and compares this technology with the existing ones (silane technologies and Elkem Solar silicon (ESS) production process) in terms of energy consumption, environmental safety and the process scalability.It is shown that the proposed technology is environmentally safe, scalable and has low power consumption. The final product of this technology is multicrystalline silicon, ready for silicon wafer production.

Safety, Economic, and Environmental Optimization Applied to Three Processes for the Production of Solar-Grade Silicon

In this work, we present the multiobjective optimization of the Siemens Process, the Intensified FBR Union Carbide Process, and the Hybrid Process (the three processes to obtain solar-grade silicon...

Production of high purity silica by microfluidic-inclusion fracture using microwave pre-treatment

Demand for high purity silica used in component manufacture is set to outstrip current supply in the near future. As such, alternative processing routes to feed-stock materials suitable for use in lighting and solar cell fabrication are required, without having to rely on reject material from semi-conductor manufacture. In this work, we report a facile, environmentally friendly method of producing quartz powder with a total residual impurity level of 30 ± 3 ppm from whole pebbles having an initial impurity level of 158 ± 22 ppm. This has been achieved using a metallurgical upgrading process incorporating microwave pre-treatment, crushing and milling, High Intensity Wet Magnetic Separation (HIWMS) and acid leaching. This process yielded a quartz powder having an 80% reduction in residual impurities compared to the untreated quartz pebbles. Pre-treatment of whole quartz pebbles in a multimode microwave cavity for 10 min yielded a reduction of the residual elemental impurity content associated with micro-fluidic inclusion sites containing calcium, potassium and sodium of 84, 78, and 50% respectively. Statistically significant reduction in residual aluminium phases was also observed (83%) compared to the as received material to below the IOTA® specification for Ultra High Pure Quartz produced by Sibleco. Mechanistically, this has been achieved by selectively heating impurity containing micro-fluidic inclusion sites. Resulting in their explosive decrepitation and enabling removal of the impurities in subsequent processing steps. It has been concluded that natural quartz pebbles can be upgraded through a combination of microwave treatment, magnetic and chemical refinement to produce a viable feedstock for the subsequent production of solar grade silicon.

Purification of Diatomaceous Earth Using Leaching and Liquid-Liquid Extraction Processes to Produce High Purity Silica for Solar-Grade Silicon

Recently, the demand for solar-grade silicon (SOG-Si) with 6-7N (99.9999~9%) level of purity is dramatically increasing due to its popularity of photovoltaic power generation. The conventional process, Siemens method, is majorly used for SOG-Si production, which, however, consumes huge amount of energy and time. So, there is a need to develop an alternate production process for SOG-Si. As the cost-effective process, we are now focusing on a wet chemical process for purification of silica, followed by electrochemical reduction of silica to SOG-Si [1]. This purification process consists of two main steps: (i) acid leaching including dissolution & precipitation treatment of diatomaceous earth as an abundant Si source and (ii) liquid-liquid extraction for precise elimination of lighter element impurities that are possible to influence the semiconductor property of Si. Aim of the present work is to increase the extraction efficiency of each process to be optimized and combining them to complete the whole process. First, we optimized the acid leaching process. This process performs pH adjustments of strong alkaline solution with diatomaceous earth dissolved to separate impurity-rich phase and silica. It has been shown that more than 90% of the heavy element (Fe, Al, etc.) impurities precipitate by decreasing pH [2]. However, since NaOH was used in the preparation of initial alkaline solution, much amount of Na impurities turned out to be included in the final silica product. For this, we employed the mixture of NaOH and Tetramethylammonium hydroxide (TMAH) to lower the concentration of Na in the alkaline solution preparation. Experimental results suggested that some amount of Na ion was necessary to be included in the solution in order to provide sufficient dissolution rate of silica. This led us to the optimal concentration of NaOH and TMAH in the mixture: 0.05 M NaOH and 1.40 M TMAH. Further purification, experiment results concluded that this mixture effectively prevented the inclusion of Na impurity, resulting in 5N level of purity when the process was repeated 3 times. In spite of the achievement shown above, the lighter element, in particular, B is hard to be eliminated from the leaching process, because the pH dependence of solubility of B is similar to that of Si. To further eliminate this impurity, the liquid-liquid extraction process is employed. Since our previous results indicated that the effective mixing of aqueous and organic phase leads to the best efficiency of B elimination, we first designed flow type reactor with 3D printing technology. The reactor was designed to have 2 characteristic structures: a throat like structure that shows gradually diverging part after converging (converging/diverging structure) near Y shaped inlets and obstacles structure on downstream [3]. The high-speed camera demonstrated the reactor offered a fully dispersed flow regime, which leaded to the efficient solution mixing. Based upon these results, we carried out a model experiment, using silica powder of 99.9 % purity with trace amount of B was dissolved in NaOH solution (aqueous phase) and injected into one of the inlet of the liquid-liquid extraction reactor. The extractant, 2,2,4-trimethyl-1,3-pentanediol (TMPD) solution (organic phase) was injected into the other inlet. The extraction was repeated 5 times at 380 ml/min and the efficiency was found to be 99.7% leaving less than 0.1 ppm of B in silica (7N purity). From the above results, it is concluded that we have achieved efficient process to prepare 7N level silicon, by combining optimized leaching and liquid-liquid extraction with designed reactor. This could be a promising method to produce SOG-Si with lower cost than the present process at industries.

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.