In the previous article “Photovoltaics: Materials Used and How Their Efficiency and Cost Can Be Improved”, we discussed the growth of the photovoltaics market. We found out that the main driver for this growth was the sinking costs for photovoltaic cells.
Given that the vast majority of photovoltaic cells are made with silicon (94% as of 2015 according to the US Department of Energy), decreasing the costs of silicon production is essential for the photovoltaics market. In order to achieve this objective, research and development efforts are focused on the reduction of energy consumption and manufacturing losses, which also helps to minimize the ecological footprint of photovoltaic energy.
In this article, we will present the two main silicon production methods – the Siemens process and the Metallurgical route – and explain how energy consumption differs throughout them.
Different Purity Grades of Silicon
Silicon is purified in different steps starting from the raw material, which is usually quartz sand. Different processes lead to different levels of energy consumption and different levels of purity. The most common impurities are Fe, Al, Ti, Mn, C, Ca, Mg, B, P. The standards in concentration limits vary according to the elements and the authors for the solar grade but we can sum up the overall concentration limits as:
Producing Metallurgical Grade Silicon
In most cases, the silicon used for solar cells is derived from metallurgical grade silicon. This metallurgical grade silicon is produced by reducing silica from quartz sand with carbon: SiO2(l)+2C(s)=Si(l)+2CO(g).
Next, carbon electrodes are often used in a furnace to create temperatures of around 2000°C, so that many impurities become volatile. Liquid silicon is then collected at the bottom of the furnace, drained and cooled, resulting in metallurgical grade silicon.
The Main Routes for Silicon Production
The two most important chains of processes to purify silicon from metallurgical grade silicon (MG-Si) are the Siemens process and the Metallurgical route:
The Siemens Process
The Siemens Process is the dominant process for the production of silicon for electronics and
photovoltaics from MG-Si. It similar to distillation, so that silicon reacts with HCl gas in a reactor at about 300°C: Si(s)+3HCl(g)=SiHCl3(g)+H2(g).
Then the trichlorosilane gas will thermally decompose on heated silicon rods (>1300°) in a hydrogen atmosphere: SiHCl3(g)+H2(g)=Si(s)+3HCl(g).
The silicon produced with this process is going to be very pure electronic-grade silicon (EG-Si). But it comes with very high energy costs and with important security measures due to the handling of hydrogen and hydrochloric acid. Such expenses are necessary to fit the requirements of the electronics industry.
However, the purity requirements for the silicon for solar cells are less strict. Thus, less expensive and less energy intensive ways are currently being developed and used to produce silicon for the photovoltaic market. They are usually grouped under the category: “Metallurgical route”.
The Metallurgical Route
The term “Metallurgical route” groups different purification methods where the silicon stays at liquid or solid Si phase during the purification process. It’s made of a cycle of different steps:
Most impurities can be removed through a process called directional solidification. At thermodynamical equilibrium between the solid phase and the liquid phase of silicon, impurities tend to concentrate in the liquid phase. The silicon melt is heated electromagnetically at fusion temperature. As a result, the upper part of the crucible is filled with electromagnetically stirred liquid silicon whereas the lower part is filled with solid silicon.
The process is regulated in such a way that the solidification front moves upwards. This tends to concentrate the impurities more and more at the upper part of the crucible. The upper part of the melt is going to be removed after the silicon has cooled down entirely. It is thus important to optimize the process in such a way as to minimize the losses of silicon to be removed and the energy consumption.
This can be achieved by concentrating as much as possible the impurities at the top of the crucible and by heating and stirring the melt in a shorter time. For this purpose, the combination of experiments and numerical simulations are critical.
This process works well for all impurities except boron and phosphorus which need other types of processes.
Vacuum removal of phosphorus
This process is based on the fact that under vacuum, phosphorus tends to volatilize much more than silicon from liquid silicon. Silicon is being electromagnetically heated to a liquid state and stirred under vacuum. The challenges are to obtain a sufficient vacuum and the right thermodynamical data to calibrate the process properly.
Boron removal with gas and plasma
Two similar processes are being used and researched to remove boron: plasma and cold gas processes. In both cases, silicon is electromagnetically heated to the liquid state and stirred to homogenize the silicon in temperature and composition. Oxygen and hydrogen atoms are going to react with the boron in the liquid silicon, which is going to be removed predominantly in the form of HBO(g). The silicon atoms are also going to react with the oxygen atoms but to a lesser extent. This decreases the boron concentration in the liquid silicon.
The challenges are to minimize the energy consumption (by minimizing the heating time) and the silicon losses. In this regard, the main areas of research are simulations and modeling of the complex transport phenomena involving silica aerosols, plasma modeling and control of the oxygen concentration in the bath, as well as increasing the precision of the thermodynamical data.
Relative to cold gases, plasma processes increase the energy consumption per time unit, due to the high temperatures, but decrease some problems of obstruction by silica aerols.
Removing impurities with slag refining
Slag refining is a process that competes with the other processes presented for boron removal. In this process, a slag usually made of CaO-SiO is put on the top of heated liquid silicon. It absorbs the boron from the liquid silicon.
Compared to the cold gas and plasma processes, the advantage is that other impurities are removed. However, the disadvantage is the consumption of large quantities of slag that must be clean from impurities, especially from boron. Current research is focusing on different slag compositions and thermodynamical data.
From Solar-Grade Silicon to Solar Panels
The silicon is crystallized into a single crystal (higher performance) or into a multicrystalline ingot. The crystallization into monocrystals directly in the furnace is being investigated as this method could reduce costs for more efficient monocrystal silicon photovoltaic panels.
The last step is cutting, and that’s where a lot of silicon losses happen. The silicon rods are cut in thin slices with different techniques, such as with laser or diamond blades. This causes further silicon losses which have to be recycled. Also, the slices from the cylinder rods have to be square-shaped which causes further losses. Current research is focusing on giving a rectangular shape to the silicon rods to minimize those losses, which would reduce the cost of the final solar panels.
Silicon refining is a large and very active field of research which has a major potential to reduce the costs and ecological footprint of photovoltaic energy. At each stage of the silicon production process for photovoltaic cells, there are areas where manufacturing losses and energy requirements can be reduced.
The Siemens process, producing very pure silicon, is still dominant. However, the Metallurgical route continues to gain ground because it’s generally cheaper and offers the greatest potential for energy and costs savings in the production process.
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 CECCAROLI, Bruno, OVRELID, Eivind, et PIZZINI, Sergio (ed.). Solar silicon processes: technologies, challenges, and opportunities. CRC Press, 2016
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