Why are photovoltaic cells made of sand and gravel?

Why are photovoltaic cells made of sand and gravel?

  1. Why is it said that photovoltaic cells are produced using sand and gravel?

There are many kinds of photovoltaic cells produced now, but more than 90% are monocrystalline silicon photovoltaic cells or polycrystalline silicon photovoltaic cells. Both monocrystalline silicon and polycrystalline silicon are produced from industrial silicon, and the raw material of industrial silicon is sand and gravel. Therefore, photovoltaic cells are produced with sand and gravel as raw materials, but in order to ensure product quality, reduce costs and simplify the process, the general industry uses not ordinary sand and gravel, but some sand and gravel with high silicon content. These sands are commonly known as silica, but the names are different, and some are also called quartz sand (see Figure 1).

Figure 1 - Silica
Figure 1 – Silica

In fact, the composition of silica is not elemental silicon, but silicon dioxide. To produce photovoltaic cells, oxygen must be removed from silicon dioxide and reduced to elemental silicon. The industrial approach is to use silica and carbon as raw materials, and in an electric arc furnace, the following chemical reactions occur between silica and carbon (2):

The affinity of carbon to oxygen increases with the increase of temperature. When the temperature is higher than 1400℃, the affinity of carbon to oxygen will be greater than that of silicon. At this time, the oxygen in the quartz sand will be continuously taken away by the hot carbon. , and the silicon dioxide that loses oxygen will gradually become elemental silicon. This elemental silicon is called industrial silicon (see Figure 3).

Photovoltaic cells have high requirements on the purity of silicon, generally above 6N, while the highest purity of industrial silicon is only 2N, so it must be purified.
There are many purification methods, but the most commonly used in industry is the modified Siemens method. That is, the industrial silicon is first chlorinated to become trichlorosilane. Its method is to make industrial silicon and hydrogen chloride react as follows (4):

Figure 3 - Industrial Silicon
Figure 3 – Industrial Silicon

The obtained trichlorosilane is purified by chemical purification methods such as rectification or adsorption, and after the purity meets the requirements, the following reaction (5) occurs with hydrogen:

The silicon obtained by the reaction is polysilicon. The purity of polysilicon here is above 6N, which can be used for photovoltaic cell production. If it is to produce polycrystalline silicon photovoltaic cells, the polycrystalline silicon can be ingots and sliced, and then processed through texturing, diffusion, etching, coating, screen printing, sintering and other processes to produce polycrystalline silicon photovoltaic cells.

In the case of monocrystalline silicon photovoltaic cells, polycrystalline silicon has to be drawn into monocrystalline silicon and then sliced. The sheet-like monocrystalline silicon is then subjected to texturing, diffusion, etching, coating, screen printing, sintering and other processes to produce monocrystalline silicon photovoltaic cells. The production process of monocrystalline silicon and polycrystalline silicon photovoltaic cells is shown in Figure 6.

Figure 6 - Production flow of monocrystalline and polycrystalline silicon photovoltaic cells
Figure 6 – Production flow of monocrystalline and polycrystalline silicon photovoltaic cells
  1. What are the methods for reducing silica to silicon?

There are several ways to reduce silica to silicon.
(1) Elemental element silicon can be obtained by reducing silicon tetrafluoride with potassium metal. This is the invention of Swedish chemist Bai Zeli in 1822, and it is also the first method for human beings to obtain elemental silicon.

(2) Use alkali metals such as magnesium and aluminum as reducing agents, and use quartz sand as raw materials to produce industrial silicon. If the raw materials and reducing agents are pure, industrial silicon with 3 to 4 “9s” can be obtained.
(3) Using sodium, potassium and other alkali metals and calcium hydride to reduce silicon halide and fluorosilicate If the purity of raw materials and reducing agents are high, this method can also produce relatively pure industrial silicon.

(4) Carbon reduction method The raw materials used in this method are mainly silica and carbon, and carbon is the reducing agent. During production, silica and carbon are chemically reacted in an electric arc furnace. When the temperature is higher than 1400 ℃, the affinity of carbon for oxygen will be greater than that of silicon, so carbon will take oxygen from silica and generate carbon dioxide. Carbon dioxide is gaseous and less dense, so it will run upwards and run away from the top of the electric arc furnace. Silica in silica loses oxygen and is reduced to silicon. This reduced silicon is called industrial silicon.

The carbon reduction method is currently the method used for large-scale industrial silicon production. Industrial silicon used in general photovoltaic cell production is produced in this way. The purity of industrial silicon produced by this method is about 2 “9”, and its main impurities are iron, aluminum, carbon, boron, phosphorus, etc., of which iron content is the most. Therefore, people usually refer to industrial silicon as ferrosilicon. The advantages of this method are that it is simple, economical, has a large production capacity, and is suitable for industrial production.

  1. What is the specific process of the carbon reduction method?

The specific flow of the carbon reduction method is as follows.

(1) Raw material preparation In order to ensure product quality, industrial silicon must have strict requirements on raw materials, not all sand and gravel can be used as raw materials, so the production of industrial silicon, first of all, is the selection of raw materials to ensure that the silica entering the electric arc furnace must be qualified ( Sand and gravel with high silicon content) must not have obvious dirt, let alone other debris mixed in it. The selected raw material silica should be pulverized to ensure a certain appropriate particle size, and in order to ensure the quality, it should also be washed and screened. Because moisture affects the quality of industrial silicon, it needs to be dried. Only after drying can it truly become the raw material for the production of industrial silicon. In addition to silica, the production of industrial silicon also requires carbon, and the carbon also needs to be dried. Carbon is a reducing agent and is also very important.

(2) Ingredients After the raw materials are prepared, the next step is to weigh the proportions, and then send them to the electric arc furnace after mixing. At present, the industrialized large-scale production of industrial silicon has been automated, so the above processes are all carried out continuously.

(3) Smelting After the raw materials are fed into the furnace, the carbon electrode is inserted into the charge composed of carbonaceous reducing agent and silica, and then heated. At 1600~1800°C, carbon and silicon react chemically, carbon takes away oxygen in silica and reduces silicon.
(4) Refining Refining is to add oxygen to the reduced silicon, which can remove impurities such as calcium (Ca) and aluminum (Al) in silicon.

(5) Sampling and analysis The purpose of sampling and analysis is to check the quality of the product and classify it according to the standard.
(6) Silicon ingot At this time, the silicon is liquid, which can be cast into silicon bonds. When it is cast into ingots, it becomes the finished industrial silicon, which can also be used for the production of polysilicon. But it also needs to be pulverized before polysilicon can be produced.

(7) Waste gas treatment During production, the waste gas discharged from the electric arc furnace is dedusted, and the silicon powder in it can be recovered. The silicon powder is called micro-silica powder, which is an industrial raw material, which can be returned to the furnace and used in other productions. Silica fume can be used to produce refractory materials and catalysts, and can also be used as fillers for rubber. It can also be used as an admixture of concrete and can improve various properties of concrete.
In the process of waste gas treatment, in addition to obtaining a large amount of microsilica, a large amount of waste heat can also be obtained. The waste heat can be used for power generation and also for heating in winter.

Read more: What are solar and photovoltaic cells?

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