1.1 Classification of solar cell materials
In order to make effective use of solar energy, we must first find materials for making solar cells. The material for making solar cells is the fastest growing and most dynamic research field in recent years, and it is one of the most watched projects. The production of solar cell materials is mainly based on semiconductor materials. According to the different materials used, solar cells can be divided into silicon solar cells, multi-compound thin film solar cells, polymer multilayer modified electrode solar cells, nanocrystalline solar cells, and organic solar cells. Five categories of hybrid solar cells. Among them, silicon-based solar cells are currently the fastest growing and most mature solar cells, occupying a dominant position in applications.
1.1.1 Multicomponent compound thin film solar cells
The materials for multi-component thin-film solar cells are inorganic salts, which mainly include gallium arsenide III~V compounds, cadmium sulfide, cadmium telluride, gallium arsenide, copper indium selenium batteries, etc. Cadmium telluride thin film batteries and copper indium gallium selenium thin film batteries are both multi-element compound thin film batteries, and their photoelectric conversion rate is significantly higher than that of amorphous silicon thin film batteries. First solar, the world’s largest listed solar energy company by market value, is a typical representative of cadmium telluride thin film battery technology. In 2009, First Solar’s thin-film battery production reached 1.1GW, ranking first among global photovoltaic cell manufacturers. The photoelectric conversion rate of its photovoltaic cells increased to
11.1%, the production cost dropped to 84 cents/W. Although compared with polycrystalline silicon photovoltaic cells, the photoelectric conversion rate of cadmium telluride thin-film cells is still significantly lower, but its material consumption, production cost, manufacturing energy consumption and average selling price are much lower than the former. First Solar’s production cost and product gross profit margin are significantly better than manufacturers using crystalline silicon battery technology. However, cadmium is highly toxic and can cause serious pollution to the environment. Therefore, it is not the most ideal substitute for crystalline silicon solar cells.
The conversion efficiency of the gallium arsenide compound battery can reach 28%. The gallium arsenide compound material has a very ideal optical band gap and high absorption efficiency, strong anti-irradiation ability, and is not sensitive to heat. It is suitable for manufacturing high-efficiency single-junction solar energy. battery. However, the price of gallium arsenide materials is expensive, which limits the popularity of gallium arsenide batteries to a large extent. Copper-indium-selenium thin-film batteries (CIS) have the same conversion efficiency as polysilicon. They do not have the problem of light-induced degradation. They have the advantages of low price, good performance and simple process. The disadvantage is that indium and selenium are relatively rare elements. Therefore, this The development of similar batteries is bound to be restricted.
1.1.2 Polymer multilayer modified electrode solar cell
Polymer organic materials have good flexibility, easy production, wide material sources, low cost, etc., which are of great significance to the large-scale utilization of solar energy and the provision of cheap electricity. The principle of polymer multi-layer modified electrode solar cells is to use the different redox potentials of different redox polymers to perform multi-layer composite on the surface of conductive materials (electrodes) to make unidirectional conductive devices similar to inorganic PN junctions. The inner layer of one electrode is modified by a polymer with a lower reduction potential, and the reduction potential of the outer polymer is higher, and the direction of electron transfer can only be transferred from the inner layer to the outer layer; the modification of the other electrode is just the opposite, and the first The reduction potentials of the two polymers on each electrode are higher than the reduction potentials of the latter two polymers. When the two modified electrodes are placed in the electrolyte containing the photosensitizer. The electrons generated after the photosensitizer absorbs light are transferred to the electrode with the lower reduction potential. The electrons accumulated on the electrode with the lower reduction potential cannot be transferred to the outer polymer, and can only be returned through the external circuit through the electrode with the higher reduction potential. Electrolyte, so photocurrent is generated in the external circuit.
1.1.3 Nanocrystalline TiO2 chemical solar cell
The working principle of nanocrystalline TiO2: the dye molecule absorbs solar energy and transitions to an excited state. The excited state is unstable. The electrons are quickly injected into the adjacent TiO2 conduction band. The electrons lost in the dye are quickly compensated from the electrolyte and enter the TiO2 conduction band. The electricity finally enters the conductive film, and then generates a photocurrent through the external circuit.Take the dye-sensitized nanocrystalline solar cell as an example. This battery mainly includes a glass substrate coated with a transparent conductive film, a dye-sensitized semiconductor material, an electrode, and an electrolyte. Wait a few parts.
The advantages of nanocrystalline TiO2 solar cells are its low cost, simple process and stable performance. Its photoelectric efficiency is stable at more than 10%, and the production cost is only 1/5 to 1/10 of that of silicon solar cells. The life span can reach more than 20 years.
However, the research and development of nanocrystalline chemical solar cells have just started, so they have not yet entered the market.
1.1.4 Organic hybrid solar cells
According to the American Physicists Organization Network, the Cavendish Laboratory of the Department of Physics at the University of Cambridge in the United Kingdom has developed an organic hybrid solar cell that can increase the maximum photoelectric conversion rate by more than 25%. Solar cells are mainly based on semiconductor materials, which use photons in photoelectric materials to absorb light energy and then undergo photoelectric conversion reactions to generate electrical energy. Traditional solar cells can only absorb part of the visible light to near red light in the solar spectrum. As the temperature increases, many blue photons’ energy is lost. This ability to absorb light of different colors at one time determines that its photoelectric conversion rate will not exceed 34%.
A research team led by Professor Neal Greenham and Sir Richard of the University of Cambridge has developed an organic hybrid battery that can absorb red light while using additional blue light energy to generate greater current. Normally, solar cells can make one photon produce one electron. After adding an organic semiconductor to the solar cell, the solar cell can excite each photon to produce two electrons from the blue spectrum, thereby increasing the cell conversion efficiency to 44%. Organic hybrid solar cells can be mass-produced at low cost. But the cost of a solar power station is mostly reflected in the land, labor, and installation hardware. Therefore, even if organic solar panels are cheaper, they still need to improve their efficiency to make them more competitive. Although the photoelectric conversion rate of organic hybrid solar cells can reach more than 25%, it is still in the experimental stage.
1.1.5 Silicon solar cells
Silicon-based solar cells can be roughly divided into the following categories.
(1) Monocrystalline silicon solar cells
At present, the photoelectric conversion efficiency of monocrystalline silicon solar cells is about 15%, and the highest is 24%6. This is the highest photoelectric conversion efficiency among all types of solar cells, and the technology is also the most mature. Because the single-product silicon is generally encapsulated with toughened glass and waterproof resin, it is durable and has a service life of up to 15 years, and up to 25 years. Monocrystalline silicon solar cell is currently the fastest developed solar cell. Its structure and production process have been finalized, and its products have been widely used in space and on the ground. This solar cell uses high-purity monocrystalline silicon rods as raw materials. In order to reduce production costs, solar cells used on the ground now use solar-grade monocrystalline silicon rods, and the material performance indicators have been relaxed. The disadvantage of monocrystalline silicon solar cells is that the production cost is very high.In addition, the drawn monocrystalline silicon rods are cylindrical, and the solar cells made by cutting the picture are also wafers, and the plane utilization rate of the solar modules is low.
(2) Polycrystalline silicon solar cells
The production process of polycrystalline silicon solar cells is similar to that of monocrystalline silicon solar cells, but the photoelectric conversion efficiency of polycrystalline silicon solar cells is lower than that of monocrystalline silicon. Polysilicon with an efficiency of 17% to 19.8%. In terms of production cost, polycrystalline silicon solar cells are cheaper than monocrystalline silicon solar cells. The materials are simple to manufacture, consume less power, and have lower production costs. Therefore, they have been extensively developed. In addition, the service life of polycrystalline silicon solar cells is shorter than that of monocrystalline silicon solar cells. The production of polycrystalline silicon solar cells needs to consume a lot of high-purity silicon materials, and the manufacturing process of these materials is complicated, and the power consumption is very large, which has exceeded 1/2 of the total cost of solar cell production.
(3) Amorphous silicon thin film solar cells
Amorphous silicon thin-film solar cells are also called silicon-based thin-film solar cells. They mainly use amorphous silicon (a-Si, amorphous silicon) as the photosensitive material. The production method is completely different from that of monocrystalline silicon and polycrystalline silicon solar cells. The process can be greatly simplified. It has the advantages of light weight, low silicon consumption, low production cost, good response to low light, and good high temperature characteristics. However, it also has short life and poor stability (with the extension of time, its conversion efficiency gradually Attenuation), low photoelectric conversion rate and other shortcomings.
(4) Amorphous silicon thin film battery
Amorphous silicon thin-film batteries are cheap and easy to form large-scale production, but the photoelectric conversion rate is low and the stability is not as good as that of crystalline silicon.
(5) Solar grade silicon battery
In just a few years, China has become a major country in the production and application of crystalline silicon solar cells. In the world, especially the United States, Germany, Japan, India and Russia, the production of crystalline silicon solar cells has also increased sharply, which has led to a sharp increase in the demand for silicon materials. According to the current production technology level of China’s photovoltaic industry, about 15 tons of silicon materials are needed to produce MW silicon solar cells. If China produces 100MW silicon solar cells, it needs about 1500t of silicon material (if the thickness of the silicon wafer can be cut to 200~250μm), which is far greater than China’s silicon material supply capacity. Therefore, great efforts must be made to solve the production and supply problems of silicon materials. R&D and production of solar-grade silicon is an important solution. The impurity content of metallurgical grade silicon is too high, which affects the photoelectric conversion efficiency of the battery. If it can be purified by simple chemical or physical methods, it can be used to manufacture solar cells, which will significantly reduce the cost of solar cells. The silicon made by this method is called “solar grade silicon.” It is generally believed that a silicon material with a photoelectric conversion efficiency of 10% can be manufactured inexpensively, and it can be called “solar grade silicon”. The world’s most produced and applied solar cells are polycrystalline silicon solar cells, monocrystalline silicon solar cells and amorphous silicon solar cells. In particular, monocrystalline silicon solar cells and polycrystalline silicon solar cells account for more than 90% of the total output of solar cells in the world today. They have mature technology, stable and reliable performance, high photoelectric conversion efficiency and long service life, and they have entered industrialized mass production. Therefore, mainly analyze the three types of polycrystalline silicon thin film solar cells, monocrystalline silicon solar cells and amorphous silicon thin film solar cells.