Solar cell working principle introduction
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Solar cells can be divided into: 1, silicon solar cells; 2, the inorganic salt such as gallium arsenide III-V compounds, cadmium sulfide, copper indium selenium and other compounds as the battery material; 3, the functional polymer material preparation of solar cells ; 4, nanocrystalline solar cells and so on.
No matter what kind of material is used to make batteries, the general requirements for solar cell materials are: 1. The bandgap of semiconductor materials cannot be too wide; 2. The photoelectric conversion efficiency should be high: 3. The material itself does not cause pollution to the environment; 4, the material is easy to industrial production and material properties and stability.
Based on the above considerations, silicon is the most ideal solar cell material. This is also the main reason why solar cells are dominated by silicon materials. However, with the continuous development of new materials and the development of related technologies, solar cells based on other villages are increasingly showing attractive prospects. This article briefly reviews the types of solar cells and their research status, and discusses the development and trends of solar cells.
1 silicon solar cell
1.1 Monocrystalline silicon solar cells Silicon solar cells have the highest conversion efficiency and the most mature technology.
High-performance single-crystal silicon cells are based on high-quality monocrystalline silicon materials and related heat-generating processing processes. Nowadays, the electrical ground technology of single-crystal silicon has been almost mature. In the production of batteries, surface texturing, passivation of emission regions, and zone doping techniques are generally used. The batteries developed mainly include planar monocrystalline silicon cells and etched trenches. Gate electrode monocrystalline silicon battery. To improve the conversion efficiency is mainly the monocrystalline silicon surface microstructure processing and zone doping process. In this respect, the Fleurieu Institute of Solar Energy Systems in Flandrehof, Germany, maintains a world-leading level. The Institute used photolithographic techniques to texture the surface of the cell to create inverted pyramid structures. And put a 13nm on the surface. Thick oxide passivation layer combined with two antireflection coatings. The ratio of the width and height of the gate is increased by the improved plating process: the conversion efficiency of the battery obtained by the above is over 23%, which is a large value of up to 23.3%.
The conversion efficiency of a large-area (225cm2) single crystal solar cell prepared by Kyocera Co., Ltd. was 19.44%. The Beijing Solar Energy Research Institute also actively researched and developed the high-efficiency crystalline silicon solar cell and developed a planar high-efficiency single crystal silicon cell (2cmX2cm). The conversion efficiency reached 19.79%, and the conversion efficiency of the trench buried gate electrode crystalline silicon cell (5cmX5cm) was 8.6%. The conversion efficiency of monocrystalline silicon solar cells is undoubtedly the highest. It still occupies a dominant position in large-scale applications and industrial production. However, due to the price of monocrystalline silicon materials and the corresponding cumbersome battery process, the cost of monocrystalline silicon is high. No less, it is very difficult to drastically reduce its cost. In order to save high quality materials and find alternatives to monocrystalline silicon cells, thin-film solar cells are now being developed, with polysilicon thin-film solar cells and amorphous silicon thin-film solar cells being typical examples.
1.2 Polysilicon thin-film solar cells A typical crystalline silicon solar cell is fabricated on a high-quality silicon wafer with a thickness of 350 to 450 μm. This silicon wafer is sawn from a pulled or cast silicon ingot.
Therefore, more silicon material is actually consumed. In order to save materials, people began to deposit polysilicon films on inexpensive substrates since the mid-1970s, but due to the grain size of the grown silicon films, valuable solar cells could not be made. In order to obtain thin films of large size, people have not stopped researching and proposed many methods.
At present, polycrystalline silicon thin-film batteries are mostly prepared by chemical vapor deposition methods, including low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD) processes. In addition, liquid phase epitaxy (LPPE) and sputter deposition methods can also be used to prepare polysilicon thin film batteries. Chemical vapor deposition mainly uses SiH2Cl2, SiHCl3, SiCl4, or SiH4 as reaction gases, reacts under certain protective atmosphere to generate silicon atoms and deposits on heated substrates. Substrate materials generally use Si, SiO2, Si3N4, and the like. However, it has been found that it is difficult to form large grains on non-silicon substrates and easily form gaps between crystal grains. The solution to this problem is to use LPCVD to deposit a thin layer of amorphous silicon on the substrate and then anneal this layer of amorphous silicon to get larger grains and then on the seed. Deposition of thick polysilicon film, therefore, recrystallization technology is undoubtedly a very important part of the current technology used mainly solid-phase crystallization and the middle zone remelting recrystallization. In addition to the recrystallization process, polysilicon thin-film batteries also employ almost all techniques for the preparation of monocrystalline silicon solar cells, which significantly improves the conversion efficiency of solar cells.
The conversion efficiency of polysilicon cells fabricated on the FZSi substrate using the district hall recrystallization technology in Freiburg Institute of Solar Energy, Germany, was 19%. Japan Mitsubishi Corporation used this method to prepare batteries with an efficiency of 16.42%. The principle of the liquid phase epitaxy (LPE) method is to melt the silicon in the matrix and lower the temperature to precipitate the silicon film. US Astropower Corporation uses LPE to produce a battery efficiency of 12.2%. Chen Zheliang of the China Optoelectronics Development and Technology Center used silicon-phase epitaxy to grow silicon grains on a metallurgical-grade silicon wafer and designed a new type of solar cell similar to a crystalline silicon thin-film solar cell called "Silicon Grain" solar energy. Battery, but reports about performance have not been seen. Polycrystalline silicon thin-film batteries use less silicon than single-crystal silicon, and have no efficiency degradation issues. They may be fabricated on inexpensive substrate materials. The cost is much lower than that of single-crystal silicon cells, and the efficiency is higher than that of amorphous silicon films. Batteries, therefore, polysilicon thin-film batteries will soon dominate the solar-electricity market.
1.3 Amorphous Silicon Thin Film Solar Cells Two key issues in the development of solar cells are: improving conversion efficiency and reducing costs.
Due to the low cost of amorphous silicon thin-film solar cells and their ease of large-scale production, they are generally valued by people and developed rapidly. In fact, as early as the early 1970s, Carlson et al. had begun to work on the development of amorphous silicon cells. In recent years, its development work has developed rapidly. At present, there are many companies in the world that are producing this kind of battery product.
Amorphous silicon as a solar energy material is a good battery material, but because of its optical band gap of 1.7 eV, the material itself is insensitive to the long wave region of the solar radiation spectrum, thus limiting the amorphous silicon solar cell. The conversion efficiency. In addition, its photoelectric efficiency will be attenuated with the continuation of the illumination time, namely the so-called photo-induced recession S-W effect, making the battery performance unstable. The solution to these problems is to prepare tandem solar cells, which are made by depositing one or more Pin subcells on the prepared p-, i-, and n-layer single junction solar cells. The key issues for tandem solar cells to improve conversion efficiency and solve the instability of single-junction cells are:
1 It puts together different materials with different band gaps and improves the spectral response range.
The i-layer of the top battery is thinner, and the electric field intensity generated by light changes little, ensuring that the photo-generated carriers in the i-layer are drawn out;
3 The carrier generated by the bottom cell is about half of the single cell, and the light-induced degradation effect is reduced.
4 tandem solar cells Each subcell is connected in series. Amorphous silicon thin-film solar cells can be prepared in many ways, including reactive sputtering, PECVD, LPCVD, etc. The reaction source gas is SiH4 diluted with H2, and the substrate is mainly glass and stainless steel sheet, and the amorphous silicon is made. The thin film can be made into single-junction cells and tandem solar cells through different cell processes. At present, the research of amorphous silicon solar cells has made two major advances: the conversion efficiency of amorphous silicon solar cells with the first and third laminated structures reached 13%, setting a new record; the annual production capacity of second and third stacked solar cells reached 5MW. The single-junction solar cell produced by United Solar Energy Corporation (VSSC) has the highest conversion efficiency of 9.3%, and the maximum conversion efficiency of the triple-band triple-stack cell is 13%. See Table 1 for the highest conversion efficiency in the small area (0 .25cm2) made on battery.
There have been reports in the literature that the conversion efficiency of single-junction amorphous silicon solar cells exceeds 12.5%, and Academia Sinica adopted a series of new measures, resulting in a conversion efficiency of 13.2% for amorphous silicon cells. Domestic research on amorphous silicon thin-film batteries, especially tandem solar cells, is limited. Nanyang University's Xinhua and others used industrial materials to prepare an a-back electrode with an area of ​​20X20cm2 and a conversion efficiency of 8.28%. Si/a-Si tandem solar cells. Amorphous silicon solar cells have great potential due to their high conversion efficiency, low cost and light weight. But at the same time, due to its low stability, it directly affects its practical application. If you can further solve the stability problem and increase the conversion rate, then, amorphous silicon solar energy battery is undoubtedly one of the major development products of solar cells.
2 multi-component thin film solar cells
In order to find substitutes for monocrystalline silicon cells, in addition to developing polysilicon and amorphous silicon thin-film solar cells, they have also continued to develop solar cells with other materials. Among them, there are mainly III-V gallium arsenide compounds, cadmium sulfide, cadmium sulfide, and copper gallium selenide thin film batteries. Among the above batteries, although the efficiency of cadmium sulfide and cadmium telluride polycrystalline thin film batteries is higher than that of amorphous silicon thin film solar cells, the cost is lower than that of single crystal silicon batteries, and it is also easy to produce on a large scale. However, since cadmium is extremely toxic, it will Serious pollution to the environment, therefore, is not the ideal replacement for crystalline silicon solar cells, gallium arsenide III-V compounds and copper indium selenide thin-film batteries because of its high conversion efficiency has been the people's attention. GaAs is a III-V compound semiconductor material, and its energy gap is 1.4 eV, which is exactly the value of high-absorption solar light. Therefore, it is an ideal battery material.
The preparation of III-V compound thin-film batteries such as GaAs mainly uses MOVPE and LPE technologies. The MOVPE method for preparing GaAs thin-film batteries is affected by various parameters such as substrate dislocation, reaction pressure, III-V ratio, and total flow rate. In addition to GaAs, other III-V compounds such as Gasb, GaInP and other battery materials have also been developed. In 1998, the conversion efficiency of GaAs solar cells made by the Solar System Research Institute in Freiburg, Germany was 24.2%, which was recorded in Europe. GaInP cell conversion efficiency for the first time was 14.7%. See Table 2. In addition, the Institute also uses a stacked structure to prepare GaAs, Gasb batteries, which are two independent batteries stacked together, GaAs as the upper battery, the lower battery is Gasb, the resulting battery efficiency reached 31.1% . Copper Indium Selenide CuInSe2 referred to as CIC. The CIS material can be reduced to 1. leV, suitable for photoelectric conversion of sunlight, in addition, CIS thin film solar cells do not suffer from light-induced degradation. Therefore, CIS is also attracting attention as a material for high conversion efficiency thin-film solar cells. The preparation of CIS battery films includes vacuum evaporation and selenization. In the vacuum evaporation method, copper, indium, and selenium are vapor-deposited using respective evaporation sources, and the selenization method uses selenization of the H2Se laminated film, but this method is difficult to obtain a CIS having a uniform composition. CIS thin-film batteries have grown from the initial 8% conversion efficiency in the 1980s to the current level of about 15%. The erbium-doped CIS battery developed by Matsushita Electric Industrial Co., Ltd. has a photoelectric conversion efficiency of 15.3% (an area of ​​1cm2).
In 1995, the United States Renewable Energy Research Institute developed a conversion efficiency of 17. l% of CIS solar cells, which is by far the highest conversion efficiency of the battery in the world. It is expected that the conversion efficiency of CIS cells will reach 20% by 2000, which is equivalent to polysilicon solar cells. As a semiconductor material for solar cells, CIS has the advantages of low cost, good performance and simple process, and will become an important direction for the development of solar cells in the future. The only problem is the source of the material. Since indium and selenium are relatively rare elements, the development of such batteries is bound to be limited.
3Polymer multilayer modified electrode solar cells
The use of polymers instead of inorganic materials in solar cells is just the beginning of a research direction for solar cells to make dads. The principle is to make use of different redox potentials of different redox polymers to perform multi-layer composite on the surface of conductive material (electrode) to make a unidirectional conductive device similar to inorganic P-N junction. The inner layer of one of the electrodes is modified by a polymer with a lower reduction potential, the reduction potential of the outer layer polymer is higher, the electron transfer direction can only be transferred from the inner layer to the outer layer; the modification of the other electrode is the opposite and the first The reduction potential of the two polymers on the electrodes is higher than the reduction potential of the latter two polymers.
When two modified electrodes are placed in an electrolytic wave containing a photosensitizer. The electrons generated after the photosensitizer absorbs light are transferred to the electrode with a lower reduction potential. The electrons accumulated on the electrode with a lower reduction potential cannot be transferred to the outside polymer, and can only be transferred to the electrolysis through the external circuit through the electrode with a higher reduction potential. Liquid, so there is a photocurrent in the external circuit. Due to the flexibility of organic materials, ease of manufacture, wide source of materials, and cost advantages, it is of great significance for the large-scale use of solar energy and the provision of low-cost electric energy. However, the research on the preparation of solar cells using organic materials has only just begun. Both the service life and the cell efficiency cannot be compared with inorganic materials, especially silicon cells. Whether it can develop into a product with practical significance still needs further research and exploration.
4 nanocrystalline chemical solar cells
Silicon-based solar cells in solar cells are undoubtedly the most mature, but due to the high cost, they can not meet the requirements for large-scale promotion and application. To this end, people have been constantly exploring new technologies, new materials, and thin-film batteries, among which the newly developed nanocrystalline TiO2 crystal chemical energy solar cells have attracted the attention of scientists at home and abroad. Since Professor Gratzel of Switzerland has successfully developed nanometer TiO2 chemical solar energy battery, some domestic units are also conducting research in this area. Nanocrystalline chemical solar cells (abbreviated as NPC cells) are formed by modifying and assembling a band gap semiconductor material onto another large-gap semiconductor material, and the narrow band gap semiconductor material uses organic compounds such as transition metals Ru and Os. Sensitizing dyes, large energy gap semiconductor materials are nano-polycrystalline TiO2 and made into electrodes. In addition, NPC batteries also use appropriate oxidation-reduction electrolytes.
The working principle of nanocrystalline TiO2: The dye molecules absorb solar energy to transition to the excited state, the excited state is unstable, and the electrons are rapidly injected into the adjacent TiO2 conduction band. The lost electrons in the dye are quickly compensated from the electrolyte and enter the TiO2 conduction band. The electricity in the end enters the conductive film and then generates photocurrent through the outer loop. The advantages of nanocrystalline TiO2 solar cells lie in their low cost and simple process and stable performance. Its photovoltaic efficiency is stable at more than 10%, and the production cost is only 1/5 to 1/10 of silicon solar cells. Life can reach more than 2O years. However, as the research and development of such batteries have just begun, it is estimated that they will gradually embark on the market in the near future.
5 Development Trends of Solar Cells
From the above discussion, we can see that as a material of solar cells, III-V compounds and CIS are made from rare elements. Although the conversion efficiency of solar cells made of them is high, from the source of the material, this Solar cells in the future cannot dominate. The other two types of batteries, nanocrystalline solar cells and polymer modified electrodes have problems in solar energy, their research has just started, the technology is not very mature, and the conversion efficiency is still relatively low. These two types of batteries are still in the exploratory stage. It is impossible to replace solar cells.
Therefore, from the perspective of the conversion efficiency and the source of materials, the focus of future development is still silicon solar cells, especially polysilicon and amorphous silicon thin-film batteries. Since polysilicon and amorphous silicon thin-film batteries have high conversion efficiency and relatively low cost, they will eventually replace monocrystalline silicon cells and become the market's leading products. Increasing the conversion efficiency and reducing the cost are two major factors considered in the preparation of solar cells. It is difficult for the current silicon-based solar cells to further increase the conversion efficiency. Therefore, in addition to continuing to develop new battery materials, the focus of future research should focus on how to reduce costs. The existing high conversion efficiency solar cells are made on high-quality silicon wafers, which is the most important for manufacturing silicon solar cells. Expensive part. Therefore, it is particularly important to reduce the cost of the substrate while ensuring that the conversion efficiency is still high. It is also an urgent problem to be solved in the future development of solar cells. Recently, foreign countries have used certain technologies to obtain silicon strips as the substrate for polysilicon thin-film solar cells in order to achieve the purpose of reducing costs, and the effect is still relatively clear.
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