Introduction of Solar Cells
Solar cells are devices that directly convert light energy into electricity through the photoelectric effect or photochemical effect. Thin-film solar cells that work by photoelectric effect are the mainstream, while wet solar cells that work by photochemical effect are still in their infancy. Sunlight in the semiconductor p-n junction, the formation of new hole - electron pairs, in the p-n junction under the action of the electric field, the hole from the n area flow to the p area, the electron from the p area flow to the n area, connecting the circuit to form a current. This is the working principle of photoelectric effect solar cells. Solar cells can be divided into two categories according to the state of crystallization: crystalline thin-film type and non-crystalline thin-film type (hereinafter referred to as a-), and the former is divided into mono-crystalline and polycrystalline.
Solar photovoltaic cells are usually manufactured from either crystalline silicon or thin-film materials, with the former obtained by cutting, ingot casting, or forging, and the latter being a thin film attached to a low-priced backing. Most of the solar photovoltaic cells produced and used in the current market are made of crystalline silicon materials, accounting for about 93% in 2006; the focus of future development may be thin-film solar cells, which have more development potential due to the use of fewer materials, less weight, smooth appearance, and easy installation.
Solar Cells - Classification of Solar Cells
Solar cells according to the different materials used, solar cells can also be divided into: silicon solar cells, multi-compound thin-film solar cells, polymer multilayer modification of electrode-type solar cells, nano-crystalline solar cells of four categories.
1. Silicon solar cells
Silicon solar cells are divided into monocrystalline silicon solar cells, polycrystalline silicon thin film solar cells and amorphous silicon thin film solar cells.
(1) monocrystalline silicon solar cells
Monocrystalline silicon solar cells
At present, monocrystalline silicon solar cells have a photoelectric conversion efficiency of about 15%, the highest reached 24%, which is currently the highest photoelectric conversion efficiency of all types of solar cells, the technology is also the most mature, but the cost of production is very large, so that it can not be a large number of widespread and universal use. Use. Because monocrystalline silicon is usually encapsulated with tempered glass and water-resistant resin, it is robust and durable, with a service life of up to 15 years and up to 25 years.
Monocrystalline silicon solar cells are one of the fastest growing types of solar cells, and their construction and production processes have been finalized for use in both space and terrestrial applications. This solar cell to high-purity monocrystalline silicon rods as raw materials. In order to reduce the cost of production, now the ground application of solar cells, such as the use of solar-grade monocrystalline silicon rods, material performance indicators have been relaxed.
(2) polycrystalline silicon solar cells
Polycrystalline silicon solar panels
Polycrystalline silicon solar cells and monocrystalline silicon solar cell production process is almost the same, but polycrystalline silicon solar cell photoelectric conversion efficiency is to be reduced by a lot of its photoelectric conversion efficiency of about 12% or so (on July 1, 2004, Japan's Sharp listing efficiency of 14.8% of the world's The highest efficiency polycrystalline silicon solar cells). In terms of production costs, cheaper than monocrystalline silicon solar cells, easy to manufacture materials, saving electricity, the total cost of production is lower, and therefore get a lot of development. In addition, the service life of polycrystalline silicon solar cells is also shorter than monocrystalline silicon solar cells.
The production of polycrystalline silicon solar cells requires the consumption of a large number of high-purity silicon materials, and the manufacture of these materials process is complex, power consumption is very large, in the total cost of production of solar cells in their own more than one-half. In addition, the monocrystalline silicon rods are cylindrical, sliced to make solar cells are also round, composed of solar components plane utilization rate is low. Therefore, since the 80s, some countries in Europe and the United States into the development of polycrystalline silicon solar cells.
(3) amorphous thin-film solar cells
Amorphous silicon solar cells
Amorphous silicon thin-film solar cells and monocrystalline silicon and polycrystalline silicon solar cell production methods are completely different, the process is greatly simplified, silicon material consumption is very small, power consumption is lower, the cost is low and light weight, conversion efficiency is high, and easy to large-scale production, and it's main advantage is that the low-light conditions Its main advantage is that in low light conditions can also generate electricity, has great potential. However, the main problem of amorphous silicon solar cells is the low photoelectric conversion efficiency, the current international advanced level of 10% or so, and is not stable enough, with the prolongation of time, the conversion efficiency of its attenuation, which directly affects its practical application. If we can further solve the stability problem and improve the conversion rate, then, amorphous silicon solar cells is undoubtedly one of the main development of solar cells.
2. Multi-compound thin-film solar cells
Multi-compound thin-film solar cell materials for inorganic salts, which mainly includes gallium arsenide III-V compounds, cadmium sulfide, cadmium sulfide, and copper-imprisoned selenium thin-film batteries.
Cadmium sulfide, cadmium telluride polycrystalline thin-film battery efficiency than amorphous silicon thin-film solar cell efficiency, the cost of monocrystalline silicon batteries is lower, and is also easy to large-scale production, but due to the cadmium has a high degree of toxicity, will cause serious pollution of the environment, therefore, is not the most ideal alternative to crystalline silicon solar cells.
Gallium Arsenide (GaAs) III-V compound battery conversion efficiency of up to 28%, GaAs compound material has a very ideal optical bandgap and high absorption efficiency, resistance to irradiation, heat insensitive, suitable for the manufacture of high-efficiency monojunction battery. However, the price of GaAs materials is not expensive, thus limiting the popularity of GaAs batteries to a large extent.
CIS
Copper indium selenide thin-film batteries (CIS) are suitable for photovoltaic conversion, there is no photoluminescence degradation, and the conversion efficiency is the same as polysilicon. With the advantages of low price, good performance and simple process, it will become an important direction for the future development of solar cells. The only problem is the source of materials, due to indium and selenium are relatively rare elements, therefore, the development of such batteries is bound to be limited.
3. Polymer multilayer modified electrode-type solar cells
Polymer instead of inorganic materials in solar cells is just beginning a solar cell system dad research direction. The principle is to use different redox polymers of different redox potential, in the conductive material (electrode) surface multilayer composite, made similar to the inorganic P-N junction unidirectional conductive device. One of the inner layer of the electrode is modified by a polymer with a lower reduction potential, and the outer polymer has a higher reduction potential, so that the electron transfer direction can only be transferred from the inner layer to the outer layer; the other electrode is modified in the opposite direction, and the two polymers on the first electrode have a higher reduction potential than the two polymers on the latter. When the two modified electrodes are put into the electrolytic wave containing photosensitizer. Photosensitizer absorbs light and generates electrons that 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, but can only be returned to the electrolyte through the external circuit through the electrode with the higher reduction potential, and thus there is a photocurrent generated in the external circuit.
Because of the advantages of organic materials are flexible, easy to make, wide range of material sources, the bottom of the cost, and thus the large-scale utilization of solar energy, providing cheap electricity is of great significance. However, organic materials to prepare solar cell research has only just begun, whether it is the service life, or battery efficiency can not be compared with inorganic materials, especially silicon batteries. Whether it can be developed into a practical significance of the product, but also to be further research and exploration.
4. Nanocrystalline chemical solar cells
Nanochemical solar cells
Silicon solar cells in the solar cell is undoubtedly the most mature development, but due to the high cost, far from being able to meet the large-scale popularization and application requirements. For this reason, people have been continuously in the process, new materials, battery thin film exploration, and this newly developed nano TiO2 crystal chemical solar cells by domestic and foreign scientists. Since Professor Gratzel of Switzerland has successfully developed nano TiO2 chemical solar cells, some domestic units are also conducting research in this area. Nanocrystalline chemical solar cells (referred to as NPC cells) is a forbidden band semiconductor materials modified, assembled to another large energy gap semiconductor materials on the formation of narrow-band semiconductor materials using the transition metal Ru, as well as Os and other organic compounds such as sensitizing dyes, large energy gap semiconductor materials for the nanopolycrystalline TiO2 and made of electrodes, in addition to the NPC battery also choose the appropriate oxidation of a reductive electrolyte. Nanocrystalline TiO2 working principle: the dye molecule absorbs the sunlight energy to jump to the excited state, the excited state is not stable, the electrons quickly injected into the immediate vicinity of the TiO2 conduction band, the dye in the loss of electrons is quickly compensated from the electrolyte into the TiO2 conduction band of the electricity in the final into the conductive membrane, and then through the external circuit to produce photocurrent.
The advantage of nanocrystalline TiO2 solar cells is its cheap cost and simple process and stable performance. Its photovoltaic efficiency is stable at more than 10%, the production cost is only 1/5-1/10 of silicon solar cells. life can reach more than 2O years. But because the research and development of such batteries has just begun, it is estimated that in the near future will gradually on the market.
Solar cells - China's solar cell industry status
China attaches great importance to the research and development of solar cells, as early as during the Seventh Five-Year Plan, amorphous silicon semiconductor research has been included in the major national issues; Eighth Five-Year Plan and Ninth Five-Year Plan, China's research and development efforts focused on the development of large-area solar cells, etc. In October 2003, the National Development and Reform Commission, Ministry of Science and Technology Developed a solar energy resources development plan for the next five years, the Development and Reform Commission "Brightness Project" will raise 10 billion yuan to promote the application of solar power technology, plans to 2005, the total installed capacity of the national solar power system reached 300 megawatts.
In 2002, the relevant state ministries and commissions launched the "western provinces and districts without electricity township electricity program", through solar energy and small wind power to solve the problem of electricity in the western seven provinces and districts without electricity townships. The launch of this project has greatly stimulated the solar power industry, the domestic construction of several solar cell packaging line, so that the annual production of solar cells increased rapidly. China currently has 10 solar cell production lines, the annual production capacity of about 4.5MW, of which 8 production lines were introduced from abroad, in these 8 production lines, there are 6 monocrystalline silicon solar cell production line, 2 amorphous silicon solar cell production line. According to expert forecasts, China's photovoltaic market demand for 5MW per year, 2001 to 2010, the annual demand will reach 10MW, from 2011, China's photovoltaic market demand will be greater than 20MW.
Currently, the domestic solar energy silicon production enterprises are mainly monocrystalline silicon plant in Luoyang, Hebei, Ningjin, monocrystalline silicon base and Emei Semiconductor Materials Factory in Sichuan, etc., of which Hebei, Ningjin, monocrystalline silicon base is the world's largest solar cell production line, the production of mono-crystalline silicon solar cell production lines. Ningjin monocrystalline silicon base is the world's largest solar monocrystalline silicon production base, accounting for about 25% of the world solar monocrystalline silicon market share.
In the downstream market of solar cell materials, the current domestic production of solar cells, mainly Baoding Yingli New Energy, Wuxi Suntech, Kaifeng Solar Cells Factory, Yunnan Semiconductor Device Factory, Qinhuangdao Huamei PV Electronics, Zhejiang Zhongyi Solar Energy, Ningbo Solar Power, Kyocera (Tianjin) Solar Energy, etc., with a total annual production capacity of more than 120MW.