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Characteristics and classification of street-light-solar cells

2021-08-09 14:30:13

Characteristics of street light solar cells

Solar photovoltaic power generation has many advantages, which are very needed in future energy. ① It is not limited by region, and can generate electricity when there is sunshine; ② The power generation process is a simple physical process, without any waste gas and waste discharge, and basically has no impact on the environment; ③ Static operation of solar cells, no running parts, no wear, high reliability and no noise; ④ The generating power is determined by the solar cell and can be assembled into any size according to the required power; ⑤ It is not only easy to be used as an independent energy source, but also networked with other power sources; ⑥ Long service life (up to more than 20 years); ⑦ The solar cell has the advantages of lightweight, stable performance and high sensitivity; ⑧ The life of the sun reaches 6 billion years, so solar power generation is relatively unlimited energy. It is a general power technology, which can be used in many large or small fields, can be used in any place with sunshine, can be installed on the surface of any object, and can also be integrated into the building structure. It is easy to realize unmanned and full automation. Because of these characteristics, solar cells are widely used in space technology in various countries. Renewable energy is mainly bioenergy, and solar energy accounts for a small proportion. However, by 2050, the proportion of conventional energy and nuclear energy will drop to 47% and that of renewable energy will rise to 53%. Among renewable energy sources, solar energy (including solar thermal utilization and solar power generation) will occupy the first place, accounting for 29% of the total energy. In particular, solar power generation alone accounts for 25% of the total energy.

Characteristics and classification of street-light-solar cells 1

Classification of solar light cells

In the whole development process of solar cells, people have developed cells with different structures and materials. In terms of structure, it mainly includes homogeneous PN junction battery, Schottky (MS) battery, MIS battery, MINP battery and heterojunction battery, among which homogeneous PN junction battery plays a leading role from beginning to end; In terms of materials, there are mainly silicon solar cells, multi compound thin film solar cells, organic semiconductor thin film solar cells, nanocrystalline chemical solar cells, etc; From the aspect of material shape characteristics, it can be divided into bulk materials and thin film materials.

Crystalline silicon solar cell for outdoor lights

Crystalline silicon solar cells are divided into monocrystalline silicon solar cells and polycrystalline silicon solar cells.

Monocrystalline silicon solar cell is the solar cell with the highest conversion efficiency and the most mature technology. This is because the monocrystalline silicon material and its related processing technology are mature and stable, the monocrystalline silicon structure is uniform, the content of impurities and defects is small, and the conversion efficiency of the battery is high. In order to produce low contact resistance, the surface area of the battery requires heavy doping, and high impurity concentration will enhance the recombination rate of minority carriers in this area and make the minority carrier life of this layer very low, so it is called "dead layer". This area is the strongest light absorption area. Purple and blue light is mainly absorbed here. Usually, the thickness of N+ layer of thinned solar cell is 0.1 ~ 0.2 μm. That is, the "shallow junction" technology is adopted, and the surface phosphorus concentration is controlled below the limit value of solid solubility. In this way, the solar cell can overcome the influence of "dead layer" and improve the blue purple light response and conversion efficiency of the cell. This kind of cell is called "purple cell".

In addition, a concentration gradient of the same impurity is established between the battery substrate and the bottom electrode to prepare a p + P+ or N-N+ high-low junction to form a back electric field, which can improve the effective collection of carriers, improve the long wave response of solar cells, and improve the short-circuit current and open circuit voltage. This cell is called "back field battery". In the 1980s, green group developed the "grooved battery" by integrating the above technologies. Compared with the printing method, the efficiency of the battery is improved by 10% ~ 15%. Since the 1980s, surface passivation technology has been developed. From the thin oxide layer (< 10nm) of PESC battery to the thick oxide layer (about 110m) of perc and Perl battery, thermal oxidation surface passivation technology can reduce the surface density of States to 1010 / cm ² Below, the surface recombination speed is reduced to less than 100 cm / s. The use of various technologies has improved the conversion efficiency of monocrystalline silicon cells to 24.7%, and experts predict that the ultimate efficiency of monocrystalline silicon cells is 29%. In order to reduce the cost of the battery, while improving the conversion efficiency, people are exploring to reduce the thickness of the battery, that is, to achieve thin sheet.

Polycrystalline silicon solar cells generally use polycrystalline silicon materials specially produced for the use of solar cells. At present, the most widely used polysilicon manufacturing method is casting method, also known as casting method. Polycrystalline silicon solar cells generally use low-grade semiconductor polycrystalline silicon, and most of the polycrystalline silicon chips are cut from controlled or cast silicon ingots. Polycrystalline silicon ingot is made of defective silicon, waste secondary single crystal and metallurgical grade silicon powder in semiconductor industry. At present, with the explosive development of the output of solar cells, the above raw materials can no longer meet the needs of the solar cell industry. Now a production industry with polysilicon solar cells as the target is being formed, which will be described later.

In order to reduce the loss of silicon wafer cutting, polycrystalline silicon wafer required for solar cells is prepared directly from molten silicon. The cells prepared by this method are generally called silicon with silicon cells. There are two methods to prepare silicon: one is called EFG "fixed edge film feeding method", which is to grow octahedral polysilicon tubes in industrial applications, and then cut each side into silicon wafers; The other is called "webbed crystallization method", which is adopted by evergreen solar. The method is to limit the molten silicon with a fine carbon rod and pull it out of the molten pool. The silicon liquid limited in the two fine rods is cooled and solidified to form a silicon belt. Compared with monocrystalline silicon solar cells, polycrystalline silicon solar cells have lower cost, and the conversion efficiency is close to monocrystalline silicon solar cells. Therefore, polycrystalline silicon high-efficiency cells have developed rapidly in recent years, among which Geogia tech cells, UNSW cells, Kyocera cells, etc. Among the solar cells produced in recent years, polycrystalline silicon solar cells account for 52% more than monocrystalline silicon. It is one of the main products of solar cells. However, compared with the existing energy prices, crystalline silicon solar cells can not be widely commercialized because the power generation cost is still too high.

Thin film solar light cell

Thin film solar cells can be divided into the following categories according to the materials for preparing solar cells.

(1) Multicomponent compound thin film solar light cell

Copper indium selenium: CuInse has a band gap of 1.53ev and is regarded as an ideal photovoltaic material. It can form p-type and n-type with high conductivity only by introducing its own defects, which reduces the requirements of the cell for grain size, impurity content and defects, and the cell efficiency has reached 15.4%. The band gap can be increased by adding an appropriate amount of GA, A1 or s, which can be used to make high-efficiency single junction or laminated batteries. CulnSeis a ternary I Ⅲ - Ⅵ compound semiconductor. It is a direct band gap semiconductor material with an absorption rate of 105 / cm. The electron affinity of CulnSeis 4.58ev, which is very different from that of CDs (4.50ev) (0.08eV), which makes the heterojunction formed by them have no conduction band peak and reduces the potential barrier of photogenerated carriers. CulnSe film growth process: vacuum evaporation method, selenium treatment method of cu-1n alloy film (including electrodeposition method and chemical thermal reduction method), gas phase transport method in closed space (CCVT), spray pyrolysis method, radio frequency emission method, etc. CIS solar cell is a photovoltaic device composed of multilayer thin films deposited on glass or other cheap substrates. Its structure is: light → metal grid electrode / antireflection film / window layer (ZnO) / transition layer (CDS) / light absorption layer (CLS) / metal back electrode (MO) / substrate.

Cadmium telluride: CdTe has a direct band gap of 1.5ev, its spectral response is very consistent with the solar spectrum, and has a high absorption coefficient in the visible band, 1 μm thick can absorb 90% of visible light. CdTe is a Ⅱ Ⅵ compound. Because CdTe film has a direct band gap structure and its optical absorption coefficient is very large, the requirement for material diffusion length is reduced. The thin film semiconductor material with CdTe as absorber forms a heterojunction solar cell with window layer CDs. Its structure is: light → antireflection film (MgF) / glass substrate / transparent electrode (SnO: F) / window layer (CDS) / absorption layer (CdTe) / ohmic contact transition layer / metal back electrode. The preparation methods include sublimation, MOCVD, CVD, electrodeposition, screen printing, vacuum evaporation and atomic layer epitaxy. CdTe thin film solar cells with conversion efficiency of more than 10% have been made in various methods. Among them, the efficiency of the battery deposited with CdS / CdTe junction is 16.5%.

Gallium arsenide: the battery material has moderate band gap and stronger radiation resistance and high temperature performance than silicon. Solar cells can obtain higher efficiency. The maximum efficiency in the laboratory has reached more than 24%, and the efficiency of general Aerospace solar cells is also between 18% ~ 19.5%. The efficiency of single junction cells grown on single substrate is 36% of the theoretical efficiency of GaInP / GaAs cascade cells. Laminated solar cells with an area of 4m² and a conversion efficiency of 30.28% have been fabricated in the laboratory. At present, GaAs solar cells are mostly prepared by liquid phase epitaxy or metal organic chemical vapor deposition technology, so the cost is high and the output is limited. Reducing the cost and improving the production efficiency have become the focus of research. At present, GaAs solar cells are mainly used in spacecraft.

(2) Organic semiconductor thin film solar light cell

Organic semiconductors have many special properties and can be used to manufacture many thin-film semiconductor devices, such as field-effect transistors, field-effect electro-optic modulators, light emitting diodes, photovoltaic devices and so on. Organic semiconductors absorb photons to produce electron hole pairs with binding energy of 0.2 ~ 1.0ev, which is the boundary between p-type semiconductor materials and n-type semiconductor materials. The dissociation of electron hole pairs leads to efficient charge separation and forms what is commonly known as heterojunction solar cells. Organic semiconductors used in photovoltaic devices are roughly divided into molecular organic semiconductors and polymer organic semiconductors. Later, double-layer organic semiconductor heterojunction solar cells appeared. Organic semiconductors can be divided into soluble, insoluble and liquid crystal according to their chemical properties; Sometimes it is also divided into dyes, pigments and polymers according to monomers. For the doping of organic semiconductors, other molecules and atoms can be introduced, or they can be oxidized by electrochemical method. The impurities that can make it P-type include Cl, Br, I, NO, tcnqcn-ppv, etc; Doping alkali metal can make it n-type.

(3) Dye sensitized nano thin film solar light cell

Dye sensitized nano thin film battery is a battery invented by Dr. Michel Graetzel of the Swiss Federal Institute of technology. Nano chemical solar cells (NPC cells for short) are formed by modifying and assembling a narrow band gap semiconductor material onto another large energy gap semiconductor material. The narrow band gap semiconductor material adopts transition metal Ru and organic compound sensitized dyes. The large energy gap semiconductor material is nano multi product TiO and made into electrodes. In addition, NPC cells also select appropriate redox electrolytes. Working principle of nano polycrystalline TiO: dye molecules absorb solar energy and transition to the excited state. The excited state is unstable. Electrons are quickly injected into the adjacent TiO conduction band. The electrons lost in the dye are quickly compensated from the electrolyte. The electrons entering the TiO conduction band finally enter the conductive film, and then generate photocurrent through the external circuit. It is a new type of cell with nano titanium dioxide porous film sensitized by photosensitive dyes, which greatly improves the efficiency of photoelectrochemical cells. This cell has stable efficiency outdoors. In 1998, the efficiency of small-area cells of the Swiss Federal Academy of Sciences was 12%. Pilot tests were carried out in some countries. The specific battery efficiency is 30cm of Germany INAP × 6% at 30cm; 10cm of Australian st × 20cm is 5%. China's large-area dye-sensitized nano thin film solar cell research project with the Institute of plasma physics of Chinese Academy of Sciences as the main undertaking unit has built a small demonstration power station with a scale of 500W array, making China a leader in the world in some aspects of this research field.

Amorphous silicon is the earliest commercial thin film battery. Typical amorphous silicon( α - Si) solar cells deposit transparent conductive film (TCO) on glass substrate, and P-type, I-type and N-type three layers are deposited by plasma reaction α - Si, and then evaporate the metal electrode Al / Ti on it. Light is incident from the glass layer, and the battery current is led out through the transparent conductive film and metal electrode Al / Ti. Its structure is glass / TCO / p-I-N / Al / Ti, and the substrate can also adopt plastic film, stainless steel sheet, etc. After a large amount of hydrogen (10%) is introduced into amorphous silicon, the band gap increases from 1.1eV to 1.7eV, which has strong light absorption. In addition, a thick "intrinsic layer" is added between the thinner p layer and N layer to form a p1n structure. A layer with less impurity defects is used as the main absorption layer to form an electric field in the generation region of photogenerated carriers, which enhances the collection effect of carriers. In order to reduce the loss caused by the large transverse resistance of the top thin doped layer, the upper electrode of the battery adopts a transparent conductive film. Moreover, texture enhanced light transmission is prepared on the transparent conductive film. At present, the most used transparent conductive materials are SnO and ITO (mixture of InO and SnO), and Zao (aluminum doped zinc oxide) is considered to be a new excellent transparent conductive material. Due to the wide energy distribution of sunlight, semiconductor materials can only absorb photons with energy higher than its energy gap value, and the remaining photons will be converted into heat energy, but can not be transferred to the load through photogenerated carriers to convert into effective electric energy. Therefore, for single junction solar cells, even if they are made of product materials, the theoretical limit of conversion efficiency is only about 29%. In the past, non-standard silicon cells were mostly in the form of single junction cells. Later, double junction stacked cells were developed, which can collect photogenerated carriers more effectively. BP solar uses SiGe alloy as the bottom battery material. Because the band gap of SiGe alloy is narrow, it enhances the spectral response of the battery as the bottom battery material. Beckert uses amorphous silicon with different Ge content to make a three junction series battery with two bottom batteries, creating the highest stable efficiency of amorphous silicon battery module of 6.3%. Among thin-film solar cells, non-standard silicon cells were first commercialized and used by Sanyo Electric Company in 1980 α-Si The pocket calculator made of Si solar cells was industrialized in 1981, α-Si The annual sales volume of Si cells once accounted for 40% of the world's photovoltaic sales volume. With the continuous improvement of the performance and cost of non-standard silicon cells, their application fields are also expanding, from calculators to various consumer products and other fields, such as solar radios, street lamps, microwave relay stations, traffic crossing signal lamps, meteorological monitoring, photovoltaic water pumps, household independent power supply, grid connected power generation, etc. This part will be discussed in detail in the following chapters.

(5) Polycrystalline silicon thin film solar light cell

The research work of polycrystalline silicon thin film battery began in the 1970s, which was earlier than that of amorphous silicon thin film battery. However, at that time, people mainly focused on amorphous silicon thin film battery. After the research work of amorphous silicon thin film battery encountered difficult problems, people naturally began to pay attention to polycrystalline silicon thin film battery. Since polycrystalline silicon thin film cells use far less silicon materials than monocrystalline silicon cells, there is no problem of photoattenuation of amorphous silicon thin film cells, and it is possible to prepare them on cheap substrates. The expected cost is much lower than monocrystalline silicon cells. People hope to reduce the cost of solar cell modules to about us $1 / W. Polycrystalline silicon thin film battery can also be used as the bottom battery of amorphous silicon series junction battery, which can improve the spectral response and service life of the battery. Therefore, it has developed rapidly since 1987. Now the photoelectric performance of polycrystalline silicon thin film battery is stable, and the maximum laboratory efficiency of Astropower company has reached 16%. At present, polycrystalline silicon thin film cells are prepared by chemical vapor deposition, including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD). In addition, liquid phase epitaxy (LPE) and sputtering deposition can also be used to prepare polycrystalline silicon thin film cells. LPE growth technology has been widely used in high-quality and compound semiconductor heterostructures, such as GaAs, AlGaAs, SiGe and SiGe. Its principle is to reduce the temperature and precipitate silicon films by melting silicon in the matrix. The battery efficiency prepared by Astro power with PE can reach 12.2%. Chen Zheliang of China photoelectric development technology center used liquid phase epitaxy to grow silicon grains on metallurgical grade silicon wafers, and designed a new solar cell similar to crystalline silicon thin film solar cells, called "silicon grain" solar cells.

At present, the so-called third generation solar cell research center of the University of New South Wales, led by Professor Martin Green, is actively carrying out theoretical research and scientific experiments on ultra-high efficiency (> 50%) solar cells, focusing on how to fully collect carriers from the valence band transition to the high conduction band. At present, the batteries studied and tested mainly include superlattice cells, "hot carrier" cells, new "laminated" cells and "thermal photovoltaic" cells.

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