Research on the performance of flexible amorphous silicon solar cells

　　1. Flexible solar cells

　　Most of the above-mentioned solar cell types have rigid structures, which are encapsulated with glass materials and supplemented by frame fastening. We can imagine how unpleasant it would be to do this outdoors with a giant pane of glass on your back. Although some varieties can be prepared into flexible forms, they have great deficiencies in flexibility, damage resistance, weather resistance and lifespan. For example, SUNPOWER's flexible monocrystalline silicon products can continue to work after being slightly broken, but their flexibility can only reach 30%. That is to say, when bent to 31%, the battery will completely break and be scrapped.

　　In the outdoor flexible portable solar power supply system discussed in this article, solar cells are nothing more than the core content. In outdoor use, it is difficult for us to limit various usage methods for users. Especially in special fields, a fragile battery is not qualified for the word "portability".

　　Although cadmium telluride, CIGS and other batteries can be made into solar cell modules that are not easily damaged, they still have not solved the problems of oxygen and water isolation, and their service life is quite limited. In recent years, China's Hanergy Group has swept across European and American CIGS companies, acquired several heavyweight CIGS manufacturers, and is preparing to go big. Although they have made many achievements in rooftop power generation (BIPV), flexible solar cells have still been unable to launch products, mainly because of the water-proof packaging process.

　　Organic solar cells are also an important direction for future flexible solar cells. They are produced by printing, which is efficient and low-cost. In the future, they can not only be made flexible, but can even be directly applied as coatings. However, judging from the current cutting-edge laboratories or companies around the world, it is still only at the glove box + glass device stage, and the road to marketization is still very long.

　　Only flexible amorphous silicon solar cells have been successfully commercialized.

　　2. Flexible amorphous silicon solar cells

　　Since Bell Telephone Laboratories in the United States produced the world's first practical silicon solar cell in 1945, it has ushered in a new era of solar energy utilization for modern humans. With the vigorous research by countries around the world for decades, the development of solar energy is in the ascendant. In recent years, new solar products have been developed in an endless stream, among which flexible amorphous silicon solar cells are one of the brightest new stars.

　　2.1 Formation of flexible amorphous silicon solar cells

　　Flexible amorphous silicon solar cells are soft, transparent, and thin (about 1 micron thick). The original solar photovoltaic module (see Figure ②) is in the form of a sheet. Its manufacturing technology has been fully matured for mass production. Each KW requires silicon The material is only 0.067 kg, and it is most sensitive to the high-frequency sunlight spectrum (blue).

　　In addition to the photoelectric conversion function of traditional solar energy, flexible thin film solar cells, as the name suggests, are soft and lightweight. After packaging (solar cell chips must be packaged before normal use), the thickness is only 1.5mm and the panel with an output power of 50W weighs only It is 2.5KG, which is 1/6 of traditional solar energy. Because its surface abandons fragile and fragile glass or resin materials, it is encapsulated with a polymer film, making the product more flexible and durable, and can be bent and folded at will (traditional solar cells are encapsulated in glass, which is fragile and easily damaged) , and can even be made into any shape according to different equipment requirements. After folding, it is only about the size of a book, making it easy to carry. When working in the field, you only need to spread or hang the solar panels arbitrarily to power the load, and can be used on the back while walking.

　　Not affected by temperature

　　When solar products are used outdoors, the temperature of the solar cells inside them will not be affected.

　　Light quality. 7.0 kg/m?, the installation of solar panels will not cause excessive load on the roof, and the selection of the main roof structure will not change significantly due to the installation of solar panels.

　　Flexible and easy to bend. Membrane panels are lightweight software materials that can be bent arbitrarily according to the shape of the roof when the average roof slope is <60° and the bending plate radius is >13 m.

　　Durable

　　The solar cells are encapsulated with anti-UV polymer, so the product is particularly durable and clean. The surface of the board is smooth and does not attract dust. Any dust can be washed away by rain when it rains. The roof does not require exterior cleaning.

　　Resistant to shadowing

　　The product uses three-layer high-voltage photovoltaic technology. When encountering shading or shadow conditions, it can still convert and output more electrical energy than ordinary solar products.

　　No glass

　　The product abandons fragile glass and uses DuPont polymer to protect the solar cells.

　　2.2 Bypass diode technology

　　Using bypass diode technology that can still provide a higher photoelectric conversion rate when the sun's rays are blocked. When the solar panel is partially blocked by surrounding buildings or objects above the roof, due to the connection of the bypass diodes, only the solar panel covered by the shadow side does not participate in the work, and the remaining parts still work normally, that is, the group of panels blocked by the shadow. Current can still be generated. Once the polycrystalline silicon panel is shaded, the entire set of solar panels will cease to work.

　　2.3 Spectral absorption range

　　Flexible amorphous silicon solar cells are embedded with three layers of silicon crystals, adhered to a stainless steel film, and covered with a protective layer of vinyl acetate ethane polymer (EVA). Three-layer film bonding can thin the thickness of each sub-electron layer, increase the internal electric field of each sub-cell, increase the collection efficiency of each sub-cell, and expand the corresponding range of the spectrum. Each layer of silicon crystal can convert a specific part of the visible spectrum, thereby providing a high photoelectric conversion rate even in rainy or cloudy weather, making up for the conversion of amorphous silicon compared to polycrystalline silicon. Monocrystalline silicon panels can convert in fine weather The disadvantage of low efficiency (the conversion efficiency of crystalline silicon cells is about 17%, and that of amorphous silicon cells is about 8-10%. The high conversion efficiency is mainly reflected in the fact that the product can save a certain amount of light-receiving area, while the overall power remains unchanged).

　　2.4 Three-layer composite structure design

　　In traditional amorphous silicon cells, there are single junction, double junction, triple junction, and even as many as 5 junctions. However, considering the overall cost, the 3-knot structure is more commonly used. The so-called 3-junction structure is divided into three light-absorbing layers, each absorbing different bands of the solar spectrum. He uses P.I.N comprehensive multi-layer manufacturing to reduce the light reflection effect, and P.I.N is the main conductive layer. Therefore, it is made of amorphous and germanium elements and other material structures, so that the energy conversion rate can reach about 8.6% due to the band gap characteristics. The solar spectrum radiation electric field range covers X-ray radiation and gamma radiation. Its wavelengths are:

　　Ultraviolet (0.04-400nm)9%

　　Visible light waves (400-700nm)47%

　　Infrared waves (700-300,000) 44%

　　Microwaves and radio waves, etc.

　　Crystalline silicon solar cells have the highest absorption effect on the red visual spectrum band (such as the winter sunlight spectrum), which can reach 1100-1250w/m2 under clear sky and clear sunlight density. The amorphous solar cell made of A-SI has an absorption effect on the blue sunlight spectrum (such as in summer and cloudy days) and can achieve the highest light energy conversion efficiency in a climate with a sunlight density of only 50-400w/m2.

　　Figure ⑸ Three composite layer structure

　　The three-layer composite solar cell adopts a sandwich structure design and absorbs light of various wavelengths of the sun in layers, so it can convert and output more electrical energy than ordinary solar products.

　　The top layer uses amorphous silicon material with an optical energy level gap of 1.8eV, which is conducive to absorbing blue light.

　　The bottom layer uses amorphous silicon and 40-50% germanium alloy material. The optical energy level gap reaches 1.4eV and can absorb red light and far-red light.

　　The middle layer uses amorphous silicon and 10-15% germanium alloy material. The optical energy level gap reaches 1.6eV, which is conducive to absorbing green light.

　　The light that is not absorbed when the light source enters will be reflected back by the silver and zinc oxide (Ag/Zno) in the base layer, and will be absorbed on the way out. )

　　2.5 Environmental and climate impacts

　　Due to the seasonal relationship, the long-term sunshine density changes with the atmospheric temperature, solar sunshine density, cloud cover time and temperature (heat) effect, etc. The operating efficiency of solar cells is closely related to the product and climate (Figure ⑹, ⑺ Curve data data comparison explanation).

　　According to the output energy temperature rise and loss curve comparison table, the energy (voltage) conversion loss of polycrystalline silicon solar photovoltaic modules is as high as more than 15% when operating at a standard full sunlight amount (1000 w/m2) and an operating temperature of 60°C. Solar photovoltaic modules account for about 17%.

　　Flexible amorphous silicon solar cells, on the other hand, are not only unaffected, but also gain energy without any loss during sunlight density (600 w/m2) conditions. Therefore, under the seasonal climate of East Asia, their energy output shows signs of improvement. Therefore, the energy conversion rate of solar photovoltaic modules is directly affected by the material and structural technology as well as the regional seasonal climate and temperature.

　　2.6 Packaging features

　　The polymer adhesive film encapsulated on the surface of the flexible amorphous silicon solar cell has been scientifically designed. The texture treatment on its surface can absorb light from all angles, effectively preventing sunlight damage. Even on cloudy and rainy days when there is no sunshine, it can still generate about 30% of the power, and the overall efficiency is 20% higher than that of traditional solar cells.

　　Not only that, after packaging, flexible thin-film solar cells also have advantages that traditional solar cells cannot have, such as maintenance-free, waterproof, anti-corrosion, anti-fouling, impact resistance, pressure resistance, temperature resistance and insulation. Its surface is encapsulated with a high-strength inert fluoroplastic film. The surface is smooth and does not absorb. Even if it is contaminated, it can be washed away by rain or simple cleaning to remove dirt. TP-SOLAR flexible thin film solar cells can work normally in environments ranging from -40°C to 80°C. Even if a certain part is penetrated by a bullet, the remaining parts can still generate electricity without being affected.

　　2.7 Service life

　　Flexible amorphous silicon solar cells alone have a long service life. The entire photovoltaic panel power generation system has 80% of the electric energy output guaranteed within 20 years (because the photovoltaic panels will stop degrading after 15 years, and the photovoltaic panels can still generate electricity after that) Not less than 20% of electrical energy), how long the entire photovoltaic panel system can be used after 20 years mainly depends on the climate environment in which the building is located.

　　Electronic equipment such as controllers and inverters usually only have a service life of 5 years. After 5 years, some of the parts need to be replaced to make them work properly, just like certain parts of cars or daily appliances need to be replaced without a lot of cost. According to the outdoor system test report of the American Renewable Energy Research Laboratory: the attenuation rate is -0.74% (TP-SOLAR brand).

　　3. Comparison between flexible amorphous silicon solar cells and traditional solar cells

　　To evaluate the pros and cons of a solar cell power generation system, in addition to looking at whether it has higher photoelectric conversion efficiency, you should also refer to the system's working efficiency during the day. The photoelectric conversion efficiency of monocrystalline silicon solar cells currently available on the market is 17%, that of polycrystalline silicon is 16%, and that of amorphous silicon solar cells is only 9%. However, this does not mean that crystalline silicon solar cells are more efficient than amorphous silicon solar cells. Silicon solar cells convert more light energy into electricity throughout the day. Due to the different characteristics of the materials themselves, amorphous silicon solar cells have advantages that crystalline silicon solar cells cannot match.

　　Monocrystalline silicon solar cells generally only have photoelectric conversion capabilities at noon on a sunny day, while amorphous silicon solar cells have a longer power generation time than monocrystalline silicon and polycrystalline silicon solar cell modules, whether under strong light irradiation on a sunny day or under Under weak scattered light on sandy and rainy days, the power generation of amorphous silicon solar panels with the same power is greater than the power generation of monocrystalline silicon solar cells with the same power. Therefore, in the climate that changes throughout the year, the power generation efficiency of amorphous silicon solar panels is better than that of monocrystalline silicon solar panels.

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