Localization of the back film: the inevitable choice of photovoltaic powers
China's PV industry has experienced rapid growth in the past few years, but there are also problems such as blind investment, vicious competition, and insufficient innovation. At the same time, due to continuous improvement and breakthrough of technology, China's PV industry is emerging from its image of high energy consumption and high pollution, showing great development potential. In March 2009, in the tender for China's Dunhuang 10MW grid-connected photovoltaic power generation project, all bids were lower than 2 yuan / KWh, the lowest standard was 0.69 yuan / KWh, the second low standard was 1.09 yuan / KWh, which shows that Chinese PV companies have quite The price affordability and competitiveness are likely to be among the highest in the world in reducing the cost of photovoltaic power generation.
The just-concluded Copenhagen Climate Change Conference is an important milestone in the global response to climate change. As the carbon cost in the future becomes higher and higher, the replacement of traditional fossil energy with new energy represented by solar energy has become an inevitable trend. In order to speed up the replacement process, for the photovoltaic power generation industry, while continuously improving the technology, it is necessary to reduce costs. Solar cell module packaging materials mainly include glass, EVA film, frame, back film, junction box, silica gel, etc. At present, other packaging materials other than the back film have been highly localized in China's photovoltaic industry, greatly reducing solar cell components. Manufacturing cost per unit of power generation. However, as a kind of important solar cell module encapsulation material, the back film has high technical threshold requirements, and the related raw materials have long been restricted by the patent technology of foreign fluorine chemical giants. Today, the degree of localization is still extremely low, resulting in the domestic The backsheets used by solar cell module manufacturers are mostly imported products, which are expensive and cannot be guaranteed. Therefore, from the perspective of reducing the manufacturing cost of the solar cell module unit power generation, the localization of the back film is an inevitable choice for Chinese PV companies.
Double-sided fluorine material: high quality back film is indispensable
The solar cell back film is mainly divided into two categories: fluorine-containing back film and fluorine-free back film. The fluorine-containing back film is divided into two kinds of fluorine-containing (such as TPT) and single-sided fluorine (such as TPE); the fluorine-free back film is more adhesively laminated with multiple layers of PET. At present, the use of commercial crystalline silicon solar cell modules is 25 years, and the back film is a photovoltaic packaging material that directly contacts the external environment. It should have excellent resistance to long-term aging (damp heat, dry heat, ultraviolet) and electrical resistance. Insulation, water vapor barrier and other properties. Therefore, if the backing film cannot meet the environmental test of the solar cell module for 25 years in terms of aging resistance, insulation resistance, moisture resistance, etc., the reliability, stability and durability of the solar cell will not be guaranteed, and the solar cell panel will be Quarantine, cracking, blistering, yellowing, etc., which occur in the general climatic environment for 8 to 10 years or in special environmental conditions (plateau, island, wetland, etc.) for 5 to 8 years, causing the battery module to fall off and the battery The slippage of the chip, the reduction of the effective output power of the battery, etc., is even more dangerous. The battery component will be electrically arced at a lower voltage and current value, causing the battery component to burn and cause a fire, resulting in personal safety damage and property. loss.
Studies have shown that PET molecular backbone contains a large number of ester groups, has a good affinity with water, is prone to water plasticization, and even a small amount of water can lead to degradation of the molecular backbone. The change of aging performance of PET in the process of damp heat aging is affected by three factors: crystallinity, water plasticization and hydrolysis. All factors are active from beginning to end. Different factors play a leading role in different environments and different stages. In the early stage of aging, crystallization is the dominant factor, which increases the Young's modulus and the maximum tensile stress, but makes the material brittle and reduces the impact strength. Then water plasticization becomes the main factor, which increases the toughness of the material, but the hydrolysis reaction rises very quickly. As a main factor, it causes the PET macromolecular chain to break and the molecular weight to drop, thereby causing damage to mechanical properties. The increase in temperature will significantly accelerate the above process, so water and heat are the main reasons for the sharp decline in physical and mechanical properties of PET [1]. In addition, ultraviolet radiation also causes a significant decrease in the molecular weight and elongation of PET, and an increase in crystallinity, thereby embrittlement of the material. Therefore, the non-fluorinated back film formed by the adhesive bonding of the multilayer PET by the adhesive can not meet the environmental test and use requirements of the commercial crystalline silicon solar cell module for 25 years, such as damp heat, dry heat and ultraviolet. It is difficult to fit the package for crystalline silicon solar cell modules.
The fluorine material on the surface of the fluorine-containing back film is large in electronegativity due to fluorine, the radius of van der Waals is small, the fluorocarbon bond energy is extremely strong (up to 485 KJ/mol), and the unique fluorinated chain structure of the spiral rod-shaped molecules is tight. It is hard and has a smooth surface, which makes the fluororesin excellent in weather resistance, heat resistance, high and low temperature resistance and chemical resistance. The excellent properties of fluororesin make fluorine-containing materials (fluorinated or fluorocarbon coatings) excellent in weather resistance and ensure long-term outdoor reliability. As a backing film that acts as a barrier to the external environment and the interior of the solar cell, the air surface in contact with the outside and the bonding surface (lighting surface) combined with the EVA are subjected to major aging effects. However, as current back-film development and manufacturing companies consider the cost pressure of double-sided fluorine-containing materials for the production of the entire back film, manufacturers use EVA materials (or other olefin polymers) instead of EVA in the double-sided fluorine-containing TPT structure back film. The fluorine material of the bonding surface (lighting surface), so that the back film of the TPE structure with one side fluorine is present. After the backsheet of this type of TPE structure is bonded to the EVA film for component packaging, the back surface film cannot be subjected to long-term UV aging after the component is mounted because the illuminating surface has no fluorine-containing material to effectively protect the PET main substrate of the back film. Tests, within a few years, the components will have adverse phenomena such as yellowing of the back film, embrittlement and aging, which seriously affect the long-term power generation efficiency of the components. Therefore, the back film of the single-sided fluorine-containing TPE structure is not suitable for the crystalline silicon solar cell. The package used by the component.
Coated back film: development trend of back film materials
At present, the mainstream solar cell backsheets are mostly double-sided fluorine-containing compound backing film and coated back film. The compound type solar cell back film (TPT, KPK, etc.) is mostly formed by bonding a fluorine film such as PVF or PVDF developed by some fluorine chemical companies in Europe and America through an adhesive and a PET substrate. The compound type back film has adhesive on both sides of the inner PET substrate, and the quality level of the adhesive is different. In addition, the composite process is mixed, and the composite back film is affected by the dual factors of humidity and temperature during long-term outdoor use of the battery component. It is prone to damage such as hydrolysis of the adhesive layer, which eventually leads to interlayer peeling of the fluorine film (PVF or PVDF, etc.) and the PET substrate, and it is difficult to meet the long-term reliability requirements of the battery assembly.
At the same time, due to the limitations of manufacturing patent technology and the hydrophilic modification treatment technology on the surface of fluorine film, fluorine film products such as PVF and PVDF have not been domestically produced in China. Therefore, Chinese companies that develop and produce double-sided fluorine-containing solar cell backsheets using fluorine film such as PVF or PVDF have long been subject to foreign fluorine film manufacturers, and their back film manufacturing costs are high, and they are suitable for bonding fluorine film and PET. High-quality adhesives are mostly monopolized by foreign manufacturers and are difficult to import. Some domestic film manufacturers can only use some ordinary polyurethane, epoxy or acrylic adhesives. These adhesives are easy to age and cannot meet the 25-year durability requirements.
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