Photovoltaic modules – what types of backsheet materials are there?

What are the various backplane materials?
  1. Fluorine material

In order to make the outer layer of the polymer backsheet have good weather resistance, fluorine materials are often used as backsheet materials. Fluorine materials have a unique molecular structure, and their weather resistance, heat resistance, high and low temperature resistance and chemical resistance are All are excellent. The electronegativity of fluorine is large, the van der Waals radius is small, and the CF bond energy is as high as 439.2kJ/mol, which is the largest bond energy among the covalent bonds of polymer materials. Ultraviolet light in sunlight has a short wavelength, strong penetrating power, and great damage to materials. Almost all ultraviolet light below 290 nm is absorbed by the ozone layer in the atmosphere. The ultraviolet wavelength that can reach the surface is generally 290~400 nm. As can be seen from the picture, except for the CF bond, other molecular bonds are easily destroyed by ultraviolet light, so fluorine material is the best choice for the outer layer material of the polymer backplane. Commonly used fluorine materials are polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer (THV), etc. PVDF and PVF are used more in the industry.

Common molecular bond energies
Common molecular bond energies

Polyvinyl fluoride (PVF) was developed by DuPont and named Tedlar. PVF is commonly used in first-generation and second-generation products. The application of PVF in the backsheet of photovoltaic modules has a history of at least 25 years. Because it is monopolized by DuPont and exclusively supplied, the cost is relatively high. In recent years, the market share of PVF in the photovoltaic industry has gradually declined. The PVF processing technology is relatively complicated. When manufacturing PVF film, PVF containing latent solvent is extruded onto a stainless steel plate, and the PVF film is obtained after volatilizing the solvent. The particularity of the manufacturing process leads to many pinhole defects on the surface of the film. , so the water vapor barrier ability of PVF is not high.

Polyvinylidene fluoride (PVDF) is not easy to form a film alone, and a plasticizer needs to be added to improve its film-forming properties. Due to the addition of plasticizers, its aging resistance may be reduced. In order to ensure that the aging resistance of this material is not reduced, the French company Akema has innovatively developed a three-layer PVDF film structure. The inner and outer two layers are pure PVDF, and the middle layer is a plasticized layer of PVDF. Compared with PVF, the thickness is the same. The water vapor transmission rate of the PVDF film is about one tenth of that of PVF. Due to its high cost performance, strong processing adaptability and sufficient supply, PVDF currently occupies a relatively high market share.
Because fluorine materials have greater electronegativity, the higher the fluorine content, the lower the surface energy of the material and the worse the bonding performance. Therefore, special surface treatment is generally required to form good bonding with other materials; in addition, fluorine Material prices are also relatively high. At present, many companies are studying the use of fluorine-free materials with strong weather resistance as outer layer materials.

  1. Р Е Т

PET is polyethylene terephthalate, and PET is mainly used to make the middle layer of the backplane to provide skeleton support for the entire backplane. PET can also be modified to improve its weather resistance and be used to make the outermost layer of the backsheet.
PET has good barrier properties, heat and cold resistance, insulation and dimensional stability. Because of its biaxially stretched manufacturing process, it has excellent mechanical properties. Although PET has good barrier properties, it is prone to hydrolysis reaction due to the large amount of ester groups in its molecular main chain, which will lead to a sharp decline in mechanical properties. Therefore, many manufacturers have greatly improved its hydrolysis resistance by modifying it. However, this will increase the production cost, so the requirements for PET thickness are usually slightly reduced when choosing hydrolysis-resistant PET to achieve suitable cost performance and differentiated applications.

  1. New Backplanes and Applications

With the advancement of upstream technology and the diversification of downstream applications, various new photovoltaic module technologies have emerged one after another, and there are also more types of photovoltaic module backsheets.

3.1 Glass blue plate
Glass is an inorganic material, will not age, and is impermeable to water. The use of glass as the backplane of the module can improve the sealing, insulation, PID resistance, black line resistance, and anti-cracking performance of the module, thereby greatly enhancing the reliability of the module. The backplane selection of the components provides a good solution. Generally, the back glass does not need to use ultra-white patterned glass, but ordinary float glass can be used, and ultra-white patterned glass should be used for double-sided battery modules.

Some people in the industry believe that when glass is used as the backplane of the module, the small molecule gas such as acetic acid generated by the degradation of the macromolecules inside the module is not easily released, which will have a negative impact on the reliability of the double-glass module. In this regard, after a lot of research, Trina Solar has come to the following conclusions: the main causes of the aging of polymer materials such as EVA under natural outdoor conditions are moisture, heat, oxygen and ultraviolet radiation, and the aging process is accompanied by light Oxygen aging, photothermal aging, and hydrolysis are three interacting processes, in which water vapor plays a catalytic role to a large extent, and the double-glass module uses a glass backplane, so it can effectively prevent water vapor from infiltrating the inside of the module, thereby increasing the The aging process of the EVA material is greatly alleviated, so that the aging process will hardly release acetic acid, and no small molecular gas will be generated, so the double glass module has very high reliability.

3.2 . Conductive backplane
The conductive backsheet (see Figure 2) is a combination of hermetic protection and electronic interconnection. Its manufacture draws on printed circuit board (PCB) technology and uses a composite method to combine conventional photovoltaic module backsheets and metal foils with specific patterns. The circuits are glued together. This type of backplane is mainly used for MWT and EWT components.

Conductive backplane diagram (left) and enlarged view of partial
Conductive backplane diagram (left) and enlarged view of partial

The early conductive backplanes were generally prepared by PCB manufacturers using traditional photolithography methods to prepare metal circuits, which were very costly and did not achieve industrialization. In recent years, some photovoltaic companies have begun to use mechanical or laser processing methods to prepare metal circuits, and can use low-cost aluminum foils to replace traditional electrolytic copper foils as metal circuit layers, greatly reducing production costs, and new conductive backplanes have begun to enter the market. mass production.

Read more: What is a high-efficiency solar cell?

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