What are the bonding materials of photovoltaic modules?

What are the bonding materials of photovoltaic modules?

The photovoltaic module bonding material mainly refers to the material used in the module to protect the battery and bond the cover plate and the back plate, generally a polymer hot-melt film material. Common bonding materials mainly include EVA, PVB, epoxy resin and POE, etc. At present, EVA film occupies a dominant position in the market, and other materials are still less used in photovoltaic modules due to problems such as process and cost. When selecting the bonding material, it is necessary to consider the light transmittance of the material, the match with the refractive index of the cover material, the bonding strength, the shrinkage rate, the elongation rate, the UV resistance, the aging resistance and the vulcanization performance. Selecting an appropriate thickness of bonding material helps to improve the yield and reliability of crystalline silicon photovoltaic modules during the lamination process. Below we describe other bonding materials besides EVA.

1. POE film
POE (Polyolefin elastomer) film is a metallocene polyethylene elastomer polymerized by ethylene and 1-hexene or 1-octene under the action of a metallocene catalyst system. The earliest POE used in photovoltaics is non-cross-linked, but due to the high temperature of the modules operating outdoors (especially in high-temperature and high-irradiation areas), the POE will soften. Shear phenomena, slippage occurs, which affects component appearance and reliability. In response to this problem, POE manufacturers have optimized and modified cross-linked POE to effectively solve the above problems.

Compared with EVA, which will release acetic acid gas during long-term use, the molecular structure of POE is more stable, and there is almost no gas release, and POE has higher volume resistivity, better thermal stability, and UV aging resistance. The biggest advantage of POE is that its water vapor transmission rate is only about 1/8 of that of EVA and silica gel, which can effectively block water vapor, better protect solar cells, and inhibit the power attenuation of components. Its high volume resistivity and low water permeability are improved. One of the important characteristics of the component’s anti-PID performance. Of course, POE also has shortcomings. Its glass bonding ability is not as good as EVA, which is easy to cause interface failure, and the lamination time is long, the process window is narrow, and the lamination process is easy to cause air bubbles, resulting in poor appearance, and its raw materials are basically imported, so the price is relatively expensive. . At present, domestic and foreign companies are stepping up the research and development and application of POE. If the cost can be reduced, I believe there will be a good development prospect.

2. PVB film
PVB (Polyvinyl butyral) is polyvinyl butyral. PVB film is a translucent film. It is extruded from polyvinyl butyral resin after plasticizer plasticization. It is generally used in the glass lamination industry. . Compared with EVA, PVB has good bonding performance, high mechanical strength, and good impact resistance, and is more suitable for photovoltaic modules for construction; but PVB has high water absorption, low volume resistivity, low light transmittance, and layered The pressing process is difficult to control. Although the improved PVB has also improved the volume resistivity and light transmittance, PVB is used for packaging, usually using an autoclave. Compared with the production process using a laminator, the component yield is low. Due to the complex process and high material cost, PVB-encapsulated photovoltaic modules have not been used on a large scale in the market. At present, only a small number of companies represented by CECEP are using them. CECEP has carried out a lot of innovation and effective in PVB and production equipment. At present, it is unique in mass production and application.

3. Epoxy resin
Epoxy resin is a polymer compound containing epoxy groups in its molecular structure. It is a relatively common adhesive. It has various product forms, including one-component and two-component, and can be made into liquid. It can also be made into powder. For example, the epoxy resin adhesive used for solar cells is a two-component liquid, which is ready to use when used. The biggest advantage of epoxy resin materials is that the formula can be ever-changing, and by changing the curing agent and accelerator, it can have various properties to meet various needs.

The use of epoxy resin to encapsulate solar cell components has a simple process and low material cost. However, due to the relatively poor performance of epoxy resin against thermal oxidation aging and ultraviolet aging, only some small components, such as those with an output power below 2w, are still used. Epoxy resin is used for encapsulation. The early lawn lamps used epoxy resin encapsulated photovoltaic modules (Figure 1). The modules using this encapsulation method can be used continuously for about 2 years outdoors. With the subdivision of solar energy application products, solar energy products encapsulated with epoxy resin will also have a certain market share according to application occasions and related life requirements.

What are the bonding materials of photovoltaic modules?
Figure 1. Small power components encapsulated in epoxy resin

4. liquid silicone
Silicone is a new type of packaging material made of silicone polymers. The main chain contains inorganic Si-O bonds, and its side groups pass through silicon and organic groups R (methyl, ethoxy, phenyl, etc.) connected. The polymer chain contains both inorganic structures and organic groups. This special composition and unique molecular structure combine the functions of inorganic substances with the characteristics of organic substances, thus reflecting the unique properties of silicone polymers. .

This packaging material has good light transmittance, which can effectively improve the conversion efficiency of the module. It also has high hydrophobicity, high chemical stability and extremely low water absorption, which can ensure the module has reliable sealing and insulation performance; in addition In addition, silicone has excellent adhesion to various substrates. Since this packaging material is liquid, two-component liquid or paste silicone is commonly used, so its packaging method is completely different from the traditional lamination method. It is necessary to add silicone mixing equipment, dispensing equipment, etc., although the initial equipment investment is relatively high. Large, but in the later production process, it can shorten the production time, reduce energy consumption, and reduce costs.

Liquid silicone has not been applied on a large scale because its packaging process is not compatible with current equipment, and there are problems such as low yield and low cohesive destructive power of the material itself. At present, companies such as BYD in the industry have been insisting on the research and development of silicone components, and the warranty period is said to be up to 50 years.

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