What are the important accessories for solar photovoltaic modules?

EVA is one of the most critical packaging materials in the production process of solar modules. It covers the top of the cell and seals it in the middle to protect the cell; EVA has a high light transmittance after melting, which can improve the light transmittance. The incidence rate increases the output power of the module; in addition, in the module production process, lamination is a key link, and the parameter settings of the laminating machine are basically set around the characteristics of EVA, so EVA is very important for module production.

1. Ingredients

The main component of EVA is a copolymer of ethylene and vinyl acetate, plus various additives such as cross-linking agent, thickener, antioxidant, light stabilizer and so on.

<1>Cross-linking agent——The amount of cross-linking agent added is high, and the degree of cross-linking is high, but too much is easy to age and yellow. Therefore, for a good EVA film product, the formula is the key, by the process flow, process equipment, production environment, etc.

<2> VA content - the molecular weight (melting index) is certain, the higher the VA content, the elasticity of EVA, impact resistance, flexibility, stress crack resistance, weather resistance, adhesion, compatibility, heat sealing , Solderability, radiation crosslinking, transparency, gloss, density, etc. increase, while strength, hardness, melting point chemical resistance, flexural stress, thermal denaturation, isolation, etc. decrease.

<3> Melt index (M1)——The VA content is constant, the higher the melt index, the fluidity of the melt increases, and the viscosity of the melt, toughness, tensile strength, stress crack resistance, etc. decrease.

Note:

Ethylene and vinyl acetate are soluble in xylene, but insoluble in xylene after cross-linking curing. This characteristic is the theoretical basis for the EVA cross-linking degree experiment, which can be used to test the cross-linking degree of the laminated EVA.

·Crosslinking agent is an organic peroxide. At a certain temperature, it will decompose to generate free radicals, triggering the combination of EVA molecules, forming a three-dimensional network structure, and curing EVA. This temperature is the curing temperature of EVA. If the temperature is too high, the cross-linking agent will decompose, producing oxygen, causing the generation of air bubbles inside the component.

·Light stabilizer can improve the UV resistance of EVA.

Antioxidants can improve the antioxidant capacity of EVA and effectively prevent EVA from aging and yellowing.

2. Features

EVA has excellent flexibility, impact resistance, elasticity, optical transparency, low temperature bending, adhesion, environmental stress crack resistance, weather resistance, corrosion resistance, heat sealing and electrical properties.

EVA is a kind of hot melt adhesive, that is, at normal temperature, EVA is solid, has no viscosity, and has poor light transmittance. When the EVA is heated to a certain temperature, the EVA will melt and bond to the objects in contact with it. EVA used for solar cell encapsulation is a specially designed thermosetting hot melt adhesive, that is, a cross-linking reaction occurs while heating and melting. When the temperature is low, the speed of the cross-linking reaction is very slow, and the time required to complete the curing is relatively long, otherwise the time required is relatively short. Therefore, it is necessary to choose an appropriate lamination temperature, so that the EVA can obtain fluidity in the melt, and at the same time, the curing reaction occurs. As the reaction progresses, the degree of cross-linking increases, and the EVA loses its fluidity and acts as an encapsulation. The performance at each temperature is:

• Melting temperature (70-80°C). At this time, the EVA is heated and melted, and the fluidity is good, which is the best time for vacuuming.

· Curing temperature. At this time, the cross-linking agent contained in EVA generates free radicals, and cross-linking occurs between EVA molecules, resulting in a three-dimensional network structure, poor fluidity, and high viscosity. This temperature is suitable for laminating components to make their structure tighter and higher bond to glass and backplane.

·>Cure temperature. At this time, the cross-linking agent decomposes gas, which is easy to generate bubbles in the components, and at the same time, the cross-linking degree of EVA decreases, and it is easy to vulcanize and turn yellow, resulting in shrinkage.

3. Storage

1 EVA film has strong adsorption capacity. During use, pay attention to moisture-proof and dust-proof, and avoid contact with colored objects.

·If it absorbs moisture, it will affect the bonding strength of EVA to glass and backplane, and the moisture in it will vaporize as the temperature rises, resulting in bubbles.

·If the dust is vacuumed, it will definitely affect the light transmittance of the module, and it will lead to a decrease in the adhesion of EVA to the glass and the backplane, and even accompanied by the generation of air bubbles.

·If it comes into contact with colored and unclean objects, it is easy to be polluted.

2. Do not expose the whole roll of film with its outer packaging to the air. If the film that is cut into pieces cannot be used up on the same day, it should be covered tightly.

3 The storage environment is below 30 degrees and humidity below 60%.

4. Quality problems related to EVA

1 bubble or lack of glue.

The lamination temperature is too high, the cross-linking agent decomposes the gas, resulting in the reduction of the cross-linking degree and the generation of bubbles

·EVA absorbs moisture, which affects the degree of cross-linking and water vapor.

2 back plate protrusions

EVA shrinks (too long vacuum time, or too high lamination temperature)

3 Cells are displaced or broken

·The pressing speed is too fast and the pressure difference is too large.

· EVA shrinks.

·There is foreign matter in the module.

4 turns yellow

• The lamination temperature is too high.

• Lamination time is too long.

5. Experiments related to EVA

1. Cross-linking degree test

The reason why EVA can form a whole by lamination of battery, glass and backplane is because EVA contains a cross-linking agent, which reacts at a certain temperature and can bond other materials together. In order to check the bonding strength between the materials of the components after lamination, or to check whether the cross-linking degree of a certain brand of EVA is qualified, the EVA cross-linking degree test is usually carried out.

·Principle: EVA adhesive, in the application process, due to heat bonding and curing, part of EVA is cross-linked into gel. The uncrosslinked EVA is completely soluble in the xylene solution, while the crosslinked EVA is almost insoluble.

·Method: At a temperature of 140°C, the EVA sample was extracted with xylene solvent for about 5 hours, and the mass before and after cross-linking was weighed, so as to measure the degree of cross-linking.

·Standard: The degree of cross-linking of EVA is generally between 70% and 90%.

2 Shrinkage experiment

·Principle: EVA film is heated and melted when the temperature is higher than the melting temperature, and the external dimension changes.

·Method: Set the temperature of the laminator to 120°C, cut the EVA into a size of 100mm (horizontal)*200mm (vertical), put it on the glass velvet surface and heat it on the laminator for 3-5 minutes together with the glass, then remove it. , cool, measure its size, take the minimum value to calculate the shrinkage.

·Standard: Longitudinal (MD)<4%; Transverse (TD)<2%.

6. Precautions

1. Due to the differences in raw materials, auxiliary materials, formulas, technological processes, process equipment, and production environments, the EVA film of different manufacturers has a large gap in product quality. There is also a big difference in the setting of lamination parameters. Among them, the setting of lamination temperature is the most critical. If the temperature is too high, the cross-linking and curing is fast, and the production efficiency is high, but it is easy to produce bubbles, lack of glue, displacement and other problems. The degree of crosslinking is not good, the bond strength is also affected, and the production efficiency is too low, but the probability of bubbles, lack of glue, displacement, bumps, etc. will be smaller.

2. The favorable combination of EVA and glass can increase the transparency of the light source, which is beneficial to increase the power of the module, but the bonding strength of the flat glass is not very satisfactory, and it is not as good as the suede ultra-clear glass.

3. There is a compatibility problem between EVA and the backplane and sealant. Since the chemical composition of the EVA film of each manufacturer is not completely consistent, and the chemical composition of the surface layer of each backsheet manufacturer is also different, there may be accidents such as low bonding strength during lamination. Of course, it is not ruled out that the settings of the lamination parameters are different reasonableness, etc.

4. For laminated components, it is best to open the high temperature cloth when the temperature drops below 80°C.

5. Putting an extra layer of high temperature cloth during lamination is conducive to vacuuming (relatively prolongs the scorch time).

6. Pay attention to the degree and direction of force when the components are trimmed, otherwise it is easy to cause delamination of the corners, especially at the four corners.

7. When using EVA film, you must pay attention to the vertical and horizontal directions. Generally, the horizontal shrinkage rate is very small and can be ignored; the vertical shrinkage rate is larger, and the good film is generally controlled within 5%. When using, the longitudinal direction is the same as the welded battery string, otherwise it is easy to shift. If the shrinkage rate of the adhesive film is too large, it is easy to cause bumps and shifts.

8. Do not directly touch the EVA film with your hands, and do not pull it hard, so as not to affect the use effect.

9. When each roll of film is opened and used, it is recommended to cut off the top layer and discard it, and the layer that is close to the core paper tube is also not recommended to be used. "