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Recently, more and more component manufacturers have begun to pay attention to the phenomenon of horizontal and vertical distribution of wide dark lines on the components. This type of image called a snail or lightning bolt greatly affects the appearance of components and causes this phenomenon. The real reason is that the surface of the cell is oxidized.
The components are protected by the backplane and the frame silicone. How is the battery oxidized? And the shape of snails is very special. How does this shape like a snail crawl mark appear?
In the analysis of the snail-printed components, we found that the lines are generally accompanied by the cracking of the cell. In the EL imaging, it can be clearly seen that the cell fragments in the snail-like components are cracked. Although this cracking does not seem to have a large effect on the power attenuation of the component, the cracking of the cell itself has an effect on the power generation, which can prevent current from flowing from the finger electrode to the bus bar.
It has been reported that if the crack reaches 8% of the surface area of ​​the battery, it will have an adverse effect on its output. If the battery breaks up to 12%, a linear drop in the relative fracture area occurs. Some analysis reports indicate that the snail lines themselves have no effect on the attenuation of the components. This is because the area where the snail lines appear accounts for a very small part of the entire module, so it does not have much influence on the light transmittance. When peeling and analyzing the snails, it can be seen that the cell itself does not change color, and the discoloration position is located at the junction of the silver paste and the packaging material EVA.
Foreign articles have pointed out that the appearance of snail lines is related to water vapor, especially the water vapor that passes through the backplane, seeps through the cracks at the EVA film, and accumulates on the surface of the cell, causing the silver paste to oxidize and form black lines. The author will laminate cracked cells, do not back pack, put the components into the damp heat aging box, can not reproduce snail lines, showing that the appearance of snail lines is not only related to the unilateral effects of water vapor, I believe that with EVA glue The relationship between the membrane is large.
EVA film formulations include cross-linking agents, antioxidants, coupling agents and other additives, in which the cross-linking agent generally uses peroxide to initiate cross-linking of EVA resin, since peroxide is a highly active initiator, If there is more residual crosslinker after laminating, it will initiate and accelerate the generation of snails. EVA film additives have purity indicators, in general, the purity requirements of 99.5% or more. The impurities in the additives are mainly the by-products of the synthesis and the residues of the additives in the synthesis. They exist in the form of small molecules and have a high boiling point and cannot be eliminated from the system by laminating vacuum. Therefore, the purity of the additives is not high. , then these impurities will also affect the stability of EVA film, may cause the appearance of snail lines.
The author believes that the appearance of snails is an integrated process. The additives in the EVA film, the composition of the silver paste on the surface of the cell, the cracking of the cell, and the catalysis of water in the system all contribute to the formation of snails. The appearance of the snail pattern phenomenon is not inevitable, but it has occasional triggering factors, which also explains the fact that not all components in the same batch of components must appear.
Component manufacturers will undergo at least two EL imaging inspections at the time of shipment of the component, and the shipment will not occur until the test has been cracked. However, since the transportation process, especially during the installation process, inevitably impacts or vibrates the components, the impact will reach the battery sheet through the frame, the glass, the backboard, and the packaging material, which will cause the delicate cell sheet to have tiny cracks and bury the snail. Patterns appear hidden. In the author's investigation, there is no sufficient understanding of the components in the process of component installation and the risk of cracking increases.
Some people think that using EVA film factory will cause the cell sheet cracking, and I deliberately add 50% of different types of materials, while controlling the dispersion of the cross-linking agent when the additive and the resin are mixed, the test results surface test The local cross-linking degree of EVA film is different, and the difference in cross-linking degree can reach 30%. Even if the component laminated by EVA film in this test can not cause the cell sheet to crack, the author thinks that the addition of the back material will Increase the probability of cracking of the battery, but the chance is not great. On the contrary, some EVA film suppliers will mix EVA resins with different melting fingers, which will have a greater chance of causing cell cracks.
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