At present, the main sealing material for crystalline silicon solar cells is EVA, which is a copolymer of ethylene and vinyl acetate. Its chemical formula structure is shown in Figure 1.
EVA film is a kind of thermosetting hot melt adhesive that will undergo cross-linking reaction when heated to form a thermosetting gel resin. Figure 2 shows a schematic diagram of the cross-linking reaction of EVA. It is non-sticky and anti-sticky at room temperature, which is easy to operate. After certain conditions, it will melt and bond and cross-link and solidify, and become completely transparent. Long-term practice has proved that it has achieved quite satisfactory results in both solar cell packaging and outdoor use.
The EVA film is a linear macromolecule before being laminated. When heated, a cross-linking reaction occurs, and the cross-linking agent decomposes to form active free radicals, which initiates the intermolecular reaction of EVA to form a network structure, thereby improving the mechanical properties and resistance of EVA. Heat resistance, solvent resistance, and aging resistance.
The cured EVA has elasticity, encapsulates the solar cell group, and is bonded with the upper protective material glass and the lower protective material TPT (polyvinyl fluoride composite film) through vacuum lamination technology. On the other hand, it can increase the light transmittance of the glass after being bonded to the glass, and play an anti-reflection effect, thereby increasing the output power of the photovoltaic module.
①. The relationship between the main components of EVA and the main performance parameters
EVA film is mainly composed of EVA main body, cross-linking agent system (including cross-linking initiator and cross-linking agent), polymerization inhibitor, heat stabilizer, light stabilizer, silane coupling agent, etc. The influence of the main components of EVA on the performance of EVA is shown in Table 1.
| name | Impact on performance |
| VA content | The higher the VA content, the greater the fluidity, the lower the softening point, the better the bonding performance, and the greater the polarity. |
| Molecular weight and distribution | The higher the molecular weight, the worse the fluidity and the better the overall mechanical properties |
| Crosslinker system | Determine the curing temperature and curing time of EVA. A good cross-linking agent system can reduce the possibility of air bubbles, and at the same time, there are fewer residual free radicals, which can reduce unstable factors |
| Polymerization inhibitor | Mainly used to delay the time of the cross-linking reaction, which is conducive to the elimination of bubbles when vacuuming |
| Antioxidant | Improve the oxidation resistance of EVA |
| Light stabilizer | Improve the UV yellowing resistance of EVA, capture free radicals, and delay the aging of EVA |
| A silane coupling agent | Improve the bonding strength between EVA and glass |
②. Technical requirements of EVA film
There are two upper and lower EVA layers in photovoltaic modules. The upper EVA film not only requires high light transmittance (the transmittance of 0.5mm thick film should be greater than 91% in the wavelength range of 400~1100nm) and high UV resistance and thermal radiation resistance, but also It has good insulation performance, temperature resistance (-60℃~90℃) and reliable adhesion. In addition to the above-mentioned properties, the lower EVA film needs to have good thermal conductivity in order to dissipate the heat accumulated on the silicon cell sheet as soon as possible, and to prevent the silicon cell’s photoelectric conversion efficiency from falling rapidly. Some data indicate that the increase in temperature will lead to a decrease in battery efficiency. Without considering battery cooling, the operating temperature of solar cells can reach 70°C or higher. At this time, the actual power of the battery will be higher than the power under standard conditions. Decrease by 18%~29%. The main performance indicators of EVA are shown in Table 2.
| performance | index |
| Glass transition temperature/℃ | <-40 |
| Working temperature/℃ | -40 ~90 |
| Modulus/MPa | <20.7 |
| Hydrolyzable | 80℃, relative humidity 100%, still not hydrolyzed |
| Thermal oxidation resistance | Stable above 85℃ |
| Molding temperature/℃ | <170 |
| UV absorption and degradability | Not sensitive to wavelengths above 350nm |
| Thickness/mm | 0.1-1.0 |
| Odor, toxicity | without |
| Insulation voltage/V | ＞600 |
③. EVA film storage and use points
(1) Storage temperature is 5℃~30℃, humidity is less than 60%, avoid light, keep away from sunlight, heat source, dust and fire.
(2) The storage time for complete packaging is half a year, and the storage time is 3 months after unpacking. It should be used as soon as possible, and unused products should be repackaged in the original packaging or equivalent packaging.
(3) Do not expose the entire roll of film that has been unpacked to the air. If the film cut into pieces cannot be used up on the same day, it should be covered tightly and repackaged.
(4) Do not touch the surface of the EVA film with bare hands, pay attention to moisture and dust, and avoid contact with colored objects.
(5) The EVA film will be slightly tightened during rewinding, so do not pull it forcefully during unwinding and cutting, and leave it for half an hour after cutting to allow the film to naturally retract before being used for lamination.
(6) In the process of cutting and laying EVA film, it is best to set up a static elimination process to eliminate static electricity in each component in the assembly, so as to ensure the quality of the package assembly.
④. Common causes of EVA failure
(1) Yellowing: EVA yellowing is caused by two factors (mainly the additive system reacts with each other to turn yellow; secondly, the EVA molecule itself deacetylates under the conditions of oxygen and light to cause yellowing), so the formula of EVA Decide whether its anti-yellowing performance is good or bad.
(2) Bubbles: There are two types of bubbles: bubbles appear during lamination and bubbles appear during use after lamination. The bubbles appearing during lamination are related to the additive system of EVA, the compatibility of other materials with EVA, and the lamination process; there are many factors that cause bubbles to appear after lamination, which are generally caused by poor matching between materials.
(3) Delamination: The reason for delamination from the backplane is that the degree of crosslinking is unqualified, and the bonding strength with the backplane is poor; the reason for delamination with glass is the defect of silane coupling agent, the dirty glass, the poor performance of silica gel encapsulation, and the crosslinking. Degree is unqualified.
⑤. EVA film inspection
EVA film is very important to the final performance of photovoltaic modules. Therefore, the performance of EVA film needs to be comprehensively and carefully tested, especially the test of EVA cross-linking degree, as shown in Table 3.
| Test items | Inspection content | Detection method (use tools) |
| Package | Whether the packaging is intact; confirm the manufacturer, specifications, and shelf life | Visual inspection |
| Exterior | Check the EVA surface for black spots, stains, wrinkles, creases, stains, cavities, etc. | Visual inspection |
| size | The width error is ±2mm; the thickness error is ±0.02mm | Vernier caliper, tape measure |
| Thickness uniformity | Take 10 pieces of film of the same size and compare after weighing, the weight ratio of the heaviest and lightest film is not more than 1.5% | Electronic scale |
| Peel strength | Peel strength of EVA and TPT: Use a wallpaper knife to cut the middle of the backplane with a width of 1cm, and then use a tension meter to pull apart the TPT and EVA. The tensile force is greater than 35N. The peeling strength of EVA and glass: The method is the same as the above, and the tensile force is greater than 20N. | Tensile meter or universal testing machine |
| Degree of cross-linking | Peel off the TPT insulation layer and remove the EVA sample (the mass should be greater than 0.5g); the mesh bag is cleaned with absolute ethanol and then dried; the sample is put into the mesh bag and weighed; xylene extraction; drying and weighing; calculation according to the formula Degree of cross-linking | Drying box, electronic balance, three-necked flask, heating mantle, reflux condenser, etc. |
The degree of crosslinking of EVA can be measured by the mechanical method or the equilibrium swelling ratio method. The mechanics method is mainly introduced here, and the flowchart is shown in Figure 3 and Figure 4.
The calculation formula is
(W3﹣W1)/(W2﹣W1)x100%
In the formula: W1 is the weight of the empty mesh bag; W2 is the weight of the mesh bag with the sample; W3 is the weight of the mesh bag after the extraction and drying, without the bundled copper wire and number plate.