The crystalline silicon solar cell encapsulating material is EVA, which is a copolymer of ethylene and vinyl acetate, having the following chemical structure (CH2-CH2)-(CH-CH2)
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O
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O — O — CH2
EVA is a hot-melt adhesive, which is non-sticky and anti-adhesive at room temperature for operation. After certain conditions of hot pressing, it will be melt-bonded and cross-linked and cured, and becomes completely transparent. Long-term practice has proved that it is in solar cells. Both packaging and outdoor use are quite satisfactory.
After curing, EVA can withstand atmospheric changes and has elasticity. It will “set the under-cushion†of the crystal silicon wafer group, encapsulate the silicon wafer group, and protect the material glass with the upper layer, and the lower protection material TPT (polyvinyl fluoride composite film). , Using vacuum lamination technology to bond into one.
On the other hand, it adheres to the glass to increase the light transmittance of the glass, acts as an antireflection agent, and has a gain effect on the output of the solar cell module.
The thickness of EVA is between 0.4mm and 0.6mm. The surface is smooth, uniform in thickness, contains cross-linking agent, can be crosslinked at a curing temperature of 150°C, and a stable adhesive layer is formed by an extrusion molding process.
There are two main types of EVA: 1 rapid curing 2 conventional curing, different EVA lamination process is different
An EVA film layer with a thickness of 0.4 mm with an anti-ultraviolet agent, an antioxidant, and a curing agent is used as a sealant for the solar cell, so that it is sealed and bonded to the glass and the TPT. The EVA used for packaging silicon solar cell modules is mainly selected based on light transmission performance and weatherability.
Principle
EVA has excellent flexibility, impact resistance, elasticity, optical transparency, low temperature flexing, adhesiveness, resistance to environmental stress cracking, weather resistance, chemical resistance, and heat sealability.
The performance of EVA mainly depends on the molecular weight (indicated by the melt index MI) and the content of vinyl acetate (expressed in VA). When MI is constant, the elasticity, softness, adhesion, compatibility and transparency of VA are improved, and the content of VA is decreased, which is close to that of polyethylene. When the VA content is constant, MI decreases, the softening point decreases, and the workability and surface gloss improve, but the strength decreases, and the molecular weight increases, and the impact resistance and the stress cracking property can be improved.
Different temperatures have a greater impact on the degree of adhesiveness of EVA. The degree of adhesiveness of EVA directly affects the performance and service life of the components. In the molten state, EVA is bonded to crystalline silicon solar cells, glass, and TPT, and both physical and chemical bonding occurs during this process. Unmodified EVA is transparent, soft, has hot-melt adhesiveness, low melting temperature, and good melt fluidity. However, its poor heat resistance, easy to extend and low elasticity, low cohesive strength and poor creep resistance, easy to produce thermal expansion and contraction lead to chip fracture, making the bond delamination. Therefore, by adopting a chemical bonding method to modify EVA, the method is to add an organic peroxide cross-linking agent to EVA. When EVA is heated to a certain temperature, the cross-linking agent decomposes to generate free radicals, triggering EVA molecules. The combination of the three-dimensional network structure, resulting in cross-linking EVA adhesive curing, when the degree of bonding to more than 60% can withstand atmospheric changes, no longer thermal expansion and contraction.
The principle of measuring the degree of adhesiveness:
Xylene extracts the unlinked EVA in the sample. The remaining undissolved material is the already glued EVA. Assuming that the total amount of sample is W1 and the weight of the undissolved material is W2, then the EVA adhesiveness is W2/ W1*100%.
2. Function introduction
a) Encapsulate the battery to prevent the external environment from affecting the electrical properties of the battery.
b) Enhances the light transmission of the module.
c). Bonding the cell sheet, tempered glass, and TPT together has a certain bond strength.
3. Material introduction
EVA used as a photovoltaic module package, mainly to the following performance requirements
a). Melt index affects the melting rate of EVA.
b). Softening Point Affects the temperature at which EVA begins to soften.
c). Transmittance There are different transmission rates for different spectral distributions. Here, the transmissivity is mainly referred to in the spectral distribution of AM1.5.
d) Density The density after glueing.
e) Specific heat The specific heat after glueing reflects the magnitude of the temperature increase in the case where the EVA after glueing absorbs the same amount of heat.
f). Thermal Conductivity The thermal conductivity after the glue, reflects the thermal conductivity of the EVA after the glue.
g). The glass transition temperature reflects the low temperature resistance of EVA.
h) Tensile Tensile Strength The tensile strength of EVA after a glued joint reflects the mechanical strength against fracture after EVA bonding.
i). Fracture elongation The elongation of EVA after the bonding has reflected the elongation properties of EVA after bonding.
j). Tension Coefficient EVA tension coefficient after glueing reflects the tension of EVA after glueing.
k). Water absorption directly affects the sealing performance of the cell.
l). Adhesive ratio EVA's degree of adhesive directly affects its water resistance.
m) Peel strength Reflects the bond strength of EVA to glass.
n). Resistant to UV light: affects the outdoor life of the module.
o). Heat aging: Affects the outdoor service life of the module p). Low-temperature environment aging: Affects the outdoor service life of the module
4. Quality requirements and incoming inspection
a). Appearance inspection: EVA surface without creases, stains, flat, translucent, no stains, embossing is clear.
b) Measuring with a precision thickness gauge of 0.01mm, measuring at least five points in the direction of the amplitude, taking the average value, the thickness conforming to the agreed thickness, and allowing the tolerance to be ±0.03mm.
Measured with a precision 1mm steel ruler, the amplitude is in accordance with the agreed thickness, and the allowable tolerance is ±3.0mm.
c) Transmittance test (1) Take the film size of 50mm x 50mm, use a 50mm x 50mm x 1mm glass-loaded glass, laminate the glass/film/glass three layers.
(2) The above sample is placed in a laminator, heated to 100°C, vacuumed for 5 min, then pressurized at 0.5 MPa for 5 min; then placed in a curing oven, cross-linked and cured according to the required curing temperature and time of the product , Then remove it and cool it to room temperature.
(3) Inspect according to the provisions of GB2410.
d). Degree of cross-linking inspection (1) Apparatus and apparatus capacity 500ml to 1000ml, 24†ground round bottom flask; reflux condenser with 24†grinding mouth; electric heating jacket or electric heating oil with temperature controller Bath; vacuum oven; with 0.125mm (120 mesh) stainless steel wire mesh, cut 80mm × 40mm, folded into 40mm square, both sides of the folded into 6mm fixed, made of the top opening of the bag.
(2) Reagents Xylene: (AR grade)
(3) Sample preparation Take a piece of plastic film, and after the TPT/film/latex/glass are laminated, cure and cross-link according to the usual one-time curing process (or cure cross-linking according to the manufacturer's process requirements). Film cut into small pieces to be used.
(4) Inspection procedure l Wash, dry, and weigh the stainless steel wire mesh bag to W1 (accurate to 0.01g).
l Take a sample of 0.5g±0.01g, place it in a stainless steel mesh bag and weigh it to W2 (accurate to 0.01g).
l Seal the bag mouth and make a sample package, and weigh it as W3 (accurate to 0.01g).
l The sample was suspended in a flask under a reflux condenser with a thin wire. The flask was filled with 1/2 xylene solvent, heated to about 140°C, and the solvent refluxed at a rate of 20 drops/minute for 5 to 6 hours. 40 drops/minute.
l Cool out the sample bag, suspend to remove solvent droplets, and then put it in a vacuum oven, the temperature is controlled at 140 °C, the vacuum degree is 0.08Mpa, dry 3h, completely remove the solvent.
l Remove the sample pack from the vacuum oven, place it in a desiccator and cool it for 20 minutes, then take it out and weigh it to W4 (accurate to 0.01g)
l Calculation of results: C=[1-(W3-W4)/(W2-W1)]×100
In the formula:
C—degree of cross-linking (%)
W1 - Empty bag weight (g)
W2 - bag weight with sample (g)
W3 - sample weight (g)
W4—Pack weight (g) after solvent extraction and drying.
e) Peel strength test (1) Take two pieces of film with dimensions of 300mm x 20mm as the sample, and press TPT/film/film/glass to overlap.
(2) According to the usual curing process for curing.
(3) Inspect according to GB/T2790 regulations.
f) Resistance to ultraviolet light aging test The film was placed in an aging chamber and irradiated continuously for 100 h before visual comparison.
g) Evenness test Take 10 sheets of the same size for weighing, then compare the weight of each sheet, between the maximum and minimum must not exceed 1.5%
5 inspection rules
According to the manufacturer's factory lot number for sample sampling, the contents of the fourth chapter of the full inspection, one does not meet the inspection requirements, the lot number of products for the full inspection, if there is still does not meet the fourth chapter d), e), g) related inspection requirements , determine the batch as unqualified incoming materials.
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