Panel glass and EVA film

Panel glass

The panel glass used in the battery module is low-iron ultra-white suede or smooth tempered glass. Generally, the thickness is 3.2mm and 4mm, and sometimes 5~10mm thick tempered glass is used for building material battery components. Regardless of the thickness, the light transmittance is required to be above 91%, the wavelength range of the spectral response is 320~1100nm, and it has a higher reflectivity for infrared light greater than 1200nm.

Low iron super white means that the iron content of this glass is lower than that of ordinary glass, and the iron content (ferric oxide) is less than or equal to 150 ppm, thereby increasing the light transmittance of the glass. At the same time, viewed from the edge of the glass, this kind of glass is whiter than ordinary glass, which is greenish from the edge.

Suede means that in order to reduce the reflection of sunlight, the surface of this kind of glass is treated by physical and chemical methods to reduce reflection, so that the surface of the glass becomes fluffy, thereby increasing the amount of light incident. Some manufacturers also use sol-gel nanomaterials and precision coating technologies (such as magnetron sputtering, double-sided immersion, etc.) to coat a thin film containing nanomaterials on the glass surface. This kind of coated glass can not only significantly increase the light transmittance of the panel glass (more than 2%), but also significantly reduce light reflection, and also has a self-cleaning function, which can reduce the pollution of rain and dust on the surface of the battery panel, and keep it clean, reduce light decay, and increase power generation rate by 1.5% to 3%.

The toughening treatment is to increase the strength of the glass, resist the impact of wind, sand and hail, and play a long-term role in protecting the solar cells. The tempering treatment of the panel glass is to heat the glass to about 700℃ through a horizontal tempering furnace, and use cold air to cool it quickly and uniformly, so that the surface forms a uniform compressive stress, and the inside forms a tensile stress, which effectively improve the bending and impact resistance of the glass. After tempering the panel glass, the strength of the glass can be increased by 4 to 5 times compared with ordinary glass.

EVA film

EVA film is a copolymer of ethylene and vinyl acetate. It is a thermosetting film-like hot melt adhesive. It is non-sticky at room temperature. After certain conditions, it will melt and bond and cross-link and solidify, becoming completely transparent. It is a commonly used bonding material in the packaging of battery components at present. The appearance of the EVA film is shown in Figure 1. Two layers of EVA film should be added to the solar cell module. The two layers of EVA film are sandwiched between the panel glass, the cell sheet and the TPT back sheet film to bond the glass, cell sheet and TPT together. After being bonded to glass, it can increase the light transmittance of the glass, play an anti-reflection effect, and have a gain effect on the power output of the battery assembly.

Panel glass and EVA film
Figure 1 – The appearance of EVA film

EVA film has the advantages of flat surface, uniform thickness, high transparency, good flexibility, good hot-melt adhesiveness, melt fluidity, no adhesion at room temperature, easy cutting, and low price. The EVA film contains a cross-linking agent, which can be cross-linked at a curing temperature of 150°C. The extrusion molding process is used to form a stable adhesive layer. The thickness is generally 0.2-0.8mm, and the usual thicknesses are 0.46mm and 0.5mm. The performance of EVA mainly depends on its molecular weight and the content of vinyl acetate. Different temperatures have a relatively large impact on the degree of cross-linking of EVA, and the degree of cross-linking of EVA directly affects the performance and service life of the module. In the molten state, the EVA film is bonded to the solar cell sheet, panel glass, and TPT back sheet material. This process involves both physical bonding and chemical bonding. In order to improve the performance of EVA, EVA is generally modified by chemical cross-linking. The specific method is to add organic peroxide cross-linking agent to EVA. When EVA is heated to a certain temperature, the cross-linking agent decomposes to generate free radicals, which initiates the combination between EVA molecules to form a three-dimensional network structure, which leads to cross-linking and curing of the EVA adhesive layer. When the degree of cross-linking reaches more than 60%, it can withstand the change of normal atmospheric pressure without thermal expansion and contraction. Therefore, the EVA film can effectively protect the battery, prevent the external environment from affecting the electrical performance of the battery, and enhance the light transmittance of the battery assembly.

EVA film not only plays a role of bonding and sealing in battery modules, but also plays a vital role in the quality and life of solar cells. Therefore, the EVA film used for module packaging must meet the following main performance indicators.

(1) Curing conditions: fast curing type film, heated to 135~140℃, constant temperature for 15-20min; conventional type film, heated to 145℃, constant temperature for 30min.

(2) Light transmittance: greater than 90%.

(3) Cross-linking degree: fast curing type film is greater than 70%, conventional type film is greater than 75%.

(4) Peel strength: glass/film is greater than 30 N/cm, TPT/film is greater than 20 N/cm.

(5) Temperature resistance: high temperature 85℃, low temperature -40℃, no thermal expansion and contraction, good dimensional stability.

(6) Ultraviolet light aging resistance (1000h, 83℃); yellowing index is less than 2, no cracking, no aging, and no yellowing under long-term ultraviolet irradiation.

(7) Heat aging resistance (1000h, 85℃): the yellowing index is less than 3.

(8) Damp heat aging performance (1000h, relative humidity 90%, 85℃): the yellowing index is less than 3.

In order to make the EVA film play its due role in the battery module, pay attention to moisture and dust prevention during use, and avoid contact with colored objects; do not expose the entire roll of film with the outer packaging to the air; if the film cut into pieces cannot be used up on the same day, it should be covered tightly. If the EVA film absorbs moisture, it will affect the adhesion between the film and the glass; if it absorbs dust, it will affect the light transmittance; if it comes in contact with colored and dirty objects, the EVA film has a strong adsorption capacity and is easy to be contaminated.