Light from the surface of the component to the silicon body through the glass, sealant (usually EVA), so glass and EVA will affect the light absorption, the higher the transmittance of glass and EVA, the smaller the package loss of the component. The transmittance of conventional ultra-white tempered glass is about 92%. Currently, coated glass with antireflection coating has been introduced on the market, and the transmittance can be as high as 96%. The coated glass can generally increase the output power gain of the component by 1%, but its long-term stability. Sex and reliability require further research. The transmittance of glass from different manufacturers is very different. The higher the transmittance, the more light enters the battery, and the output power of the battery is proportional to the light intensity. In the case where the battery and other auxiliary materials are unchanged, the use of tempered glass with high transmittance increases the output power of the module and reduces the package loss.
EVA (ethylene-vinyl acetate polymer ester) is used to bond tempered glass, battery and back sheet. Because it is UV-labile, ultraviolet radiation of about 6% of sunlight can cause aging of EVA film and turtle. Cracking, yellowing, and then reduce its light transmittance, so some manufacturers of EVA will add anti-UV agent, which will cause the EVA transmission in the short-band. The high-resistance battery has a higher IQE in the short-band (<450nm) than the conventional battery, and if the EVA is applied to the short-wavelength light-cut, it will cause this part of the light to be absorbed by the high-resistance battery, so the package loss is certain. The package loss is greater than that of components made with conventional batteries. Therefore, solar cells fabricated using different processes need to select a matching EVA, find a compromise between light transmittance and UV resistance, and reduce the package loss of components without affecting reliability. In addition, some companies have proposed the use of transparent silica gel with stable chemical properties, high UV resistance and high transmittance as a component of the sealant, which can effectively avoid the problem of yellowing of the sealant and the inability of the battery to receive short-wavelength light.
The surface of the solar cell is deposited with a silicon nitride antireflection film with a refractive index of about 2.1, which has EVA and tempered glass (both refractive index of about 1.48), in order to maximize the transmittance of the module. The anti-reflection effect also needs to obtain the best matching result of the SiNx film thickness, EVA and glass thickness and the best optical anti-reflection effect, which can effectively increase the output power of the component.
The backing plate of the solar cell module is used to prevent moisture from entering the component, and a TPT (Tedler-PET-Tedler) film is often used. The reflectance curve of the conventional white TPT with the EVA contact surface is shown in Fig. 5. It can be seen that the reflectance is as high as about 80% in the medium and long wavelength bands. The white TPT film has a reflection effect on sunlight incident on the solar cell that is not absorbed by the battery, and this portion of the light is reflected to the solar cell at the interface between the air and the glass, increasing the utilization of light incident on the solar cell module. In general, the use of white TPT can increase the output power gain of the component by 1% than the black TPT, which helps to reduce the package loss of the component.
The solar cell is covered by the soldering strip and cannot absorb sunlight. Some solder ribbon companies have introduced reflective strips. The front side of the strip is silver plated and calendered into a longitudinal groove-like structure. This structure can illuminate the light incident on the strip. Reflected at an angle to the inner surface of the glass layer of the assembly, totally reflected at the glass-air interface and projected back to the surface of the cell. The captured light energy allows the component to generate additional power, which in theory can increase component efficiency by about 2%.
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