Crystalline silicon solar cells
Currently, the crystalline silicon solar cells used in the production of crystalline silicon solar cell modules are mainly 158mm monocrystalline silicon solar cells, 166mm monocrystalline silicon solar cells, and 182mm and 210mm polycrystalline silicon solar cells, with a thickness of generally 175μm.
Typically, crystalline silicon solar cell modules are composed of individual solar cells connected in series. Under ideal conditions, the output voltage of the module is the sum of the voltages of each cell, and the output current of the module is determined by the cell with the smallest output current.
To ensure good consistency in the electrical performance of individual cells used in each solar cell module, the cells must be sorted by performance during module manufacturing. Cells with large differences in electrical performance are not allowed to be connected in series in the same module. For the sake of the module's aesthetic appearance, the cells are usually also sorted by color difference during module manufacturing.
In addition, to avoid hot spots in solar photovoltaic modules when used outdoors, it is generally required that the crystalline silicon solar cells used in the production of the modules have a small reverse current. The reverse current of a crystalline silicon solar cell refers to the current flowing through the solar cell when a certain reverse voltage is applied. The figure on the right shows the test method for testing the reverse current of a solar cell.
If the reverse current of the selected crystalline silicon solar cell is too high during the production of crystalline silicon solar cell modules, the bypass diode will not provide protection when partial shading occurs, which may easily damage the solar cell module.
Low-iron patterned tempered glass
Currently, the glass used in the production of crystalline silicon solar cell modules is usually produced by rolling, with a typical thickness of 3.2mm ± 0.3mm. Within the wavelength range of the solar cell's spectral response (350～1100nm), the direct transmittance of sunlight with a standard thickness of 3mm should be greater than 91%, and it should have a high reflectivity for infrared light greater than 1200nm.
Reducing the iron content of glass can effectively increase its transmittance. The iron content (Fe2O3) of solar cell glass should not exceed 0.015%. Making the two sides of the glass into different sized velvet surfaces during the glass production process can increase the amount of sunlight entering the glass. Tempering is intended to increase the strength of the glass and provide long-term protection for the solar cells.
Tempered glass used as encapsulation material for crystalline silicon solar photovoltaic modules typically requires good resistance to mechanical impact, low curvature, and no scratches. The arc curvature of solar cell glass should not exceed 0.2%; the wave curvature should not exceed 0.3mm within any 300mm range.
To prevent tempered glass from shattering before and after encapsulation, it is generally required that each meter of tempered glass has no more than 10mm long edge defects, with a depth of no more than 2mm extending from the edge to the glass surface, and no more than one-third of the glass thickness extending from the surface to the other side. No concentrated air bubbles shorter than 1mm are allowed inside the tempered glass. For air bubbles longer than 1mm but not exceeding 6mm, there must be no more than 6 per square meter. No stones, cracks, or missing corners are allowed. Within a 50mm x 50mm area, the number of fragments must exceed 40, and a small number of long, thin fragments are permitted, with a length not exceeding 100mm. Substandard tempering can easily lead to component breakage during use.
To reduce optical reflection in crystalline silicon solar cell modules and improve power output, coated glass is increasingly being used. Photovoltaic coated glass substrates are made from 3.2mm ultra-clear tempered glass, using special nano-coatings as the main raw material and subjected to high-temperature treatment. The requirements are that the light transmittance of the coated glass is increased by more than 2% compared to the original, and that the coated glass exhibits good adhesion between the optical film and the glass substrate, good weather and corrosion resistance, good self-cleaning properties, and a long service life.
When selecting coated glass, the following experiments should be conducted:
Water resistance test: After immersion in water for 96 hours, the coated layer should not show significant changes, and the change in light transmittance before and after the test should not exceed 0.5%.
Acid resistance test: After immersion in 5% H2SO4 solution for 48 hours, the coated layer showed no significant change, and the change in transmittance before and after the test should not exceed 0.5%.
Alkali resistance test: After immersion in saturated Ca(OH)2 solution for 48 hours, the coated film should show no significant change, and the change in transmittance before and after the test should not exceed 0.5%.
In addition, tests such as salt spray resistance, artificial climate aging resistance, coating temperature change resistance, and stain resistance must be conducted to ensure that solar cell modules using coated glass can also achieve a service life of more than 25 years.
EVA
The material used for encapsulating crystalline silicon solar cells is EVA, a copolymer of ethylene and vinyl acetate. EVA is a thermosetting hot melt adhesive; it is non-sticky at room temperature for ease of handling, but under certain hot pressing conditions, it melts, bonds, cross-links, and cures, becoming completely transparent. Long-term practical experience has proven that, compared to other materials, EVA has achieved quite satisfactory results in solar cell encapsulation and outdoor applications.
The EVA thickness is between 0.4 and 0.6 mm, requiring a smooth surface, uniform thickness, and the presence of a crosslinking agent that can crosslink at a curing temperature of 150°C. A stable adhesive layer is formed using an extrusion molding process.
There are two main types of EVA: fast-curing and conventional-curing.
EVA possesses excellent flexibility, impact resistance, elasticity, optical transparency, adhesion, resistance to environmental stress cracking, weather resistance, chemical resistance, and heat sealing properties.
The cured EVA can withstand atmospheric changes and is elastic. It acts as a "top cover and bottom pad" for crystalline silicon solar cells and is bonded together with the upper protective material glass and the lower protective material TPT using vacuum lamination technology.
On the other hand, when bonded to glass, it can increase the light transmittance of the glass, thus playing a role in enhancing light transmission, and also has a boosting effect on the output of solar cell modules.
Different temperatures have a significant impact on the degree of crosslinking of EVA, which directly affects the performance and lifespan of the modules. In the molten state, EVA bonds with crystalline silicon solar cells, glass, and TPT (thermal permeable terephthalate), a process involving both physical and chemical bonding. When EVA is heated to a certain temperature, the crosslinking agent decomposes to generate free radicals, initiating the bonding between EVA molecules to form a three-dimensional network structure, leading to the crosslinking and curing of the EVA adhesive layer. When the degree of crosslinking reaches 60% or higher, it can withstand environmental changes; therefore, encapsulating solar cell modules with EVA can achieve a very long lifespan. In actual production, the degree of crosslinking of EVA is generally controlled between 85% and 95%.
Back panel
The main materials used as backsheets for crystalline silicon solar cells include TPT, TPE, and PET. TPT has a three-layer composite structure of Tedlar/Polyster/Tedlar, TPE has a two-layer structure with EVA, and PET has a single-layer polyester structure. It is used on the back of the module as a back protection and electrical insulation material.
Backsheets used in crystalline silicon solar cells require a longitudinal shrinkage rate of no more than 1.5%. Practical experience has shown that the outer protective layer of the backsheet should ideally contain fluorine, as this provides strong resistance to environmental corrosion. Furthermore, white backsheets reflect sunlight, resulting in less encapsulation loss compared to black backsheets, and their higher infrared emissivity also helps reduce the module's operating temperature.
Aluminum alloy frame
The main function of the aluminum alloy frame is to protect the glass, facilitate installation and transportation, and increase the sealing performance and overall mechanical strength of the crystalline silicon solar cell module.
The metal frame of the module is made of aluminum alloy. In order to meet the mechanical strength and other requirements of photovoltaic modules, in accordance with GB/T3190—1996 "Chemical Composition of Wrought Aluminum and Aluminum Alloys", aluminum alloy of 6063 T5 or higher is required.
To ensure a lifespan of up to 25 years for solar cell modules, the aluminum alloy surface must be treated, namely anodized, with a surface oxide layer thickness greater than 20μm. The frame used for solar cell modules should be free from deformation and scratches.
Junction Box
After the positive and negative electrodes of the crystal osmosis solar cell module are led out from the backsheet, a special electrical connection box is needed to connect them to the external circuit.
To ensure a 25-year service life, the junction box should be injection molded from engineering plastics and coated with anti-aging and UV radiation-resistant agents to ensure that the modules do not age or crack during long-term outdoor use. The terminals should be made of electrolytic copper with an outer nickel plating layer to ensure reliable electrical conductivity and connections. The junction box is bonded to the TPT surface using silicone rubber. The requirements for junction boxes used in solar cell modules are: the outer casing must have excellent anti-aging and UV resistance to meet the requirements of use under harsh outdoor environmental conditions, such as IP junction boxes for crystalline silicon solar cell modules.
