Solar photovoltaic backplane and materials used
1、 Concept and structure of photovoltaic backplane
Solar cell backplane is also called solar cell backplane film, photovoltaic backplane, photovoltaic backplane film, and solar backplane. It is widely used in solar cell (PV) modules, located at the back of the solar panel, to protect the solar cell modules from moisture erosion in outdoor environment, and to prevent oxygen from internal oxidation of the modules. It has reliable insulation, water resistance, aging resistance, high and low temperature resistance, and corrosion resistance. It can reflect sunlight and improve the conversion efficiency of components; It has high infrared reflectivity, which can reduce the temperature of the module.
The structure of the photovoltaic backplane is shown in Figure 1. It is generally divided into five layers, and the core has three layers:
(1) The outer protective layer is the weathering layer:
In order to have good weather resistance, the outer material is generally required to contain fluorine. PVF and PVDF are two well-known weather resistant polymer materials. Because the C-F bond energy existing in them is 485KJ/mol, which is the largest bond energy among the covalent bonds of organic compounds.
Only photons with wavelength less than 220 nm can dissociate the C-F bond, while these photons in the sun account for less than 5%, and are easily absorbed by the ozone layer, and very few can reach the ground. THV, ETFE and ECTFE are also used by manufacturers. PTFE with coated structure is also common.
(2) Middle layer:
As a support, it is required to be able to withstand high and low temperatures, stable mechanical properties, excellent electrical insulation, good creep resistance, fatigue resistance, friction resistance and dimensional stability, and low gas and steam permeability. Modified PET materials are generally used.
(3) Laminated adhesive layer:
The unmodified fluorine-containing film and PET have poor adhesion with EVA, so it is necessary to use modified fluorine-containing materials or EVA, PE and PA films with strong adhesion.
Figure 1 Structure of photovoltaic backplane
2、 Classification of photovoltaic backplane
According to the membrane classification of the backplane, it can be divided into three types:
1. Glued composite back plate film
Composite fluorine film or EVA adhesive film on both sides of PET polyester film with three-layer structure.
2. Coating back plate film
Fluorine resin is coated on both sides of PET polyester film, and the film is formed after drying and curing.
3. There are also a few manufacturers that use the crosslinking reaction method to produce composite film or EVA film on both sides of PET through the reaction of crosslinking agent.
According to the different classification of materials, the backplane can be divided into FPF (represented by TPT), KPK, FPE (represented by TPE), KPE and multi-layer PET backplane, TAPE (aluminum layer is added between T layer and P layer), TFB (PVF/PET/fluorine-containing adhesive layer), KFB (PVDF/PET/fluorine-containing adhesive layer), BBF (THV/PET/EVA), FFC (modified PTFE coated on both sides of PET), KPC (PVDF/special treatment PET), KPF (invented by Suzhou Saiwu, fluorine-coated film technology, the structure is PVDF/PET/fluorine-coated film) PPC (special treatment PET/weather resistant PET), etc.
3、 Chemical materials used in photovoltaic backplane
PVF, the scientific name of PVF, is a copolymer formed by extrusion of the copolymer of fluorine and fluorocarbon molecules. The most famous PVF film maker is DuPont, whose registered trademark TEDLAR is the most widely used PVF film in the photovoltaic industry. T in TPT or TPE refers to TEDLAR. The disadvantage is that the surface of the coated film is prone to pinholes, and the film has poor water vapor barrier ability. In addition, PVF material itself has low fluorine content, and PVF film needs to have sufficient thickness to ensure its performance.
PVDF, the scientific name of polyvinylidene fluoride, has a density of 1.3~1.4 times that of PVF, and has one more F in its molecular structure, so it has better weather resistance and barrier properties than PVF. However, PVDF is difficult to form. It needs to add about 20~30% acrylic plasticizer (such as PMMA and other acrylic materials), which is easy to cause local aging, low transverse fracture elongation, and further decline and embrittlement after thermal aging and damp heat aging. There is a risk of cracking under long-term outdoor stress aging.
PET is polyethylene terephthalate, also known as polyester film, which is milky white or light yellow highly crystalline polymer. Long-term use temperature can reach 120 ℃, short-term use can withstand 150 ℃ high temperature and - 70 ℃ low temperature. However, it is easy to hydrolyze in high temperature and humidity environment, and it is easy to undergo photodegradation reaction under ultraviolet light.
PE is polyethylene, which is polymerized from ethylene and is a widely used polymer material. Good chemical stability, low water absorption and excellent electrical insulation.
EVA, namely ethylene-vinyl acetate copolymer, has good chemical stability, small water absorption, high transparency, high adhesion with various interfaces, low melting point and easy flow, and is suitable for various glass laminating processes.
Polyamide, namely nylon, has good comprehensive properties, including mechanical properties, heat resistance, abrasion resistance, chemical resistance and self-lubrication, low friction coefficient, certain flame retardancy, easy processing, and is suitable for filling and strengthening modification with glass fiber and other fillers, improving performance and expanding the application range.
THV is a new fluorine-containing material developed by American Dyneon Company in the 1980s. When it is compounded with other materials into a multi-layer structure, its flexibility is very prominent. Another important feature of THV is that it is easy to bond with other materials without surface treatment. The composite manufacturing process of backplane and the use of silica gel to paste the junction box are very simple, especially suitable for occasions where the backplane is required to be soft.
The ETFE film developed by Asahi Corporation of Japan is completely self-produced from raw materials to film, with good weather resistance and chemical stability, but the market consumption is not large.
ECTFE was successfully developed by DuPont in 1946. It is a 1:1 alternating copolymer of ethylene and trifluoroethylene. It has the chemical corrosion resistance of typical fluoroplastics. No solvent can corrode ECTFE or cause stress cracking at 120 ℃, and it has high weather resistance and barrier resistance. In the commercial photovoltaic cell backplane, some of the backplane products use ECTFE as the weather resistant material for the outer layer.
10. PCTFE, PTFE, FEVE
Backplates made of PCTFE, PTFE and FEVE materials are mostly prepared by coating method. The back plate is coated with fluorocarbon coating on PET film instead of fluoroplastic film, which has better economy and shorter production cycle. China is also the third country after the United States and Japan to possess the synthetic technology of fluorocarbon coatings and realize industrialization.
A simple comparison of the performance of main fluorine-containing materials is shown in Table 1.
Table 1 Comparison of characteristics of various fluorine-containing materials
*Oxygen index (OI) refers to the lowest oxygen concentration required for flame combustion of materials in oxygen-nitrogen mixed gas flow under specified conditions. It is expressed as the volume percentage of oxygen. High oxygen index indicates that the material is not easy to burn, while low oxygen index indicates that the material is easy to burn.
4、 Key technical parameters of photovoltaic backplane
Because of the importance of the backplane, the component manufacturer is very strict with the backplane test. Even the backplane manufacturers of first-line brands often fail the test.
The main reasons why the backplane failed to pass the test include DH2000 (humidity and heat aging 85 ℃, 85% humidity, 2000 hours) failure, yellowing after UV test, TC200 (temperature cycling - 40 ℃ to+85 ℃) failure, and low adhesion with EVA. The specific classification and proportion are shown in Figure 2. Among the backplanes that failed the test, there are products from first-line backplane manufacturers. Among them, DH2000 and UV are the two test items that are most likely to fail, so the current component manufacturers have increasingly high requirements for the water permeability and yellowing coefficient of the backplane.
Figure 2 Reasons for failure of photovoltaic backplane test
5、 Development trend of photovoltaic backplane
The internal driving force driving the development of the photovoltaic backplane industry is mainly performance and cost.
From the performance point of view, the backplane of high demand components basically uses fluorine-containing materials to protect PET. The only difference is that the fluorine-containing materials are different. Whether fluorine materials are directly coated on PET in the form of film or coating is the two main directions of the development of PET backboard.
From the perspective of cost, the cost of coating is much lower than that of coating, and the fluorine film technology is mainly monopolized by foreign enterprises. In the case of continuous decline in the profit margin of photovoltaic products, the development of coated backplane products is the only way for photovoltaic backplanes. However, the adhesion and stability of the coated backplane still need to be improved.
The final trend of backplane should be to find a material with good weather resistance to replace PET backplane.