The photovoltaic (PV) industry is experiencing unprecedented growth as the global focus and demand for clean energy grows. As a key component of PV modules, PV backsheet materials play a critical role in improving photovoltaic conversion efficiency, reducing costs, and addressing environmental challenges.
However, despite the promising market, competition within the industry is becoming increasingly intense, especially in terms of the variety and quality of PV backsheet materials.
Introduction of photovoltaic backsheet
Backsheets are divided into inorganic backsheets i.e. glass and polymer organic backsheets according to the material used. If the backsheet is also made of glass, it is a double-glass module, otherwise it is a single-glass module, where the glass side of the module faces the sun directly and the backsheet is located at the back of the module.
At present, single-glass modules are the main photovoltaic modules, but the development trend is that the proportion of double-glass modules will gradually increase, because double-glass modules can be double-sided power generation, which improves the power generation efficiency.
The backsheet is located in the outermost layer of the backside of the PV module, which is in direct contact with the external environment over a large area. In order to apply to various climatic environments and service time, the backsheet needs to meet a variety of performance requirements
It is difficult for a single material to meet these performance requirements, so the backsheet generally adopts a composite structure, in which PET film is used as the base film and fluorine film or other plastic films are adhered to both sides of the base film through adhesives.
PET film has excellent insulation and mechanical properties, but the weather resistance is poor. Fluorine resin film includes polyvinyl fluoride (PVF) film and polyvinylidene fluoride (PVDF) film, which have excellent insulation, weather resistance and barrier properties, and the main role of adhesive is to make PET base film and fluorine film, polyolefin film adhesive composite.
According to the production process, the backsheet is divided into laminated and coated type. The fluorine material in the form of fluorine film is laminated (generally pressed) on PET base film by adhesive, which is the composite backsheet with complete fluorine film and therefore superior overall performance, but high cost;
The fluorine resin is directly coated on the PET base film through a special process, and the adhesive is mixed with the fluorine resin, which is a coated backsheet with slightly inferior performance to the composite backsheet, but with price advantages.
Solar backsheets can be divided into double-sided fluorine film backsheets, single-sided fluorine film backsheets and non-fluorine backsheets according to their fluorine content, which are suitable for different environments due to their weathering properties.
In general, the degree of weather resistance to the environment is in the order of double-sided fluorine film backsheet, single-sided fluorine film backsheet, and fluorine-free backsheet, and their prices are generally lower in that order.
Double-sided fluorine film composite backsheet can withstand severe cold, high temperature, sand, rain and other harsh environments due to its superior weather resistance, and is usually widely used in the plateau, desert, Gobi and other areas;
Single-sided fluorine film composite backsheet is a reduced type of double-sided fluorine film composite backsheet. Compared with double-sided fluorine film composite backsheet its inner layer has poorer UV resistance and heat dissipation, and it is mainly applied to roofs and areas with mild UV rays.
Status of backsheet companies
The global PV industry chain is basically in China, and so is the backsheet, which was dominated by foreign companies, especially Japanese companies, in the early days, but with the rapid rise of Chinese companies, foreign backsheet companies have gradually reduced their market share and withdrawn from the market due to the need to adapt to rapid cost reduction and declining profitability. The production and sales volume of Toppan and Iso are also at a standstill.
The major domestic backsheet manufacturers include Suzhou SAIVO Applied Technology Co.
In the early days, TPT and KPK backsheets were the main products, but PVF and PVDF films were expensive, especially PVF films were basically only produced by DuPont in the U.S. Therefore, through research and development, domestic enterprises replaced fluorine films with fluorine-containing coatings on the one hand, and replaced composite processes with coating processes on the other hand, which significantly reduced the cost while meeting the performance of PV modules.
For example, the Fluorine skin film of SAIVO Technology, FFC fluorine coating of Zhonglai, C fluorine resin of Foster, etc. Further, M film of Mingguan New Material and E film of Lekai Film directly replace the fluorine film with polyolefin film.
Backsheet component materials
1、PET base film
PET is a crystalline polymer made of purified terephthalic acid (PTA) and ethylene glycol (MEG) condensed by heating under the action of catalyst, that is, polyester chips, long-term use can withstand high temperatures up to 120 ℃, short-term use can withstand 150 ℃ high temperature, -70 ℃ low temperature, and the mechanical properties of high and low temperatures have little impact on its.
PET film is made from polyester chips by drying, melting, extruding, casting and stretching, which is called CPET if it is only one-way stretching and mainly used for packaging; BOPET if it is stretched in both directions, and now more than 95% of PET films are BOPET, of which the medium and high-end PET films are mainly used for optical films and photovoltaic backsheet films.
The production of medium and high-end PET films has the following characteristics:
(1) Equipment needs to be imported and the cycle time is long.
The key processes of base film production include the polycondensation reaction of PTA and MEG to produce polyester chips, extrusion and biaxial stretching, and the best performance of polycondensation reaction and extrusion equipment is produced by Oerlikon Barmag in Germany, and Bruckner in Germany is the best manufacturer of biaxial stretching equipment, so the production line composed of equipment from these two German companies produces the best quality products. However, the output of the two companies is effective, the delivery cycle is long, and the installation and commissioning time is also long.
(2) High technical barriers.
Even if the same formula is changed, the process parameters will change and the performance will change.
In order to apply the new backing structure, the base film companies are working hard to develop high weathering transparent PET base film.
2、Fluorine film
PVF film produced by the United States DuPont, its trademark Tedlar, so referred to as T film, its TPT backsheet structure is the earliest used, has been through more than 30 years of practical application, proved its structure, stable performance, can meet the module more than 25 years of time requirements. However, T-membrane is expensive, so the market size is decreasing, mainly for large ground power plants.
PVDF membrane was developed to replace PVF membrane and was first produced by Arkema in France. In recent years, as the technology level of domestic fluorine membrane manufacturers has improved and production costs have gradually decreased, domestic fluorine membranes are gradually accepted by backsheet companies in the market and their market share has gradually increased. According to the IPO documents of SAIFO Technology and Mingguan New Material, Jiaxing Gaozheng, Hangzhou Fu Film and Zhongtian Technology have entered their top five suppliers.
In order to further reduce costs, companies have developed fluorinated coatings.
Since fluorine-containing plastic is not easy to decompose and incineration will produce toxic gas, so the recycling of backsheet is difficult to handle. Now the enterprises are developing environmentally friendly backsheet, including replacing fluorine film with polyolefin film, etc.
3、Adhesive
Adhesives are mainly used for the bonding of fluorine film and PET base film in the composite backsheet, as well as coated on the surface of the base film to directly form a functional film. The technical barriers to the production of adhesives required for solar backsheet are high, and the market is mainly occupied by a few foreign companies such as Bostic of Arkema (France), Henkel (Germany) and Toyo Textile Co.
The production of adhesives in China is mainly based on low-grade adhesive products, and only very few companies produce high quality adhesives that can be applied to fluorine films and PET base films, such as Sailwood Technology, which is developed from a backsheet adhesive manufacturer to a backsheet manufacturer.
Because of the ability to control the cost and the use of different scenarios, the domestic backsheet manufacturers and foreign companies embarked on a different technical path, the domestic centralized large-scale ground photovoltaic power plants, the 25-year service life of strict requirements, so the majority of domestic backsheets containing fluorine, in order to reduce costs, the production process from the composite fluorine film improved to coated with fluorine coating.
For example, Coveme (Italy) has completely abandoned fluoropolymer and adopted PET base film, and used UV-irradiation stabilized polyester film (P) on the outermost side and polyethylene film (E) on the inner layer to form PPE backsheet, the structure of such backsheet is Modified polyester/polyester/polyethylene.
For example, the APO backsheet developed by DSM in the Netherlands uses polyolefin PO, which has excellent water vapor barrier properties, as the base material, and PA12, which has excellent UV resistance, and uses co-extrusion to form the backsheet.
With the energy crisis brought about by the Russia-Ukraine conflict, the demand for PV in Europe has risen sharply, and Chinese companies are also grasping the production of non-fluorine backsheets. In short, in the production of backsheet, regardless of technology or production, domestic enterprises are ahead of foreign enterprises.
4、R&D direction
(1) Environmental protection: The main purpose of the massive use of new energy is to reduce emissions and protect the atmosphere, so the parts used in the manufacture of photovoltaic modules should also be environmentally friendly, but the fluorine film used in the backsheet is difficult to degrade, making it difficult to recycle, so with the rapid development of photovoltaic power generation, the recycling of the backsheet is an urgent problem to be solved.
(2) Cost reduction: Cost reduction and efficiency improvement in all aspects of the PV industry chain is the main driving force for PV development and will always be the main theme of PV development, while fluorine film is not only environmentally friendly but also expensive, so it is urgent to develop new materials to replace fluorine film.
(3) Development of backsheet for different use scenarios: for example, the backsheet used in the desert areas of western China should have better heat dissipation, UV resistance, resistance to sand and abrasion, and resistance to day and night temperature difference, etc.; while the backsheet used in eastern coastal applications should have lower water vapor transmission rate, better resistance to hydrolysis and salt spray, etc.
(4) Applicable to the needs of double-sided photovoltaic module power generation: double-sided power generation is the trend, and its proportion is getting higher and higher, and module manufacturers generally use glass backsheet, although the transparent backsheet can replace the glass backsheet, but the transparent backsheet has not been tested after a long time of use, so how to convince the module enterprises to use the transparent backsheet is a long way to go.
