According to the different requirements of the battery modules, the back plate material can have a variety of choices. Generally there are toughened glass, plexiglass, aluminum alloy, TPT composite film and so on. The tempered glass backsheet is mainly used to make double-sided light-transmitting building material type battery modules, which are used for photovoltaic curtain walls, photovoltaic roofs, etc. The price is relatively high, and the weight of the modules is also large. In addition to this, the most widely used TPT composite membrane is currently. Commonly seen white coverings on the back of battery modules are mostly this type of composite film, as shown in Figure 1. According to the different requirements of the battery modules, the backplane film can have a variety of options. Backsheet membranes are mainly divided into two categories: fluorine-containing backsheets and fluorine-free backsheets. The fluorine-containing backsheet is divided into two types: double-sided fluorine (such as TPT, KPK, etc.) and single-sided fluorine (such as TPE, KPE, etc.); and the fluorine-free backsheet is mostly made by bonding multiple layers of PET glue with an adhesive. At present, the service life of battery modules is required to be 25 years, and the backsheet, as a photovoltaic packaging material that directly contacts the external environment in a large area, should have excellent long-term aging resistance (damp heat, dry heat, ultraviolet), electrical insulation resistance, and water vapor barrier properties. Therefore, if the backplane film cannot meet the 25-year environmental test of battery modules in terms of aging resistance, insulation resistance, and moisture resistance, the reliability, stability and durability of the solar cell will eventually be unable to be guaranteed. This makes the battery module use for 8-10 years in ordinary climates or 5-8 years in special environmental conditions (plateaus, islands, wetlands), which will cause undesirable conditions such as delamination, cracking, blistering, and yellowing, and cause the battery module to fall off, the battery slice to slip, the effective output power of the battery is reduced, etc. What’s more dangerous is that the battery assembly will experience electric arcing under the condition of lower voltage and current value, which will cause the battery assembly to burn and trigger a fire, causing personal safety damage and property loss.
At present, considering the cost pressure caused by double-sided fluorine-containing materials on the whole back plate film and module products, some back plate film and module manufacturers have adopted EVA materials (or other olefin polymers) to replace the fluorine materials in the inner layer of the back plate film with the structure of “material polyester bottle material” with double-sided gas, and introduced a single-sided fluorine-containing composite adhesive film composed of three layers of “fluorine material polyester EVA”. After the back film of this structure is bonded with the EVA adhesive film for module packaging, because there is no fluorine-containing material on the light surface to effectively protect the pet main substrate of the back film, the back film of module installation cannot withstand long-term ultraviolet radiation. Within a few years, the back film of the module will turn yellow, embrittlement and aging, which will seriously affect the long-term power generation efficiency of the module. However, because this type of backsheet film uses less fluorine material, its performance is not as good as TPT, but its cost is about 2/3 of TPT, and its adhesion to EVA is also good, so it is often used in the packaging of some small modules.
TPT (KPK) is the abbreviation of “fluorine film-polyester (PET) film-fluorine film” composite material. This composite film combines the advantages of “Plastic King” fluoroplastics with good aging resistance, corrosion resistance, moisture resistance, and excellent mechanical properties, high insulation properties and water vapor barrier properties of polyester film. Therefore, the composite TPT (KPK) film has the characteristics of airtightness, good strength, good weather resistance, long service life, no change under the lamination temperature, and strong bonding with the bonding material. These characteristics are suitable for encapsulating photovoltaic cell modules. As the back sheet material of the cell module, it can effectively prevent the corrosion and influence of various media, especially water, oxygen, corrosive gas, etc., on EVA and cells.
In addition to TPT (KPK), common composite materials also include TAT (that is, a composite film of Tedlar and aluminum film) and TIT (that is, the composite film of Tedlar and iron film) and other composite films with a metal film sandwich structure in the middle. These composite films have the characteristics of high strength, flame retardancy, durability, self-cleaning, and good heat dissipation. The white composite film can also reflect the sunlight, improve the conversion efficiency of the battery module, and has a strong reflection performance on infrared rays, which can reduce the working temperature of the battery module under strong sunlight.
At present, double-sided fluorine-containing backsheets are divided into two categories: film type and coating type according to different production processes. The film-coated back sheet is made by bonding and compounding PVF (polyvinyl fluoride), PVDF (polyvinylidene fluoride), ECTFE (Chlorotrifluoroethylene-ethylene copolymer) and THV (tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer) and other fluoroplastic films through an adhesive and PET polyester film as the base material. The coating type backsheet is made of fluorine-containing resin such as PTFE (polytetrafluoroethylene) resin and CTFE (Chlorotrifluoroethylene) resin, PVDF resin and FEVE (fluoroethylene vinyl ether copolymer) as the main resin. The coating method is coated on the PET polyester film and cured.