Photovoltaic modules – a detailed introduction to the structure and function of backsheet materials

What is the structure and function of backplane material?

The outer material on the back of the photovoltaic module is called the back plate, which is the key component of the photovoltaic module. It isolates the interior of the module from the external environment, realizes electrical insulation, and enables the module to operate outdoors for a long time. The reliability and service life of components are also closely related to the quality of backplane. If the backplane material fails, the packaging material inside the module will be directly exposed in the harsh outdoor environment, causing hydrolysis of packaging material, corrosion and delamination of battery and welding strip, rapidly reducing the power output and service life of the module, and seriously leading to insulation failure of the module, leading to fire and casualty accidents. Therefore, excellent backplane materials should have good mechanical stability, insulation, moisture barrier, adhesion, heat dissipation, environmental aging resistance (ultraviolet, high temperature, damp heat and chemicals), and add certain light reflection function to enhance power generation efficiency. Generally, the appropriate backplane can be selected according to the different requirements of components and applications.

Backplanes with different structures have different functions, and appropriate backplanes can be selected according to different use areas. Components using fluorine-containing backplane can be used in areas with strong ultraviolet rays; Components using hydrolysis resistant pet can be used in high temperature and high humidity areas; The traditional white backplane can enhance the reflection of light, so as to improve the power generation efficiency of components; The black back plate can meet the aesthetic requirements of buildings such as roofs; The glass back plate can be made into transparent components, which is suitable for building daylighting and agricultural greenhouses. Black and transparent backplanes have no light reflection function. Compared with the components of white backplane, the output power will be reduced by 2% ~ 3%.

At present, the market puts forward extremely high requirements for the backplane. Because the installation address of the components is unknown, the backplane will be required to have the function that can meet all the requirements of the use environment during production, resulting in high cost. However, the reality is that in the application process of photovoltaic modules for decades, its operating environment is relatively single, and there is no need to use materials that can meet all climatic conditions. The best solution is to develop and select differentiated backplanes to meet different climatic conditions, which can reduce the cost to a certain extent and select materials and products with better cost performance.

  1. Structure and function
Structure of back plate material
Structure of back plate material

As mentioned above, photovoltaic modules have high requirements for backplane. At present, only one single polymer material can not meet the requirements of all projects. Generally, polymer backplane is composed of multiple layers of materials with different functions. The typical three-layer structure of backplane is usually divided into outer layer, middle layer and inner layer, and the functions of these three layers are different. The outer layer of the back plate generally adopts weather resistant fluorine layer or modified weather resistant pet, which is directly exposed to the external environment. It not only needs to have good weather resistance and durability, that is, maintain good mechanical stability and appearance integrity under the conditions of wet, heat, UV, cold and heat cycle, wind and rain, and reliably bond with the junction box and edge sealant, In addition, it should be able to withstand the high temperature of up to 150 ℃ during the lamination process of components, as well as mechanical scratch during installation and handling. The middle layer of the back plate mainly provides the guarantee of mechanical performance, electrical insulation performance and barrier performance. PET polyester material is commonly used in the middle layer. This material has poor resistance to ultraviolet light and damp heat aging. Therefore, there are more and more modified high barrier hydrolysis resistant pet.

The inner layer of the back plate mainly ensures the reliable bonding between the back plate and the component packaging materials. At the same time, because the sunlight will shine on this layer through the glass, the inner layer also needs to have certain weather resistance and UV resistance. In addition, if the inner layer has high reflectivity, the output power of the module can be improved.

Polymer backboards can be divided into fluorine-containing backboards (such as TPT backboards, kPa backboards, etc.) and non fluorine-containing backboards (such as pet backboards, polyamide backboards, etc.) according to materials. Before 2006, double-layer fluorine-containing backboards were widely used in the market. Later, with the rapid development of solar energy industry, cost competition became more and more fierce, and backboards made of non fluorine materials began to appear in the market, For example, the pet backplane with modified hydrolysis resistant polyester as the outer layer and the backplane with polyamide (commonly known as nylon) as the outer layer.

Polymer backplane can be divided into composite backplane, coated backplane and coextrusion backplane according to its production process. The three-layer materials of the composite backplane are generally formed into a film separately, and then the three-layer composite is made by glue, such as kpk backplane; Generally, the upper and lower sides of the middle pet are coated with coatings, and the coatings used are mostly fluorine-containing coatings, such as CPC type backplane; Coextrusion backplane is made by extruding several layers of polymer (typically three layers) materials from the die of the extruder at the same time. Generally, the processing properties of these layers of materials are similar, such as AAA backplane. However, this backplane has a serious cracking problem in practical application, so it has been stopped at present.

Read more: Electrical performance of photovoltaic modules

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