Backplane knowledge

Published on.

2022-11-25 13:28

What's the backboard?

Understanding the backplane in principle

  topic of discussion

  -  The history of the backplane's evolution

  - Understanding the logic of changes in the structure and material of backsheets from the background of the era in which they were born and their specificity.

  - Understand the design philosophy and reliability evaluation method of the backplane based on its function.

  - Discuss what makes a good backsplash.

  What is a backsheet - diagram of the position of a backsheet in a PV module


  【Logic of the backplane】

Essential purpose of the backsheet: insulation + water repellent

                               (Protecting people) (Protecting the battery's electronics - a central factor in the reduced efficiency of electricity generation from components)

  ◆ So-called 25-year life of the backplane: The above-mentioned "insulation + water resistance" function will not be lost within 25 years, or as little as possible.

  The backsheet is the "clothing" wrapped around the battery to accomplish the above-mentioned mission, at the cost of lowering the heat dissipation of the battery, lowering the power generation efficiency, and premature failure of the semiconductor.

  If the "clothing" is not made of good material or workmanship, the slit between the backplane and the aluminum frame will be ripped open by thermal expansion and contraction, or by stress rebound contraction, which will increase the amount of water vapor and decrease the power generation efficiency. The shrinkage force will also lead to hidden cracking of the battery cells.

  - The added value of this garment is the increased power generation if the inner side of the back panel is more reflective of light.



  The evolution of the world's representative backplane


  Conclusion: No backsheet has a probability of not failing for more than 25 years, but the backsheet is constantly evolving in response to the increase in battery power, the thinning of cells and the extension of battery life.

  With backplane types changing so rapidly, how can we predict their long-term reliability?

  Methods for determining the long-term reliability of organic materials.



  Track record in the construction, automotive and outdoor high-voltage electrical industries


  Can a doctor diagnose a human (organic) and conclude that the person will live to be 90 years old by taking blood pressure, doing a CT scan, measuring enzyme levels, etc.? The current medical development, the diagnosis of many diseases can not yet do. By the same token, it is not scientific to say that a certain backplane will not fail in 25 years, it can only be understood as a "concept penetration" marketing tool of commercial marketing. Medical research techniques have also expanded from phenotypic measurements, to the cellular, DNA level (mechanistic principles of research, as well as investigating family history and investigating commonalities in populations (historical methodology + probability studies). The current practice within the PV industry of ignoring the latter two by simply expanding and enhancing phenotypic test metrics for backsheets is divorced from the current state of the science of the degree of organic material science.

  From the design principle of backplane structure and its reliability assurance measures


  Why do different types of backplanes exhibit different performance?


  Let's understand the design principles of the backplane structure and the nature of the materials used to construct each layer.

  In fact, a single sheet of PET can take care of the core function of the backsheet, or PET is the most important layer in the composition of the backsheet.



  However, PET can not withstand the UV, water, sand, chemicals and other negative energy erosion, direct exposure to the atmosphere when 6 to 7 years later, aging, cracking, chalking, then it bears the insulation, water resistance capacity will decline.



  Why use fluorine film on both sides?

  So, you need to isolate and protect the PET with a fluorine film. Why fluorine? It is because fluorine is the most stable chemical structure in organic matter, it can withstand the erosion of UV, water, sand, chemicals and other negative energy, and then can bear the isolation effect.



  UV blocking function of outer and inner layer


  How fluorine membranes block UV light

  However, fluorine itself is transparent, can not block UV, is dependent on the white TiO2 particles dispersed in the fluorine film to isolate. In fact, as long as the choice of the right particle size, TiO2 laminated thickness as long as 0.4u or more can fully block UV, in order to block UV, there is no need for such a thick fluorine film now. The fluorine membrane made of the current thickness because (1) to improve the fluorine membrane water resistance; (2) the film process conditions limit.


  Water blocking function of the outer layer


  Water-blocking and water-ageing problems of the backplane

  In fact, the water resistance of fluorine film is relatively poor, which is the most important index to judge the quality of fluorine film. Although the water resistance of the backplane depends on the PET film, but the fluorine film's water resistance can be improved to extend the PET itself to resist hydrolysis to avoid early aging and the loss of PET to bear the insulation, water resistance, etc.



  Test method: Infrared method (ASTM F1249), test conditions 38℃, 90%RH Unit: (g/m2*day)

  Conclusion 1: The 3-layer structure of Kynar PVDF fluorine film has the same water resistance as PVF initially, but better after UV aging.

  Conclusion 2: All fluorine films have excessive water vapor permeability, so it is not enough to prevent water aging of PET by fluorine films alone

  Selection of intermediate core material PET


  The hydrolysis resistance of PET is an important factor in the life of the backsheet.

  ●TPT, KPK, TPE, KPE all use PET with 24h hydrolysis resistance.

  ●Safer KPf uses PET for photovoltaic installation, with a hydrolysis resistance of 48h.

  ●Sailwood also offers customized and specific hydrolysis grade PET, such as 60h PET for Toshiba.

  Elongation at break after 48h of PCT with different PET hydrolysis resistance grades (%)



  1) For ordinary water-resistant grade PET, PCT is brittle after 48h test, without any mechanical strength.

  (2) PCT test conditions are 121°C, 100% RH, 2 atm.

  (3) Hydrolysis resistance grade is the time to reduce the elongation at break of PET to 10% after aging under PCT conditions.

  Wind and sand resistance of the outer layer of the back panel

  All fluorine films with a thickness of 20u or more can pass the 150L test.


  Selection of fluorine film for the outer layer of the backplane

  Why is PVDF the dominant fluorine film?

  Why are triple-layer membranes better than single-layer membranes?



  CONCLUSION: Monolayer structured PVF and monolayer structured PVDF are at the same level.

  Three-layer structure PVDF is superior.

  Selection of the inner layer



  - Fluorofilm technology and the birth of KPF, which overcame the shortcomings of E-films.

  - UV resistance and UV-blocking ability of the inner layer meets the performance of KPK and TPT, but at a much lower cost.



  Fluro-skin: Direct film production method. It is a technology in which a fluorine resin mixed with TiO2 is applied directly to the PET surface using a coating method, and a film is formed. The film adheres tightly to the PET without the need for an adhesive.

  One step filming technology, which is used for direct film application on backsheet, has been pursued by many senior international polymer material manufacturers, and SAYWOOD has applied for patents, KPF structure patents and product trademarks.


  Not only the choice of materials, but also the manufacturing process of the backsheet affects long-term reliability.

  ●Changes in peel force at high and low temperatures (adhesive design)

  ●Control of cross-link density of adhesives (manufacturing process)

  ●Control of solvent residues (manufacturing process)

  ●Tension control range when laminating (manufacturing process)

  Uniformity control of glue thickness




  sum up

  Principles considered in the selection of the backplane

  ●Choosing the most appropriate type of backsheet for the environment in which it will be used will optimize the cost per unit of electricity LCOE ●The factors to be considered are the type of climate, the effect of chemicals, the action of machinery, the amount of wind, wind grit, ice, snow, moisture and rain.

  ●The choice of the inner PET layer, the material of the glue layer and the lamination process are also critical. As a comprehensive applied materials company with chemical background, Saiwu has the knowledge, experience and R&D, production and quality control capabilities to provide customers with world-class backsheets.

  ●The KPF structure backsheet is the backsheet that can meet all the requirements in terms of performance and cost performance.

  ●For high humidity environment, CPC backsheet is not recommended, but PPF structure backsheet is recommended for low price and reliability. The reasons are as follows



      Poor wetting and leveling of the coating, which is in direct contact with PET without glue as buffer layer.

  Due to the poor CTE match between the coating and PET, the difference between the two is 3-4 times, the shear force between the coating and PET will cause the coating and PET to fall off easily during the long-term thermal expansion and contraction process.

  The PET composite structure of the backsheet is widely used, such as Sharp, T, H and other companies, and has been used for many years to meet the basic requirements.

  Reliability simulation

  1) Establishment of an independent dedicated lab from raw material introduction to component end application testing

  2) Set up an independent evaluation and test center from the organizational structure to intervene and supervise the quality of backplane development, and simulate the performance of backplane in various actual application environments.

  3) Translate into backplane development quality control documents to ensure that new products are validated and meet performance requirements.



  Real-world environmental design requirements for the backplane

  (1) Water vapor has a significant impact on module performance (gridline corrosion, worming, etc.), and also requires water permeability of double-glass modules - i.e., edge sealing or low water permeability encapsulation materials.

  (2) The inner layer of the backsheet still requires good UV resistance, even with a layer of UV cut-off EVA.

  (3) Fluorine film on both sides or outer layer of fluorine film + PET is still the most stable combination of backsheet materials, no bulk defects, such as cracking, embrittlement, yellowing, etc., have been found on power plants in several typical applications.

  4) The design and development of the backplane must fully consider the influence of various environments, even extreme environments, such as the influence of low temperature environment on the backplane, which puts forward comprehensive requirements on the research and development capabilities and configuration of the backplane, the backplane is not a simple manufacture of glue + film lamination.

  The control of critical points in the backplane manufacturing process is supported by chemical expertise and coating technology, which is a major part of quality and reliability.


  sum up

  1. Both KPK and KPF backplanes are suitable for terrestrial power stations installed in harsh environmental and climatic conditions.

  2. If you want to increase the amount of weathering margin, the K-layer choose Arkema production of 3-layer structure of 30μm PVDF film is more appropriate, the cost is also lower.

  3. The reason why PVF is not considered the only suitable is that the membrane is at the same level as monolayer structured PVDF and is not worth its market price. This is the same background that all domestic state-owned power plant investors, except Three Gorges, have opened up to K, not to mention the reason why almost all module manufacturers around the world have abandoned T. It is also the foundation of the backsheet demand business that has made SAYWU the number one backsheet in China and the world.

  4. as cell power has increased, cells have become thinner, demands on module power have increased, and demands on module life have increased, the industry has raised the benchmark for judging any material to think in terms of LCOE. Based on LCOE, TPT/KPK is the backward backsheet and KPF is the most advanced backsheet available today, but it is not the end of the line, and backsheet technology is still evolving.

  5.The backsheet is a composite material, not only the outer layer, but also the inner PET layer, the glue layer and the lamination process are very important. As a comprehensive applied materials company with chemical background, SAYWOOD has enough knowledge, experience and R&D, production and quality control capabilities to provide world class backsheets for CLP.