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Core Material for Solar Cell Upgrade:Polyolefin Elastomers (POE)

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Due to their outstanding performance, polyolefin elastomers (POE) products are widely used in toughening and modifying automotive materials, construction, electronics and electrical appliances, daily necessities, medical equipment, and so on.

In recent years, it has been widely used in high-performance photovoltaic film, and it has become a new material with great development prospects to widely replace traditional rubber and some plastics.

In this informational guide, we’ll cover polyolefin elastomer from:

  • What are polyolefin elastomers (POE)?
  • Properties of polyolefin elastomers (POE)
  • Application of polyolefin elastomers (POE)
  • POE Vs. EVA
  • POE Vs. EPE
  • Future Trends

What is Polyolefin Elastomers POE)?

POE is the full name of the Polyolefin Elastomer, that is, polyolefin elastomer, earlier known as ultra-low density polyethylene (ULDPE). There are two main categories: ethylene-propylene copolymer and ethylene/a-olefin copolymer. And in the field of photovoltaics, it usually refers to ethylene-a-olefin copolymer. Currently known a – alkene copolymerization monomers in PO are 1-butene (4C), 1-hasene (6), and 1-octene (8C).

The absence of unsaturated double bonds in the molecular structure of POE allows POE to have excellent physical and mechanical properties such as high elasticity, high strength, and high elongation as well as excellent weathering properties.

Ethylene-Octene Copolymer & Ethylene-Butene Copolymer
Ethylene-Octene Copolymer & Ethylene-Butene Copolymer

Polyolefin Elastomers (POE) Material Characteristics

The basic characteristics of POE elastomers are high octene mass fraction (greater than 20%), low density, very narrow relative molecular weight, a certain degree of crystallinity, its structure of crystalline PE exists in the side chain of the amorphous copolymer monomer, the crystalline PE chain links as a physical cross-linking to withstand the load, and non-crystalline ethylene and octene long chain to provide elasticity. Its specific properties are as follows:

  • Outstanding low-temperature impact resistance
  • Low specific gravity and cleanliness
  • Outstanding heat sealability
  • Excellent compatibility with various base polymers
  • Excellent flexibility and puncture resistance
  • Very high inorganic filler properties
  • Excellent elongation and high elasticity
  • Good light transmission
  • Excellent electrical insulation properties

POE Material

Polyolefin Elastomers (POE)Application in Solar Panels

POE material is one of the core auxiliary materials of solar panels, mainly used for encapsulation film, in addition to common photovoltaic encapsulation materials such as EVA film, EPE film in the cost of the component accounted for about 4-6%, although not high, its service life of the component, photoelectric conversion efficiency is very obvious.

The PV module structure from bottom to top is glass, encapsulation film, battery sheet, encapsulation film, and backsheet/glass, the photovoltaic adhesive film will be the battery sheet with the top cover below the pad sealing method, and the main role is to protect the solar cell sheet, so that photovoltaic modules in the operation of the process of the external environment, to extend the service life of photovoltaic modules.

At the same time, to maximize the sunlight through the film to reach the surface of the battery sheet, to improve the power generation efficiency of photovoltaic modules. Therefore, photovoltaic encapsulation film is required to have the pros:

  • high transmittance
  • UV resistance
  • weather resistance
  • moisture barrier
  • good electrical characteristics

POE Vs. EVA Material: Properties Comparison

Compared with EVA film, POE film has a higher water vapor barrier rate, weather resistance, and stronger anti-PID performance.

Its water vapor transmission rate is only 1/8 of that of EVA film, which can effectively reduce the PID effect, and it is mainly used for the encapsulation of monocrystalline PERC (emitter and back-side passivated cell) bifacial solar panels and N-type solar cell modules.

Performance indicators EVA POE
Density 0.96 g/cm3 0.88 g/cm3
Polarity Polar Non-polar
Acidity/Alkalinity Acidic Neutral
WVTR (Water Vapor Transmission Rate, 38°C, 90%RH) 20-40 g.m2/day 2-5 g.m2/day
UV Stability Weak Strong
Crosslinking Rate (under the same formula) Fast Slow

 

POE VS. EVA
POE VS. EVA Modulus Difference

Graph showing :

  • The change in modulus of uncross-linked EVA and POE as the temperature increases.
  • When cross-linking is complete, the modulus of EVA is generally higher than that of POE.
  • Higher modulus films put more stress on thin wafer cells

Compared to EVA films, POE films are superior :

-First of all, in terms of volume resistivity, volume resistivity is one of the key electrical performance indicators of the film, usually the higher the volume resistivity, the more effective the material used as an electrical insulating component. At high temperatures, the volume resistivity of Polyolefin Elastomers (POE) is 1-2 orders of magnitude higher than that of ordinary EVA film. POE film can more effectively reduce the occurrence of PID phenomenon in the module and improve the safety and reliability of the module.

Secondly, in terms of water vapor transmission rate, POE belongs to non-polar materials, can not form hydrogen bonds with water molecules with excellent water vapor barrier, circumventing the weakness of EVA film adsorption of water vapor, water vapor transmission rate of one-tenth of the EVA material, which can effectively prolong the use of the module cycle and reduce power degradation.

-At the same time in the UV aging performance, EVA film with the extension of the aging time, its yellowness index gradually climbed, while POE film due to the saturated structure of the macromolecular chain, the molecular structure contains relatively few tertiary carbon atoms, with more excellent aging resistance and UV resistance.

POE Vs. EPE Material: Performance Differences 

We will discuss the performance differences between two different encapsulation materials, POE and EPE materials.

EPE film is a three-layer composite film with an “EVA-POE-EVA” structure.

It is a co-extruded POE film that combines the high water resistance and anti-PID performance of POE film with the lamination characteristics of EVA film used in double-glass components.

This film is ideal for encapsulating PV modules, such as PERC double-face dual glass and N-type double-face double-glass, that demand high weather resistance.

Stability

  • EPE: additives are easy to migrate to EVA, and EPE performance will change with longer storage time.
  • POE: additives are easy to migrate to the surface.

Low-temperature performance

  • EPE: internal stress increases, and solar panels perform poorly at low temperatures under snow load.
  • POE: meet the outdoor demand, better performance of solar panels.

Thickness controlling

  • EPE: uneven
  • POE: no thickness control problem, more stable PID performance.

Uniformity of crosslinking degree

  • EPE: EVA is higher than POE
  • POE: no uniformity problem

Performance difference

  • EPE: quality risk
  • POE: A low cross-linking degree of POE will produce empty glue, bubbles, aging delamination, and other problems.
Risks to the quality of EPE
Risks to the quality of EPE

N-Type Cells Drive POE Demand Growth

 

At present, the photovoltaic industry is dominated by P-type batteries. Still, the conversion efficiency of P-type batteries is close to the limit, the future as N-type battery technology breakthroughs and yield enhancement, is expected to take over the P-type battery as the next generation of mainstream technology.

N-type solar cells put forward higher requirements for the encapsulation film: N-type batteries with a silver paste containing aluminum, the film requires higher water-blocking properties, and N-type batteries PN junction and P-type batteries on the contrary, the front side is more likely to produce PID phenomenon. POE does not contain acetate, which ensures that it has no acid decomposition in hot and humid environments, so it can meet the needs of N-type batteries for anti-PID, acid resistance, water vapor resistance, and so on.

The feasibility of EPE for TOPCon solar cell encapsulation is great. EPE products should first solve the problem of poor cross-linking of the POE layer, especially the problem of reduced cross-linking of additives after migrating from the POE layer to the EVA layer.

More details about N-type and P-type solar energy cells can be found in our previous article: History of Photovoltaic Cells from P-Type to N-Type Solar Panels

More details about TOPCon solar cell can be found in our previous article: What Is A TOPCon Solar Cell?

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