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Why PVDF Is the Best Solar PV Backsheet Material?

The solar PV backsheet is the outer layer of the photovoltaic module, directly contacting the external environment. The performance of the backsheet directly determines the overall performance and lifespan of the module. Therefore, photovoltaic modules typically require the backsheet to have excellent temperature resistance, good insulation properties, durability, and corrosion resistance.

To meet strict requirements on solar PV backsheet, choosing the right and high-quality backsheet material is very important. In this article, we will introduce one of the backsheet materials “PVDF” and explain why it is the best solar PV backsheet material.

Basic Characteristics of PVDF

  1. Physical Properties: Low friction coefficient and wear resistance.
  2. Mechanical Properties: Good mechanical strength; high toughness compared to other fluoroplastics; good impact strength and abrasion resistance.
  3. Heat Resistance: Poor thermal conductivity; usable within a temperature range between -50°C and 150°C; thermal decomposition temperature above 316°C.
  4. Combustion Properties: Self-extinguishing and flame-retardant; limiting oxygen index of 44%.
  5. Chemical Stability: Excellent chemical stability; not corroded by acids, bases, strong oxidizers, or halogens at room temperature. Only fuming sulfuric acid, strong bases, ketones, and ethers can cause swelling or partial dissolution. Strongly polar organic solvents like dimethylacetamide and dimethyl sulfoxide can dissolve it into a colloidal solution.
  6. Electrical Properties: Excellent electrical performance; dielectric constant (60~106Hz) ranges from 610 to 810; slightly lower volume resistivity; currently known as one of the most piezoelectric polymers; high insulation properties.
  7. Weather Resistance: Excellent UV resistance and aging performance; strong resistance to gamma radiation; while fluoroplastics generally have poor radiation resistance, PVDF is an exception; low permeability to most gases and liquids; has antifungal properties.

Photovoltaic Module's Requirements on Solar PV Backsheet

As one of the key photovoltaic module components, solar PV backsheet must meet a series of vigorous performance requirements to ensure long-term stability and efficiency of the photovoltaic module.

Ideal solar PV backsheets should possess the following key characteristics:

1. Insulation Properties

The backsheet must have excellent insulation properties to ensure electrical safety and prevent electrical leakage. Effective insulation can prevent current loss and protect the components from electrical faults.

2. Protective Functions

The backsheet needs to provide comprehensive protection, including:

  • UV Resistance: It should effectively block ultraviolet (UV) radiation to prevent UV-induced aging and deterioration of the backsheet material.
  • Waterproofing: It must have excellent waterproof performance to prevent moisture from penetrating the component and causing corrosion and electrical faults.
  • Gas Barrier: It should prevent oxygen and other gases from affecting the photovoltaic module, avoiding chemical reactions that could damage cell performance.
  • Mechanical Strength: The backsheet should have sufficient mechanical strength to withstand external impacts and physical wear, protecting the component from physical damage.

3. Reflective Properties

The backsheet should also have appropriate reflective properties to optimize light utilization. Effective reflectivity can enhance the light absorption capability of the photovoltaic module, thereby increasing the module’s efficiency.

4. Long-Term Durability

The photovoltaic backsheet needs to withstand up to 25 years of environmental exposure, ensuring it continues to function effectively under extreme weather conditions. As an encapsulating material, the backsheet must not only protect the solar module and its components but also isolate the cell junction box to prevent any potential environmental impacts.

Composite Backsheet Structure

Since a single material often struggles to meet all the performance requirements simultaneously, photovoltaic backsheets typically use a multi-layer composite structure. A typical composite backsheet structure is a “sandwich” layered configuration, usually consisting of three layers:

  • Outer Layer: Facing the external environment, this layer primarily provides excellent resistance to environmental erosion and protects the middle layer.
  • Middle Layer: This layer offers outstanding insulation and barrier properties, preventing moisture, gases, and other environmental factors from affecting the inner layers.
  • Inner Layer: In contact with the photovoltaic module’s EVA (ethylene-vinyl acetate copolymer) layer, it must have good UV resistance and excellent adhesion to the EVA material.

Basic Principles of Weather Resistance for Fluorine-Containing Materials in Backsheets

Fluorine-containing materials exhibit many unique and superior properties, making them commonly used in the outer layer of composite backsheet structures.

This is primarily due to C-F bonds between fluorine and carbon atoms. Fluorine is known for having the highest electronegativity among all elements, and its C-F bonds are very short and have a high bond energy (up to 485.6 kJ/mol), which is stronger than the typical C-H bonds (413 kJ/mol) and C-C bonds (347 kJ/mol). This makes the C-F bonds difficult to break.

UV's Impact on Backsheet Material

Ultraviolet (UV) radiation can destroy materials because its energy is sufficient to break chemical bonds. However, the C-F bonds in fluorinated materials have extremely high bond energy, making them resistant to UV damage. Most UV radiation in sunlight is absorbed by the Earth’s ozone layer, with less than 5% of UV energy reaching the Earth’s surface, so these UV rays have minimal impact on fluorine-containing materials.

Additionally, fluorine atoms carry strong negative charges and repel each other, which forms a robust shielding layer around the main carbon chain structure of the material. This unique arrangement of fluorine atoms provides fluorinated materials with exceptional weather resistance, high-temperature resistance, low-temperature resistance, and chemical corrosion resistance, ensuring the stability and reliability of photovoltaic modules for long-term outdoor use.

Advantages of PVDF Compared to Other Fluorine-Containing Materials

Currently, the mainstream fluorine-containing solar PV backsheet materials are PVDF and PVF. Following are the performance comparisons between the two.

1. Higher Fluorine Content

PVDF has a higher fluorine content than PVF, which enhances its UV resistance and chemical durability. PVDF also has better density, providing stronger resistance to wind and sand, making it more suitable for harsh outdoor environments.

2. Stronger Flame Retardancy

PVDF exhibits superior flame retardancy, which helps to reduce the likelihood of fire incidents.

3. Superior Wear Resistance and Mechanical Properties

Given that photovoltaic stations typically require a 25-year lifespan, the performance of protective films over the long term is crucial. Experiments by PV-Tech have shown that under aging conditions, PVDF has higher breakdown voltage and better abrasion resistance compared to PVF. Both types have similar performance regarding yellowing and mechanical properties.

4. Better Barrier Properties

PVDF’s moisture permeability is only one-tenth of that of PVF, making PVDF more effective at blocking moisture and gases. This excellent barrier performance is critical for the long-term protection of solar PV backsheets, ensuring that internal components are not affected by moisture and other environmental factors.

5. Poor Thermal Conductivity

The poor thermal conductivity of PVDF is generally considered an advantage in solar PV backsheets. This is because low thermal conductivity helps reduce the transfer of heat from the photovoltaic module to the backsheet, thereby lowering the temperature of the backsheet material and internal components.

This thermal insulation feature contributes to the overall stability and lifespan of the photovoltaic module, preventing overheating and damage to the solar cells. Therefore, although low thermal conductivity might be seen as a disadvantage in some applications, it helps maintain the long-term performance and reliability of solar PV backsheets.

Other fluorine-containing materials, such as PTFE, are also commonly used in solar PV backsheets. However, PTFE has poor processability, making molding and secondary processing difficult, and requires specialized equipment and techniques for installation. Additionally, the material and processing costs of PTFE are generally higher than those of PVDF and PVF, which limits its application in projects.

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