Solar Photovoltaic Backsheets and Their Material Composition

I. Concept and Structure of PV Backsheets

Solar cell backsheets, also known as PV backsheet films, are critical components in photovoltaic (PV) modules. Positioned on the rear side of solar panels, they protect the module from moisture ingress, oxygen penetration, and environmental degradation in outdoor conditions. These backsheets ensure reliable insulation, waterproofing, aging resistance, thermal stability (-40°C to +85°C), and corrosion resistance. They also reflect sunlight to enhance conversion efficiency and reduce module temperature through high infrared reflectivity.

Structure of PV Backsheets (typically five-layer design with three core layers):

1. 1.Outer Weather-Resistant Layer:
   • Requires fluorinated materials (e.g., PVF, PVDF) for superior UV resistance. C-F bonds (485 kJ/mol) provide exceptional durability, as only <5% of sunlight photons (<220 nm wavelength) can disrupt them. Alternatives include THV, ETFE, ECTFE, and coated PTFE.
2. 2.Middle Support Layer:
   • Thermally and mechanically stable modified PET with low gas permeability, excellent electrical insulation, fatigue resistance, and dimensional consistency.
3. 3.Lamination Adhesive Layer:
   • Modified fluoropolymer films or high-adhesion materials (EVA, PE, PA) to enhance bonding between non-reactive fluorinated films/PET and encapsulants.

II. Classification of PV Backsheets
By Film Type:

1. 1.Adhesive-Laminated Backsheets: PET core with fluorinated/EVA films (tri-layer).
2. 2.Coated Backsheets: PET coated with fluororesin on both sides (cured via drying).
3. 3.Crosslinking Backsheets: PET with crosslinked composite/EVA films.

By Material:

• Fluoropolymer-based: FPF (TPT), KPK, FPE (TPE), KPE, TFB (PVF/PET/fluoroadhesive), KFB (PVDF/PET/fluoroadhesive), BBF (THV/PET/EVA), etc.
• Advanced composites: FFC (modified PTFE-coated PET), KPC (PVDF/specialized PET), KPF (PVDF/PET/fluoroskin), PPC (treated PET/weather-resistant PET).

III. Key Chemical Materials in PV Backsheets

1. 1.PVF (Polyvinyl Fluoride):
   • Dominated by DuPont’s Tedlar®. Offers UV resistance but requires thick layers due to lower fluorine content (~29%). Prone to micropores and moderate moisture barrier performance.
2. 2.PVDF (Polyvinylidene Fluoride):
   • Higher fluorine density (42%) than PVF, with superior weather/barrier properties. Requires acrylic plasticizers (e.g., PMMA), risking localized aging and brittleness under long-term stress.
3. 3.PET (Polyethylene Terephthalate):
   • Cost-effective with high-temperature tolerance (120°C long-term, 150°C short-term) but vulnerable to hydrolysis and UV degradation.
4. 4.PE (Polyethylene) & EVA (Ethylene Vinyl Acetate):
   • PE: Chemically inert, low water absorption.
   • EVA: High adhesion, transparency, and flowability for lamination processes.
5. 5.PA (Polyamide/Nylon):
   • Balanced mechanical strength, wear/chemical resistance, and flame retardancy.
6. 6.Advanced Fluoropolymers:
   • THV: Flexible, self-adhesive; ideal for bendable backsheets.
   • ETFE: Weather-resistant (Asahi Glass), limited market adoption.
   • ECTFE: Developed by DuPont; solvent-resistant and high-barrier.
   • PCTFE/PTFE/FEVE: Used in coated backsheets for cost efficiency.

IV. Critical Technical Parameters
Key failure modes during testing:

• DH2000: 85°C/85% RH aging (2000h) failures (40% of cases).
• UV-Induced Yellowing (30% of failures).
• TC200 Thermal Cycling (-40°C ↔ +85°C) degradation (20%).
• Low EVA Adhesion (10%).

Increasingly stringent requirements:

• Water Vapor Transmission Rate (WVTR): <0.5 g/m²/day.
• Yellowness Index (ΔYI < 3 after 3000h UV exposure).

V. Industry Trends

Integration of recyclable materials to meet circular economy demands.

1.Material Innovation:

Shift from fluorinated films (e.g., Tedlar®) to direct fluorocoating technologies to reduce costs and dependency on imported films.

2.Performance Challenges:

Coated backsheets face adhesion and durability gaps compared to laminated counterparts.

3.Future Outlook:

Development of non-PET alternatives with superior weather resistance (e.g., advanced fluoropolymer composites, ceramic-reinforced substrates).