Why Silicone for PV Encapsulation
The dominant encapsulant in photovoltaic modules for the past four decades has been ethylene-vinyl acetate (EVA). EVA is inexpensive and processable by standard lamination equipment, but its long-term stability has become a limiting factor as module lifetime warranties extend from 20 to 30+ years:
- Conventional EVA degrades under UV + humidity: acetic acid release → corrosion, browning, delamination within 10–15 years
- Silicone: no UV degradation, no acetic acid, hydrolytic stability at 85 °C/85% RH for 3000+ h, service life 25–30 years
- Trade-off: silicone encapsulants cost 3–5× more than EVA; used for high-reliability bifacial, glass-glass, and concentrator PV modules
The acetic acid liberated by EVA photodegradation corrodes silver busbars and aluminum frames, and the yellowing of degraded EVA directly reduces optical transmittance and module power output. Silicone encapsulants eliminate both failure modes.
Product Forms
Silicone encapsulants are available in three product forms suited to different module assembly processes:
| Form | Cure System | Process | Application |
|---|---|---|---|
| Two-component RTV-2 gel | Pt addition cure | Liquid dispensing | Junction box potting, edge sealing |
| LSR film | Pt addition cure | Lamination | Cell encapsulant film (replacing EVA) |
| One-component RTV-1 | Condensation | Bead dispensing | Frame sealing, busbar coating |
LSR films are the direct EVA replacement in standard laminator equipment. They are calendered to 400–600 µm thickness, supplied in rolls, and laminated at 120–150 °C with Pt-catalyzed addition cure. Cure time in the laminator is typically 10–20 minutes, comparable to EVA but at lower temperature, reducing risk of cell cracking.
Key Performance Requirements
IEC 61215 and IEC 61730 qualification testing defines the durability requirements that encapsulants must satisfy:
| Property | Test | Requirement |
|---|---|---|
| Transmittance (400–1100 nm) | UV-Vis | >90% |
| Damp heat | IEC 61215: 85 °C/85% RH, 1000 h | No delamination |
| Thermal cycling | IEC 61215: −40 °C to +85 °C, 200 cycles | No cracking |
| Volume resistivity | ASTM D257 | >10¹³ Ω·cm |
Silicone encapsulants exceed IEC damp-heat requirements with substantial margin — 3000+ h testing at 85 °C/85% RH shows no transmittance loss or delamination in well-formulated systems. The low modulus of silicone gel also accommodates differential thermal expansion between silicon cells and glass or backsheet, reducing thermomechanical stress during thermal cycling.
Refractive Index and Light Trapping
Optical design considerations are relevant because the encapsulant sits directly between the front glass and the active silicon cell:
- Silicone encapsulant RI 1.41–1.43 (vs EVA 1.48) → slightly more reflection at cell interface, offset by UV stability gains
- Methyl-phenyl silicone resin formulations: RI tunable to 1.50–1.54 for better RI matching with silicon cell (RI 3.5 at 1000 nm, effective encapsulant RI less critical due to textured cell surface)
For standard mono-facial modules with textured cell surfaces, the RI difference between silicone and EVA has minimal impact on module efficiency (typically <0.3% relative). For concentrator photovoltaic (CPV) systems with flat-surface multi-junction cells and high optical concentration, RI matching is more critical and phenyl-modified silicone formulations with RI 1.50+ are preferred.
Related Reading
RTV-2 Silicone Rubber · LSR Liquid Silicone Rubber · Methyl-Phenyl Silicone Resin