Commercial EPE encapsulants show notable material differences, particularly in the composition and structure of the polyolefin core layer
The study suggests that EPE encapsulation can provide improved resistance to moisture penetration compared with conventional EVA materials used in PV modules
While compatible with existing module manufacturing processes, the long-term reliability of EPE encapsulants still requires further investigation
Encapsulation materials play a major role in protecting solar cells inside PV modules. These polymer layers provide electrical insulation, mechanical stability, and protection against moisture, heat, and ultraviolet radiation. Ethylene-vinyl acetate (EVA) has been the dominant encapsulation material used due to its low cost and compatibility with module manufacturing processes.
However, new developments in cell technologies are introducing additional reliability requirements for polymer materials used in PV modules. Alternative encapsulants, such as polyolefin elastomers (POE), have been introduced to overcome the challenges of EVA. In addition, co-extruded EVA-polyolefin-EVA (EPE) encapsulants have been developed to combine the processability of EVA with the improved moisture barrier and electrical insulation properties of polyolefin.
Researchers from the Polymer Competence Center Leoben and collaborating institutes investigated EPE encapsulants used in solar modules. The results are presented in a research paper titled ‘Is EPE the future of PV encapsulation? A comprehensive material-level assessment’, published in Solar Energy Materials and Solar Cells. The study evaluates the chemical, optical, thermal, and thermo-mechanical properties of these materials.
For the study, 4 EPE products (EPE-1, EPE-2, EPE-3, and EPE-4) were analyzed alongside reference EVA and POE materials.
The researchers first analyzed the chemical composition of the encapsulants. Infrared spectroscopy showed that all tested EPE films used similar EVA outer layers with vinyl acetate content between roughly 25% and 29%. However, significant variations were identified in the polyolefin core layers. One encapsulant used an ethylene acrylate copolymer, while the others relied on ethylene α-olefin copolymers with varying polymer structures.
Optical measurements revealed that EPE-2 - 4 exhibited visible-light transmittance between 90.5% and 93%. EPE-1 showed lower transmission of about 86.5%. The lower transmission was linked to higher crystallinity in the polyolefin layer, which created optical haze after lamination.
A major advantage of EPE encapsulation is improved resistance to moisture penetration. WVTR measurements show that EPE films provide a much stronger moisture barrier than EVA. The tested EPE encapsulants exhibited WVTR values between 3.1 and 11.7 g m⁻² day⁻¹ per 100 µm, compared with about 35 g m⁻² day⁻¹ for EVA. Some EPE variants even achieved barrier performance comparable to or better than single-layer polyolefin materials.
This improvement results from the multilayer structure. The polyolefin core acts as the primary moisture barrier, while the EVA layers maintain compatibility with conventional lamination processes used in module manufacturing.
Microscopic analysis of laminated mini-modules showed that the encapsulant layers generally wet the solar cell interconnections well and formed defect-free laminates without visible bubbles. However, lamination caused structural changes within the multilayer films. In particular, the polyolefin layer often experienced a significant reduction in thickness due to its lower viscosity and slower curing process. In some cases, thickness reductions of up to 75% above busbars were observed.
Such changes could affect both moisture barrier performance and mechanical reliability. Optimizing lamination pressure and processing parameters may therefore be necessary when integrating EPE encapsulants into module manufacturing lines.
Thermal characterization indicated that most EPE formulations (EPE-2 to EPE-4) undergo crosslinking reactions similar to EVA and POE encapsulants. In contrast, EPE-1 contains a non-crosslinking polyolefin core. This results in higher crystallinity and stronger moisture-barrier performance, but also reduced optical transmission.
The co-extruded structure also appeared to improve dimensional stability compared with pure EVA films. Reduced thermal expansion mismatch within the module stack could help lower mechanical stresses that contribute to cell cracking or delamination during thermal cycling.
In addition, the EVA layers in EPE films still contain vinyl acetate groups that may degrade into acetic acid under environmental exposure. Since EVA typically accounts for 42-64% of the total film volume, this mechanism may still influence long-term reliability.
The study shows that EPE encapsulants improve moisture barrier performance while maintaining compatibility with established module manufacturing processes. However, long-term reliability data under environmental stresses such as humidity and temperature cycling remain limited.
