While bifacial modules can deliver a game-changing 20-30% power boost for solar projects, they carry a hidden risk few datasheets mention. Certain bifacial designs can degrade at over 2% per year—more than double the rate of their monofacial counterparts. The billion-dollar difference between a high-performing asset and a premature failure often comes down to a single, overlooked component: the encapsulant.
Investments are often lost in the gap between academic material science and real-world production. This guide from PVTestLab bridges that gap. We decode the critical interactions between encapsulants, cells, and glass that define bifacial reliability, moving beyond marketing claims to offer actionable, data-driven insights from our full-scale R&D production line.
The Bifacial Dilemma: Higher Gains vs. Accelerated Degradation
The value of bifacial technology is clear: by capturing reflected light (albedo) from the rear side, modules can significantly outperform their nameplate rating. Installations over highly reflective surfaces can yield up to 30% more energy.
But this higher performance introduces new material stresses. The transparent rear side exposes the module’s internal components to UV radiation, humidity, and thermal cycling from both sides. This dual exposure can accelerate aging, leading to alarming degradation rates of 1.46% to 2.30% annually if the material stack is not perfectly optimized. In these failures, the primary culprit is often a mismatch between the encapsulating polymer and the other module components.
At PVTestLab, our work in Prototyping & Module Development has shown that longevity hinges not on a single „best“ material, but on a compatible system—and the encapsulant is the heart of that system.
PID Resistance and the Encapsulant-Cell Interface
Potential-Induced Degradation (PID) is a primary failure mode in bifacial systems, capable of slashing power output by over 30%. It occurs when a voltage potential difference drives ion migration between the solar cell and the module frame, effectively short-circuiting parts of the cell.
In a bifacial module, this risk is amplified. The transparent rear allows moisture and other contaminants to create conductive pathways, making the encapsulant choice all the more critical.
EVA vs. POE: A 1000x Difference in Protection
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Ethylene Vinyl Acetate (EVA): The industry incumbent, EVA is cost-effective and well-understood. However, its polymer structure makes it relatively permeable to moisture. More importantly, its volume resistivity is low, meaning it provides little resistance to the ion leakage that causes PID.
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Polyolefin Elastomer (POE): POE is inherently more resistant to moisture due to its non-polar chemical structure. Crucially, its volume resistivity is approximately 1,000 times greater than EVA’s. This makes it an electrical insulator that effectively stops the ion migration at the heart of PID.
Our lamination trials consistently demonstrate that under the high-voltage conditions typical of modern solar farms, modules using POE encapsulants show negligible PID effects, while EVA-based modules require careful cell selection and other additives to pass the same tests.
PVTestLab Recommendation:
For any utility-scale or long-term bifacial project, specifying POE or a POE-based hybrid encapsulant is the single most effective way to mitigate PID risk. Always demand to see the module’s specific PID test results, not just a pass/fail on the datasheet.
Moisture Ingress and Encapsulant-Interconnection Corrosion
Moisture is the enemy of module longevity. When water vapor penetrates a module, it accelerates the corrosion of delicate cell interconnections and solder bonds, leading to increased series resistance and eventual power loss.
The Acetic Acid Problem with EVA
When EVA is exposed to heat and humidity over time, it can undergo hydrolysis, breaking down and releasing acetic acid. This corrosive byproduct directly attacks the silver paste on cell fingers and the metallic ribbons connecting the cells. We see this in our accelerated aging chambers as „snail trails“ and delamination, which are visual indicators of ongoing chemical breakdown.
POE’s Inherent Water Resistance
POE’s non-polar polymer chains are naturally hydrophobic—they repel water. This structure is stable and does not produce corrosive byproducts, even after thousands of hours of damp-heat testing. This stability is essential for protecting the n-type TOPCon and HJT cells commonly used in high-efficiency bifacial modules, which are particularly sensitive to moisture and acidic environments. Our comprehensive Material Testing & Lamination Trials confirm POE provides a more robust and hermetic seal over the module’s 25-year life.
PVTestLab Recommendation:
The lamination process itself is key to preventing moisture ingress. By fine-tuning temperature, pressure, and curing times, we can optimize the cross-linking of the polymer to create a superior barrier. For bifacial modules intended for humid or coastal environments, POE is the superior choice for preventing long-term corrosion.
UV Stability and Encapsulant-Glass Compatibility
For a bifacial module, transparency is everything. Any loss of light transmission on either the front or rear side directly reduces energy yield. The encapsulant must remain crystal clear for decades, even under intense UV exposure.
EVA Yellowing vs. POE Clarity
Traditional EVA formulations contain additives that are susceptible to degradation from UV radiation, causing a distinct yellowing or browning over time. While this impacts monofacial modules, it’s far more damaging to bifacial performance, as it chokes off the light reaching the rear cells and significantly reduces the bifacial gain.
POE, by contrast, demonstrates exceptional UV stability. Its chemical backbone is inherently resistant to being broken down by UV light. In our side-by-side environmental chamber tests, POE samples retain over 99% of their initial transparency, while some EVA formulations can lose several percentage points—a loss that directly impacts long-term energy yield.
PVTestLab Recommendation:
A systems approach is vital. The encapsulant must be compatible not only with the glass but also with any anti-reflective or anti-soiling coatings. Our Process Optimization & Training programs emphasize validating the entire material stack—from glass coating to encapsulant to backsheet—to ensure no adverse chemical reactions compromise transparency and adhesion over the module’s lifetime.
The Next Generation: EPE and the Rise of Hybrid Encapsulants
While POE offers superior performance, its processing characteristics and cost have sometimes been a barrier to adoption. This has led to the development of co-extruded, multi-layer encapsulants like EPE (EVA-POE-EVA).
EPE sandwiches a core layer of PID-resistant POE between two outer layers of EVA. This clever design offers a „best of both worlds“ solution:
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Performance: The central POE layer provides the critical PID resistance and moisture barrier needed for bifacial longevity.
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Processability: The outer EVA layers offer excellent adhesion to both the solar cells and the glass/backsheet, leveraging EVA’s well-known lamination properties.
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Cost-Effectiveness: It provides near-POE level performance at a price point closer to traditional EVA.
Our testing shows that well-formulated EPE encapsulants can pass the most stringent PID and damp-heat tests, making them a compelling and cost-effective solution for next-generation bifacial modules.
Frequently Asked Questions
If POE is superior, why does anyone still use EVA?
Cost and process familiarity are the primary reasons. EVA has been the industry standard for decades, and production lines are highly optimized for its use. However, as the performance gap widens—especially for high-voltage bifacial systems—the small upfront cost saving of EVA is quickly erased by higher degradation and lower lifetime energy yield. The shift to POE and EPE is an investment in long-term asset value.
Is EPE a compromise or a true high-performance solution?
EPE is a strategic innovation, not a compromise. It leverages the strengths of two different materials to solve specific challenges. The key is in the quality of the formulation and the co-extrusion process. A high-quality EPE from a reputable supplier, validated through rigorous testing like that done at PVTestLab, offers a robust and cost-effective pathway to bifacial reliability.
What tests should I ask a module manufacturer for?
Don’t settle for the standard IEC certification. For bifacial modules, ask for extended reliability test data, specifically:
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PID Test: Request the results after 192 hours at 85°C, 85% relative humidity, and the system’s maximum voltage. Degradation should be less than 5%.
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Damp Heat Test: Ask for data after 2,000 hours, not just the standard 1,000. Look for stable power output and check for any signs of delamination in the test report.
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UV Exposure Test: Inquire about performance after extended UV sequencing to validate the long-term stability of the encapsulant and backsheet.
Your Path from Concept to Bankable Production
Choosing the right material combination is the most critical decision in developing a reliable bifacial module. Moving from a datasheet to a durable, high-yielding product requires testing under real industrial conditions.
The full-scale production line and expert engineering support at PVTestLab provide the applied research environment needed to validate your materials, optimize your lamination process, and build prototypes that perform in the field, not just in the lab.
Ready to validate your next-generation module design? Contact a PVTestLab process engineer to discuss your project and schedule a trial on our R&D line.