Imagine a state-of-the-art bifacial solar module, fresh off the production line. It passes every initial quality check flawlessly—no bubbles, no defects. But two years later, out in the field, a subtle weakness emerges. Moisture creeps in, and the delicate layers start to separate at the edges. This is delamination, a silent failure that compromises performance and undermines the entire project’s bankability.
What went wrong? The answer often lies hidden in a component that makes up less than 5% of the module’s cost but is 100% responsible for its structural integrity: the encapsulant.
Choosing the right encapsulant for Glass-to-Glass (G2G) bifacial modules is a critical decision in modern solar manufacturing. While the industry standard, POE (Polyolefin Elastomer), is known for its durability, cost-effective alternatives like EPE (a composite of EVA-POE-EVA) are gaining traction.
But does saving on upfront cost introduce a hidden risk of long-term failure? We conducted a study to find out, focusing on the single most important factor for G2G module longevity: the adhesive bond between the encapsulant and the glass. The results might change how you think about your bill of materials.
The Unique Challenge of Glass-to-Glass Modules
Traditional solar modules have a glass front and a polymer backsheet. Bifacial modules, designed to capture light from both sides, often use a more robust Glass-to-Glass (G2G) structure. This design offers superior mechanical strength and protection against the elements.
However, it also presents a unique challenge. Unlike a polymer backsheet, glass is impermeable. Any moisture that penetrates the edges cannot easily escape, creating a permanently humid microenvironment inside the module.
This is where the encapsulant’s role becomes paramount. It must create an incredibly strong, long-lasting bond to two glass surfaces, acting as a weatherproof seal that can withstand decades of thermal stress and moisture exposure. If that bond weakens, delamination is inevitable.
Meet the Contenders: POE vs. EPE
Two main types of encapsulants dominate the conversation for G2G modules:
- POE (Polyolefin Elastomer): The proven champion. POE is inherently resistant to moisture and doesn’t produce corrosive byproducts. Its chemical structure allows it to form a very stable, long-term bond with glass. Its main drawback? Higher cost.
- EPE (EVA-POE-EVA): The cost-effective challenger. EPE is a co-extruded, three-layer film. It features a central core of POE for moisture resistance, sandwiched between two outer layers of EVA (Ethylene Vinyl Acetate). The outer EVA layers provide strong adhesion to cells and glass, and the overall structure is cheaper to produce than pure POE.
On paper, EPE seems like a brilliant compromise. But the real question is how that layered structure holds up over 25+ years in the field.
Putting Adhesion to the Test: Our Research Approach
To simulate decades of harsh, humid conditions, we used the industry-standard Damp Heat (DH1000) test. This involves placing test laminates—simple Glass/Encapsulant/Glass sandwiches—in an environmental chamber at 85°C and 85% relative humidity for 1,000 hours.
We created samples using both POE and EPE, varying the lamination parameters (time and temperature) to see how these affected the outcome.
Before and after the DH1000 exposure, we measured the adhesion strength using a 90° peel test. This test quantifies the force required to pull the encapsulant away from the glass surface, measured in Newtons per centimeter (N/cm). The industry generally accepts a minimum of 60 N/cm as a strong bond.
The Moment of Truth: What 1,000 Hours of Damp Heat Revealed
Our comparative study revealed a definitive—and critical—performance gap between the two materials.
Initial Adhesion: A Deceptively Strong Start
Before the DH test, both encapsulants performed exceptionally well. Fresh out of the laminator, both POE and EPE samples delivered peel strengths well over 100 N/cm, far exceeding the industry benchmark.
Based on this initial data alone, one might conclude that EPE is a perfectly suitable, cost-effective substitute for POE. But this is why accelerated aging tests are non-negotiable in solar technology.
The Great Divide: Performance After Aging
After 1,000 hours of intense heat and humidity, the story changed completely.
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POE adhesion actually increased after the DH test. This might seem counterintuitive, but it’s a known phenomenon. The silane coupling agents within the POE formulation use the energy from the heat and moisture to form stronger, more permanent covalent bonds with the glass surface. The material essentially becomes more integrated with the glass over time.
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EPE adhesion plummeted by an average of 48%. This catastrophic failure reveals the material’s hidden weakness. The outer EVA layers in the EPE film are susceptible to hydrolysis. Under heat and humidity, the vinyl acetate in the EVA breaks down, producing acetic acid. This acid then attacks and degrades the very silane coupling agents responsible for creating the bond with the glass. The adhesive foundation is chemically eaten away from the inside out.
As our PV Process Specialist, Patrick Thoma, notes, „The initial peel strength of EPE can be deceptive. It’s the post-DH degradation that reveals the inherent chemical instability of its EVA layers against glass—a risk that process optimization alone cannot eliminate.“
Can a „Perfect“ Lamination Process Save EPE?
We also tested whether adjusting lamination parameters could mitigate EPE’s degradation. We ran trials with longer times (up to 900 seconds) and higher temperatures (up to 165°C).
While optimizing these parameters did improve the initial cross-linking and adhesion for both materials, it could not prevent the severe drop in EPE’s adhesion after the DH test. The fundamental chemical reaction—the hydrolysis of EVA—is an inherent property of the material. No amount of process fine-tuning can change that.
This underscores a critical point: robust module design starts with choosing the right materials. Rigorous material testing is the only way to validate that your choices can withstand the test of time.
The Takeaway: Balancing Cost vs. Long-Term Reliability
For manufacturers and developers of Glass-to-Glass bifacial modules, the conclusion is clear.
While EPE offers an attractive upfront cost reduction, it introduces a significant long-term reliability risk due to its poor adhesion durability in humid conditions. The potential cost of warranty claims, reputational damage, and underperforming assets far outweighs the initial savings on encapsulant.
POE, despite its higher price, demonstrates superior and even improving adhesion over time, making it the safer, more reliable choice for G2G applications intended to last 25 years or more.
This is a classic example of why moving beyond datasheet values to build and validate new solar module concepts under real-world conditions is essential for de-risking new technology.
Frequently Asked Questions (FAQ)
What is delamination and why is it so bad?
Delamination is the separation of the layers within a solar module. It breaks the protective seal, allowing moisture and air to penetrate, which can lead to rapid corrosion of solar cells and interconnectors, causing severe power loss and complete module failure.
What is a Damp Heat (DH) test?
The Damp Heat test is an accelerated aging test standard in the PV industry (defined by IEC 61215). By exposing modules or materials to prolonged high temperature (85°C) and high humidity (85% RH), it simulates the aging effects of several decades in a harsh outdoor environment.
Can’t you just use a sealant at the edge of the module to prevent moisture?
Edge sealants add a secondary line of defense, but they are not a substitute for a high-performance encapsulant. Over time, sealants can also degrade, and they cannot protect the module if the primary encapsulant-glass bond itself is chemically unstable.
Is EPE always a bad choice?
Not necessarily. EPE may be a suitable option for traditional modules with breathable backsheets, where moisture has a pathway to escape. However, our study indicates it is a high-risk choice specifically for Glass-to-Glass constructions where moisture gets trapped.
How can I test my own materials or module designs?
The best way is to conduct structured experiments using industrial-scale equipment that mimics real production conditions. This involves creating test laminates, running them through controlled lamination cycles, and performing accelerated aging tests followed by quantitative analysis like peel strength testing. If you have questions about setting up a test plan, our PVTestLab engineers are a great resource.
From Lab Insights to Production Reality
Choosing the right materials is foundational to a reliable and profitable solar project. This study highlights the importance of looking beyond initial specifications and investing in applied research that validates long-term performance.
By understanding the chemical interactions at the material level, manufacturers can avoid costly field failures and build products that truly stand the test of time.
Ready to de-risk your next module design? Explore our material testing and lamination trials to get data-driven confidence in your bill of materials.