EVA vs. POE: The Hidden Chemical Battle Degrading Your HJT & TOPCon Modules

Imagine investing in the latest high-efficiency solar modules—Heterojunction (HJT) or TOPCon—only to see their power output decline faster than expected. You check for the usual suspects like shading, soiling, and faulty inverters, but the real culprit is invisible: a slow-motion chemical reaction happening deep inside the module.

This isn’t a hypothetical scenario. It’s a critical challenge facing module developers and asset owners today—one where the very components designed to protect advanced solar cells could be silently undermining their performance. The issue lies in the complex relationship between the solar cell and its encapsulant, the clear polymer that holds everything together.

The Promise and Peril of Advanced Cell Technologies

To understand the problem, it helps to first appreciate what makes HJT and TOPCon cells so special. Unlike traditional PERC cells, these next-generation technologies achieve their record-breaking efficiencies by using ultra-thin, delicate surface layers.

One of the most critical of these is the Transparent Conductive Oxide (TCO) layer. Think of it as a transparent superhighway for electrons, incredibly efficient at collecting electricity generated by the cell without blocking sunlight. However, this high performance comes at a cost: TCO layers, often made of Indium Tin Oxide (ITO), are extremely sensitive to chemical corrosion, particularly from acids.

And this chemical vulnerability is the crux of the issue.

More Than Just Glue: The Critical Role of Your Encapsulant

The encapsulant is the unsung hero of a solar module. This polymer layer, typically EVA (Ethylene Vinyl Acetate) or POE (Polyolefin Elastomer), fulfills several vital roles:

• Adhesion: Bonding the glass, cells, and backsheet into a single, durable unit.
• Protection: Cushioning the fragile cells from mechanical stress and moisture.
• Optical Clarity: Allowing maximum sunlight to reach the cells.

For decades, EVA has been the industry workhorse—cost-effective, well-understood, and reliable for traditional solar cells. But it has a chemical quirk. During the lamination process, and over its lifetime when exposed to heat and humidity, EVA releases a small but significant byproduct: acetic acid.

For robust PERC cells, this was rarely a problem. But for the delicate TCO layers in HJT and TOPCon cells, it’s a recipe for degradation.

This is where POE comes in. As a polyolefin, POE is chemically inert. It doesn’t break down to create acidic byproducts, making it inherently more compatible with these sensitive cell architectures.

Acetic Acid vs. TCO: A Microscopic Battlefield

When an HJT or TOPCon cell is encapsulated with EVA, a slow but relentless chemical attack begins. The acetic acid released by the EVA corrodes the TCO layer, damaging its conductivity. This process, often called TCO corrosion, creates „dead zones“ on the cell where electricity can no longer be efficiently extracted.

The effects become dramatically clear during industry-standard damp heat testing, where modules are subjected to 85°C and 85% relative humidity for over 1,000 hours to simulate decades of harsh outdoor exposure.

At PVTestLab, our applied research programs have repeatedly demonstrated this phenomenon. In controlled experiments, we build identical bifacial HJT modules where the only variable is the encapsulant—one using EVA, the other POE.

The results are stark.

After damp heat testing, electroluminescence (EL) imaging, which reveals the active areas of a solar cell, tells the full story. The module made with POE looks nearly identical to its pre-test state. The EVA module, however, is riddled with dark patches, particularly around the cell edges and busbars, indicating severe power loss from TCO corrosion.

This degradation is especially pronounced on the rear side of bifacial modules, where the TCO layer is more exposed. The result is a significant drop in both performance and long-term reliability.

The Case for POE: Preserving Long-Term Performance

The evidence points to a clear conclusion: for HJT, TOPCon, and other advanced cell technologies with sensitive TCO layers, POE is the superior encapsulant to ensure long-term performance and prevent acid-induced degradation.

While POE can present unique challenges during lamination due to its different melting and curing behavior, these are solvable engineering problems. The risk of TCO corrosion from EVA, however, is a fundamental material incompatibility that cannot be easily engineered away.

„We’ve seen promising module designs fail reliability tests not because of the cell technology, but because of a mismatch with the encapsulant. Choosing POE for sensitive cells like HJT isn’t just a material swap; it’s a strategic decision to protect the module’s core asset—its power-generating capability.“
— Patrick Thoma, PV Process Specialist

Here’s a quick breakdown to guide your thinking:

EVA (Ethylene Vinyl Acetate)

• Pros: Low cost, mature processing, widely available.
• Cons: Releases acetic acid, which corrodes TCO layers in HJT/TOPCon cells, leading to significant long-term power degradation.

POE (Polyolefin Elastomer)

• Pros: Chemically stable, no acidic byproducts, excellent moisture resistance, and proven compatibility with sensitive cell layers.
• Cons: Higher cost, requires more precise lamination process control.

For any organization building reliable PV module prototypes with next-generation cells, validating the encapsulant choice is not just a recommendation; it’s essential for bankability.

Frequently Asked Questions About Encapsulant Choice

1. What exactly is a TCO layer?
   A Transparent Conductive Oxide (TCO) is a razor-thin film of material that is both optically transparent and electrically conductive. In HJT and TOPCon cells, its crucial role is to collect electrons from the cell’s surface and guide them to the metal contacts with minimal power loss.

2. Is EVA always a bad choice for solar modules?
   Not at all. For conventional cell technologies like PERC, which do not have sensitive TCO layers in direct contact with the encapsulant, EVA remains a very reliable and cost-effective choice. The issue is specific to the chemical sensitivity of newer cell types.

3. Why is TCO corrosion worse for bifacial modules?
   Bifacial modules generate power from both sides, which means both front and rear surfaces of the cells have active, sensitive layers. The rear TCO layer is often more vulnerable to encapsulant-related corrosion, leading to a disproportionate loss in bifacial gain over the module’s lifetime.

4. Are there „acid-free“ EVA formulations?
   Yes, some manufacturers offer low-acid or acid-scavenging EVA formulations. While these can mitigate the problem to some extent, they don’t eliminate the fundamental risk of acid generation. For sensitive cells, POE’s inherent chemical inertness provides a more robust and permanent solution.

5. How can I test for this problem in my own module design?
   The most effective method is through accelerated aging tests. Conducting comprehensive material validation using a structured damp heat test (85°C / 85% RH), combined with pre- and post-test EL imaging and performance measurements, will quickly reveal any encapsulant-TCO incompatibility.

From Lab Insight to Factory Floor: Your Next Step

The transition to higher-efficiency cell technologies like HJT and TOPCon is exciting, but it introduces new rules for module manufacturing. Long-held assumptions about materials like EVA must be re-evaluated to ensure the final product is as reliable as it is powerful.

Understanding the chemical interaction between your encapsulant and your cells is the first step toward building a truly durable, high-performing solar module. The journey from a promising concept to a bankable product begins with making informed material choices backed by real-world data.

If you are developing a new module or evaluating materials, the logical next step is to move from theory to practice in a controlled, industrial-scale environment. Testing your specific combination of cells, encapsulants, and backsheets is the only way to guarantee long-term reliability and secure your investment.