Imagine two brand-new, high-efficiency Heterojunction (HJT) solar modules. They look identical, pass every initial quality check, and promise decades of superior performance. But fast-forward five years. One is performing as expected, a reliable workhorse of clean energy. The other has suffered a mysterious and significant drop in power output, its high-tech promise quietly fading under the sun.
What went wrong? The culprit wasn’t a faulty cell or a bad connection. It was a silent chemical reaction—an invisible incompatibility between the HJT cell and the encapsulant chosen to protect it.
HJT technology is at the forefront of solar efficiency, but its unique structure also harbors a specific vulnerability that many manufacturers are only now beginning to fully appreciate. This isn’t a story about cutting corners; it’s about understanding the complex chemistry that dictates whether a module thrives or fails.
The Power and Problem of HJT’s Unique Structure
To understand the risk, we need to look at what makes HJT cells so special. Unlike traditional PERC cells, an HJT cell is a high-tech sandwich: a core of crystalline silicon layered with ultra-thin films of amorphous silicon and, crucially, a Transparent Conductive Oxide (TCO) layer.
Think of the TCO layer as the cell’s nervous system. Its job is to collect all the electricity the cell generates and transport it efficiently with minimal loss. It’s a key ingredient in HJT’s record-breaking efficiency.
But this high-performance layer is also incredibly delicate and highly susceptible to corrosion from acidic compounds—a problem that can turn a module’s protective encapsulant into its worst enemy.
When Protection Turns to Poison: The Encapsulant’s Role
Every solar module uses encapsulant films—typically made of EVA (Ethylene Vinyl Acetate) or POE (Polyolefin Elastomer)—to laminate the components together. This material has three critical jobs:
- Cushioning: Protect the fragile cells from mechanical stress.
- Bonding: Hold the glass, cells, and backsheet together for decades.
- Optical Clarity: Allow maximum sunlight to reach the cells.
For years, EVA has been the industry standard because it’s cost-effective and reliable. When used with HJT cells, however, it can introduce a catastrophic flaw. Under the heat and humidity a module experiences in the field, certain EVA formulations can undergo hydrolysis, releasing small amounts of acetic acid.
To a standard solar cell, this might not be a major issue. But to the sensitive TCO layer in an HJT cell, acetic acid is highly corrosive.
„The TCO layer is fundamental to HJT’s performance, but it’s chemically vulnerable,“ explains Patrick Thoma, PV Process Specialist at PVTestLab. „When acidic components from an encapsulant attack this layer, they increase the module’s series resistance. It’s like trying to send water through a rusty pipe—the flow is restricted, and you lose a significant amount of power. This degradation is often irreversible.“
This chemical attack results in two silent killers of HJT module performance:
- TCO Corrosion: The acid literally eats away at the conductive layer, reducing the fill factor and leading to a direct loss of power.
- Adhesion Loss: The chemical reaction can also weaken the bond between the encapsulant and the cell surface. Over time, this can lead to delamination, allowing moisture to penetrate deeper into the module and cause further damage.
Because of this risk, many HJT module manufacturers turn to POE encapsulants, which are inherently non-acidic and have a much higher resistance to water vapor. But the story doesn’t end there. Not all POE films are created equal. Additives, primers, and processing variations can still create compatibility issues. Assuming any POE is a „safe“ choice without proper validation is a gamble.
How to Predict the Future: The Power of Validation Testing
You can’t see TCO corrosion with the naked eye. It doesn’t show up in a standard factory flash test. The damage accumulates slowly, becoming apparent only after modules have been in the field for months or years—when it’s far too late and incredibly expensive to fix.
So, how do you prevent it? You have to fast-forward time.
This is where a structured testing protocol becomes essential. By simulating a module’s 25-year lifespan under harsh, controlled conditions, you can expose these hidden incompatibilities before they ever reach a customer. The gold standard for this is the damp heat test.
The process involves building complete modules through solar module prototyping, using the exact combination of cells, encapsulants, and other materials planned for production. These modules are then placed in a climate chamber and subjected to grueling conditions—typically 85°C and 85% relative humidity—for 1,000 to 2,000 hours.
This intense environment accelerates the chemical reactions that would take years to unfold in the field. After the test, the real investigation begins.
Reading the Signs of Failure
Once a module has undergone the damp heat test, it’s analyzed using several methods to get a complete picture of its health:
- I-V Curve Measurement (Flasher Test): This is the bottom line. By comparing the module’s power output before and after the test, you can precisely quantify any performance degradation. A significant drop in fill factor is a classic red flag for TCO corrosion.
- Electroluminescence (EL) Imaging: This is like an X-ray for a solar module. It reveals hidden defects invisible to the eye. In a module suffering from TCO corrosion, the EL image will show darkened areas or patterns of inactivity where electricity is no longer being collected efficiently.
- Visual Inspection: Specialists look for physical signs of material failure, such as delamination, bubbles between the layers, or yellowing of the encapsulant.
Combining these data points provides a clear, undeniable verdict on whether an encapsulant is truly compatible with an HJT cell. This type of rigorous encapsulant material testing replaces guesswork with certainty. Instead of hoping for the best, manufacturers can make material choices based on hard evidence, ensuring the long-term reliability and bankability of their products. By leveraging accelerated aging tests, you aren’t just testing a module; you’re securing its future.
Frequently Asked Questions (FAQ)
What exactly is a TCO layer in an HJT cell?
The Transparent Conductive Oxide (TCO) layer is an ultra-thin film of material (like Indium Tin Oxide, or ITO) that is both optically transparent and electrically conductive. In HJT cells, it’s applied to the amorphous silicon layers to efficiently collect the electrical current generated by the cell and move it to the metal contacts.
Why is EVA encapsulant often incompatible with HJT cells?
Standard EVA encapsulants can release acetic acid as a byproduct when exposed to heat and moisture over time. This acid is highly corrosive to the TCO layers in HJT cells, triggering a chemical reaction that degrades the layer’s conductivity and causes significant power loss.
Is POE always a safe choice for HJT modules?
While POE is generally a much safer choice because of its non-acidic nature and low water vapor transmission rate, it’s not a guaranteed solution. Different POE formulations contain various additives or primers that could still react with the cell surface. Therefore, every specific POE material should be validated through testing to confirm its long-term compatibility.
What does a damp heat test (85°C / 85% RH) simulate?
The damp heat test is an accelerated stress test designed to simulate the cumulative effect of decades of exposure to hot, humid environments. The high temperature and humidity accelerate aging processes like corrosion, hydrolysis, and delamination, allowing engineers to assess the long-term durability of a module in a matter of weeks instead of years.
How long does it take to see TCO corrosion during a test?
The onset of detectable corrosion depends on the severity of the incompatibility between the encapsulant and the cell. In some cases, initial signs of power degradation and changes in EL images can appear after just 500 hours in a damp heat chamber. A standard 1,000 or 2,000-hour test is typically sufficient to provide a definitive conclusion about long-term reliability.
The First Step to a Stronger Module
The exceptional efficiency of HJT technology represents a major leap forward for the solar industry. But unlocking its full potential requires a deeper understanding of the materials inside the module. The choice of encapsulant is not a minor detail—it is a critical decision that directly impacts a module’s power output, lifespan, and financial viability.
Moving beyond assumptions and committing to data-driven validation is the first and most important step toward building HJT modules that deliver on their promise for decades to come.