Heterojunction (HJT) technology is a marvel of solar engineering, promising some of the highest efficiency ratings on the market. But this high performance comes with a hidden vulnerability—one that standard certification tests may not be rigorous enough to expose. The vulnerability lies within a microscopic layer critical to performance: the Transparent Conductive Oxide (TCO).
Imagine spending years developing a cutting-edge HJT module, only to discover that its performance silently degrades in the field, far faster than predicted. The culprit? Moisture, which triggers a slow, creeping corrosion that can cripple your module from the inside out.
Understanding and preventing this failure mode is the difference between a high-efficiency concept and a bankable, long-lasting solar asset.
HJT’s Achilles’ Heel: A Closer Look at the TCO Layer
To appreciate the risk, it helps to understand the architecture of an HJT cell. Unlike traditional cells, HJT uses a crystalline silicon wafer sandwiched between ultra-thin layers of amorphous silicon. A transparent, electrically conductive layer—the TCO—is then applied to the surface to extract the electricity efficiently.
Think of the TCO layer as the cell’s nervous system. It’s responsible for collecting the electrons generated by sunlight without blocking the light itself. It’s a delicate balancing act, and this layer is notoriously sensitive.
![A diagram showing the layers of an HJT solar cell, highlighting the TCO and encapsulant layers.]
The primary threat to the TCO layer is moisture. When water vapor penetrates the module, it can trigger a chemical reaction with the TCO material, typically Indium Tin Oxide (ITO), causing oxidation. This corrosion degrades the layer’s conductivity, creating dead zones in the cell and leading to significant power loss. Worse, it can lead to delamination, where the layers of the module begin to peel apart.
The Encapsulant’s Critical Role: Shield or Saboteur?
Your first line of defense against this moisture invasion is the encapsulant—the polymer material sealing the solar cells between the glass and the backsheet. But not all encapsulants are created equal. In fact, the wrong choice can actively accelerate the degradation you’re trying to prevent.
For years, Ethylene Vinyl Acetate (EVA) has been the workhorse encapsulant for conventional solar modules. But for HJT, it carries a significant risk. During lamination and over its lifetime, EVA can release acetic acid as a byproduct. This acid creates a highly corrosive environment right next to the sensitive TCO layer, dramatically speeding up its decay.
This vulnerability has led innovators to Polyolefin Elastomer (POE) encapsulants. POE is a better choice for HJT modules because it’s both chemically stable and acid-free—releasing no corrosive byproducts—and it provides a superior moisture barrier. With a much lower Water Vapor Transmission Rate (WVTR) than EVA, it acts as a far better shield against humidity.
Choosing between these materials isn’t just a component swap; it’s a fundamental decision that impacts the long-term viability of your new solar module concepts.
Why Standard Certification (IEC 61215) Is Not Enough
So, you’ve chosen a high-quality POE, and your module has passed the standard IEC 61215 certification tests, including the 1000-hour Damp Heat (DH) test. You’re safe, right?
Not necessarily.
While essential, these standard tests represent a baseline for safety and performance, not a guarantee of 25+ year reliability in the field. For sensitive technologies like HJT, the 1000-hour DH test may not be long or stressful enough to reveal slower-acting corrosion mechanisms. A module can pass the test, be certified for sale, and still contain a hidden vulnerability that will only appear after several years of real-world operation.
To truly understand a module’s resilience, you need to go beyond the standard and subject it to more extreme, targeted stress profiles designed to accelerate aging.
Uncovering the Truth: The Power of Sequential Stress Testing
This is where advanced reliability testing comes in. One of the most effective methods for assessing TCO stability is a sequential test combining two powerful stressors: Humidity Freeze (HF) and Damp Heat (DH).
Here’s how it works:
- Humidity Freeze (HF): The module is subjected to repeated cycles of high humidity followed by deep-freezing temperatures. This process mimics harsh environmental swings and creates microscopic stresses or cracks in the module’s layers.
- Extended Damp Heat (DH): After being „primed“ by the HF cycles, the module is then placed in a climate chamber for an extended Damp Heat test (e.g., 2000 hours at 85°C and 85% relative humidity). The high heat and humidity aggressively push moisture through any potential weaknesses created during the HF phase.
This „one-two punch“ is far more revealing than a standalone DH test. The HF cycles can open the door for moisture, and the extended DH test shoves it through, directly attacking the TCO layer.
The results can be stark.
![A side-by-side comparison of two HJT modules after testing. One shows severe TCO corrosion (’snail trails‘), the other looks pristine.]
The module on the left used an encapsulant with poor moisture resistance, leading to widespread corrosion, often visible as „snail trails“ or discoloration. The module on the right, using a superior POE encapsulant validated through rigorous lamination trials, shows no signs of degradation.
But visual evidence is only half the story. To quantify the actual power loss, we need to look deeper with Electroluminescence (EL) imaging. EL testing passes a current through the module, causing it to light up. Healthy, active areas shine brightly, while damaged or inactive areas appear dark.
![An Electroluminescence (EL) image showing dark, inactive areas on a degraded HJT cell, corresponding to the visible corrosion.]
This EL image reveals the true extent of the damage. The dark patches correspond directly to the corroded TCO, showing areas of the cell that are no longer producing power. This is the kind of data that transforms an observation („it looks discolored“) into a quantifiable business problem („we’ve lost 8% of our power output“). This level of validation requires access to real industrial equipment capable of simulating and measuring these effects accurately.
FAQ: Your Questions on HJT Module Reliability Answered
What exactly is a TCO layer?
It’s an ultra-thin, transparent film in a solar cell that is also electrically conductive. Its job is to collect electrons generated by sunlight and move them to the metal contacts without blocking light from reaching the silicon absorber layer.
Why is EVA encapsulant a risk for HJT modules?
EVA can release acetic acid over time, especially under heat and humidity. This acid is highly corrosive to the TCO layer, a key component of HJT cells, leading to premature degradation and power loss.
What does WVTR mean for an encapsulant?
WVTR stands for Water Vapor Transmission Rate. It’s a measure of how quickly water vapor passes through a material. For solar module encapsulants, a lower WVTR is better, as it means the material provides a more effective barrier against moisture.
What’s the difference between Damp Heat (DH) and Humidity Freeze (HF) testing?
Damp Heat (DH) testing exposes a module to constant high heat and high humidity to test its resistance to moisture ingress over time. Humidity Freeze (HF) testing subjects a module to cycles of humidity and sub-zero temperatures, which tests its ability to withstand mechanical stresses from expansion and contraction. Combining them sequentially is a powerful way to accelerate aging.
Are „snail trails“ always a sign of power loss?
While they don’t always cause immediate, catastrophic power loss, „snail trails“ are a clear indicator that moisture has penetrated the module and a chemical reaction is occurring. It’s a significant warning sign of underlying degradation that will very likely lead to reduced performance and reliability over the module’s lifetime.
From Data to Durability: Building Resilient HJT Modules
The incredible efficiency of HJT technology holds immense promise for the future of solar energy—a promise that can only be realized if the modules are built to last. Protecting the sensitive TCO layer from moisture-induced corrosion is not just a technical detail; it is fundamental to ensuring long-term performance, bankability, and investor confidence.
Relying on standard certifications alone is not enough. Adopting advanced, sequential stress tests that push materials to their limits gives module developers and material suppliers the critical data needed to make informed choices, validate their designs, and build a truly resilient product. Understanding these failure modes is the first step toward innovating with confidence.