You’ve done everything right. You sourced cutting-edge Heterojunction (HJT) cells, known for their chart-topping efficiency and excellent performance. You designed a state-of-the-art module. Yet, when the final flash test results come in, the numbers are disappointingly low. The power output is a fraction of what you calculated.
What went wrong?
The answer might lie in a step that many consider routine: lamination. In the world of HJT modules, the lamination process isn’t just about bonding layers together; it’s a delicate chemical balancing act. Get it wrong, and you can silently destroy the very efficiency you paid a premium for. The culprit is an invisible threat known as chemical degradation, and its primary target is the heart of the HJT cell.
The Achilles‘ Heel of High-Efficiency HJT Cells
To understand the risk, we first need to look at what makes an HJT cell so special. Unlike conventional PERC cells, an HJT cell features a crystalline silicon wafer sandwiched between ultra-thin layers of amorphous silicon. On top of this sandwich sits a critical component: the Transparent Conductive Oxide (TCO) layer.
Think of the TCO layer as the cell’s transparent superhighway for electricity. It’s designed to be optically transparent (letting sunlight through) and electrically conductive (guiding generated electrons out). This layer, often made of Indium Tin Oxide (ITO), is fundamental to the cell’s high performance.
Unfortunately, this TCO layer is also incredibly sensitive. It’s chemically fragile and can be easily damaged by certain industrial chemicals—especially acids.
When the TCO corrodes, it’s like putting potholes on that electrical superhighway. Its resistance increases, impeding the flow of electrons. The result is a direct loss in fill factor and overall module efficiency. The cell is still there, but its ability to perform at its peak has been permanently crippled.
The Invisible Threat: Chemical Outgassing During Lamination
So, where do these damaging chemicals come from? They are often released from the very materials used to protect the cells—the encapsulant and the backsheet—in a process called outgassing.
During lamination, the module stack is heated under vacuum to cure the encapsulant, turning it from a soft film into a durable, protective layer. This heating process, however, can trigger chemical reactions.
For decades, the industry standard encapsulant has been Ethylene Vinyl Acetate (EVA). While cost-effective and reliable for traditional modules, EVA has a significant drawback for HJT technology. As it cures, it releases acetic acid as a byproduct. For most solar cells, this isn’t a major issue. But for the sensitive TCO layer in an HJT cell, this acidic vapor is corrosive poison.
This is why many HJT module developers have switched to Polyolefin Elastomer (POE) encapsulants. POE is a more chemically stable material that does not produce acidic byproducts during curing, making it a much safer choice for HJT cells.
However, the encapsulant isn’t the only potential source of trouble. Backsheets, adhesives, and even cleaning residues can also release volatile organic compounds (VOCs) when heated, posing a further risk to the TCO. That’s why a holistic approach to material testing and validation is not just a good idea—it’s essential for success.
From Problem to Process: How to Protect Your TCO Layer
Preventing TCO degradation isn’t about finding a single „magic“ material. It’s about ensuring the compatibility of all your materials and fine-tuning your process to mitigate risks. This requires a two-pronged strategy: meticulous material qualification and intelligent process control.
Step 1: Pre-Qualify Your Bill of Materials (BOM)
Before you ever attempt to build a full module, every component must be evaluated. That means going beyond the supplier’s datasheet. You need to understand how your specific encapsulant and backsheet behave under real-world lamination conditions.
Does the backsheet release unexpected volatiles at higher temperatures? Does the POE you’ve selected interact negatively with other components? Answering these questions through structured trials can save you from costly failures down the line and is a cornerstone of robust solar module prototyping.
Step 2: Master the Lamination Vacuum Cycle
Once you have qualified your materials, the next line of defense is the lamination process itself. Simply swapping EVA for POE is not enough. The key lies in managing the outgassing, ensuring any volatile compounds are removed from the chamber before they have a chance to reach and react with the TCO layer.
This is where a multi-stage vacuum becomes critical. Instead of a single, long vacuum phase, an optimized cycle uses several stages of pumping down and holding. This technique effectively „breathes“ volatile gases out of the module stack before the temperature ramps up and the encapsulant fully melts. This sophisticated approach to lamination process optimization is one of the most powerful tools for preserving HJT cell integrity.
The Proof Is in the Performance: Validating Cell Integrity
How do you know if your strategy worked? The definitive proof comes from post-lamination analysis, specifically through Quantum Efficiency (QE) measurements.
QE testing measures how efficiently a solar cell converts photons of specific light wavelengths into electrons. For HJT cells, TCO degradation shows up as a distinct signature: a significant drop in QE response in the blue part of the light spectrum (short wavelengths).
Blue light is absorbed very close to the surface of the cell, right where the TCO layer is. Any damage to this layer immediately impacts its ability to collect the electrons generated by these high-energy photons.
By comparing the QE of a cell before and after lamination, you get a clear, data-driven verdict. If the blue response remains strong, your lamination process was a success. If it has dropped, it’s a clear sign that damaging chemical reactions occurred and your process needs refinement.
Frequently Asked Questions (FAQ) about HJT Lamination
What exactly is a Transparent Conductive Oxide (TCO) layer?
It’s a very thin, optically transparent film on the surface of an HJT solar cell that is also electrically conductive. Its job is to collect the electrons generated by sunlight and transport them to the metal contacts with minimal energy loss.
Why is EVA a risk for HJT cells but less so for others?
Standard PERC cells have a protective silicon nitride layer that is more robust against the acetic acid released by EVA. HJT’s TCO layer is far more chemically reactive and susceptible to acid-induced corrosion, which increases electrical resistance and lowers efficiency.
Is POE always the better choice for HJT modules?
Generally, yes. POE encapsulants are chemically inert and do not produce acidic byproducts, making them the industry standard for HJT modules. However, different POE formulations can have varying properties, so material qualification is still crucial.
Can’t I just use a longer vacuum time to remove the gases?
Not necessarily. A simple, long vacuum phase might not be effective. A multi-stage vacuum cycle, which alternates between pumping and holding, is much more efficient at drawing out volatiles from the entire module layup before the encapsulant fully liquefies and traps them.
What does a „drop in the blue spectrum“ of a QE test indicate?
It’s a classic sign of TCO degradation. Blue light (shorter wavelengths) is absorbed near the cell’s surface. If the TCO layer at the surface is damaged, its ability to transport electrons generated by that blue light is impaired, leading to a lower QE reading in that specific spectral range.
Your Bridge from Lab Concept to Industrial Reality
The incredible potential of HJT technology can only be realized when every step of the manufacturing process is executed with precision. Lamination, far from being a simple assembly step, is a critical process where the module’s final performance is ultimately decided.
Success requires a deep understanding of material interactions and the ability to control the process environment with scientific rigor. By combining careful material selection with optimized lamination recipes, you can protect the delicate TCO layer and ensure your high-efficiency cells deliver on their promise.
Developing and validating these processes shouldn’t require building an entire pilot factory. If you’re ready to test your materials and perfect your lamination recipes in a real industrial environment, you can rent our R&D production line and work alongside our process engineers to turn your innovative concepts into reliable, high-performance solar modules.