You’ve made the strategic decision to use Heterojunction (HJT) cells in your latest module design, banking on their impressive efficiency and superior performance. The first prototypes roll off the line, but when you put them through the flasher, the results are disappointing. The Open-Circuit Voltage (Voc) is lower than you calculated. Where did that precious voltage go?
Chances are, it didn’t just vanish. It was likely lost during the final, critical step of production: lamination.
This is a common and frustrating scenario for innovators working with HJT technology. The very process designed to protect the solar module can inadvertently degrade its most sensitive components. But this isn’t a dead end—it’s an engineering challenge that, with the right approach, can be overcome to unlock the full potential of your HJT modules.
The Power and Problem of HJT’s Unique Structure
To understand the challenge, you first need to look at what makes HJT cells so special. Unlike conventional PERC or TOPCon cells, HJT cells feature ultra-thin layers of amorphous silicon (a-Si:H) that „passivate“ the crystalline silicon wafer surface.
Think of these passivation layers as perfect insulation. They prevent electrons from getting lost at the surface, which is a major reason HJT cells achieve such high voltages and efficiencies.
However, this high-performance design comes with a crucial trade-off: a very low thermal budget. The delicate a-Si:H layers are sensitive to heat. If exposed to temperatures above approximately 180°C, these layers can degrade and lose their ability to passivate effectively. When this happens, electrical losses increase, and the cell’s Voc drops—permanently.
Why Standard Lamination Profiles Fall Short
On a typical production line, laminating PERC or TOPCon modules involves peak temperatures between 165°C and 175°C. This has been the industry standard for years, as it works perfectly for curing the encapsulant materials (like EVA or POE) that bind the module together.
Here’s the problem: that 165-175°C window is right in the danger zone for HJT cells.
Applying a standard PERC temperature profile to an HJT module is like trying to bake a delicate soufflé at the temperature you’d use for a pizza. You might cook the outside, but you’ll ruin the sensitive structure inside. The result is a module with compromised performance from day one.
The Cross-Linking Conundrum: A Delicate Balancing Act
So, why not just turn down the heat?
While lowering the temperature protects the HJT cell, it creates another critical problem with the encapsulant. Materials like Polyolefin Elastomer (POE), favored for its durability, require a specific amount of thermal energy to achieve „cross-linking.“
Cross-linking is the chemical process where individual polymer chains link together to form a strong, stable, and durable network. It’s what transforms a soft, pliable sheet of POE into a resilient, protective layer that can withstand decades of harsh weather.
For a module to be reliable, the industry benchmark is a degree of cure, or cross-linking level, of over 85%. If the lamination temperature is too low or the time too short, the encapsulant won’t cure properly. This can lead to serious long-term field issues like delamination, moisture ingress, and premature module failure.
This creates the central paradox of HJT lamination:
- Too Hot: You damage the HJT cell and lose Voc.
- Too Cold: The encapsulant doesn’t cross-link, and you sacrifice long-term reliability.
The Solution: A Low-Temp, Long-Hold Temperature Profile
The key to solving this paradox isn’t just reducing the peak temperature; it’s redesigning the entire temperature profile. The solution lies in a „low-temperature, long-hold“ cycle.
Instead of a quick ramp-up to a high peak temperature, this optimized approach involves:
- Lower Peak Temperature: Reducing the maximum temperature to a safer level for HJT cells, typically between 155°C and 160°C.
- Longer Holding Time: Extending the duration the module spends at this peak temperature.
This strategy ensures the encapsulant receives the total thermal energy it needs for full cross-linking, but without the damaging temperature spike that degrades the HJT passivation layers.
By carefully balancing time and temperature, you can satisfy both requirements: a healthy cell and a fully cured encapsulant.
Validation is Everything: Moving from Theory to Reality
Developing the perfect lamination recipe isn’t guesswork; it requires precise, repeatable experiments in a controlled environment. When working on new solar module concepts, every variable matters.
The validation process involves two critical measurements:
- Post-Lamination Voc Measurement: Immediately after lamination, the module is tested in a AAA Class flasher to measure its electrical parameters. Any significant drop in Voc compared to pre-lamination cell data is a clear sign of thermal damage.
- Degree of Cure Analysis: A sample of the encapsulant is taken from the finished module and analyzed—often using Differential Scanning Calorimetry (DSC)—to precisely measure its cross-linking level.
The goal is to find the processing sweet spot where Voc degradation is minimized (ideally <0.5%) while the degree of cure is maximized (>85%). This methodical approach, combining electrical testing with deep encapsulant material testing, is the only way to guarantee both initial performance and long-term reliability.
Frequently Asked Questions (FAQ)
What exactly is Voc degradation in this context?
Voc degradation refers to the permanent loss of Open-Circuit Voltage in the solar cell caused by thermal damage to its passivation layers during lamination. It’s a direct indicator that the cell’s peak performance potential has been compromised.
Can’t I just use a special low-temperature encapsulant?
Low-temperature encapsulants exist, but they often come with their own trade-offs, such as higher material costs, different processing requirements, or less extensive field data on long-term reliability. Optimizing the process for a proven, cost-effective material like POE is often a more robust and economical solution.
How much Voc loss is considered significant?
Even a 1% loss in Voc can be significant when multiplied across thousands of modules in a utility-scale solar plant. Best practice aims for Voc degradation of less than 0.5% to ensure the module performs as close as possible to its theoretical maximum.
Does this lamination profile affect other module components?
Yes, which is why a holistic approach is crucial. The time and temperature profile must also be suitable for the backsheet, junction box adhesives, and any other polymers in the module stack. A successful recipe considers the entire Bill of Materials (BOM).
Your Path to Optimized HJT Lamination
Manufacturing high-efficiency HJT modules successfully requires more than just sourcing great cells. It demands a process that respects their unique thermal limitations without compromising the structural integrity essential for a 30-year lifespan.
The transition from a standard lamination process to an optimized HJT profile is a solvable engineering challenge. By adopting a data-driven approach based on rigorous testing and validation, you can confidently produce modules that deliver on the full promise of HJT technology.
If you are navigating the complexities of HJT module production or evaluating new materials, getting the process parameters right from the start is essential. To discuss your specific project and challenges, feel free to consult with our process engineers.