You’ve invested in Heterojunction (HJT) cells, chasing that extra percentage of efficiency that promises to set your solar modules apart. They represent the pinnacle of PV technology—incredibly efficient but also famously delicate, with thinner wafers and materials that are more sensitive to heat.
But what if the very process designed to protect these cells is secretly damaging them?
The lamination cycle, a critical step that encases cells for 25 years of service in the field, can introduce invisible mechanical stresses. These stresses create microcracks, tiny fractures that degrade power output and compromise long-term reliability. For HJT technology, where low-temperature lamination is non-negotiable, this risk is magnified. The question is, how do you see this invisible damage before it becomes a costly problem?
The Challenge: Lamination in the Dark
Traditional solar module manufacturing often treats lamination as a „black box.“ You place the components in, run a standard cycle, and check the final product. If the module passes a final flash test, it’s deemed a success.
But this approach misses a critical detail: what is happening to the cells during the process? HJT cells, with their thin wafers and temperature-sensitive amorphous silicon layers, cannot withstand the high temperatures used for standard EVA encapsulants. This forces the use of low-temperature thermoplastic polyolefins (TPO), which require a completely different set of process parameters.
Without a precise, optimized lamination cycle, you risk:
- Uneven Pressure Distribution: Causing localized stress points that can initiate cracks.
- Thermal Shock: Rapid temperature changes that cause the cell and surrounding materials to expand and contract at different rates.
- Encapsulant Flow Issues: Improperly melted TPO can fail to cushion the cells, transferring mechanical stress directly to the silicon.
The result is a web of microcracks. Invisible to the naked eye, you might not know they exist until performance issues arise months or years later.
Seeing the Invisible: The Power of Electroluminescence (EL) Imaging
This is where advanced diagnostics come in. Electroluminescence (EL) imaging functions like an X-ray for solar cells. By passing a current through a cell, it illuminates, revealing any inactive or damaged areas as dark spots or lines. These patterns expose defects like microcracks, broken finger contacts, and other issues hidden from view.
The real breakthrough, however, lies in a simple but powerful methodology: comparing EL images before and after lamination.
This pre-and-post analysis transforms lamination from a black box into a transparent, data-driven engineering challenge. It provides undeniable proof of any damage induced during the cycle.
(Image: A side-by-side comparison of an EL image of HJT cells before lamination (left) and after a non-optimized lamination cycle (right). The post-lamination image clearly shows new dark lines and star-shaped patterns, indicating fresh microcracks.)
As the images show, the cell on the left is clean. The one on the right, after being subjected to a poorly controlled lamination process, is riddled with new cracks. This is lamination-induced stress made visible.
„The difference between a high-performing HJT module and a failing one often comes down to a few degrees Celsius or a few seconds of pressure difference during lamination. Without pre- and post-lamination EL data, you’re flying blind.“
— Patrick Thoma, PV Process Specialist
From Seeing to Solving: How EL Data Optimizes Your Process
Identifying the problem is only half the battle. The true value of EL imaging lies in using that data to systematically optimize the lamination process. Each type of crack tells a story about the stress it experienced.
(Image: A detailed EL image showing close-ups of different microcrack typologies. Annotations point out linear cracks, star cracks at busbar intersections, and edge cracks, with a brief explanation of their likely causes.)
By analyzing the location, shape, and severity of these cracks, engineers can correlate them directly to specific process parameters.
- Linear Cracks: Often suggest mechanical stress from uneven pressure or issues with the laminator’s membrane.
- Star Cracks: Typically appear at solder points or busbar intersections, indicating concentrated pressure that wasn’t properly cushioned by the encapsulant.
- Edge Cracks: Can point to handling issues or stress concentration at the cell’s perimeter during the pressure cycle.
Armed with this visual data, you can begin a structured optimization process. This involves conducting carefully designed Material Testing & Lamination Trials where you systematically adjust one variable at a time:
- Temperature Ramp-Up/Cool-Down Rates: Are you heating or cooling too quickly and causing thermal shock? Slowing the rates can give materials time to expand and contract uniformly.
- Pressure Application: Is pressure applied too soon, too late, or too aggressively? Modifying the pressure curve can prevent mechanical stress while the TPO encapsulant is still solidifying.
- Dwell Times: Are you holding temperature and pressure for the optimal duration to ensure the TPO flows correctly and fully encapsulates the cells without stressing them?
By comparing the pre- and post-lamination EL images from each trial, you can quantify the impact of every adjustment. The goal is to find the process window that results in zero new microcracks, ensuring the full potential of your HJT cells is preserved. This methodical approach is fundamental to successful Prototyping & Module Development.
Your Path to a Stress-Free Lamination Process
Unlocking the full efficiency of HJT technology requires moving beyond standard procedures and embracing a more scientific approach to production. While lamination-induced stress is a significant threat, it is solvable—and pre- and post-lamination EL analysis is the key.
By making the invisible visible, you can protect your investment, ensure long-term module reliability, and confidently deliver the high-performance products your customers expect.
Frequently Asked Questions (FAQ)
What exactly is a microcrack in a solar cell?
A microcrack is a tiny, often microscopic fracture in the silicon wafer of a solar cell. Though invisible to the naked eye, it disrupts the flow of electrons, creating an inactive area that reduces the cell’s overall power output. Over time, thermal cycling in the field can cause these cracks to grow, leading to further power degradation.
Why are HJT cells more sensitive to lamination stress than PERC cells?
HJT cells are built on very thin n-type monocrystalline silicon wafers, making them mechanically more fragile than the thicker wafers typically used for PERC. Additionally, the low-temperature passivation layers of amorphous silicon on HJT cells are sensitive to thermal stress, requiring specialized low-temperature lamination cycles that create their own unique process challenges.
Can you see microcracks without EL imaging?
No, in most cases, you cannot. Microcracks are too small to be seen with the naked eye. While some very large cracks might be visible (which would likely cause the cell to shatter), the ones that degrade performance are only detectable with diagnostic tools like Electroluminescence (EL) or Photoluminescence (PL) imaging.
What is TPO and why is it used for HJT lamination?
TPO stands for Thermoplastic Polyolefin. It’s a type of encapsulant that, unlike traditional EVA, does not require a chemical cross-linking (curing) process. This allows it to be processed at much lower temperatures (around 150-165°C), which is essential for protecting the temperature-sensitive layers of HJT cells.
Does every new microcrack mean the module will fail immediately?
Not necessarily, but it is a significant reliability risk. A small, isolated microcrack might cause only a minor initial power loss. However, it represents a weak point in the cell. Under real-world conditions with daily temperature fluctuations, the crack can propagate, leading to a much larger inactive area and accelerating the module’s power degradation over its lifetime.