The Invisible Threat: Why Your Lamination Process Might Be Breaking Your Best TOPCon & HJT Cells

You’ve done everything right. Your team has integrated the latest ultra-high-efficiency TOPCon or HJT cells into a new module design. Initial flash tests look promising, and the datasheets predict outstanding performance. But after lamination, something is wrong: power output is lower than expected, and faint, spiderweb-like patterns appear on Electroluminescence (EL) images—the telltale sign of microcracks.

What happened?

The culprit is often an invisible force that conventional quality control can miss: mechanical stress. As the solar industry pushes for higher efficiency, cells are becoming thinner and more fragile than ever. The very lamination process designed to protect them can become their biggest threat. It’s the new reality of working with advanced cell technologies, where yesterday’s standard operating procedures can lead to today’s costly failures.

The Double-Edged Sword of Ultra-Thin Cells

TOPCon (Tunnel Oxide Passivated Contact) and HJT (Heterojunction) cells are marvels of photovoltaic engineering, pushing module efficiency well over 22%. A key part of their design is the use of incredibly thin silicon wafers, often just 120–130 micrometers (µm) thick—a significant reduction from the 180 µm standard of previous generations.

This thinness helps reduce material costs and improve performance, but it comes at a price: extreme brittleness. These cells are far more susceptible to mechanical pressure during manufacturing, especially during lamination, where heat and pressure bond the module layers together.

The result? Microcracks. These tiny, often invisible fractures can:

• Disrupt electrical flow: Create inactive or „dead“ areas within the cell, which appear as black spots on EL images.
• Reduce power output: A single, well-placed microcrack can compromise the performance of an entire cell string.
• Pose long-term reliability risks: Over time, thermal cycling and environmental stress can cause these small cracks to grow, leading to significant power degradation in the field.

The challenge is that this damaging pressure isn’t uniform. It concentrates in microscopic “hotspots” that standard laminator sensors can’t detect. So, how do you fight an enemy you can’t see?

Making the Invisible Visible: A Methodology for Mapping Stress

At PVTestLab, we operate on a simple principle: you can’t solve a problem you can’t measure. To protect these fragile, high-efficiency cells, we first need to visualize where and why stress occurs during lamination. Our methodology combines two powerful tools: pressure mapping films and high-resolution EL imaging.

Step 1: Laying the Trap with Pressure Mapping Film

Before the module layup goes into the laminator, we strategically place a specialized, ultra-thin pressure-sensitive film (like Fuji Prescale film) directly onto the solar cells. The film is designed to change color permanently based on the amount of pressure applied.

This film acts as a silent witness during the lamination cycle, capturing a precise, high-resolution map of every pressure point the cells experience.

Step 2: Revealing the Pressure „Hotspots“

After the lamination cycle, the film is removed and analyzed. The results are often startling. Instead of a uniform color indicating even pressure, we see a detailed map of peaks and valleys. Sharp, red spots instantly reveal the hidden hotspots where pressure concentrated to dangerous levels.

These hotspots are commonly caused by:

• Cell interconnect ribbons: The edges of solder ribbons can create sharp pressure lines.
• Overlapping materials: Any slight overlap between glass, encapsulant, or backsheet can create a high-pressure ridge.
• Busbars and junction boxes: These components naturally create uneven surfaces.
• Debris or imperfections: Even a tiny foreign particle can act like a needle, focusing immense pressure on a small area of the cell.

Step 3: Connecting the Dots with EL Imaging

The pressure map shows where the stress occurred; the EL image reveals the consequence. By comparing the pressure map to a post-lamination EL image of the module, the connection becomes undeniable. The red hotspots on the film align perfectly with the microcracks and dark areas visible in the EL scan.

This one-to-one correlation is the „aha moment.“ It provides concrete, visual evidence that links specific layup features and process parameters directly to cell damage. We are no longer guessing; we are seeing the exact cause and effect.

Actionable Guidelines for Minimizing Cell Stress

Armed with this data, we can move from diagnosing the problem to preventing it. Here are three key areas where this methodology provides actionable insights for module designers and manufacturers.

1. Rethink Your Layup and Interconnection Design

The way components are arranged is critical. The pressure map data allows you to test and validate design choices before committing to mass production.

• Ribbon geometry: Using flatter, wider ribbons can distribute pressure more evenly than round or thick wires.
• Gap management: Ensure there are no unintentional overlaps between layers of encapsulant or backsheet. Precision is key.
• Busbar integration: Analyze how busbar connections create pressure points and explore alternative designs or cushioning materials.

Testing these variables is a core part of our solar module prototyping services, allowing developers to build modules with minimal inherent stress from the start.

2. Choose Your Materials Wisely

Not all materials behave the same under pressure. The encapsulant, in particular, plays a crucial role in cushioning the cells.

• Encapsulant flow & hardness: Softer encapsulants like certain POEs may provide better cushioning than more rigid EVAs, absorbing pressure differentials more effectively.
• Backsheet & glass conformity: The flexibility and uniformity of your backsheet and the flatness of your glass can also influence how pressure is distributed.

Conducting structured material testing and lamination trials is essential to compare how different material combinations perform under real-world lamination conditions.

3. Fine-Tune Your Lamination Parameters

The laminator isn’t a simple oven; it’s a precision tool. While pressure mapping doesn’t change the machine’s programming, it provides the feedback needed to optimize its settings.

• Pressure & dwell times: Is it possible to achieve a robust bond with slightly less membrane pressure or a different ramp-up speed? Data from pressure films can help you find the sweet spot.
• Temperature profiles: The viscosity of the encapsulant changes with temperature. Adjusting the heating cycle can influence how the material flows and cushions the cells.

„With ultra-thin cells, you can’t assume your old lamination recipe will work,“ notes Patrick Thoma, a PV Process Specialist at PVTestLab. „You have to prove it with data. The pressure map doesn’t lie, and it gives you the evidence needed to build a more robust and reliable process.“

Frequently Asked Questions (FAQ)

Q: What exactly are microcracks and why do they matter so much?
A: Microcracks are tiny fissures in the silicon wafer of a solar cell. While sometimes harmless, they can sever the cell’s electrical grid, creating inactive „dead zones“ that no longer produce power. This directly reduces the module’s overall efficiency and can worsen over time, posing a significant long-term reliability risk.

Q: What makes TOPCon and HJT cells more fragile than traditional PERC cells?
A: It comes down to their thickness. At just 120–130 µm, they are about 30% thinner than their 180 µm PERC predecessors, making them physically more brittle and less able to withstand mechanical force. It’s much like how a thin pane of glass is more fragile than a thick one.

Q: Can I detect mechanical stress without using special films?
A: Not directly. Standard laminator controls measure the chamber’s overall pressure, not the microscopic pressure points on the cells themselves. You only see the results of the stress—the microcracks in a post-lamination EL image—by which point the damage is already done. The films allow you to proactively identify the source of the stress.

Q: Is this problem only for brand-new module designs?
A: No. Any manufacturer switching from thicker PERC cells to thinner TOPCon or HJT cells in an existing module design is at risk. An old process that was perfectly safe for more robust cells may not be suitable for the newer, more fragile technology, making re-validation crucial.

Q: How much power is typically lost from these microcracks?
A: It varies widely depending on the number, location, and severity of the cracks. A few minor cracks might cause a negligible drop, but a network of cracks across several cells in a string can lead to a power loss of 3-5% or even more, completely negating the efficiency gains of the advanced cells.

From Guesswork to Guarantee

The transition to ultra-thin TOPCon and HJT cells is a major leap forward for the solar industry, but it demands a higher level of process control. Relying on outdated assumptions is no longer an option.

By making invisible stresses visible, you can shift from reactive problem-solving to proactive design. Quantifying pressure helps you understand the root causes of cell breakage, enabling you to build a lamination process that protects your most valuable components. This data-driven approach is the key to unlocking the full potential of high-efficiency modules, ensuring they deliver on their promise—not just in the lab, but for decades in the field.

Ready to build a more resilient manufacturing process? Discover how our process optimization services can help you scale successfully and turn today’s challenges into tomorrow’s competitive advantage.