You’ve upgraded to high-efficiency N-Type TOPCon cells and you’re expecting a significant leap in module power and performance. Yet, when the final flash test results come in, they’re not quite hitting the mark. The efficiency is a little lower than spec, and the Fill Factor (FF) is disappointingly soft. You check the stringer, the layup, the materials—everything seems perfect.
What if the problem isn’t in the components, but in a process step you’ve always taken for granted? What if the damage is happening in the final few minutes after the main lamination cycle is complete?
For years, including a „hold time“ or „dwell time“ after lamination, where the module rests at high temperature before moving to the cooling press, has been common practice in module manufacturing. With older cell technologies like PERC, this was rarely an issue. But our research shows that for the highly sensitive passivation layers of TOPCon cells, these few minutes can be devastating.
The Hidden Vulnerability of High-Efficiency Cells
N-Type TOPCon (Tunnel Oxide Passivated Contact) cells are an incredible leap forward in solar technology, enabling higher efficiencies and lower degradation. However, their advanced structure—particularly the ultra-thin tunnel oxide and polysilicon layers—makes them more sensitive to certain chemical and thermal stresses during production.
Think of it like the difference between a rustic stoneware bowl and a delicate piece of porcelain. Both are strong, but the porcelain requires a much more controlled and gentle firing process to achieve its superior quality without cracking. TOPCon cells are the porcelain of the solar world.
Two critical indicators of a solar cell’s health are:
- Fill Factor (FF): This measures how efficiently the cell can extract the power it generates. A high FF means the cell is performing close to its theoretical maximum.
- Shunt Resistance (Rsh): This indicates unwanted alternative paths, or „leaks,“ for the electrical current. High shunt resistance is good—it means the current is flowing where it should. Low shunt resistance is a classic sign of cell damage or process-induced defects.
When these values drop, the module’s power output suffers directly. The challenge is that this degradation can be triggered by process parameters once considered safe.
A Data-Driven Look at Hold Time Degradation
To investigate how lamination hold time affects TOPCon cells, our process engineers at PVTestLab conducted a controlled experiment. We manufactured a series of identical glass-glass modules using the same N-Type TOPCon cells and the same EVA encapsulant.
The only variable we changed was the dwell time—the period the module was held at the lamination temperature of 155°C after the main cycle, before being transferred to the cooling press. We tested hold times of 0, 3, 6, and 9 minutes.
The results were immediate and alarming.
The Visual Evidence: What the EL Image Reveals
An Electroluminescence (EL) test is like an X-ray for a solar module, revealing hidden defects, microcracks, and areas of inactivity. The images below show the same module area before lamination and after a lamination cycle with a 9-minute hold time.
The „before“ image on the left is clean and uniform, showing a healthy, high-performing cell. The „after“ image on the right is riddled with dark areas. This darkening is a direct visualization of shunting—areas where the cell is effectively short-circuiting and losing power. This damage is irreversible.
The Numbers Don’t Lie: A Sharp Drop in Performance
The visual degradation in the EL images lined up perfectly with the electrical data we measured. We tracked both Fill Factor and Shunt Resistance across the different hold times.
The graph tells a stark story:
- At 0 minutes hold time (immediate cooling): The module maintained an excellent Fill Factor of 82.1% and a healthy Shunt Resistance of 11.2 kΩ.
- After just 3 minutes of hold time: The Fill Factor dropped to 81.1%, and the Shunt Resistance was more than halved to 5.7 kΩ. This represents a significant performance loss from a seemingly minor process tweak.
- At 9 minutes of hold time: The results were catastrophic. The Fill Factor plummeted to 79.5%, and the Shunt Resistance collapsed to just 2.2 kΩ. This represents an absolute FF loss of 2.6%—a massive hit to the module’s final power rating and profitability.
„The data clearly indicates that certain encapsulant formulations can interact negatively with the sensitive TOPCon passivation layer under prolonged thermal stress,“ notes Patrick Thoma, a PV Process Specialist at PVTestLab. „This chemical interaction damages the cell structure, creating shunts that drain the module’s performance. The process window for TOPCon is simply much tighter than it was for PERC.“
This experiment reveals a critical reality for modern module manufacturing: processes that were once standard are now potential failure points. Optimizing your production line for TOPCon isn’t just about handling new cells; it’s about re-evaluating every step of the process, especially thermal cycles. This is where dedicated solar module prototyping and validation become essential.
Key Takeaways for Your Production Line
The transition to new technologies like TOPCon offers incredible opportunities, but it demands a more precise and data-driven approach to manufacturing. Here’s what this research means for you:
- Eliminate Unnecessary Hold Times: The safest approach for TOPCon lamination is to move the module to the cooling press immediately after the curing cycle is complete. Every extra minute spent at peak temperature risks irreversible degradation.
- Rethink Your Process Parameters: Don’t assume your legacy PERC lamination recipe will work for TOPCon. A thorough lamination process optimization is crucial to define a new, narrower process window that protects cell performance.
- Material Compatibility is Paramount: Not all encapsulants behave the same way. The interaction between your specific TOPCon cell supply and your chosen encapsulant (whether EVA, POE, or something else) must be validated. Comprehensive PV module material testing can identify the ideal material combination before you commit to mass production.
Ultimately, building high-quality TOPCon modules means understanding and respecting their unique sensitivities. By focusing on data-driven process control, you can unlock their full performance potential and avoid the silent, costly trap of post-lamination dwell time.
Frequently Asked Questions (FAQ)
What is a TOPCon solar cell?
TOPCon (Tunnel Oxide Passivated Contact) is a next-generation solar cell technology known for its high efficiency and low-temperature coefficient. It enhances traditional N-type cells with an ultra-thin layer of silicon dioxide (tunnel oxide) and a layer of highly doped polysilicon, which significantly reduces recombination losses and boosts performance.
What is Fill Factor (FF) in a solar module?
Fill Factor measures the quality of a solar cell. It’s the ratio of the maximum power a cell can produce to the theoretical power (the product of its open-circuit voltage and short-circuit current). A higher Fill Factor, typically expressed as a percentage, indicates lower internal power losses and higher overall efficiency.
Why is Shunt Resistance (Rsh) so important?
Shunt Resistance measures the opposition to unintended current leakage across the solar cell. A high Rsh is desirable because it means the generated electricity is flowing through its intended path to the module’s output. A low Rsh indicates that current is leaking through defects or shunts, which dissipates energy as heat and significantly reduces the cell’s efficiency and Fill Factor.
Is this degradation problem specific to certain encapsulants?
While our test was conducted with a specific EVA encapsulant, the underlying principle applies more broadly. The chemical composition of any encapsulant (including different types of EVA and POE) can interact differently with the sensitive TOPCon layers under thermal stress. It is crucial to validate the compatibility of your specific cell and encapsulant combination.
How can I test my own lamination process for this issue?
The most effective way is to run a controlled experiment (a Design of Experiments, or DOE) similar to the one described. This involves producing a series of small modules or single-cell laminates where you vary only one parameter at a time, such as hold time or temperature. By carefully measuring the electrical characteristics (like IV curves and Rsh) and inspecting the cells with EL imaging before and after, you can pinpoint the exact conditions that cause degradation. Facilities like PVTestLab provide the ideal environment for such detailed process validation.