You’ve made the switch. The new, high-efficiency TOPCon cells have arrived, promising a significant jump in module power output. The team is excited, management is optimistic, and the datasheets look fantastic. But after the first few batches run through your stringing line, the electroluminescence (EL) tests come back with alarming results: a field of tiny, dark lines—microcracks—scattered across the cells.
Suddenly, the promise of higher efficiency is overshadowed by the reality of lower yield and reliability concerns. What went wrong? The stringer parameters were the same ones you’ve trusted for years with PERC cells.
This scenario is becoming increasingly common in the solar industry. While TOPCon (Tunnel Oxide Passivated Contact) technology is a brilliant leap forward in photovoltaic efficiency, it brings a new set of mechanical challenges that can catch even experienced manufacturers off guard. The very innovations that make these cells better at converting sunlight into electricity also make them more susceptible to physical stress.
Understanding the Shift: From PERC to TOPCon
For years, PERC (Passivated Emitter and Rear Cell) has been the undisputed workhorse of the solar industry. It’s reliable, well-understood, and cost-effective. PERC technology improved upon traditional cells by adding a passivation layer on the rear surface, which helped capture more light.
TOPCon technology takes this a step further by introducing an ultra-thin tunnel oxide layer and a layer of highly doped polysilicon. This sophisticated structure dramatically reduces recombination losses, pushing cell efficiencies to new heights.
But from a mechanical perspective, the architecture presents a crucial difference:
- PERC Cells: Feature screen-printed aluminum back-surface fields (Al-BSF), which are fired at high temperatures to create a relatively robust structure.
- TOPCon Cells: Employ a more delicate, layered structure for their advanced passivation. This architecture, while electronically superior, can be less forgiving of mechanical stress.
It’s this fundamental structural difference that explains why your trusted stringing process might be failing your new TOPCon cells.
The Stringing Process: The First Major Stress Test
The journey from a single cell to a full module is fraught with mechanical stress, and the first hurdle is the stringing process. Here, cells are soldered together with ribbons at high temperatures to form the panel’s electrical circuits. This process subjects each cell to a combination of intense thermal stress from rapid heating and cooling, and mechanical pressure from hold-down clamps and bonding tools.
While a robust PERC cell can typically withstand these forces, a more sensitive TOPCon cell can suffer its first, often invisible, damage during this critical step.
A Head-to-Head Comparison: Putting Cells to the Test
We recognized that module developers needed clear, unbiased data on this issue. Anecdotal reports are helpful, but repeatable, industrial-scale testing provides the certainty needed to adapt processes confidently.
At PVTestLab, we conducted a controlled experiment to quantify the difference in microcrack susceptibility between M10 n-type TOPCon cells and M10 p-type PERC cells.
The Setup:
- Equipment: A standard, industrial-grade stringer.
- Parameters: We used the exact same soldering temperature, hold-down pressure, and process speed for both cell types.
- Goal: To isolate the cell technology as the only variable and measure the resulting microcrack incidence.
The Results:
The findings were definitive. Under identical process conditions, the TOPCon cells developed microcracks at a significantly higher rate than their PERC counterparts.
The data paints a clear picture: the standard „one-size-fits-all“ approach to stringing is no longer sufficient. What works for PERC can induce damaging levels of stress in TOPCon cells.
„We’re seeing that the process window for TOPCon is much narrower,“ notes Patrick Thoma, PV Process Specialist at PVTestLab. „The cells are less tolerant of thermal and mechanical inconsistencies. A process that was ‚good enough‘ for PERC can quickly lead to yield loss with TOPCon if it isn’t precisely optimized.“
These aren’t just theoretical numbers. An EL image reveals what this damage looks like in practice—a web of fractures that compromises the cell’s long-term performance and durability.
From Data to Action: How to Adapt Your Process for TOPCon Cells
This data isn’t a verdict against TOPCon technology; it’s a roadmap for process adaptation. The efficiency gains are real and valuable, but they must be unlocked with smarter manufacturing.
Based on our findings, here are the key areas to focus on when adjusting your stringing process for TOPCon cells:
- Reduce Thermal Shock: TOPCon’s delicate layers are more sensitive to rapid temperature changes. Experiment with slightly lower soldering temperatures and optimized heating/cooling profiles to minimize thermal stress.
- Optimize Mechanical Pressure: The pressure from hold-down tools and soldering heads is a primary culprit. Systematically reduce and test pressure settings to find the minimum force required for a reliable solder joint without flexing the cell.
- Ensure Uniform Support: Verify that the cell support system on your stringer is perfectly flat and free of any debris. Even a tiny particle can create a pressure point that initiates a crack.
- Validate with Data: Don’t rely on guesswork. Implementing changes requires a structured approach. Running small, controlled batches and using high-resolution EL testing to verify the results is critical. This is a core part of the solar module prototyping phase, allowing you to perfect your recipe before scaling up.
It’s also important to remember that stringing is just one step. The stresses introduced here can be magnified later on, making a holistic approach to lamination process optimization essential for ensuring the final module’s quality and durability.
Frequently Asked Questions (FAQ)
Q1: What exactly is a microcrack?
A microcrack is a tiny, often invisible fissure in a solar cell caused by thermal or mechanical stress. While you can’t see it with the naked eye, it disrupts the flow of electrons, reducing the cell’s efficiency. Over time, environmental factors like temperature swings can cause these cracks to grow, leading to dead cell areas and significant power loss in the module.
Q2: Does this mean PERC is a better technology than TOPCon?
Not at all. TOPCon offers superior efficiency and performance potential. This data simply highlights that it is a different technology that requires a more refined manufacturing process. The challenge isn’t the cell itself, but rather adapting our production methods to handle its unique mechanical properties.
Q3: Are all TOPCon cells equally fragile?
No. There can be variations in fragility based on the cell manufacturer, wafer thickness, and specific cell architecture. However, the general trend of increased sensitivity compared to PERC holds true across the board. That’s why testing and validating your specific cell supply is crucial.
Q4: Can you fix a cell after a microcrack has formed?
Unfortunately, no. Once a microcrack occurs, the damage is permanent. The only solution is prevention—optimizing your handling and stringing processes to avoid creating them in the first place.
The Path to High-Efficiency, High-Yield Production
Transitioning to new cell technologies like TOPCon is essential for driving the solar industry forward. But innovation doesn’t stop at the cell level; it must extend to the production line.
Ultimately, you cannot treat a high-performance TOPCon cell like a standard PERC cell and expect optimal results. Success requires a commitment to process engineering—a willingness to challenge old assumptions and use data to build new, more precise manufacturing protocols.
By understanding the unique vulnerabilities of next-generation cells and systematically adapting your processes, you can unlock their full efficiency potential while maintaining the high yield and reliability your customers demand.
Ready to explore how your materials and processes stand up to the rigors of real-world production? The journey begins with controlled, data-driven testing.