You’ve done everything by the book. You selected high-efficiency bifacial PERC cells, chose a promising new encapsulant, and your initial flash tests show fantastic power output. But six months after deployment, the performance reports come in, and the numbers are lower than projected. The culprit? A silent performance killer known as LeTID.
You might blame the cells or the materials, but what if the root cause lies hidden in a place few think to look: the minutes your module spent inside the laminator?
The truth is, the lamination process isn’t just about gluing components together. It’s a delicate chemical reaction that can fortify your modules against degradation—or unknowingly leave them vulnerable. Understanding this link is the first step toward building solar modules that perform well not just on day one, but for their entire 25-year lifespan.
Understanding the Key Players: LeTID, PERC, and Encapsulants
Before we dive into the lamination process, let’s meet the key players.
LeTID: The Silent Performance Killer
Light and elevated Temperature Induced Degradation (LeTID) is a degradation mechanism affecting certain types of solar cells, most notably Passivated Emitter and Rear Contact (PERC) cells. It occurs when modules are exposed to sunlight and high temperatures (above 50°C), causing a gradual drop in power output that can reach up to 10% in some cases. The frustrating part is that this degradation happens over the first few years of operation, long after the module has left the factory.
Bifacial PERC Cells: The Double-Sided Powerhouse
Bifacial PERC cells are the current workhorse of the solar industry. They can capture light from both the front and back sides, boosting energy yield. However, their advanced design involves high concentrations of hydrogen, introduced during manufacturing to „passivate“ or heal defects in the silicon. While this hydrogen is essential for high efficiency, it’s also a key ingredient in the LeTID equation.
Encapsulants: The Protective Shield (POE vs. EPE)
The encapsulant is the polymer material surrounding the solar cells, protecting them from moisture, mechanical stress, and electrical shock. The two common choices today are:
- POE (Polyolefin Elastomer): Known for its excellent durability and very low water vapor transmission rate (WVTR), making it a premium choice for bifacial modules.
- EPE (Ethylene-vinyl Acetate / Polyolefin / Ethylene-vinyl Acetate): A multi-layer encapsulant that combines the cost-effectiveness of EVA with some of the protective benefits of POE.
Many manufacturers are exploring EPE as a way to balance cost and performance. However, as we’ll see, the choice of encapsulant is only half the story.
The Lamination Process: More Than Just Glue and Heat
At its core, lamination uses heat and pressure to bond the glass, encapsulant, cells, and backsheet into a single, durable unit. The specific combination of temperature and time used to achieve this is called the curing profile.
A typical datasheet might recommend a curing temperature of, say, 155°C for 12 minutes. But what actually happens during those 12 minutes? The encapsulant polymer chains cross-link, transforming from a soft film into a tough, transparent gel.
Here’s the „aha moment“: this curing process does more than just solidify the encapsulant. The heat and duration of the lamination cycle can influence the behavior of hydrogen within the PERC cells. It can either help the hydrogen stay in place to passivate defects or cause it to move in ways that later contribute to LeTID. This transforms the laminator from a simple assembly machine into one of your most powerful tools for influencing long-term module reliability.
Unpacking the Research: How Curing Profiles Affect EPE and POE
To understand this effect, our engineering team conducted a study to see how different lamination curing profiles impacted LeTID in modules made with identical bifacial PERC cells but different encapsulants (EPE vs. POE).
We tested two distinct curing profiles:
- Low & Slow: 155°C for 550 seconds.
- High & Fast: 165°C for 350 seconds.
Both profiles are within the manufacturers‘ recommended processing windows. After lamination, the modules were subjected to an accelerated LeTID test (75°C at 10 Amps) for nearly 500 hours to measure their power degradation.
The results were eye-opening.
Key Finding 1: EPE is Highly Sensitive to the Curing Profile
The modules built with the EPE encapsulant showed dramatically different LeTID behavior depending on the lamination recipe.
- The „High & Fast“ (165°C) profile resulted in nearly 4.5% power degradation.
- The „Low & Slow“ (155°C) profile resulted in only 1.2% power degradation.
This is the same cell type and the same encapsulant material, yet a simple 10°C change in the lamination recipe led to a 3.3% difference in long-term performance. The slower, lower-temperature cure appears to create a more stable hydrogen passivation environment within the cell, making it far more resistant to LeTID.
Key Finding 2: POE Shows Greater Process Robustness
In contrast, the modules built with the POE encapsulant were remarkably stable regardless of the curing profile. Both the „High & Fast“ and „Low & Slow“ profiles resulted in minimal degradation of around 0.5%. This demonstrates POE’s inherent material advantage in creating a stable environment for PERC cells.
Data clearly illustrates the impact of the lamination recipe, showing a significant performance gap between the two EPE modules, which were identical except for their curing profile.
„It’s not enough to choose the right material; you have to apply it with the right process,“ notes Patrick Thoma, PV Process Specialist at PVTestLab. „Our data shows that the same EPE encapsulant can perform like a premium material or a liability, and the only difference is the lamination recipe. This level of process control is where reliability is truly born.“
This research highlights a critical risk for manufacturers. If you’re using EPE without validating your specific curing profile, you could be unknowingly locking in significant future degradation. It’s a perfect illustration of why solar module prototyping under real industrial conditions is so crucial.
What This Means for Module Manufacturers
These findings have direct, actionable implications for anyone involved in solar module design and production.
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Datasheets Are a Starting Point, Not a Guarantee. The recommended curing window on a material datasheet is broad. The optimal point within that window depends on your specific module design, cell technology, and even the type of laminator you use. You must find your own „sweet spot.“
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Process Validation is Non-Negotiable. Simply switching to a new, cost-effective EPE encapsulant without rigorous testing is a massive risk. Conducting structured material lamination trials allows you to compare different curing profiles and measure their impact on LeTID, ensuring you select a recipe that maximizes reliability.
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Think in Systems, Not Components. A module’s final performance isn’t just the sum of its parts; it’s the result of how those parts interact during manufacturing. The combination of PERC Cell + EPE Encapsulant + Curing Profile creates a unique system that must be evaluated as a whole.
Frequently Asked Questions (FAQ)
What exactly is LeTID again?
LeTID is a form of power loss in certain solar cells, especially PERC, that occurs when modules are exposed to both sunlight (light) and high temperatures (elevated temperature). It typically appears in the first 1-3 years of field operation.
So, is POE always a better choice than EPE?
From a pure LeTID-resistance standpoint, our study shows POE is more robust and less sensitive to the manufacturing process. However, EPE can deliver excellent, reliable performance if the lamination process is perfectly optimized. The choice often comes down to a balance of cost, manufacturability, and the willingness to invest in process validation.
How do I know if my lamination process is optimized?
The only way to know for sure is through controlled testing. This involves building small batches of modules (prototypes) using different curing profiles and then subjecting them to accelerated stress tests (like thermal cycling, damp heat, and LeTID testing) to measure and compare their long-term performance.
Can’t I just use the settings recommended by my laminator or encapsulant supplier?
Those recommendations are excellent starting points. However, they are generic by nature. They don’t account for the specific type of PERC cells you’re using or the unique thermal properties of your module design. Fine-tuning the process for your specific bill of materials is essential for unlocking maximum reliability.
What is „hydrogen passivation“?
Think of it like patching tiny invisible holes in the solar cell’s silicon structure. Hydrogen atoms are used during cell manufacturing to „passivate“ or neutralize electronic defects that would otherwise trap energy and reduce efficiency. The stability of this passivation is key to preventing degradation like LeTID.
Your Path from Research to Reliability
The connection between lamination curing profiles and LeTID is no longer a mystery. It’s a clear, measurable, and—most importantly—controllable factor in the long-term reliability of your solar modules.
Ignoring this crucial process step means leaving performance and bankability to chance. By embracing a data-driven approach, you can turn your lamination process from a potential source of risk into a powerful competitive advantage.
The journey to optimizing performance begins with understanding your process variables. If you’re ready to move from theory to practice, exploring a controlled environment for process optimization and validation is the logical next step toward building modules that stand the test of time.