Your newly laminated solar module looks perfect. The glass is flawless, the cells are aligned, and it passes the initial flash test with flying colors. But hidden beneath that pristine surface, a ticking time bomb could be compromising its long-term performance and reliability.
This hidden threat isn’t a crack or a snail trail; it’s an army of microscopic bubbles, invisible to the naked eye and often missed by standard quality checks. For years, these defects were typically blamed on „trapped air“—a simple mechanical issue. But the real story is often more complex and rooted in the chemistry of the materials themselves.
This is the world of material outgassing, a subtle but critical phenomenon that high-resolution Electroluminescence (EL) imaging is finally bringing to light.
More Than Just Trapped Air: The Hidden World of Micro-Bubbles
When bubbles appear in a laminated module, the first suspect is usually the vacuum process in the laminator. It’s a logical assumption: if the vacuum isn’t strong enough, air gets trapped between the layers. While this certainly happens, it overlooks a more insidious culprit: gases released from the module’s own components during the heating process. This phenomenon, known as outgassing, is a chemical issue, not a mechanical one.
Think of it like baking bread. If you don’t let the dough rise properly and release its gases before it forms a crust in the oven, you end up with unwanted air pockets. Similarly, if the gases from your module materials don’t have time to escape before the encapsulant cures, they become permanently trapped as micro-bubbles.
What is Material Outgassing in Solar Module Lamination?
During the lamination cycle, materials undergo intense heat and pressure. This process is designed to cure the encapsulant (like EVA or POE), bonding everything into a durable, weatherproof package. However, this same heat can trigger chemical reactions in various materials, causing them to release gases.
Common sources of outgassing include:
- Encapsulants: Additives within some solar encapsulants can vaporize during heating.
- Backsheets: Certain polymers in backsheets can release moisture or volatile organic compounds (VOCs).
- Flux Residues: Leftover flux from the cell stringing process can vaporize, creating gas pockets around the solder ribbons.
The critical factor is timing. The lamination recipe must be perfectly tuned to allow these gases to escape before the encapsulant cross-links (hardens). If the encapsulant cures too quickly, it traps the gas, leading to the formation of micro-bubbles that can plague the module for its entire lifespan.
Seeing the Invisible: Why High-Resolution EL Is a Game-Changer
Standard visual inspection can’t see these bubbles. Even low-resolution EL imaging might miss them or misinterpret their signals, which is why high-resolution EL is such an essential diagnostic tool.
Electroluminescence works by applying a current to the solar module, causing the cells to light up—similar to how an LED works. A specialized camera then captures this infrared light. Any inactive or blocked area, such as a crack or bubble, cannot transport current and therefore appears as a dark spot.
Micro-bubbles block the flow of electrons to the cell emitter, creating distinct, circular dark spots on the EL image. A high-resolution image delivers the clarity needed not only to detect these bubbles but also to analyze their pattern, size, and location—clues that are vital for pinpointing the root cause.
From Diagnosis to Solution: Reading the Story in the EL Image
A high-resolution EL image is more than just a picture of defects; it’s a detailed map that tells a story about your materials and process.
As our PV Process Specialist, Patrick Thoma, often says, „High-resolution EL doesn’t just show you a problem; it tells you a story about your process. The location and pattern of the bubbles are fingerprints that point directly to the source, whether it’s the flux on your ribbons or the chemistry of your backsheet.“
By analyzing the image, an experienced engineer can identify the likely culprit:
- Bubbles concentrated along solder ribbons? This strongly suggests outgassing from flux residue.
- A uniform, scattered pattern across the entire cell? The source is more likely the encapsulant or the backsheet.
- Bubbles appearing only at higher temperatures? This indicates a material that releases gas late in the lamination cycle.
This level of insight is crucial when building new solar module prototypes or qualifying new materials. It transforms guesswork into a data-driven process, allowing for precise adjustments to the lamination recipe—like adding a dwell time for degassing or adjusting the temperature ramp rate. This is the core of effective lamination recipe validation.
Why You Can’t Afford to Ignore Micro-Bubbles
A few microscopic bubbles might seem harmless, but they are a silent yield killer. These tiny defects create thermal mismatches in the cell. The area under the bubble cannot dissipate heat effectively, leading to the formation of hotspots.
Over years of thermal cycling in the field, these hotspots can:
- Accelerate the degradation of the encapsulant and cell.
- Increase the risk of backsheet cracking.
- Lead to a gradual but irreversible loss of power output.
- Potentially compromise the long-term safety and bankability of the module.
In short, catching micro-bubbles caused by outgassing isn’t just about passing initial quality control; it’s about ensuring the 25-year performance and reliability of your product.
Frequently Asked Questions (FAQ)
What’s the difference between a micro-crack and a micro-bubble on an EL image?
A micro-crack typically appears as a sharp, dark line, often following the crystal grain structure of the cell. A micro-bubble, on the other hand, usually shows up as a round or oval-shaped dark spot because it blocks current in a circular area.
Can standard quality control find these bubbles?
Not reliably. Visual inspection will miss them entirely. Standard-resolution EL might not have the detail to distinguish them from other noise, and flash tests only measure initial performance, not the long-term reliability risks that bubbles introduce.
Does every material outgas?
Many materials used in solar modules have the potential to outgas to some degree. The key is understanding when and how much they outgas. This knowledge makes it possible to develop a lamination process that safely vents these gases before the module is sealed. Proper material compatibility testing is essential for this.
How do I fix an outgassing problem?
The solution starts with an accurate diagnosis using high-resolution EL. Once the source material is identified, the fix typically involves modifying the lamination recipe. This could mean increasing the vacuum time, slowing the temperature ramp-up, or introducing a „dwell“ phase at a specific temperature to allow gases to escape before the encapsulant cures.
Your Next Step: From Awareness to Action
Understanding the difference between trapped air and material outgassing is the first step toward building more robust and reliable solar modules. The next is to use the right tools to see what’s really happening inside your laminate.
High-resolution EL imaging provides the data-driven insights you need to optimize your materials and processes. By turning invisible threats into actionable data, you can move beyond simply identifying defects to proactively preventing them, ensuring your products deliver on their promise of long-term performance and value.