Why Your Field Data Doesn’t Match Your Factory Specs: A Deep Dive into Module-Level LeTID & LID Validation

Imagine this: your new line of high-efficiency PERC solar modules passes every quality check. The flash test data is perfect. Yet, six months after installation, you get a call from the field. The plant’s output is inexplicably 4% lower than projected.

You check the weather data, the inverters, and the installation quality. Everything seems fine. But the culprit may be hiding within the solar cells themselves: a sneaky form of degradation that was accidentally “reset” during your manufacturing process.

This scenario is becoming increasingly common as manufacturers adopt advanced cell technologies like PERC. The good news is that it’s entirely preventable. The solution isn’t just about how the cells are treated, but how you validate that treatment after the module is fully assembled.

The Hidden Threats: A Quick Introduction to LID and LeTID

To understand the problem, we need to get familiar with two key terms: LID and LeTID. Think of them as the invisible growing pains of modern solar cells.

Light Induced Degradation (LID) is the classic, well-understood power loss that occurs within the first few hours or days of a module’s life. In traditional P-type cells, it’s primarily caused by the interaction of boron and oxygen under sunlight, which creates defects that reduce efficiency.

Light and elevated Temperature Induced Degradation (LeTID) is a more severe and complex phenomenon, particularly prominent in PERC (Passivated Emitter and Rear Cell) technology. Unlike LID, LeTID can take months or even years to fully manifest in the field, causing a slow but significant power drop of up to 10% in some cases. It’s triggered by a combination of light and higher operating temperatures (above 50°C), making it a major concern for long-term energy yield.

Cell manufacturers are well aware of these issues. To combat them, they often perform a „regeneration“ process—a controlled treatment of heat and light at the cell level designed to stabilize the material and mitigate future degradation.

So, if the cells are already treated, why is there still a problem?

The Lamination Effect: Where a Perfect Cell Meets an Imperfect Reality

The critical issue arises during module lamination. For many manufacturers, this is the „aha moment“: the very process designed to protect the cells for 25+ years can inadvertently undo the protective measures taken by your cell supplier.

During lamination, the module sandwich (glass, encapsulant, cells, encapsulant, backsheet) is heated to temperatures around 150-170°C to cure the encapsulant (like EVA or POE) and create a durable, weatherproof package.

However, this thermal budget can be high enough to break the stable chemical bonds formed during the cell-level regeneration process. The lamination cycle can effectively „reset“ the cell’s susceptibility to LeTID. The cell that entered the laminator was stable, but the cell inside the finished module is now vulnerable once again.

This leaves you with a critical blind spot: your incoming quality control on the cells was perfect, but the outgoing performance of your module is compromised. You can’t just trust the cell supplier’s datasheet; you must verify performance after your own production process.

A Blueprint for Certainty: How to Validate Your Regeneration Process

The only way to guarantee that your LeTID mitigation strategy is working is to test it at the finished module level. This involves a comparative process that simulates real-world conditions to reveal any hidden degradation potential.

An effective validation protocol looks like this:

Step 1: Establish a Control Group.

First, produce a set of test modules using the potentially vulnerable cells without implementing any specific in-line regeneration step. These modules represent your baseline—the „worst-case scenario“ for degradation.

Step 2: Produce Your Test Group.

Next, produce an identical set of modules, but this time, integrate your planned in-line regeneration process. This could be a specific thermal treatment or light-soaking step within your production line.

Step 3: Conduct Accelerated Stress Testing.

Both sets of modules are then subjected to a controlled stress test that simulates the conditions known to trigger LeTID. This typically involves placing the modules in a climate chamber and exposing them to elevated temperatures (e.g., 75°C) and electrical current for several hundred hours. The modules‘ power output (Pmax) is measured precisely before, during, and after the test using a class AAA solar simulator.

Step 4: Analyze and Compare the Data.

After the stress test, you compare the degradation levels between the control and test groups.

A successful outcome is clear: the control group shows significant degradation (e.g., 5% power loss) while the test group remains stable (e.g., <1% power loss). This confirms your in-line regeneration process is effective and has successfully stabilized the cells through the lamination process and beyond.

Conversely, if both groups show significant degradation, it’s a clear sign that your regeneration process isn’t working as intended and needs to be optimized.

This data-driven approach moves you from assumption to certainty. It provides objective proof that your modules will perform as expected in the field, protecting your brand’s reputation and ensuring your customers‘ projects deliver the promised return on investment.

It’s Not a One-Time Fix

This validation isn’t a „set it and forget it“ activity. Any change to your Bill of Materials (BOM) or process parameters requires re-validation. A new brand of encapsulant, a different backsheet, or even a faster lamination cycle can alter the thermal and chemical environment within the module.

These changes can impact the effectiveness of your regeneration step. That’s why leading manufacturers build this validation into their standard R&D and quality assurance workflows, especially when qualifying new materials. Access to a full-scale R&D line for prototyping and module development becomes essential for running these comparative trials without disrupting mass production. At the same time, a deep understanding gained from material testing and lamination trials is crucial for predicting how new components will interact and affect long-term stability.

Frequently Asked Questions (FAQ)

What is the main difference between LID and LeTID?

LID (Light Induced Degradation) occurs quickly, usually within the first few hours or days of light exposure, and is primarily linked to Boron-Oxygen complexes. LeTID (Light and elevated Temperature Induced Degradation) is a much slower process that can take months or years, triggered by both light and heat, and is often linked to hydrogen’s role within the cell structure—especially in PERC cells.

Why can’t I just trust my cell supplier’s regeneration data?

Your cell supplier’s data is valid for the state of the cell before it enters your production line. The high temperatures used in the module lamination process can reverse the effects of the cell-level regeneration, making the cell susceptible to degradation again. You must validate the final, laminated product.

What equipment is needed to test for LeTID at the module level?

A robust LeTID test setup requires a climate chamber capable of maintaining precise temperature and humidity control, a power supply to inject current into the module (simulating sunlight), and a high-precision solar simulator (flasher) to accurately measure the module’s power output before and after the stress test.

From Uncertainty to Unshakeable Reliability

The gap between factory specifications and real-world field performance represents one of the biggest risks in the solar industry today. Relying solely on cell-level data for phenomena like LeTID is like navigating without a map—you might be heading in the right direction, but you can’t be certain.

By implementing a rigorous, module-level validation protocol, you replace assumption with data. You prove that your manufacturing process not only assembles high-quality components but also preserves their performance and stability for the long term. This commitment to process validation is the ultimate foundation for building modules that are not just powerful on day one, but reliably productive for decades to come.