Imagine a brand-new solar farm, its panels gleaming under the sun, performing exactly as predicted. For the first year, everything is perfect. But then, slowly and silently, the energy output begins to drop—not by a lot, but by a few percent more than expected. This mysterious decline continues, eating into revenue and raising difficult questions about long-term performance.
What’s happening? The culprit is likely a phenomenon known as Light and elevated Temperature Induced Degradation, or LeTID. It’s a silent yield killer affecting one of the most common solar technologies on the market today. For years, the industry has struggled to reliably predict its impact.
The good news is that it’s a problem that can be understood, measured, and, most importantly, solved before a single module leaves the factory.
What Exactly is LeTID (and Why Should You Care)?
LeTID is a degradation mechanism that primarily affects PERC (Passivated Emitter and Rear Cell) solar modules. Unlike initial Light Induced Degradation (LID), which typically stabilizes within the first few days of operation, LeTID is a slow-burn problem. It can take months or even years of field operation—requiring both sunlight and heat—to fully manifest, causing power losses that can exceed 5% or more.
For asset owners, this means a direct hit to the project’s financial model and return on investment. Module manufacturers, meanwhile, face a massive risk of warranty claims and damage to their brand reputation. Because LeTID is so slow to appear, standard quality control tests like flash tests or simple light soaking often miss it completely.
This has created a critical industry challenge: how do you test for a problem that takes years to show up? The answer lies in accelerating the process under controlled conditions.
The Challenge: Why Standard Tests Fall Short
The core difficulty in tackling LeTID lies in the lack of a universally accepted, standardized testing protocol. Without one, manufacturers are left guessing. A test that is too weak might fail to trigger the degradation, giving a false sense of security. Conversely, a test that is too harsh could cause unrelated damage, leading to unnecessary and expensive changes in materials or design.
This uncertainty is precisely why a reliable, repeatable, and accelerated testing methodology is no longer a „nice-to-have“—it’s an essential tool for ensuring product bankability and long-term performance.
Building a Reliable, Accelerated LeTID Test: The PVTestLab Protocol
After extensive research into the kinetics of LeTID, we developed a protocol that doesn’t just find the problem but helps pinpoint the solution. It’s based on a simple principle: by exposing modules to the right combination of light and heat in a controlled environment, we can replicate years of field degradation in just a few hundred hours.
Here’s how it works.
Step 1: The Setup – A Precision Climate Chamber
It all starts in a climate chamber capable of maintaining precise conditions. We subject the test modules to a constant current, simulating the energy flow from sunlight, while holding the cell temperature at a consistently elevated level. This specific combination of electrical load and heat is the catalyst that kicks the LeTID degradation mechanism into high gear.
Step 2: The Test Cycle – Triggering and Tracking Degradation
Before the test begins, each module undergoes a baseline performance measurement using a AAA Class flasher. The module then enters the climate chamber for the accelerated stress test. We periodically remove the module to measure its performance and track the power loss over time.
For a typical, non-optimized PERC module, the results are often dramatic. The test data reveals that the module loses a significant percentage of its initial power. This curve reveals the maximum potential degradation, giving us a clear picture of the module’s vulnerability to LeTID in the field.
Step 3: The „Aha Moment“ – Finding the Stable State
One of LeTID’s key characteristics is that after the initial degradation, a module can sometimes enter a „regeneration“ phase where it recovers some of the lost power. Our test continues until the module’s performance fully stabilizes, ensuring we capture the true, long-term steady-state power output. This is the number that matters for predicting real-world energy yield.
Beyond Detection: How Lamination Can Mitigate LeTID
Identifying a problem is only half the battle. The real breakthrough comes from using this test data to solve it. Our research has revealed a powerful correlation between the module lamination process and LeTID susceptibility. By carefully adjusting parameters during this critical production step, we can effectively „immunize“ the module against severe degradation.
Through our prototyping & module development services, we began experimenting with different lamination recipes—adjusting temperatures, curing times, and pressure profiles. The goal was to find a combination that would create a more stable final product without requiring costly changes to the bill of materials.
The results were remarkable. By implementing an optimized lamination process, we could drastically reduce the impact of LeTID on the exact same type of PERC cells.
A comparison between a standard module and one produced using our optimized process shows a clear difference: the optimized module exhibits minimal degradation, proving that the solution lies in process control.
What a „LeTID-Stable“ Module Looks Like
After applying the optimized lamination recipe, we put the new module through the exact same accelerated test. The final result is a module that can be trusted to perform reliably for decades.
This level of stability is achievable. It’s the direct result of combining a deep understanding of the degradation mechanism with rigorous material testing & lamination trials. By validating the entire process, manufacturers can move forward with confidence, knowing their product is built to last.
FAQ: Your LeTID Questions, Answered
What causes LeTID at a cellular level?
While the exact physics are complex, LeTID is widely believed to stem from the interaction of hydrogen with boron-oxygen defects within the silicon of the PERC cell. Heat and light provide the energy needed to activate this process, which reduces the cell’s efficiency.
Is LeTID the same as standard LID?
No. Standard Light Induced Degradation (LID) typically occurs within the first hours or days of sun exposure and stabilizes quickly. LeTID is a much slower process that requires both light and elevated temperatures (typically above 50°C) to manifest and can continue for months or years.
Can any PERC module be affected by LeTID?
Yes, virtually all p-type PERC cells have some susceptibility to LeTID. However, the severity of the degradation depends heavily on the specific cell manufacturing process and, as we’ve shown, the module assembly and lamination process.
How long does an accelerated LeTID test take?
A comprehensive test to trigger, measure, and confirm the stabilization of a module typically takes several hundred hours in the climate chamber. While this is a significant time investment, it’s far faster and less expensive than discovering the problem after years of field operation.
The Path to Predictable Performance
LeTID is no longer an unpredictable ghost in the machine. It’s a measurable and manageable phenomenon. For module developers and manufacturers, the path to building truly reliable, long-lasting PERC modules is not about finding a magic new material, but about mastering the production process.
By using a data-driven, accelerated testing protocol, it’s possible to design for stability from day one. Understanding how small adjustments can lead to massive gains in long-term performance is the key to delivering on the promise of clean, dependable solar energy for decades to come.
If you’re exploring ways to validate your module design or looking for data-driven insights, our engineers are available to discuss your challenges in a process optimization & training consultation.