Imagine this: You’ve invested in a state-of-the-art solar project using the latest high-efficiency TOPCon modules. The initial performance is fantastic, exceeding expectations. But after the first year under the hot sun, the energy yield begins to dip mysteriously—not by a lot, but just enough to notice. You check for soiling, shading, and inverter issues, but everything seems fine.
What you might be witnessing is a silent degradation mechanism that affects even the most advanced solar cells: LeTID, or Light and elevated Temperature Induced Degradation. It’s a challenge that can undermine a solar asset’s long-term performance and bankability, but it’s one that can be neutralized before a single module leaves the factory. The solution is a precise, carefully validated manufacturing step: post-lamination thermal treatment.
What is LeTID and Why Does It Matter for TOPCon?
As the solar industry pushes the boundaries of efficiency, technologies like TOPCon (Tunnel Oxide Passivated Contact) have become leaders. They promise more power from the same footprint—a huge advantage for developers. However, this advanced cell architecture can be susceptible to LeTID.
The name itself tells the story:
- Light: The degradation is triggered by sunlight.
- Elevated Temperature: The effect is accelerated by heat, conditions common in real-world solar installations.
- Induced Degradation: The module’s power output slowly decreases over time.
Think of it as an invisible flaw that only reveals itself under working conditions. Within the first one to three years of operation, LeTID can cause a performance loss of 2% to 6%. For a multi-megawatt solar farm, that’s a significant and unexpected loss of revenue. This degradation stems from complex interactions involving hydrogen within the silicon cell structure, which can be unintentionally introduced during manufacturing.
The Challenge: Predicting and Preventing a Future Problem
For a module manufacturer or a project developer, the key challenge is guaranteeing 25-year performance. How can you be confident in your product’s long-term stability when a hidden degradation mechanism could be lurking?
The answer is to be proactive. Instead of letting degradation happen slowly in the field, we can trigger and then heal it in a controlled factory environment. This is done through a specific „annealing“ or thermal treatment process applied to the module after lamination. But getting the recipe right is a delicate science. Too little heat for too short a time, and the LeTID effect won’t be neutralized. Too much, and you risk damaging the module’s encapsulants or other materials.
This is where controlled, data-driven testing becomes essential.
The Solution in Practice: Validating Post-Lamination Thermal Treatments
A post-lamination thermal treatment is essentially a carefully controlled „baking“ process. After a module is laminated, it is placed in a climate chamber and subjected to a specific temperature profile for a set duration. This process stabilizes the atomic structure within the solar cells, effectively „immunizing“ the module against future LeTID effects.
The goal is to find the perfect balance—the optimal temperature and time that neutralizes the degradation mechanism without introducing any new problems. This validation demands an environment where every variable can be controlled and measured, a fundamental requirement to build and validate new solar module concepts using real industrial equipment.
At PVTestLab, our applied research focuses on simulating real-world aging and validating these treatments. By subjecting modules to accelerated stress tests—exposing them to light and heat in a climatic chamber—we can measure the impact of LeTID and confirm the effectiveness of different annealing cycles.
The PVTestLab Findings: Data-Driven Certainty
Our tests on high-efficiency TOPCon modules have delivered clear, compelling results.
- Untreated TOPCon Modules: When subjected to our accelerated LeTID stress test, these modules showed a measurable power loss of over 3%—a clear indicator of what would happen in the field over the first few years.
- Thermally Treated TOPCon Modules: Modules from the same production batch that underwent a validated annealing cycle showed virtually no degradation under the exact same stress test. Their performance remained stable.
The data confirms that a correctly implemented thermal treatment effectively mitigates LeTID. However, the process is not one-size-fits-all. The optimal treatment depends heavily on the specific cell technology, encapsulants, and other materials used in the module. This variability makes it critical to conduct structured experiments on all components—from encapsulants and glass to foils and cell interconnections—to ensure holistic module stability.
Why You Can’t ‚Set and Forget‘ Annealing Processes
A common misconception is that a single annealing „recipe“ can be applied to all TOPCon modules. In reality, every module’s Bill of Materials (BOM) is unique. A change in the cell supplier, EVA/POE encapsulant, or backsheet can alter how the module responds to thermal stress.
„Finding the right thermal treatment protocol is not a theoretical exercise; it’s an empirical process,“ notes Patrick Thoma, PV Process Specialist at PVTestLab. „You need to test, measure, and validate. Each new module design requires a dedicated effort to analyze and fine-tune process parameters for lamination, curing, and interconnection to guarantee long-term stability without compromising material integrity.“
That’s why having access to a full-scale R&D production line is so valuable. It allows manufacturers and material suppliers to test different annealing profiles under real industrial conditions, ensuring that the chosen process is both effective against LeTID and safe for all module components.
Frequently Asked Questions (FAQ)
What exactly is LeTID?
LeTID stands for Light and elevated Temperature Induced Degradation. It’s a performance loss that occurs in certain high-efficiency solar cells, such as PERC and TOPCon, after exposure to sunlight and heat during field operation. It typically manifests in the first 1-3 years.
Is LeTID the same as LID (Light Induced Degradation)?
No, they are different mechanisms. Classic LID primarily affects p-type silicon and happens quickly, often stabilizing within hours or days of sun exposure. LeTID is a slower, more complex process that can continue for months or even years before stabilizing.
Does the thermal treatment process damage the solar module?
If done correctly, no. The key is validation. The temperature and duration must be carefully selected to be high enough to neutralize LeTID but low enough to avoid damaging the encapsulants (like EVA or POE) and other polymer components in the module. This is why precise process control and testing are non-negotiable.
Why can’t manufacturers just do this testing in their own factory?
While some large manufacturers have in-house R&D, many need a flexible, cost-effective way to validate new materials or processes without interrupting their main production lines. An external R&D center like PVTestLab provides access to a complete, industrial-scale production line and process experts on-demand, de-risking innovation and accelerating time-to-market.
Which module types are most affected by LeTID?
Historically, multi-crystalline PERC cells were highly susceptible. However, the phenomenon has also been widely observed in mono-crystalline PERC and, as discussed, is a critical factor to manage in modern, high-efficiency TOPCon and HJT cells.
From Research to Reliability: Your Next Step
LeTID is no longer an unknown risk but a manageable engineering challenge. For module developers and material suppliers, the path to creating stable, reliable, and bankable TOPCon products is paved with data. By proactively validating post-lamination thermal treatments, you can eliminate the silent yield killer before it ever becomes a problem for your customers.
Understanding and controlling these subtle manufacturing steps is what separates good modules from great ones. It transforms a high-efficiency prototype into a product that delivers on its performance promise for decades to come.
Ready to explore how applied process validation can de-risk your next-generation module technology? The journey from the laboratory to real-world production starts with a single, validated test.