Imagine this: your new solar power plant is performing beautifully, exceeding expectations for the first few months. Then, a slow, mysterious decline in power begins. It’s not soiling, not shading, and it’s happening across hundreds of modules. You might be witnessing one of the most subtle and costly degradation mechanisms in modern solar technology: LeTID.
This isn’t a rare defect; it’s a known phenomenon affecting some of the most common solar cells on the market. But because it appears long after standard quality checks are complete, it often goes undetected until it’s already impacting your bottom line. The key to preventing this isn’t just about choosing the right components—it’s about understanding and verifying their long-term stability before they ever reach the field.
What is This „Hidden Threat“? Unpacking LeTID
LeTID stands for Light and elevated Temperature Induced Degradation. It’s a performance-sapping effect that can cause power losses of up to 10% in certain types of high-efficiency solar modules, primarily those using PERC (Passivated Emitter and Rear Cell) technology.
What makes LeTID so challenging is its timing. Unlike Light Induced Degradation (LID), which typically stabilizes within the first few days of operation, LeTID can take months or even years to fully manifest under real-world conditions. The degradation process is slow, insidious, and can be easily mistaken for other performance issues.
This delayed onset means modules can pass all standard factory tests with flying colors, only to begin underperforming once installed on-site. For asset owners and investors, this creates significant uncertainty in long-term energy yield predictions and financial models.
The Science Behind the Slump: Why Does LeTID Happen?
At its core, LeTID is a complex interaction of light, heat, and materials within the solar cell. Research points to a primary culprit: hydrogen.
During the manufacturing of PERC cells, hydrogen is intentionally introduced to „passivate,“ or repair, microscopic defects in the silicon, which boosts initial efficiency. However, under the combined stress of sunlight and elevated operating temperatures—conditions common in sunny climates—this hydrogen can become mobile.
Instead of helping, these mobile hydrogen atoms can interact with metal impurities or other defects in the silicon, creating complexes that effectively trap charge carriers. This reduces the cell’s ability to convert sunlight into electricity, leading to a gradual drop in power output.
The tricky part? This degradation isn’t always permanent. Over a much longer period, the module can enter a „regeneration“ phase and slowly recover some of its lost power. But this recovery is often slow and unpredictable, making it an unreliable factor for project planning. The only true solution is to prevent it from happening in the first place.
Why Standard Tests Fall Short
So, why isn’t this caught by rigorous IEC certification standards? The reason is simple: standard tests were not designed to detect such a slow-moving degradation mechanism. Tests like damp-heat or thermal cycling focus on structural integrity and specific failure modes, but they often lack the duration and specific conditions—a combination of light and high temperature—needed to trigger and map the full LeTID cycle.
This leaves a critical gap in quality assurance, a gap that can only be closed with a more specialized, data-driven testing approach.
A Proactive Approach: Mapping the Full Degradation and Recovery Curve
Instead of waiting for problems to appear in the field, we can proactively simulate years of operation in a controlled laboratory environment. This is where a methodical, data-driven approach becomes essential for de-risking technology choices.
The process involves subjecting modules to a controlled „light soaking“ test at elevated temperatures. Here’s how it works:
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Baseline Measurement: The module’s initial performance (its I-V curve) is precisely measured under standard test conditions.
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Controlled Stress: The module is then placed in a climate-controlled chamber. Inside, it’s exposed to continuous illumination (around 1000 W/m², simulating peak sunlight) and a consistently high temperature (e.g., 75°C) for hundreds of hours.
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Periodic Mapping: At regular intervals, the module is cooled and its performance is re-measured. This creates a series of data points that map the degradation curve over time.
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Observing Regeneration: The test continues well past the point of maximum degradation. By continuing to monitor the module, we can also map the subsequent regeneration phase, providing a complete picture of its long-term behavior.
This detailed mapping provides invaluable insights. It doesn’t just tell you if a module degrades; it tells you how much, how fast, and whether it can recover. This level of analysis is fundamental for successful solar module prototyping, ensuring new designs are robust against long-term field conditions.
From Lab Data to Real-World Solutions
The goal of this intensive testing isn’t just to find problems—it’s to validate solutions. Cell manufacturers have made significant strides in developing LeTID-resilient PERC cells by modifying their production processes, such as optimizing firing temperatures or adjusting the composition of the passivation layers.
However, a cell’s inherent stability can be influenced by how it’s integrated into the final module. The lamination process, for example, subjects the cells to specific thermal and mechanical stresses that could potentially influence their susceptibility to LeTID.
This is why the only way to be certain is to test the final, complete module. This allows developers to:
- Verify Supplier Claims: Confirm that the cells used are truly „LeTID-free“ at the module level.
- Compare Components: Test modules built with cells from different suppliers side-by-side.
- Optimize Processes: Ensure that in-house manufacturing processes are not inadvertently contributing to the problem.
As our lead PV process specialist, Patrick Thoma, often emphasizes, „You cannot optimize what you cannot measure. LeTID testing replaces assumptions with hard data, allowing developers to de-risk their material choices and production processes before scaling.“
FAQ: Your LeTID Questions Answered
Is LeTID the same as LID (Light Induced Degradation)?
No. While both cause power loss, LID occurs within the first few hours or days of light exposure and stabilizes quickly. LeTID is a much slower process that emerges after hundreds or thousands of hours of operation at elevated temperatures.
Does LeTID affect all solar panels?
It primarily affects multicrystalline and monocrystalline PERC solar cells, which have become the industry standard for their high efficiency. As cell technologies evolve, ongoing testing is crucial to identify and mitigate similar long-term degradation effects.
Can LeTID be fixed once it happens in the field?
The regeneration phase is a natural recovery process, but it’s slow, unpredictable, and may not fully restore the initial power output. The most effective strategy is prevention through careful material selection and rigorous pre-production testing.
How long does a proper LeTID test take?
A comprehensive test to map both degradation and the onset of regeneration can require 500 to 1,000 hours of continuous light soaking under controlled conditions, making it a specialized test that goes beyond standard certification protocols.
Why is temperature so important in LeTID testing?
Elevated temperature acts as an accelerator. It speeds up the chemical reactions involving hydrogen that cause degradation, allowing us to simulate years of field operation in a compressed timeframe.
Building Confidence in Long-Term Performance
LeTID represents a significant challenge to the solar industry’s promise of 25+ years of reliable power generation. It is, however, a manageable one. By moving beyond standard tests and embracing a more proactive, data-driven validation process, manufacturers and developers can build modules with confidence.
Understanding the nuances of how materials behave under real-world stress is key to creating more durable, reliable, and profitable solar projects for decades to come.