Imagine a brand-new utility-scale solar farm, commissioned and performing perfectly to spec. But a few months later, after the first hot, sunny season, its energy output mysteriously dips. The drop isn’t catastrophic, but it’s real, measurable, and was missed by standard certification tests. The culprit is a hidden phenomenon known as LeTID: a silent drain on performance.
Light and elevated Temperature Induced Degradation, or LeTID, is one of the most significant long-term reliability challenges facing modern solar technology. It’s a subtle but persistent form of power loss that affects the workhorses of the industry: Passivated Emitter and Rear Cell (PERC) modules.
Understanding LeTID isn’t just an academic exercise; it’s essential for ensuring the bankability and long-term value of solar assets. Let’s explore what it is, why it’s so tricky to detect, and how an advanced testing protocol can bring this behavior out of the shadows.
What Is LeTID, and Why Does It Affect PERC Cells?
At its core, LeTID is a degradation process that occurs when PERC solar cells are exposed to sunlight at temperatures above 50°C. Think of it as a hidden weakness within the cell material that activates only under real-world operating conditions.
The mechanism is linked to a complex interaction involving hydrogen and trace metallic impurities within the silicon. When energized by heat and light, this defect complex becomes active, reducing the cell’s ability to convert sunlight into electricity efficiently. This can lead to power losses of up to 10% in the initial years of operation.
Since both multi-crystalline and mono-crystalline PERC cells are susceptible, this issue impacts a vast majority of the modules being installed today. The challenge is that this degradation isn’t a simple, one-way street.
The Tricky Nature of a Power-Draining Ghost
What makes LeTID so difficult to manage is its unique degradation and regeneration cycle. Unlike other forms of degradation that cause a steady decline, LeTID causes an initial power drop, which can then be followed by a period of partial recovery, or „regeneration,“ as the module continues to operate.
This behavior creates a moving target for performance analysis. A module might lose 6% of its power and then recover 3%, leaving a net loss that defies simple prediction.
This curve shows why standard industry certifications, like those under IEC 61215, often fail to capture the full picture. Their testing cycles may not be long enough or configured with the right conditions to trigger the full degradation and subsequent regeneration phases. A manufacturer could unknowingly use a cell and material combination with high LeTID susceptibility, pass standard tests, only to discover the problem after modules have been deployed in the field for months.
Beyond Standard Tests: A Modern Protocol for Clarity
To accurately measure and mitigate LeTID, we need to go beyond standard protocols and replicate its full lifecycle in a controlled environment. This requires an accelerated test that maps the entire degradation and regeneration curve, providing clear, actionable data.
An advanced LeTID test protocol involves placing complete solar modules inside a climate chamber where illumination and temperature can be precisely controlled.
Here’s what that looks like in practice:
- Baseline Measurement: The module’s initial performance (Pmax, Voc, Isc) is measured under Standard Test Conditions (STC) using a Class AAA flasher. This establishes a precise „before“ picture.
- Accelerated Stress Phase: The module is placed in the climate chamber and exposed to a constant current, which simulates illumination (e.g., 1000 W/m²), at a consistently elevated temperature (e.g., 75°C).
- Cyclical Monitoring: The test runs for an extended period, often over 160 hours. At specific intervals, the module is cooled and its performance is remeasured. This repeated process allows engineers to plot the power loss and subsequent recovery over time, fully mapping the LeTID curve.
By controlling the environment, we can compress months or years of field exposure into a matter of days. This level of precise analysis is fundamental to any comprehensive Quality & Reliability Testing program designed to ensure long-term field stability.
Why Your Choice of Materials Is the First Line of Defense
The data from this advanced testing reveals a critical insight: LeTID is highly dependent on the specific materials used in a module. The choice of encapsulant (like EVA or POE), the properties of the silicon wafer, and the cell manufacturing process all play a significant role in how severely a module is affected.
Some materials may contribute to higher degradation, while others can help suppress it, making testing a powerful tool for innovation. By running comparative tests, manufacturers can:
- Validate Cell Batches: Ensure that cells from a new supplier meet long-term reliability standards.
- Compare Encapsulants: Determine which encapsulating polymer offers the best protection against LeTID for a specific cell technology.
- Optimize Module Design: Inform the entire process of Prototyping & Module Development by selecting a „bill of materials“ that is inherently resilient.
For example, running identical LeTID tests on two modules—each with a different encapsulant—can provide objective proof of which material offers better long-term performance. The goal is to make data-driven decisions long before a module reaches mass production. Detailed analysis, such as high-resolution Electroluminescence imaging, can even reveal how LeTID affects individual cells.
This proactive approach, centered on rigorous Material Testing & Lamination Trials, transforms reliability from a hope into a measurable, engineered outcome.
Frequently Asked Questions About LeTID
What actually causes LeTID?
LeTID is caused by a defect complex in PERC silicon wafers, primarily involving hydrogen atoms. When activated by light and heat, these defects „trap“ electrons, reducing the cell’s efficiency and power output.
Is LeTID permanent?
Not entirely. LeTID is characterized by a degradation phase followed by a partial recovery or „regeneration“ phase. However, the recovery is often incomplete, and in some cases, the degradation can reappear. Accurate testing is essential for understanding a module’s final, stable power output.
Which types of solar panels are most affected?
LeTID is most prominent in both multi-crystalline and mono-crystalline PERC (Passivated Emitter and Rear Cell) solar modules. As PERC technology dominates the current market, this is a widespread industry challenge.
Can you see LeTID with the naked eye?
No. LeTID is an invisible, performance-based degradation. It can only be detected through precise power measurements (flash testing) and specialized imaging techniques like Electroluminescence (EL), which can show variations in cell activity.
Building for the Future with Deeper Insights
LeTID is no longer an unknown ghost in the machine. It is a well-understood phenomenon that can be measured, managed, and mitigated with the right approach. By moving beyond baseline certifications and embracing advanced, accelerated testing protocols, manufacturers and developers can build confidence in their products, protect their investments, and ensure their solar projects deliver on their promise for decades to come.
The journey begins by asking the right questions about your materials and implementing a reliable, data-driven method for finding the answers.