Imagine this: You’ve commissioned a state-of-the-art solar farm using high-efficiency PERC modules. You account for the initial, expected power drop from Light Induced Degradation (LID)—a well-understood phenomenon. But then, months or even years later, performance continues to slide, dipping far below projections. You’re left staring at a balance sheet that doesn’t add up, wondering what went wrong.
You may be looking at the handiwork of a far more complex and costly problem: Light and elevated Temperature Induced Degradation, or LeTID.
This isn’t your standard degradation. It’s a slow-acting mechanism that can erode a module’s power output by 10% or more, striking at the heart of a project’s financial viability. For module manufacturers, it poses a latent threat that can lead to massive warranty claims. For asset owners, it’s a silent thief of long-term returns.
The challenge is that LeTID is notoriously difficult to pin down. But by understanding how to test for it properly, you can move from uncertainty to action.
Not All Degradation is Created Equal: LID vs. LeTID
To understand LeTID, it helps to first understand its more famous cousin, LID.
Light Induced Degradation (LID)
Think of this as the initial „settling in“ period for most common p-type solar cells. When first exposed to sunlight, a reaction involving boron and oxygen creates defects that slightly reduce the module’s efficiency. The good news is that LID happens quickly—within the first hours or days of operation—and then stabilizes. It’s a predictable, one-time toll that engineers account for in performance models.
Light and elevated Temperature Induced Degradation (LeTID)
This is a different beast entirely.
- It’s Slow: LeTID unfolds over hundreds or thousands of hours.
- It Needs Heat: It’s triggered by a combination of charge carriers (from light) and elevated operating temperatures, typically above 50°C.
- It’s Deceptive: The degradation can be severe, but under certain conditions, the module might appear to „heal“ or regenerate, only to degrade again later.
This unpredictable cycle makes LeTID a major headache. It’s particularly prevalent in high-efficiency PERC (Passivated Emitter and Rear Cell) technology—the very technology designed to boost solar panel output. The culprit is believed to be the behavior of hydrogen within the solar cell, which, under heat and carrier injection, can form performance-killing defects.
While you can plan for LID, LeTID introduces a level of long-term risk that can undermine the bankability of a solar project if left unchecked.
The Challenge: How Do You Test for a Ghost?
Testing for LeTID in the real world is incredibly difficult. Is a module’s power loss due to LeTID, soiling, other degradation modes, or just a cloudy month? Outdoor testing is too slow and riddled with variables to provide a clear answer.
To truly understand a module’s susceptibility, you need a controlled, repeatable laboratory test that isolates the LeTID mechanism. The goal isn’t just to see if it degrades, but to understand the rate and magnitude of that degradation. This requires moving beyond sunlight to a more precise, controllable trigger.
A Protocol for Precision: The Current Injection Method
At PVTestLab, we’ve refined a protocol that replaces the variability of sunlight with the precision of electronics. Using controlled current injection in a dark, thermally regulated environment, we can reliably trigger and measure LeTID, providing clear and actionable data.
Here’s how it works:
Step 1: Establish the Baseline
Before any stress is applied, we fully characterize the module. We bring it to a stabilized state and perform initial measurements under Standard Test Conditions (STC). This includes:
- I-V Curve Measurement (Flasher): A high-precision flash test determines the module’s starting power (Pmax), voltage (Voc), and current (Isc).
- Electroluminescence (EL) Imaging: An EL image acts like an X-ray, revealing any hidden cell cracks, soldering defects, or other potential issues that could interfere with the test results.
This initial data set is our „Point Zero“—the reliable benchmark against which all future degradation will be measured.
Step 2: Current-Induced Stress in a Controlled Climate
This is the core of the test. The module is placed inside a climate chamber where we can control the ambient conditions with extreme precision.
Instead of using lamps, we inject a forward current through the module. This forces charge carriers to flow through the cells, closely mimicking the effect of sunlight but without the variables of spectral mismatch or non-uniform illumination. Simultaneously, we raise the chamber temperature to 75°C, providing the second critical ingredient for triggering LeTID.
Expert Insight from Patrick Thoma, PV Process Specialist at PVTestLab:
„Outdoor testing gives you a result, but controlled lab testing gives you an answer. By replacing sunlight with precise current injection, we strip away the environmental noise and isolate the LeTID mechanism itself. This is how you move from observing a problem to actually fixing it at the cell processing level.“
Step 3: Cyclical Testing and Measurement
The stress isn’t applied continuously. Instead, the test is broken into cycles, such as 100-hour intervals. After each interval, the stress is paused, the module is cooled to 25°C, and a new set of I-V and EL measurements are taken.
Plotting the power loss after each cycle reveals the degradation curve. Does it drop sharply? Does it degrade slowly and steadily? Does it show signs of regeneration? This detailed timeline is crucial for understanding the behavior of the specific cells and materials used in the module. This is where robust reliability and quality testing provides its true value—by creating a predictable performance model.
Step 4: Analysis and Action
The final degradation curve tells the story. For companies working on solar module prototyping, this data is gold. They can compare the LeTID susceptibility of different cell batches, test new passivation techniques, or evaluate how changes to the lamination process might impact long-term stability.
By quantifying the LeTID risk upfront, manufacturers can refine their cell processes, select more stable materials, and confidently stand behind their long-term performance warranties.
Frequently Asked Questions (FAQ)
What exactly is a PERC solar cell?
PERC stands for Passivated Emitter and Rear Cell. It’s an advancement on conventional cell architecture that adds a special dielectric layer to the back of the cell. This layer reflects light that would otherwise pass through, giving it a second chance to generate electrons, and also reduces electron recombination. The result is a significant boost in cell efficiency.
Is LeTID a problem in all solar modules?
LeTID is most prominent in p-type multicrystalline and monocrystalline PERC cells. However, other technologies can exhibit different forms of long-term degradation. The key is having a testing methodology that can be adapted to isolate and characterize these specific mechanisms.
Can LeTID be reversed?
Interestingly, yes, to some extent. The degradation-regeneration cycle is a known characteristic of LeTID. Under certain conditions (e.g., lower temperatures and illumination), a module can „recover“ some of its lost performance. However, this recovery may not be permanent, and the module can degrade again. A thorough test protocol will map out this entire cycle.
How long does a typical LeTID test take?
Because the mechanism is slow to manifest, a comprehensive LeTID test typically runs for 500 to 1,000 hours of stress testing. The intermediate measurement points are crucial for building an accurate degradation model.
How is this different from standard IEC certification tests?
Standard IEC tests (like Damp Heat and Thermal Cycling) are designed to test for known failure modes related to module construction and material durability—things like delamination, junction box failure, or frame integrity. LeTID, by contrast, is a cell-level performance degradation mechanism. It requires a specific, targeted test protocol like the one described here, which is not yet a standard part of the IEC 61215 series.
From Characterization to Confidence
LeTID may be a complex challenge, but it’s not an unsolvable one. By moving from the variability of real-world conditions to the controlled environment of the lab, we can replace ambiguity with data. A precise, repeatable testing protocol transforms LeTID from a mysterious threat into a quantifiable characteristic that can be engineered and managed.
For the solar industry to continue its incredible growth trajectory, trust in long-term performance is non-negotiable. Understanding and mitigating degradation mechanisms like LeTID is fundamental to building that trust, one reliable, high-performing module at a time.