A solar module passes every standard quality check. Its flash test results are perfect, and a visual inspection shows nothing amiss. Yet, a few years into its life in the field, the module’s power output drops mysteriously, far faster than expected. What went wrong?
The culprit is often a hidden degradation mechanism invisible to standard tests: Light and elevated Temperature Induced Degradation (LeTID). This defect silently chips away at the performance of high-efficiency PERC solar cells. But what if you could spot this problem before it ever leaves the lab? It turns out, you can—by changing the temperature.
First, What Exactly Are LeTID and EL Imaging?
Before we dive into the solution, let’s get acquainted with two key concepts—the villain and the hero of our story.
The Villain: Light and elevated Temperature Induced Degradation (LeTID)
LeTID is a significant reliability concern for modern PERC (Passivated Emitter and Rear Cell) solar modules. It degrades a module’s performance when exposed to sunlight and operating temperatures, sometimes causing a power loss of over 5%.
The tricky part is that LeTID doesn’t happen overnight. It develops over hundreds or thousands of hours under specific conditions, making it incredibly difficult to catch during initial quality control. A module can look perfectly healthy at the factory, only to reveal its weakness after years in the field.
The Hero: Electroluminescence (EL) Imaging
Electroluminescence imaging is like an X-ray for solar modules. Applying a small electrical current causes the solar cells to light up. A specialized camera then captures this light, revealing a detailed map of the module’s health. Healthy areas glow brightly, while defects like micro-cracks, faulty cell connections, or inactive zones appear as dark spots or patterns. It’s a powerful, non-destructive way to see what the naked eye cannot.
The Problem: A Standard EL Image Isn’t Enough
You might think a standard EL test would be enough to find LeTID. Unfortunately, it’s not that simple.
At room temperature, the early signs of LeTID are often completely invisible. The defect can lie dormant, masked by the normal operation of the cell. A perfect-looking EL image can come from a module that is, in fact, highly susceptible to future degradation. This creates a huge challenge for manufacturers who need to guarantee long-term performance and for researchers developing new materials.
How do you find a problem that reveals itself only under specific, real-world conditions? You have to recreate those conditions in a controlled, measurable way.
Seeing the Invisible: The Power of Temperature-Dependent EL
Research has shown that the physical mechanism behind LeTID is highly sensitive to temperature. The unique spatial patterns caused by LeTID, which fade away at other temperatures, become clearly visible at specific elevated temperatures (e.g., 70°C). In fact, the degradation process is even reversible at temperatures above 140°C.
By harnessing this behavior, we can create a powerful diagnostic technique: Temperature-Dependent Electroluminescence Imaging.
The process is straightforward:
- Establish a Baseline: First, an EL image is taken at a standard room temperature (e.g., 25°C), giving us a baseline of the module’s condition.
- Introduce Heat: The module is placed in a climate-controlled chamber and carefully heated to an optimal temperature for revealing LeTID, often around 70°C.
- Capture the „Hot“ Image: A second EL image is taken while the module is held at this elevated temperature.
- Analyze the Difference: The two images are compared. Suddenly, patterns that were invisible at 25°C emerge in the 70°C image, providing a clear „fingerprint“ of LeTID.
[IMAGE 1: A side-by-side comparison of an EL image at room temperature (showing minimal defects) and one at 70°C (clearly revealing the honeycomb-like LeTID pattern).]
The image on the left, taken at room temperature, might pass a standard quality check. But the image on the right, taken at 70°C, tells a completely different story. These distinct honeycomb-like patterns are a classic signature of LeTID, signaling the potential for significant power loss over the module’s lifetime.
„Standard EL gives you a snapshot, but temperature-dependent EL gives you the full story. By changing the temperature, we can essentially ‚activate‘ the LeTID signature, making the invisible visible. It allows our clients to validate new materials or process changes with incredible confidence.“ – Patrick Thoma, PV Process Specialist
Why This Matters for Solar Innovation
This advanced diagnostic method isn’t just an academic exercise; it’s a tool that accelerates progress and reduces risk across the solar industry. By identifying LeTID susceptibility early, innovators can make smarter, faster decisions.
- For Material Manufacturers: When developing new encapsulants or backsheets, you can get rapid feedback on how your materials interact with PERC cells. This is a crucial part of comprehensive material testing and lamination trials.
- For Module Developers: You can validate the long-term stability of new module designs without waiting months or years for field data. This de-risks the process of prototyping new solar module concepts and ensures new technologies are built to last.
- For Process Engineers: It provides a clear, data-driven way to fine-tune manufacturing processes to mitigate LeTID, leading to more robust and reliable products.
[IMAGE 2: A close-up shot of an engineer at PVTestLab analyzing EL images on a large monitor, pointing to specific patterns.]
Ultimately, being able to see LeTID in the lab means fewer surprises in the field. It transforms quality assurance from a simple pass/fail check into a predictive science.
Frequently Asked Questions About LeTID and EL Testing
What exactly causes LeTID?
While the exact mechanisms are still a subject of intense research, LeTID is widely believed to be linked to the presence of excess hydrogen within the silicon of PERC cells. Under light and heat, this hydrogen can interact with crystal defects, reducing the cell’s efficiency.
Is LeTID damage permanent?
LeTID has a complex behavior. The degradation can be partially or fully reversed if the module is exposed to very high temperatures (e.g., above 140°C) or kept in the dark for an extended period. However, it can reappear once the module returns to normal operating conditions. The goal of mitigation is to create a stable state that prevents degradation from occurring in the first place.
Can’t I just use a standard flasher test to find this?
A flasher measures a module’s power output at a single moment under standard conditions (STC). It tells you what the performance is, but not why. A module susceptible to LeTID may have a perfect flash test result before degradation sets in. Temperature-dependent EL is a diagnostic tool that reveals the underlying vulnerability, predicting future performance issues that a flasher test would miss.
From Detection to Prevention
Understanding how to detect hidden defects like LeTID is the first step toward building the next generation of reliable, high-performance solar technology. It shifts the industry from a reactive approach—analyzing failures after they happen—to a proactive one, where potential problems are designed out of the system from the very beginning.
By leveraging advanced diagnostics, innovators can shorten development cycles, reduce investment risk, and bring more robust products to a market that demands long-term reliability.
Ready to see how advanced diagnostics can fit into your R&D process? Explore how applied research and process optimization can accelerate your next project.