You’ve invested in high-efficiency solar cells—PERC, TOPCon, or another advanced technology—to build a next-generation module. The initial flash tests look fantastic, and the datasheets promise years of high performance. But weeks or months after installation, a strange phenomenon can emerge: the module’s power output begins to drop, sometimes significantly, with no obvious cause.
This isn’t a traditional defect. It’s a stealthy degradation mechanism known as LeTID, or Light and Elevated Temperature Induced Degradation. LeTID is one of the most critical challenges facing modern solar module manufacturers, and testing for it is no longer optional—it’s essential for ensuring long-term reliability and bankability.
The Silent Yield Killer: Understanding LeTID
Most people in the solar industry are familiar with Light Induced Degradation (LID), a well-known effect that causes a small, predictable power loss in the first few hours of a module’s life. LeTID, however, is different.
Light and Elevated Temperature Induced Degradation (LeTID) is a slower, more severe degradation process that primarily affects high-efficiency cell technologies like PERC (Passivated Emitter and Rear Cell) and its successors, like TOPCon. Unlike standard LID, it requires two specific triggers working together:
- Light: The module must be exposed to sunlight (or simulated light).
- Elevated Temperature: The effect accelerates significantly when cell temperatures rise above 50°C, a common occurrence during field operation.
The degradation can continue for months or even years, leading to power losses of 6% or more before it eventually stabilizes or, in some cases, begins to recover. For a utility-scale solar project, a hidden 6% loss is a massive blow to financial projections.
Why PERC and TOPCon are Susceptible to LeTID
Ironically, the very innovations that make PERC and TOPCon cells so efficient also make them vulnerable to LeTID. These technologies use advanced passivation layers to reduce electron recombination, which boosts voltage and overall efficiency.
The prevailing theory is that the interaction between these passivation layers and excess hydrogen introduced during manufacturing creates the potential for LeTID. When the module is exposed to both light and heat, these hydrogen atoms can form defects that „trap“ charge carriers, reducing the module’s power output.
The severity of LeTID varies significantly from one cell to another. It depends heavily on the quality of the silicon wafer, the specific cell manufacturing process, and the mitigation strategies applied by the producer. Different module types can exhibit wildly different degradation profiles under the same stress conditions.
This variability is precisely why testing is so critical. You can’t assume a cell is LeTID-free just because it’s a PERC or TOPCon design. You have to verify its performance with a controlled, scientific protocol.
How to Test for LeTID: A Step-by-Step Protocol
Quantifying LeTID is a complex process, requiring specialized equipment and a meticulous, multi-stage approach to simulate real-world conditions in a compressed timeframe. Our engineers follow a rigorous protocol to deliver clear, repeatable results.
Step 1: Establish a Precise Baseline
Before any stress is applied, the module must be fully characterized. This involves stabilizing the module to eliminate any initial LID effects and then measuring its maximum power (Pmax) under Standard Test Conditions (STC) using a Class AAA solar simulator. This initial Pmax value serves as the critical baseline for all subsequent measurements.
Step 2: Controlled Stress in a Climate Chamber
The core of the test takes place inside a large-scale, precision climate chamber. The module is subjected to a constant, controlled stress designed to accelerate the LeTID effect. Typical conditions are:
- Constant Illumination: A light source provides a consistent irradiance, often around 1000 W/m², to simulate full sunlight.
- Elevated Temperature: The chamber maintains a steady module temperature, typically 75°C, to speed up the degradation process.
This controlled environment is crucial. By eliminating variables like weather and changing daylight hours, we can ensure that observed degradation is due to the LeTID effect and not external factors.
Step 3: Interim Measurements and Degradation Tracking
The stress test is a long-duration procedure, often running for hundreds of hours. At predefined intervals (e.g., every 100-200 hours), the module is carefully cooled down and its performance is re-measured in the flasher.
This methodical process of stress and measurement allows us to plot the degradation curve over time, offering deep insights into the module’s behavior: how quickly it degrades, whether it stabilizes, and if it begins to recover. Answering these questions is fundamental to any solar module quality and reliability testing program.
Step 4: Reaching Stabilization
The test continues until the module’s power output stabilizes. „Stabilization“ is typically defined by the IEC TS 63342 standard as a degradation rate of less than 1% over the final 200 hours of the test. The entire process can take 500 hours or more, but it’s the only way to truly understand the maximum potential power loss from LeTID.
What the Data Tells You: From Test Results to Actionable Insights
Completing a LeTID sensitivity test provides more than a pass/fail result; it delivers actionable intelligence that directly impacts your business.
- For Material & Cell Suppliers: It validates the effectiveness of your LeTID mitigation strategies and provides third-party proof of your product’s long-term stability.
- For Module Developers: It allows you to compare cells from different suppliers with objective data, reducing the risk of sourcing a low-quality batch. This is a crucial step in solar module prototyping and development.
- For Project Financiers & Insurers: It provides the bankability data needed to confidently warranty a module for 25-30 years, knowing that hidden degradation won’t jeopardize the project’s return on investment.
LeTID testing transforms uncertainty into a known variable, allowing you to innovate with confidence.
Frequently Asked Questions about LeTID Testing
What’s the difference between LID and LeTID?
LID (Light Induced Degradation) typically occurs within the first hours or days of light exposure and results in a relatively small, one-time power loss. LeTID is a much slower process, triggered by both light and heat, that can unfold over months or years and cause more severe degradation.
How long does a typical LeTID test take?
Because the degradation process is slow, a comprehensive test often requires 500 to 1,000 hours of stress application in a climate chamber to ensure the module has reached a stable state.
Can LeTID be reversed?
Yes, in many cases, the degradation from LeTID can be partially or fully reversed through a process called regeneration. However, the stability of this recovery can vary, making it crucial to test the underlying sensitivity first.
Does LeTID affect all PERC and TOPCon cells equally?
No. Susceptibility to LeTID is highly dependent on the quality of the silicon wafer and the specific recipes used during cell manufacturing. Two PERC cells that look identical on a datasheet can have vastly different LeTID behaviors.
Don’t Let Hidden Degradation Undermine Your Innovation
The push for higher efficiency is driving the solar industry forward. But with every new technology comes new challenges. LeTID is a serious, but manageable, risk for anyone working with PERC, TOPCon, and other advanced cell concepts. The key is to move from assumption to verification.
Understanding the specific degradation profile of your components is the first step toward building more reliable, bankable solar modules. By quantifying LeTID, you can protect your investment, validate your suppliers, and deliver a product that truly stands the test of time. Learn how our applied research and testing environment can provide the data you need to innovate with confidence.