You’ve invested in bifacial PERC technology, drawn by the promise of higher energy yields and cutting-edge efficiency. Your modules are designed to capture sunlight from both sides, pushing the boundaries of power generation. But what if that very advantage concealed a hidden vulnerability—a slow, silent degradation that standard quality checks might miss?
This phenomenon, known as Light and elevated Temperature Induced Degradation (LeTID), is a critical challenge for modern solar module developers. It’s a subtle power thief that can erode a module’s performance over time, particularly affecting the advanced cell architectures that define today’s high-efficiency market.
Understanding LeTID isn’t just an academic exercise—it’s essential for the long-term bankability and reliability of your solar innovations.
First, A Quick Refresher: What Makes Bifacial PERC Special?
Before we dive into the problem, let’s appreciate the technology. PERC (Passivated Emitter and Rear Cell) technology revolutionized solar cells by adding a dielectric passivation layer to the back of the cell. Think of it as a near-perfect internal mirror. Photons that would otherwise pass through the cell and be wasted are reflected for a second chance at generating electrons.
Bifacial PERC takes this a step further. By using a transparent backsheet or glass-on-glass design, the module can capture reflected light (albedo) from the ground, boosting energy production by as much as 25% depending on the surface.
This combination of higher front-side efficiency and back-side generation is why bifacial PERC has become a dominant force in the industry. But this boost in energy generation also creates unique internal conditions within the cell.
The Hidden Culprit: Light and elevated Temperature Induced Degradation (LeTID)
Unlike the initial, well-understood Light Induced Degradation (LID) that often stabilizes within hours or days, LeTID is a more complex and prolonged beast.
LeTID is a performance loss in specific types of solar cells that occurs under the combined influence of sunlight (carrier injection) and high operating temperatures (typically above 50°C).
The root cause is widely believed to be the interaction of excess hydrogen atoms within the silicon wafer, which creates defects that trap electrons and reduce the cell’s efficiency. This degradation can be significant, with studies showing potential power losses ranging from 2% to over 6% in the field.
The most challenging part? It can take months or even years of field operation to fully manifest, making it a serious long-term reliability risk.
Why Are Bifacial PERC Modules More Susceptible?
Here’s the „aha moment“: the very feature that makes bifacial modules so powerful also makes them more vulnerable to LeTID.
Because they absorb light from both sides, the cells operate under a state of higher minority carrier injection. In simpler terms, the cell is more „energized“ throughout the day compared to a standard monofacial module. This constant high-energy state acts as a catalyst, accelerating the hydrogen-related reactions that cause LeTID.
The technology designed for superior performance inadvertently creates the ideal conditions for this specific degradation mechanism to thrive.
You Can’t Fix What You Can’t See: A Framework for Characterizing LeTID
The good news is that LeTID is not an unsolvable mystery. Guided by standards like IEC TS 63342, the industry has developed a rigorous framework for testing this long-term issue in a controlled, measurable laboratory setting.
A proper LeTID test doesn’t just look for degradation; it characterizes the module’s entire behavior—its susceptibility, its stabilization point, and its potential for recovery.
As PV Process Specialist Patrick Thoma often notes, „You can’t optimize what you can’t measure. A structured LeTID test gives you the precise data needed to distinguish between a cell-level issue and a material interaction, which is the foundation for any real solution.“
The process involves a precise, multi-stage sequence:
1. Establish a Stable Baseline (Pmax initial)
First, the module is conditioned to eliminate any initial, temporary degradation (like standard LID). This ensures the starting measurement of its maximum power (Pmax) is stable and accurate.
2. The Degradation Phase (Accelerated Stress)
The module is then placed in a climate chamber and subjected to the conditions that trigger LeTID: a high temperature of 75°C and an electrical current that simulates intense sunlight. This phase can run for hundreds of hours, with periodic measurements taken to track the rate of power loss until it stabilizes.
3. The Recovery Phase (Annealing)
Once maximum degradation is reached, the stress conditions are removed. The module is then kept in darkness at a lower temperature. This „annealing“ step reveals whether the degradation is permanent or if the module can recover lost performance—a crucial factor for understanding the real-world financial impact.
A controlled climate chamber at PVTestLab, where modules undergo precise temperature and illumination cycles to accurately characterize LeTID behavior.
This structured approach transforms a slow, unpredictable field issue into a clear, data-driven report on a module’s long-term stability.
Turning Insights into Action: How to Mitigate LeTID
The goal of characterizing LeTID isn’t just to pass a test; it’s to gather the intelligence needed to build more resilient products. The results from a controlled test directly inform your two primary levers for mitigation:
1. Cell Processing
The data might reveal an issue within the solar cells themselves. Armed with this information, you can work with your cell supplier to refine their manufacturing processes, such as optimizing hydrogenation steps to reduce the excess hydrogen that fuels LeTID.
2. Material Selection
Sometimes, the interaction between the cell and surrounding materials (like encapsulants) can influence degradation. LeTID testing is a vital part of comprehensive material testing, helping you select encapsulants and backsheets that contribute to long-term stability.
By proactively testing, you can validate new solar module concepts before they reach mass production, ensuring they are built to withstand real-world stressors. This is a critical step in overall product reliability testing and helps protect your brand’s reputation for quality.
Frequently Asked Questions (FAQ)
What does PERC stand for again?
PERC stands for Passivated Emitter and Rear Cell. It’s a solar cell technology that incorporates a passivation layer on the rear surface to increase efficiency by reflecting photons back into the cell.
What is the main difference between standard LID and LeTID?
LID (Light Induced Degradation) typically occurs within the first few hours or days of light exposure and stabilizes quickly. LeTID (Light and elevated Temperature Induced Degradation) is a much slower process that requires both light and heat, and it can take hundreds or thousands of hours to fully manifest and stabilize.
Is LeTID damage permanent?
Not always. The „annealing“ or recovery phase of a LeTID test shows how much of the lost performance can be regained. Some modules show significant recovery, while others experience more permanent degradation. This variability is why testing is so important.
Do all bifacial PERC modules suffer from LeTID?
No. Susceptibility to LeTID depends heavily on the specific cell manufacturing process, particularly how hydrogen is managed within the silicon wafer. High-quality cell producers have developed techniques to mitigate it, but validation through testing is the only way to be certain.
Why test at 75°C? Isn’t that hotter than normal operation?
Yes, and that’s the point. The high temperature is used to accelerate the degradation mechanism in a controlled laboratory environment. It allows testers to replicate in a few hundred hours what might take years to occur in the field, providing rapid and actionable feedback for product development.
The Path to More Reliable Solar Energy
Bifacial PERC technology represents a massive leap forward for the solar industry. But this advanced technology also demands a deeper understanding of its long-term behavior. LeTID is not a reason to abandon this technology, but rather a call to embrace a more sophisticated approach to design and validation.
By understanding the mechanisms behind it and employing advanced testing frameworks to characterize it, manufacturers can proactively mitigate the risks. This commitment to deep process knowledge ensures that the next generation of high-efficiency modules delivers on its promise of more power, reliably, for decades to come.