You’ve invested in high-efficiency PERC solar cells, expecting top-tier performance from day one. Your datasheets promise impressive power output. But what if a critical step in your own manufacturing process was silently chipping away at that performance before your modules ever see the sun?
This isn’t a hypothetical scenario. It’s a subtle but significant challenge known as Light and elevated Temperature Induced Degradation (LeTID), and it’s directly linked to the thermal budget of your lamination cycle. For many, this connection is a blind spot—a hidden variable impacting bankability, warranty claims, and the long-term energy yield of a solar project.
Today, we’re pulling back the curtain on this phenomenon. We’ll explore how a few degrees of heat during lamination can trigger significant power loss in the field and, more importantly, how you can use data to turn this vulnerability into a competitive advantage.
Understanding the Players: PERC Cells and LeTID
For years, Passivated Emitter and Rear Cell (PERC) technology has been a game-changer, pushing solar module efficiency to new heights. By adding a special passivation layer to the rear of the cell, PERC architecture significantly improves light capture and electron flow.
This advanced design, however, comes with unique sensitivities—chief among them, a susceptibility to LeTID.
What exactly is LeTID?
In simple terms, LeTID is a degradation mechanism that causes a solar cell to lose power when exposed to both light and elevated temperatures, typically above 50°C. Unlike some other forms of degradation, LeTID can take hundreds or even thousands of hours to fully develop before it begins to partially recover, making it difficult to detect with standard quality control checks.
The culprit is believed to be an excess of hydrogen atoms within the silicon, which can interact with crystal defects under heat and light to create „traps“ that prevent electrons from being collected. The result? A measurable drop in power output during the module’s crucial first years in the field.
But the conditions for LeTID aren’t just created in the field—they can be primed and accelerated right inside your factory. The lamination process, with its high temperatures, is the primary suspect.
The Lamination Process: A Delicate Balancing Act
Lamination is where a solar module truly becomes a robust, weatherproof unit. The goal is to perfectly cure the encapsulant (like EVA or POE) to bond the glass, cells, and backsheet into a single, durable package that can last for decades.
This requires a specific amount of thermal energy—a „thermal budget“ defined by temperature and time. But for PERC cells, this process is a double-edged sword. While you need enough heat to ensure proper cross-linking of the encapsulant for long-term reliability, too much heat can „charge“ the cells with the thermal energy that fuels LeTID later on.
To better understand this relationship, we put it to the test. At PVTestLab, we designed an experiment to isolate the impact of lamination temperature on LeTID.
Using identical PERC cells and a bill of materials (BOM), we manufactured several mini-modules on our full-scale R&D production line. We changed only one variable—the peak lamination temperature—creating distinct batches set at 145°C, 148°C, and 150°C. These modules were then subjected to a standardized LeTID stress test to measure their power degradation over time.
Expert Commentary:
„We often see manufacturers follow a ‚one-size-fits-all‘ recipe for lamination provided by their encapsulant supplier,“ notes Patrick Thoma, PV Process Specialist at PVTestLab. „But that recipe is designed for the material, not for the specific cell technology. Our data shows that failing to account for the cell’s thermal sensitivity can lead to unnecessary and preventable performance loss.“
The Data Tells the Story: Visualizing the Impact of Thermal Stress
The results of the LeTID test were clear and dramatic. The modules laminated at higher temperatures showed significantly greater power loss, especially in the early stages of the test.
Let’s break down what this data reveals:
- Immediate Impact: The modules laminated at 150°C experienced a sharp power drop of nearly -2.8% after just 200 hours. In contrast, the 145°C batch degraded by only -1.7%—a full percentage point less.
- Accelerated Degradation: Even a small 3°C increase from 145°C to 148°C resulted in a noticeably faster rate of degradation.
- Long-Term Implications: While all modules eventually show signs of stabilization or recovery, that initial, steep power loss is what impacts project financing and energy yield models. A module that starts its life with a 2-3% deficit is already behind, and this loss can affect its performance for years.
This experiment provides concrete proof of the link between lamination and LeTID. The thermal budget isn’t just a process setting—it’s a critical lever for controlling the initial and long-term performance of your PERC modules.
The Goldilocks Principle: Finding Your Optimal Lamination Recipe
So, is the answer to simply turn down the heat? Not quite.
Lowering the temperature too much could lead to under-curing of the encapsulant, which creates a far worse problem of delamination and moisture ingress down the line. The goal isn’t to find the lowest temperature, but the optimal temperature.
This is where a data-driven approach becomes essential. The „just right“ thermal budget is a precise balance between two competing needs:
- Material Reliability: Ensuring the encapsulant is fully cured to meet adhesion and durability standards.
- Cell Performance: Minimizing the thermal stress on the PERC cells to mitigate LeTID.
Achieving this balance requires moving beyond generic datasheets to conduct structured lamination trials on your specific combination of cells, encapsulants, and backsheets. By optimizing lamination parameters, you can pinpoint a process window that delivers robust modules with minimal initial degradation—a cornerstone of developing any reliable, high-performing new solar module concept.
Frequently Asked Questions (FAQ)
What are the root causes of LeTID?
LeTID is primarily linked to the interaction of excess hydrogen with boron-oxygen complexes and other defects within the p-type silicon wafer, a process activated by heat and light. The specific chemical reactions are complex, but the outcome is the formation of recombination centers that reduce the cell’s efficiency.
Is LeTID a permanent degradation?
LeTID is largely reversible, but the recovery process can be very slow under normal field conditions. The degradation often peaks and then slowly anneals over time. However, the initial power loss during the first 1-3 years is significant for the financial viability of solar projects.
Do all PERC cells suffer from LeTID?
The severity of LeTID varies significantly depending on the cell manufacturer, the quality of the silicon wafer, and the specific cell fabrication processes used (e.g., how hydrogen is introduced and controlled). However, it is a known vulnerability across most conventional p-type PERC technologies.
How can I test for LeTID in my own modules?
Standard LeTID testing involves placing modules in a climate chamber, exposing them to a controlled temperature (e.g., 75°C) and injecting a forward current to simulate sunlight. Power (Pmax) measurements are taken at regular intervals to map the degradation and subsequent recovery curve.
My encapsulant supplier provides a recommended curing temperature. Why should I deviate from it?
A supplier’s recommendation is designed to guarantee the chemical properties of their material, such as gel content or degree of cross-linking. It doesn’t account for the performance of the solar cell inside. The optimal process fine-tunes that recommendation to achieve both material reliability and minimal cell degradation.
From Data to Action: Securing Your Module’s Performance
The promise of high-efficiency solar technology is only realized when every step of the manufacturing process is aligned with performance. As we’ve seen, the thermal budget of the lamination cycle isn’t just a minor detail—it’s a powerful control point for mitigating LeTID and ensuring your PERC modules deliver on their promises from day one.
Understanding this connection is the first step. The next is to leverage it. Adopting a scientific approach to process validation allows you to move beyond generic recipes and develop a finely tuned, data-driven lamination strategy that protects your product’s performance, enhances its bankability, and builds a reputation for quality and reliability in a competitive market.