If you’re evaluating solar modules for a large-scale project, you’ve already moved past the simplistic „0.5% annual degradation“ figure. You know that long-term asset performance isn’t a straight line; it’s a complex interplay of materials, cell architecture, and electrochemical physics.
The real risks to your project’s bankability lie in specific, accelerated degradation mechanisms—the kind standard datasheets often fail to address. Understanding the difference between Potential-Induced Degradation (PID), Light-Induced Degradation (LID), and Light and Elevated Temperature Induced Degradation (LeTID) is no longer just an academic exercise. It’s what separates a project that meets its pro-forma expectations from one that suffers unforeseen yield loss.
This guide provides a scientific framework to help you move from awareness to action. We will explore the physics behind each mechanism, detail the industrial protocols used to test for them, and present proven material and process-level strategies for mitigation. The central question we’ll answer is: How can you quantify and prevent these degradation mechanisms to guarantee module lifetime and project value?
A Comparative Overview of Key Degradation Mechanisms
Not all degradation is created equal. Each mode has a distinct physical origin, timescale, and impact on performance. Understanding these differences is the first step in effective risk management.
Mechanism: PID (Potential-Induced)
Power Loss: Up to 30%
Affected Technology: All crystalline silicon
Root Cause: Sodium ion migration causing shunting
Mechanism: LID (Light-Induced)
Power Loss: 1.5 – 3% (initial)
Affected Technology: p-type c-Si (Al-BSF, PERC)
Root Cause: Boron-Oxygen complex formation
Mechanism: LeTID (Light & Elevated Temp)
Power Loss: 4 – 6%
Affected Technology: p-type multi-c-Si PERC
Root Cause: Hydrogen-related defect activation
Potential-Induced Degradation (PID): The Silent Yield Killer
PID is one of the most severe degradation modes, capable of reducing module power by up to 30% within the first few years of operation. It’s a system-level issue driven by the high voltage potential between the cells and the module frame—a common condition in the negative string of modern utility-scale arrays.
The Electrochemical Process
The mechanism is fundamentally about ionic leakage. Standard soda-lime glass contains sodium ions (Na+), which can migrate under certain conditions. High negative voltage and humidity drive these positive ions through the encapsulant—typically EVA—causing them to accumulate on the cell surface. This accumulation creates parasitic shunt paths, effectively short-circuiting parts of the cell and draining power.
PVTestLab’s Testing Protocol: Stressing for Failure
To identify susceptibility to PID, you can’t rely on standard flash tests; you need to simulate the harsh conditions that trigger the mechanism. At PVTestLab, we follow the IEC 62804 standard in our fully climate-controlled production environment.
Inside a climatic chamber, modules are subjected to high temperature (85°C) and high relative humidity (85% RH). Simultaneously, a high negative voltage (-1000V or -1500V) is applied between the active cell circuit and the grounded frame. This accelerated stress test reveals in just 96-192 hours what might take years to manifest in the field.
Detection and Mitigation Strategies
PID is clearly identifiable through electroluminescence (EL) imaging, which reveals a characteristic darkening of cells near the frame—a direct visualization of the shunting.
Mitigation involves two primary strategies:
- System-Level: Implementing negative pole grounding can reduce the voltage stress that drives PID.
- Material-Level (The Proactive Solution): The most robust defense is built directly into the module. By conducting Material Testing & Lamination Trials, we can prove the superior resistance of certain materials. For example, using POE (Polyolefin Elastomer) encapsulants instead of traditional EVA dramatically reduces PID risk. POE has much lower ionic conductivity, effectively blocking the path for sodium ions to reach the cell.
Light-Induced Degradation (LID): The Initial Power Drop
LID is a well-known phenomenon that causes a rapid, permanent power loss of 1.5% to 3% within the first few hours or days of sun exposure. While the percentage is smaller than that of PID, it directly impacts the „nameplate“ power rating of a module. If not properly accounted for, every module in your project will underperform from day one.
The Physics of Boron-Oxygen Complexes
This degradation mode is specific to p-type silicon wafers, which are doped with boron. Under illumination, mobile oxygen atoms in the silicon lattice bind with boron atoms to form Boron-Oxygen (B-O) complexes. These complexes act as powerful recombination centers, trapping charge carriers (electrons and holes) generated by sunlight before they can be collected, thus reducing the cell’s efficiency.
Quantifying LID with Light Soaking
The test for LID is straightforward but requires precise control. First, a module’s initial power is measured with a AAA class flasher. The module is then exposed to a controlled dose of light (a process called „light soaking“) at a specific temperature, which accelerates B-O complex formation until the power output stabilizes. A final power measurement reveals the true, post-LID performance. This stabilization process is essential for ensuring that the power you buy is the power you get.
The Definitive Solution: Shifting to n-Type Architectures
LID is a fundamental material problem, not a process flaw. The most effective mitigation strategy is to eliminate one of its key ingredients: boron. This is achieved by moving from p-type to n-type silicon wafers, which are doped with phosphorus instead.
N-type architectures like TOPCon and HJT are inherently immune to B-O complex LID. By working with our team on Prototyping & Module Development, developers can validate new n-type module designs and quantify their stability advantage over legacy p-type products.
LeTID: The Long-Term Challenge for PERC Technology
Light and Elevated Temperature Induced Degradation (LeTID) is a more insidious mechanism primarily affecting p-type multi-crystalline PERC cells. It occurs much more slowly than LID but can result in a more severe power loss of 4-6%, sometimes appearing months or even years into a module’s life. The degradation can also be followed by a slow regeneration phase, making it complex to predict.
Unraveling the Hydrogen Defect Mechanism
The scientific consensus points to hydrogen as the key culprit in LeTID. During manufacturing, hydrogen is used to passivate defects in the silicon, especially at the rear AlOx/SiNx passivation stack in PERC cells. Under illumination at elevated operating temperatures (above 65°C), however, this same hydrogen can be released, forming performance-degrading defects within the silicon bulk.
Standardizing the Test: The IEC TS 63342 Protocol
Due to its complex nature, testing for LeTID requires a more rigorous protocol, now standardized in IEC TS 63342. The test involves subjecting modules to a sequence of illumination and dark annealing cycles at elevated temperatures to trigger both the degradation and regeneration phases. This allows for a full characterization of the module’s long-term stability under realistic operating conditions.
Process Control as the Primary Defense
Mitigating LeTID requires precision process control and material selection. Strategies focus on:
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Optimizing Firing Profiles: Fine-tuning the high-temperature firing step to create a more stable hydrogen configuration.
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Controlling Hydrogen Content: Carefully managing the amount of hydrogen introduced during the deposition of passivation layers.
This is where PVTestLab’s full-scale R&D line becomes invaluable. We enable manufacturers to test different firing recipes and material combinations, using our integrated process engineering support to find the optimal parameters that ensure LeTID stability without sacrificing peak efficiency.
The Next Frontier: Emerging Degradation in Advanced Technologies
The race for higher efficiency never stops, but new technologies bring new reliability challenges. We are actively researching novel degradation modes that are critical for de-risking next-generation modules.
One significant emerging trend is a new form of degradation in TOPCon cells, where sodium ingress (similar to PID) can degrade the ultra-thin tunnel oxide and polysilicon layers on the rear side, leading to significant efficiency loss.
„Predicting long-term reliability is no longer about checking off a few standard tests. It’s about understanding the deep interplay between new materials, advanced cell structures, and process variables. Our research is focused on developing test protocols that expose these future failure modes before they become field issues.“
— Patrick Thoma, PV Process Specialist, PVTestLab
From Mechanism to Mitigation: A Proactive Approach to Reliability
Moving beyond a simple degradation rate empowers you to make smarter procurement and design decisions. Instead of reacting to field failures, you can proactively select materials and specify module designs that are inherently resistant to the most damaging degradation mechanisms.
By leveraging PVTestLab’s applied research environment, you gain access to the tools, processes, and expertise needed to validate these decisions under real industrial conditions. You can compare encapsulants, test new cell technologies, and optimize lamination cycles to build a module that is truly designed for a 30-year life.
Frequently Asked Questions (FAQ)
Can PID, LID, or LeTID effects be reversed?
LID is generally permanent. PID can often be partially or fully reversed in the field if the environmental drivers (humidity, voltage) are removed, but the underlying susceptibility remains. LeTID is known for its degradation-regeneration cycle, but full recovery is not guaranteed and can take years. Prevention is always the superior strategy.
How do bifacial modules affect these degradation risks?
Bifacial modules, especially glass-glass constructions, can have different PID susceptibilities depending on their design. Because both sides are active, material choices for both the front and rear encapsulant become critical. LeTID and LID remain driven by the cell technology (e.g., p-type PERC) and are largely independent of the bifacial design.
If POE is better for PID, why is EVA still used?
EVA has been the industry standard for decades; it is well-understood, widely available, and generally lower in cost. However, for high-voltage systems where PID is a major concern, the slight premium for a high-performance POE encapsulant is often a worthwhile investment to protect the asset’s long-term energy yield.
Why can’t I just rely on the manufacturer’s datasheet and warranty?
Datasheets provide performance data under standard test conditions (STC), not after long-term environmental stress. Warranties offer financial protection, but they don’t prevent yield loss or the operational complexities of diagnosing and processing claims. Independent testing provides objective, third-party validation that the product will perform as expected in the real world.
Ready to move beyond datasheets and quantify the true long-term stability of your solar modules? Contact our process engineers to discuss a tailored testing program for your specific materials or module technology.