You’ve probably heard the good news: n-type solar module technologies like TOPCon and HJT are significantly more resistant to the classic Potential-Induced Degradation (PID) that plagued older p-type PERC cells. It was a major step forward for long-term energy yield and reliability.
But as the industry has closed the door on one problem, two new, more subtle ones have appeared.
N-type modules aren’t entirely immune to PID. Instead, they experience unique degradation modes that are harder to detect and demand a more sophisticated understanding to solve. For material suppliers, module developers, and quality engineers, failing to distinguish between these new modes—shunting (PID-s) and polarization (PID-p)—can lead to costly misdiagnoses and flawed designs.
Let’s break down what’s really happening inside these advanced cells and how to identify the true culprit behind power loss.
A Quick Refresher: What is Potential-Induced Degradation (PID)?
At its core, PID is a performance-degrading phenomenon caused by a combination of high voltage, high temperature, and high humidity. In a large solar array, a significant voltage difference exists between the solar cells and the module’s grounded frame. Over time, this voltage stress can cause stray electrical currents to flow, triggering chemical and physical changes that diminish the cell’s performance.
For years, the industry battled this issue in p-type modules. Now, with n-type architectures, the game has changed. While more robust, their unique cell structures have introduced new pathways for degradation.
The New Faces of PID: Shunting (PID-s) vs. Polarization (PID-p)
Understanding the difference between these two failure modes is the first step toward building truly reliable n-type modules. They may both cause power loss, but their underlying mechanisms—and their potential for recovery—are worlds apart.
PID-s: The Permanent Damage of Cell Shunting
Think of PID-s as a permanent electrical short-circuit within the cell. It’s the primary PID concern for n-type TOPCon modules.
The mechanism involves positive ions, typically sodium (Na+) from the module’s front glass, migrating through the encapsulant and into the delicate layers of the solar cell. Research shows these ions can create alternative current paths, or „shunts,“ that divert energy away from the intended circuit.
The result is a direct and often irreversible loss of performance. Key indicators include:
- A significant drop in the module’s maximum power (Pmax).
- A severe reduction in the Fill Factor (FF) and shunt resistance (Rsh).
- The damage is typically permanent and non-recoverable.
PID-p: The (Sometimes) Reversible Threat of Surface Polarization
PID-p, on the other hand, is more like a temporary shield. This mode is the primary PID concern for HJT modules, though it can also affect TOPCon cells.
Instead of ions physically penetrating the cell, they accumulate on its surface, typically at the interface between the silicon and the anti-reflective coating. This buildup of electrical charge creates a polarization effect that disrupts the cell’s electrical field, making it harder to extract power efficiently.
The good news? This effect is often temporary. Key indicators are:
- A drop in Pmax and Fill Factor, particularly the pseudo Fill Factor (pFF).
- The effect is often fully or partially reversible by „resetting“ the module—for example, by applying a reverse voltage in a controlled environment.
How We Uncover the Truth: The PID Testing and Diagnostic Process
You can’t distinguish between shunting and polarization just by looking at a power reading. It requires a specific, multi-step diagnostic process that simulates harsh field conditions and reveals what’s happening inside the cell.
Step 1: Simulating the Worst-Case Scenario
To see how a module will perform after 25 years in the field, we must accelerate the degradation process in a controlled chamber. Based on IEC standards, this involves subjecting the module to a punishing environment:
- Damp-Heat: 85°C and 85% relative humidity.
- High Voltage: A negative voltage up to -1500V is applied to the active cell circuit relative to the grounded frame.
- Duration: The test typically runs for 96 to 192 hours to simulate years of stress.
The quality of the materials used in the solar module lamination process is critical. The choice of encapsulant (like EVA or POE) and front glass can either accelerate or inhibit the ion migration that leads to PID.
Step 2: Visualizing the Damage with Electroluminescence (EL)
After the stress test, the first diagnostic step is Electroluminescence (EL) imaging. By running a current through the module in a dark room, we cause the cells to emit light. Healthy areas glow brightly, while damaged or inactive areas appear dark.
EL imaging is excellent for spotting PID hotspots—you can literally see which cells are failing. However, it doesn’t always tell you why they are failing. Since both PID-s and PID-p can cause cells to go dark, the next step is crucial.
Step 3: Differentiating the Culprit with Dark I-V Analysis
This is where the real detective work begins. A „Dark I-V“ test measures the current-voltage (I-V) characteristics of the module without any light, allowing us to isolate electrical behaviors that are normally hidden during operation.
Here’s how it helps us distinguish the two failure modes:
- A module with PID-s (shunting) will show a significant increase in „leakage“ current when a reverse voltage is applied. This is the classic signature of an internal short-circuit.
- A module with PID-p (polarization) will not exhibit this same shunting behavior. Its Dark I-V curve will look different, reflecting the surface charge effect rather than a physical shunt.
By comparing the Dark I-V curves before and after the stress test, we can definitively diagnose the root cause of power loss.
Is the Damage Reversible? The Critical Question of Recovery
Once we’ve identified the problem, the next question is: can it be fixed?
- For PID-p, the answer is often yes. By applying a positive voltage to the module for a period (a „recovery“ or „regeneration“ step), the accumulated surface charges can be dispersed, and much of the initial power loss can be restored.
- For PID-s, the outlook is far worse. Because shunting involves a physical change to the cell structure, the damage is largely irreversible. No amount of reverse voltage will fix these internal short-circuits.
This distinction is vital. For a material developer, proving a new encapsulant prevents irreversible PID-s is a massive competitive advantage. And for anyone designing a new module, this knowledge is fundamental to securing a 25-year performance warranty. Rigorous testing is the only way to get the data needed for confident solar module prototype development.
Frequently Asked Questions (FAQ) about N-Type PID
Q1: What exactly causes PID in solar modules?
PID is caused by voltage stress between the solar cells and the module frame, amplified by high temperatures and humidity. This stress drives the movement of ions (like sodium from the glass) into or onto the solar cells, causing degradation.
Q2: Why are TOPCon and HJT modules susceptible to different types of PID?
It comes down to their unique cell structures. TOPCon’s polysilicon-oxide layer can be vulnerable to ion penetration that creates shunts (PID-s), while HJT’s amorphous silicon layers are more susceptible to surface charge accumulation (PID-p).
Q3: Can better materials prevent PID?
Absolutely. Using low-sodium glass and advanced encapsulants like POE, which have high volume resistivity and low water vapor transmission rates, can dramatically reduce the risk of both PID-s and PID-p.
Q4: How can I know if my modules are at risk?
The only way to know for sure is through certified laboratory testing. A proper PID test subjects modules to standardized stress conditions (e.g., 85°C, 85% RH, -1000V or more) and uses diagnostic tools like EL and Dark I-V to analyze the results.
Q5: Is PID covered by module warranties?
Most tier-1 manufacturers now offer PID-resistant modules and include this resistance in their performance warranties. However, the terms can vary, so it’s essential to check the fine print and ensure the modules have been certified by a reputable third-party lab.
From Lab Insights to Production Reality
The era of n-type dominance is here, but so is the need for a deeper understanding of its reliability challenges. Simply knowing that TOPCon and HJT are „better“ is not enough. True innovation comes from knowing precisely why they fail and engineering solutions to prevent it.
Distinguishing between irreversible shunting and recoverable polarization isn’t just an academic exercise—it’s essential for developing the next generation of durable, high-performance solar modules.
If you’re developing new materials or module designs and need to validate their PID resistance, there’s no need to guess. Our team is here to help you get clear, actionable data. Contact our process engineers to discuss your testing needs.