Imagine a performance chart for a brand-new solar module. The power curve is smooth and predictable, just as you’d expect. But then you spot something odd: a small, unusual dip and recovery in the curve, almost like a hiccup. It might seem like an insignificant data anomaly, but what if that tiny imperfection is the first whisper of a catastrophic failure?
This anomaly points to a specific type of Potential Induced Degradation (PID) known as PID-s, or shunting. It’s a silent threat that can undermine a module’s efficiency and bankability long before the damage becomes obvious. Stopping it requires recognizing the earliest clues—and one of the most telling is the distinctive „double hump“ signature in the module’s current-voltage (I-V) curve.
At PVTestLab, we’ve learned this electrical signature is more than a curiosity; it’s a direct link to a physical defect that can be visualized, providing a critical early warning of material and process issues.
Understanding the Silent Threat: What is PID-s?
Potential Induced Degradation (PID) is a loss of performance in solar modules caused by a high voltage difference between the solar cells and the module’s frame. While several types of PID exist, PID-s is particularly insidious.
It occurs when charged particles (specifically, sodium ions from the module’s glass) migrate into the solar cell under high voltage and humidity. This migration creates tiny, unintended electrical pathways, or „shunts,“ within the cell. Think of these shunts as microscopic short circuits. They don’t stop the cell from working entirely, but they leak electricity, draining power and converting it into heat.
Initially, the power loss is minimal. But as these shunts multiply, they can lead to significant underperformance, localized hotspots, and eventually, irreversible damage. The challenge is catching the degradation before it reaches that point.
The Electrical Footprint: Finding the „Double Hump“ in I-V Curves
The most reliable way to assess a module’s electrical performance is by measuring its I-V (Current-Voltage) curve. Like an EKG for the module, this curve shows how much current it produces at different voltages. A healthy module generates a smooth, consistent curve.
However, a module affected by early-stage PID-s shows a peculiar anomaly. As voltage is swept during the measurement, certain shunted cells behave erratically, causing a distinct dip and recovery in the curve. This creates the telltale „double hump“ or „S-shape“ signature.
This isn’t just a random glitch; it’s the electrical footprint of shunted cells being bypassed as the voltage sweeps across the module string. The appearance of this double hump is a major red flag that a degenerative process has already begun.
Connecting the Dots: From an Electrical Curve to a Physical Flaw
So, why does this shunting create such a specific shape? Research points to a clear correlation between the electrical behavior and the physical state of the cells. When a cell becomes shunted, it begins to act more like a resistor than a power generator, especially at certain voltages.
When an I-V curve is measured, it records the performance of the entire string of cells. The non-shunted cells behave normally, but the shunted cells create a bottleneck. The „dip“ in the curve represents the point where these shunted cells actively drain power. The subsequent recovery, or second „hump,“ occurs as the measurement effectively bypasses these faulty cells.
This insight holds real diagnostic power. Identifying this pattern during the solar module prototyping phase lets developers pinpoint weaknesses in a new design or material combination long before mass production. It transforms a potential field failure into a valuable data point for improvement.
Making the Invisible Visible with Electroluminescence (EL)
While the I-V curve tells us that a problem exists, Electroluminescence (EL) imaging shows us where it is. EL testing involves running a small current through the module in a dark environment, causing the silicon cells to light up, or emit photons. It’s like an X-ray for a solar module, revealing cracks, defects, and inactive areas that are invisible to the naked eye.
When PID-s is the culprit, EL imaging reveals a distinct visual pattern: the shunted areas appear as dark, snowflake-like or starburst shapes on the cell.
These dark patterns are the physical manifestation of the electrical shunts—the exact locations where power is being lost. By correlating the two tests, the connection becomes clear: the „double hump“ in the I-V curve is the direct result of these „snowflake“ shunts in the cells.
Why Early Detection is a Game-Changer
Ignoring the double hump signature is like ignoring a small crack in a dam. At first, the impact is negligible. Over time, however, the degradation accelerates, leading to:
- Reduced Energy Yield: The module consistently produces less power than specified, impacting the financial returns of a solar project.
- Safety Risks: Severe shunting can create localized hotspots that damage the module’s backsheet and create a fire hazard.
- Bankability Concerns: Modules known to be susceptible to PID-s are considered a higher risk by investors and insurers, making projects harder to finance.
The materials used in a module, particularly the encapsulant, play a critical role in preventing PID. To confirm that an encapsulant can resist ion migration, it must be thoroughly tested under real-world lamination conditions—this is essential to prevent shunts from ever forming.
„The double hump isn’t just a graph anomaly; it’s the first whisper of a much larger problem. By linking it to the EL snowflake pattern, we can move from reactive failure analysis to proactive process and material optimization. It gives innovators the foresight to build more resilient, reliable modules from day one.“
— Patrick Thoma, PV Process Specialist
Frequently Asked Questions (FAQ)
What exactly is PID-s?
PID-s stands for Potential Induced Degradation of the shunting type. It’s a failure mode where electrical shunts form within a solar cell due to the migration of ions (like sodium) from the glass, driven by a high voltage potential. These shunts act like leaks, draining power from the cell.
Can PID-s be reversed?
In some cases, early-stage PID can be partially reversed by applying an opposite voltage at high temperatures, though this is often not practical or fully effective in the field. The shunting type (PID-s) is generally considered more difficult to reverse and often leads to permanent damage, making prevention by far the best strategy.
Is the „double hump“ I-V curve always caused by PID-s?
While the double hump is a classic indicator of PID-s, other issues that create inconsistent cell performance—such as mismatched cells, severe cracking, or partial shading during a test—can also cause it. However, when the curve is correlated with the characteristic „snowflake“ pattern in an EL image, the diagnosis of PID-s is almost certain.
What materials are most susceptible to causing PID-s?
The primary factors are the encapsulant material (EVA films with lower volume resistivity are more susceptible), the type of solar cell, and the sodium content in the front glass. Modern module designs often use PID-resistant encapsulants (like certain POEs) and glasses to mitigate this risk.
From Diagnosis to Durability
Recognizing the „double hump“ I-V signature and correlating it with EL snowflake patterns is more than an academic exercise. It’s a powerful diagnostic tool that equips module developers, material suppliers, and manufacturers to catch a destructive problem at the earliest possible stage.
By understanding these subtle clues, you can make more informed decisions about material selection, process parameters, and quality control. This proactive approach is the key to building next-generation solar modules that are not only powerful on day one but also durable enough to perform reliably for decades.