You’ve done everything right. You selected top-tier PERC or TOPCon cells, specified a high-quality encapsulant, and followed industry-best practices for module lamination. Yet, after potential-induced degradation (PID) testing, the power output drops more than expected.
Your first instinct is to blame the encapsulant or the glass—the usual suspects in classic PID.
But what if the real culprit is hiding within the solar cell itself?
This scenario is becoming increasingly common as module technology advances. A subtle, often misdiagnosed degradation mode known as PID-s, or shunt-induced PID, is a critical challenge for manufacturers of high-efficiency modules. Unlike conventional PID, which stems from sodium ion migration affecting the anti-reflective coating, this defect directly attacks the cell’s electrical integrity. Misdiagnosing it can lead to costly and ineffective solutions.
Understanding the difference isn’t just an academic exercise; it’s essential for protecting your yield, reputation, and bottom line.
A Quick Refresher: What is „Classic“ PID?
For years, the solar industry has battled Potential Induced Degradation (PID). Classic PID occurs when a high voltage difference between the solar cells and the grounded module frame creates a leakage current. This current mobilizes positive ions (typically sodium from the glass) that travel through the encapsulant and disrupt the electrical properties of the cell’s front surface, specifically the anti-reflection and passivation layers.
The primary symptom of classic PID is a drop in the cell’s open-circuit voltage (Voc) and an increase in recombination, which reduces the cell’s ability to generate power efficiently.
The New Challenge: High-Efficiency Cells and PID-s
PERC (Passivated Emitter and Rear Cell) and TOPCon (Tunnel Oxide Passivated Contact) technologies achieve their impressive efficiency gains through highly sophisticated cell architectures. They feature advanced passivation layers on the rear side that are incredibly effective at minimizing electrical losses.
However, this complexity also introduces new vulnerabilities. These delicate layers can be susceptible to damage under high voltage stress, leading to a different form of degradation: PID-s (Shunt Degradation).
PID-s occurs when high system voltage creates localized defects or „shunts“ on the cell’s surface. Think of a shunt as a tiny electrical short circuit—an unintended path for current to leak, bypassing the intended circuit. Instead of generating power, this energy is lost as heat, severely compromising the cell’s performance.
The most dangerous part? The initial power loss looks a lot like classic PID, leading teams down the wrong diagnostic path.
The Diagnostic Dilemma: How to Tell Them Apart
If both degradation modes cause power loss, how can you pinpoint the true root cause? The answer lies not in if a module loses power, but in how it loses power. This requires a more sophisticated diagnostic protocol that goes beyond standard PID testing.
This advanced protocol combines targeted stress testing with a forensic analysis of the solar cell’s I-V curve—the fundamental signature of its performance.
Step 1: Controlled High-Voltage Stress Testing
The goal is to intentionally induce the degradation mechanism in a controlled, measurable environment. This isn’t your standard PID test.
- The Setup: The test involves placing a single-cell laminate or mini-module in a climate chamber under standard damp-heat conditions (e.g., 85°C and 85% relative humidity).
- The Stressor: A high negative voltage, often as high as -2000V, is applied to the cell for a set duration (typically 48 to 96 hours). This extreme voltage accelerates the degradation that cells might experience over decades in the field.
This step forces either classic PID or PID-s to manifest clearly. After the stress test, the real diagnostic work begins.
Step 2: Forensic I-V Curve Analysis
The I-V curve provides a wealth of information about a cell’s health. By comparing the curve before and after the stress test, we can identify the specific failure mode.
Here’s what to look for:
- Indicator of PID-s (Shunt Degradation): The tell-tale sign of PID-s is a significant drop in the cell’s Fill Factor (FF) and Shunt Resistance (Rsh). The shape of the I-V curve will change dramatically, becoming less „square.“ The Voc, however, might remain relatively stable. This signature indicates the cell is leaking current through newly formed shunts.
- Indicator of Classic PID: Classic PID primarily attacks the cell’s ability to build voltage. This shows up as a noticeable drop in Open-Circuit Voltage (Voc) and a corresponding increase in saturation current (J01). The Fill Factor might also decrease, but the primary hit is to the Voc.
By analyzing these specific parameters, you can definitively separate the two degradation modes. For manufacturers looking to refine their production, this insight is invaluable. It’s a core component of the solar module prototyping and development services that de-risk new technologies before they reach mass production.
From Diagnosis to Action: Why This Matters
Identifying PID-s as the root cause completely changes your troubleshooting strategy.
- If you find classic PID, your focus should be on the module materials: evaluating different encapsulants (PID-resistant EVA or POE), testing the sodium content of the glass, or improving lamination parameters.
- If you find PID-s, the problem lies within the cell itself. The investigation must turn to the cell manufacturing process. Are the passivation layers robust enough? Is the firing temperature for the metallization pastes optimized? Could contaminants be creating weak spots on the cell surface?
Consider this real-world example: A TOPCon module manufacturer was experiencing a 6% power loss in PID testing. They spent months evaluating new encapsulants with no improvement. An advanced diagnostic test revealed that while Voc dropped by only 1%, the Fill Factor had plummeted by 12%. The problem wasn’t the encapsulant; it was PID-s caused by micro-defects in the cell’s sensitive TOPCon layer.
This level of detailed lamination trials and material testing gives manufacturers the clarity to solve complex production challenges, saving them time, money, and materials.
Frequently Asked Questions (FAQ)
What specifically causes PID-s in PERC and TOPCon cells?
The advanced rear-side passivation layers in these cells are incredibly thin and complex. High voltage stress can cause localized dielectric breakdown in these layers, creating physical pathways for current to leak—forming shunts. The problem can be exacerbated by contaminants or inconsistencies in the cell production process.
Can a module suffer from both classic PID and PID-s at the same time?
Absolutely. In fact, it’s quite common for both mechanisms to occur simultaneously, which is why a detailed I-V curve analysis is so critical. Without it, you might solve for one problem (e.g., by changing the encapsulant) while the other continues to degrade your module’s performance.
Is this high-voltage diagnostic test destructive?
Yes. This is a highly accelerated stress test designed to force failure modes to appear for engineering and research purposes. It is a tool for characterization, process validation, and quality assurance, not a non-destructive test for releasing finished modules.
What equipment is needed to perform this diagnostic protocol?
Performing this test requires a specialized setup, including a programmable climate chamber, a precision high-voltage power supply, and a Class AAA solar simulator with I-V curve tracing capabilities. This combination enables the repeatable and accurate measurements needed to distinguish between PID and PID-s.
The Path to More Robust Modules
As solar technology pushes the boundaries of efficiency, degradation mechanisms become more complex. Simply measuring power loss is no longer enough. To build truly reliable and durable high-efficiency modules, manufacturers must look deeper and understand the precise mechanisms behind degradation.
The distinction between PID-s and classic PID is a case in point. By adopting a more sophisticated diagnostic approach, you can stop guessing and start making targeted, effective improvements to your cell technology and module design. This knowledge is the foundation for creating next-generation solar products that stand the test of time.
Ready to move from research to reality? Explore how applied research and process optimization can help you validate, troubleshoot, and perfect your solar module technology.