Your new bifacial solar modules look incredible. Gleaming glass on both sides, they’re ready to capture sunlight from the sky and reflected light from the ground. These modules represent the pinnacle of PV technology, promising higher energy yields and superior performance. Yet a hidden threat could be silently undermining their long-term power output, starting from the day they’re installed.
The culprit is a specific degradation that targets the very feature making these modules special: their glass back. It’s called rear-side Potential-Induced Degradation (PID), and your choice of encapsulant—the polymer layer that bonds the cells to the glass—is your single most important line of defense.
Let’s explore why this degradation occurs and how a deep understanding of material science can protect your investment for decades to come.
What is PID, and Why is it a Special Problem for Bifacial?
Potential-Induced Degradation (PID) is a well-known issue in the solar industry. In simple terms, it’s a power loss that occurs when a large voltage difference exists between the solar cells and the module’s frame. This voltage stress can trigger unwanted electrical currents, creating tiny short circuits within the cells that sap their power. This specific failure is often called PID-shunting (PID-s).
While PID is primarily a front-side issue in traditional monofacial modules, the glass-on-glass construction of bifacial modules creates a unique vulnerability on the rear side.
Unlike a traditional module with a polymer backsheet, the rear glass in a bifacial module can be a source of mobile ions—specifically, sodium ions (Na+). Under the high negative voltage common in modern solar arrays, these positively charged sodium ions are drawn from the glass, through the encapsulant, and into the solar cell.
When these ions reach the cell, they disrupt its electrical properties, creating shunts that drain power. The result? A gradual, irreversible decline in the module’s energy output, negating the very bifacial gain you invested in.
Your Encapsulant’s Superpower: Volume Resistivity
This is where the encapsulant becomes the hero of the story. Think of it as a security guard standing between the sodium-rich glass and the sensitive solar cell, with the crucial job of blocking these damaging ions. Its effectiveness is measured by a property called volume resistivity.
Volume resistivity is a material’s inherent ability to resist the flow of electricity. A material with high volume resistivity acts as a powerful electrical insulator, while one with low resistivity allows current to pass more easily.
Imagine two dams:
- A low-resistivity dam is like one made of porous, leaky concrete. Water (or in our case, ions) can easily find paths to seep through.
- A high-resistivity dam is like one made of solid, reinforced, non-porous concrete. It forms an impenetrable barrier, holding the water back indefinitely.
In the world of solar encapsulants, the two most common materials are Ethylene Vinyl Acetate (EVA) and Polyolefin Elastomer (POE). When it comes to volume resistivity, they are worlds apart.
At an operating temperature of 60°C, research shows the volume resistivity of POE is a monumental 100 times higher than that of standard EVA.
The exceptionally high resistivity of POE makes it that strong, non-porous dam, effectively blocking sodium ion migration. Standard EVA, by contrast, offers a much easier path for these ions, leaving the cells far more susceptible to rear-side PID-s.
„When evaluating encapsulants for bifacial modules, looking at volume resistivity is non-negotiable. It’s a direct indicator of the material’s ability to prevent ionic migration from the rear glass. POE’s inherently high resistivity provides a robust, built-in defense against PID-s, which is critical for ensuring the 25-year-plus lifetime of these advanced modules.“ — Patrick Thoma, PV Process Specialist
The Proof is in the Process: Validating Encapsulant Performance
A material’s datasheet is a great starting point, but it doesn’t tell the whole story. An encapsulant’s true performance is revealed only after it’s been subjected to the heat and pressure of the lamination cycle. Process parameters like temperature, pressure, and curing time all influence the material’s final, cross-linked properties.
This is where robust Material Testing & Lamination Trials become essential. Laminating coupons or full-sized modules under real-world manufacturing conditions allows you to verify the encapsulant’s post-processing properties. This validation step is crucial before committing a material to mass production.
Furthermore, building test modules allows you to perform accelerated aging tests (like damp heat and PID testing) to confirm that the encapsulant delivers the long-term protection required. This hands-on approach to Prototyping & Module Development moves beyond theory to provide the empirical data needed to de-risk your investment and ensure your final product is built to last.
Beyond PID: What High Resistivity Means for Long-Term Reliability
The benefits of a high-resistivity encapsulant like POE extend beyond preventing rear-side PID. This superior electrical insulation also contributes to the module’s overall safety and stability. By reducing leakage currents—a critical safety parameter—it helps maintain the electrical integrity of the entire system over its operational lifetime.
Choosing an encapsulant is not just a material decision; it’s a long-term reliability strategy. By prioritizing high volume resistivity, you build a more resilient, durable, and higher-performing bifacial module—one that will deliver on its promise of superior energy yield for decades.
Frequently Asked Questions (FAQ)
Is EVA always a bad choice for bifacial modules?
Not necessarily. „PID-free“ EVA formulations exist that include special additives to improve resistivity and block ionic migration. However, POE’s resistance is an inherent property of its chemical structure, not dependent on additives. This makes POE a fundamentally more stable and often preferred choice for high-voltage bifacial applications where long-term reliability is paramount.
What is volume resistivity measured in?
Volume resistivity is typically measured in Ohm-meters (Ω·m) or Ohm-centimeters (Ω·cm). The higher the number, the better the material is at resisting electrical current.
Can rear-side PID be reversed?
In some cases, a portion of the power loss from PID can be recovered by applying a high reverse voltage to the module—a process called „recovery“ or „healing.“ However, this is not always 100% effective and doesn’t address the root cause. Prevention through proper material selection is always the most effective and economical strategy.
Does the type of rear glass affect PID?
Absolutely. Using low-sodium or sodium-free glass can significantly reduce the source of the migrating ions. However, these glass types can be more expensive. Using a high-resistivity encapsulant like POE provides a robust solution even with standard float glass, making it a powerful and cost-effective preventive measure.
Your Next Step: From Theory to Tangible Results
Understanding the science of rear-side PID is the first step. The next is applying that knowledge to create a truly reliable product. The interaction between cells, encapsulant, and glass is complex, and the lamination process is where these materials are fused into a single, high-performance unit.
Ensuring your materials are well-chosen and your processes are optimized is the key. By moving from theoretical datasheets to hands-on validation, you can dial in the perfect lamination parameters to maximize your encapsulant’s performance and lock in decades of module stability. This is how you build bifacial modules with confidence, knowing they are protected against the hidden threats that compromise long-term performance and ROI.