The 25% Promise vs. Reality: Is This Hidden Flaw Sabotaging Your Bifacial Module Gains?
You’ve embraced bifacial solar module technology for a reason: the promise of more energy. By capturing sunlight from both sides, these modules can boost energy yield by 10-25%—a significant leap in efficiency. But what if the very feature that gives them this advantage, their transparent rear side, is also a hidden vulnerability?
There’s a specific, often overlooked failure mode lurking in glass-glass bifacial modules: rear-side Potential Induced Degradation, or PID. It’s a silent thief of performance that standard testing protocols can completely miss, potentially turning your high-efficiency investment into an underperforming asset.
Let’s explore what’s happening at a microscopic level and, more importantly, how to ensure your modules are truly built to last.
What is Potential Induced Degradation (PID)? A Quick Refresher
Before diving into the problem specific to bifacial modules, let’s cover the basics. Think of Potential Induced Degradation (PID) as a slow, steady electrical leak. In a large solar array, a high voltage potential exists between the solar cells and the grounded module frame.
Under certain conditions, like high heat and humidity, this voltage difference can create a pathway for leakage currents to flow from the cell to the frame. This current flow triggers electrochemical reactions that degrade the solar cell’s performance, causing a gradual yet significant loss of power. The ions within the module materials—particularly sodium ions (Na+) from the glass—are the primary culprits in this degradation process.
A simplified view of PID: High voltage potential creates leakage currents that shuttle performance-degrading ions to the solar cell surface.
For traditional monofacial modules with conductive backsheets, the industry has largely solved this „classic PID“ by using PID-resistant materials. But the move to glass-glass bifacial designs changed the rules of the game.
The Bifacial Twist: Why the Rear Side Changes Everything
In a glass-glass bifacial module, the conductive polymer backsheet is replaced with a second sheet of glass. While this improves durability, it fundamentally alters the electrical field within the module and creates a new, more insidious pathway for PID.
Here’s the „aha moment“: when the rear side of a bifacial module is illuminated, it generates its own voltage.
This rear-side voltage is positive relative to the module’s surface. In a typical system with a negative-grounded inverter, the cells are already at a high negative potential, so the positive voltage from rear-side illumination effectively reduces the voltage difference needed to trigger PID.
It’s like giving the degradation a head start. Conditions that might be safe for a monofacial module can become damaging for a bifacial one, initiating PID from the rear side—a place no one was previously looking.
This is a critical blind spot because standard PID tests defined by IEC 62804 only stress the front side of the module. These tests aren’t designed to detect this unique bifacial failure mechanism, leaving a massive gap in quality assurance.
Seeing is Believing: How Rear-Side PID Manifests
The damage from rear-side PID isn’t just theoretical; it’s visible and measurable. The most common sign is a significant drop in the module’s maximum power output (Pmax).
Electroluminescence (EL) imaging makes the damage starkly clear. EL tests are like an X-ray for solar modules, revealing inactive or underperforming areas of a cell. A healthy bifacial module shows uniformly bright cells under an EL test. After being subjected to a rear-side PID test, however, that same module will reveal darkened cells or entire sections that no longer generate power.
Image Description: Electroluminescence (EL) images of a bifacial module, one before and one after a rear-side PID test, showing darkened, inactive cell areas. On the left, a healthy module before testing. On the right, the same module after a rear-side PID test shows severe degradation in multiple cells, rendering them inactive.
The Solution is in the Encapsulant: EVA vs. POE
The key to preventing rear-side PID lies in the encapsulant—the polymer layer that surrounds the solar cells and bonds them to the glass. The choice of this material is the single most important factor in a module’s defense against PID.
Traditionally, many manufacturers use EVA (Ethylene Vinyl Acetate). While cost-effective, standard EVA has two major weaknesses:
- Lower Volume Resistivity: It provides an easier path for leakage currents to flow.
- Chemical Byproducts: The curing process for EVA can produce acetic acid, which accelerates ion migration and corrosion over time.
A superior alternative for bifacial modules is POE (Polyolefin Elastomer). POE offers significantly higher volume resistivity, making it a much better electrical insulator. Think of it as building a dam instead of a leaky fence. It’s also inherently more resistant to water vapor and doesn’t produce corrosive byproducts, effectively blocking the pathways for sodium ion migration.
Choosing the right material is critical, which is why rigorous Encapsulant Material Testing is no longer a „nice-to-have“—it’s essential for long-term reliability.
The data speaks for itself. In controlled tests simulating rear-side PID, modules using EVA encapsulants can lose 20% or more of their power, while those with POE often exhibit negligible degradation under the same conditions.
Image Description: A graph plotting power loss (%) over time (hours) for modules with EVA and POE encapsulants during a rear-side PID test. Test results comparing power loss in bifacial modules. The module with POE encapsulant remains stable, while the EVA-based module suffers catastrophic power degradation.
How We Test for the Unseen: Replicating Rear-Side PID
You can’t fix a problem you can’t reliably measure. Properly validating a bifacial module’s resistance to this threat requires a specialized testing protocol.
At PVTestLab, we replicate the worst-case field conditions in a controlled environment. The process involves:
- Placing the module in a climate chamber set to 85°C and 85% relative humidity to accelerate aging.
- Applying a high negative voltage bias (e.g., -1000V or -1500V) to the interconnected cells, simulating the electrical stress in a real-world system.
- Simultaneously illuminating the rear side of the module with a controlled light source (e.g., 200 W/m²).
This combination of heat, humidity, voltage stress, and rear-side irradiance is the only way to accurately trigger and analyze this specific failure mode.
„Standard tests give a false sense of security,“ notes Patrick Thoma, PV Process Specialist at PVTestLab. „You have to recreate the specific electrical and environmental cocktail that causes rear-side PID. Only then can you confidently validate your material choices and module design before going to mass production.“
This process isn’t just about finding failures; it’s a core part of Solar Module Prototyping Services that prevent them from reaching the field in the first place.
Frequently Asked Questions (FAQ)
Why don’t standard PID tests (IEC 62804) catch rear-side PID?
Standard tests were designed for monofacial modules and only apply stress to the front side without rear-side illumination. Consequently, they completely miss the unique conditions that trigger PID in bifacial designs.
Does this mean all EVA encapsulants are bad for bifacial modules?
Not necessarily. There are newer, „PID-free“ EVA formulations with higher resistivity. However, POE is inherently more resistant due to its fundamental polymer chemistry, offering a greater safety margin, especially for high-voltage (1500V) systems. The only way to know for sure is to test the specific combination of materials in your module design.
Can rear-side PID actually happen in the field?
Yes. The conditions for rear-side PID—high temperature, high humidity, and rear-side illumination (especially from highly reflective surfaces like sand, snow, or white roofs)—are common in many parts of the world. Lab testing simply accelerates what could happen over several years in the field.
How much power loss can rear-side PID cause?
The degradation can be severe. In accelerated tests, power loss can exceed 30% in just 96-192 hours. In the field, this would translate to a significant underperformance of the entire solar plant over its lifetime.
Your Next Step to Building More Reliable Modules
The promise of bifacial technology is real, but so are its hidden risks. The extra energy gain is only valuable if it’s sustained over the 25+ year lifespan of the module. Ensuring long-term performance starts with understanding the unique failure modes of your design and selecting materials, like POE, that are proven to resist them.
Don’t let a hidden flaw undermine your innovation. By testing for the specific challenges of bifacial technology, you can build modules that not only promise higher yields but reliably deliver them.
If you’re developing a new module design and want to ensure its long-term reliability, our team is here to help. Contact PVTestLab to discuss your project with an engineer and validate your design with real, industrial-scale data.