Imagine investing in high-performance bifacial solar modules, banking on that extra 15-20% energy boost from the rear side. For years, everything looks great. But slowly, almost invisibly, the power output begins to drop, eating into your return on investment. The culprit isn’t shading or soiling; it’s a hidden degradation mechanism attacking your modules from the one place few thought to look—the rear.
Bifacial technology has revolutionized solar energy, but it has also introduced new challenges. The transparent backsheets that make bifacial gain possible can, under certain conditions, create a new pathway for an old enemy: Potential-Induced Degradation (PID). And standard industry tests may be completely blind to it.
First, What Exactly is PID?
Think of Potential-Induced Degradation (PID) as a slow, silent leak in your module’s performance. It’s an electrical phenomenon that occurs within high-voltage solar systems. Over time, a voltage difference between the solar cells and the module frame can cause ions to migrate, creating leakage currents that „short-circuit“ parts of the cell. This permanently reduces the cell’s ability to generate power.
For years, the industry has focused on PID occurring on the front of modules, where the cells face the glass and the grounded aluminum frame. Standard PID tests (like IEC 62804) are designed to detect this specific vulnerability. But what happens when the rear side is no longer just a passive shield but an active, power-generating surface?
The Bifacial Twist: A New Battlefield for an Old Enemy
The ingenuity of a bifacial module lies in its ability to capture light from both sides. To achieve this, traditional opaque backsheets are replaced with highly engineered transparent backsheets. These materials must be optically clear, UV-stable, and provide decades of protection from the elements.
(Image: Diagram of a bifacial module with labels for front glass, cells, encapsulant, and transparent backsheet)
This design change creates a new electrical environment. The rear-side cells are now much closer to the external environment, separated only by the encapsulant and the transparent backsheet. This raises a critical question: If the rear surface gets wet from rain or morning dew, can it trigger the same PID mechanism that degrades the front side?
The answer, alarmingly, is yes. Research, including our own lab findings, confirms that certain transparent backsheet materials can be susceptible to PID on the rear side, especially under the damp-heat conditions common in many climates.
Why Standard PID Tests Are Missing the Real Risk
A standard PID test typically involves placing the module face-down on a conductive plate or foil and applying a high negative voltage to the cells. This simulates the stress on the front glass surface.
This method is perfectly fine for traditional modules, but it’s dangerously inadequate for bifacial ones. It completely ignores the electrical properties of the transparent backsheet and what happens when it’s exposed to moisture and high voltage in the real world.
The various polymers used in transparent backsheets have dramatically different surface resistance properties. While they are all good insulators when dry, some can become significantly more conductive when wet. This moisture creates an unintended electrical pathway, allowing leakage currents to flow and initiating PID from the rear. This is a critical factor to consider during the prototyping and module development phase, as a poor material choice can undermine an otherwise excellent design.
Designing a Test for Reality: Seeing What Others Can’t
To uncover this hidden threat, we had to go beyond the standards. At PVTestLab, we developed a custom test setup specifically designed to simulate real-world PID stress on the rear side of bifacial modules.
Instead of the standard test, our method involves applying voltage stress directly to the backsheet under controlled damp-heat conditions (85°C and 85% relative humidity). A conductive, wet fleece is placed on the rear surface to simulate the presence of moisture, mimicking the effect of rainfall or dew in the field.
(Image: Photo or diagram of a custom rear-side PID test setup, showing the module, the conductive layer on the back, and connections)
„We realized that to guarantee long-term bifacial gain, we had to replicate the worst-case scenario for the rear side,“ explains Patrick Thoma, PV Process Specialist at PVTestLab. „It’s not just about what happens in a perfect lab; it’s about what happens in a humid field after 15 years. You can’t find that answer with a standard test.“
This approach allows us to see how different backsheet and encapsulant combinations truly perform. The goal of these advanced material testing and lamination trials is to identify these vulnerabilities before they reach mass production, saving manufacturers from costly warranty claims and reputational damage.
What We’ve Learned: Not All Backsheets Are Created Equal
The results from our specialized testing have been eye-opening. We’ve found that some transparent backsheet materials show severe degradation after just 96 hours, while others remain completely stable.
(Image: A graph comparing EL images of a module before and after rear-side PID testing, showing the degradation)
Our key findings include:
- Material Matters Most: The specific polymer composition of the transparent backsheet is the single most important factor in determining its resistance to rear-side PID.
- The System is Key: The interaction between the backsheet and the encapsulant (like POE or EVA) also plays a crucial role. A good combination can enhance durability, while a poor one can accelerate degradation.
- Early Detection is Crucial: Identifying a susceptible material combination during the R&D stage is infinitely cheaper and easier than dealing with widespread field failures.
How to Protect Your Bifacial Investment: Actionable Takeaways
Whether you’re developing a new module or sourcing one for a project, understanding this risk is paramount. Here’s what you can do:
- Question the Datasheet: When a supplier provides PID test results, ask specifically if they have performed tests on the rear side under damp-heat conditions. Don’t assume a standard „PID-free“ certification covers this risk.
- Test the Complete Package: Don’t evaluate materials in isolation. The entire bill of materials—cells, encapsulant, and backsheet—must be tested as a complete system to reveal potential negative interactions.
- Prioritize Applied Testing: Simulations and material specs are a starting point, but they are no substitute for physical testing under real-world stress conditions. Investing in comprehensive quality and reliability testing early on is the best insurance against long-term performance loss.
Your Bifacial PID Questions Answered
What exactly is „bifacial gain“?
Bifacial gain is the extra energy produced by the rear side of the module from capturing reflected and scattered light from the ground (albedo). This can increase a module’s total energy output by 5-25% or more, depending on the installation site.
Is rear-side PID a problem for glass-glass bifacial modules too?
Glass-glass modules are generally considered highly resistant to PID because glass is an excellent insulator and moisture barrier. The risk discussed here is specific to bifacial modules using a polymeric transparent backsheet, which has electrical and physical properties very different from glass.
How long does a proper rear-side PID test take?
A typical test cycle runs for at least 96 hours, with some extending to 192 hours or more to assess long-term stability. The module’s performance (via IV curve and EL imaging) is measured before and after the test to quantify any degradation.
Can PID be reversed?
In some cases, mild PID can be partially reversed by applying a high positive voltage to the module at night, but this requires specialized equipment and is not always effective. Prevention through robust material selection and design validation is by far the better strategy.
Why is humidity so important in these tests?
Humidity is the catalyst. While the high voltage creates the potential for degradation, it’s the moisture on the backsheet surface that lowers its electrical resistance and creates the conductive path for leakage currents to flow. Testing without humidity doesn’t replicate the primary real-world failure mechanism.
The Future is Bright—and Thoroughly Tested
Bifacial technology holds immense promise for driving down the cost of solar energy and increasing efficiency. But realizing that promise depends on our ability to anticipate and mitigate new failure modes.
The hidden risk of rear-side PID is a perfect example of why bridging the gap between laboratory research and industrial reality is so important. By understanding the unique stresses that new designs will face in the field, we can build the next generation of solar modules to be not only more powerful but also more durable and reliable for decades to come.
If you’re exploring new module designs or materials, understanding these hidden risks is the first step toward building a truly bankable product.