You’ve invested in bifacial solar module technology for one compelling reason: more energy.
The promise of capturing sunlight from both the front and the back—the „bifacial gain“—is a powerful advantage for achieving higher efficiency and a lower levelized cost of energy (LCOE).
But what if that bonus gain was silently being eroded by a hidden threat that standard quality tests completely miss?
A unique form of degradation has emerged in many bifacial modules, particularly those using advanced cell concepts like HJT. It’s called Potential Induced Degradation on the rear side, or PID-rear, and it can undermine the very performance advantages that make bifacial technology so attractive.
First, A Quick Refresher: What is PID?
Imagine Potential Induced Degradation (PID) as a slow, invisible power leak in your solar modules. This degradation occurs when a large voltage difference exists between the solar cells and the grounded module frame, especially in hot and humid conditions. This electrical stress can cause ion migration—tiny charged particles moving where they shouldn’t—which reduces the cell’s ability to generate power.
For years, the industry has focused on front-side PID, and standard certification tests like IEC 62804 were designed to catch it. But bifacial modules changed the game.
The Bifacial Blind Spot: Why Standard PID Tests Fall Short
A standard PID test applies a voltage potential to the active front side of the cells to see if they degrade. But what happens when the back of the module also has an electrically active, conductive surface?
This is precisely the case for many high-efficiency bifacial designs, such as heterojunction (HJT) cells, which often use a Transparent Conductive Oxide (TCO) layer on the rear side. This TCO layer creates a new, unintended pathway for electrical stress.
In a typical solar array, the module frame is grounded (0V), while cells at the end of a string can have a negative potential of up to -1000V or even -1500V. This creates a massive voltage difference between the rear-side TCO layer and the grounded frame, triggering a degradation mechanism that standard tests were never designed to detect: PID-rear.
How PID-rear Works: A Look Inside the Module
The mechanism for PID-rear is a perfect storm of materials and electrical forces:
- Negative Voltage Bias: The cells on the rear of the module operate at a high negative voltage relative to the grounded frame.
- Conductive Pathway: The TCO layer provides a surface for this electrical potential to accumulate.
- Environmental Stress: High heat and humidity, common in many climates, accelerate the movement of damaging ions (like sodium from the glass) through the encapsulant and into the solar cell.
The result is a gradual and permanent loss of power originating from the rear side, directly impacting your bifacial gain and the overall bankability of your solar asset.
Uncovering the Threat: A Test Protocol for PID-rear
Because standard PID tests don’t apply voltage to the rear of the module, they simply cannot detect this problem. Properly validating a bifacial module design requires a specialized test setup—one that simulates the real-world conditions that trigger PID-rear.
At PVTestLab, we have modified our climate chambers specifically for this challenge. Here’s how the process works:
- Simulating Reality: The module is placed in the chamber with the front side facing a grounded, conductive plate.
- Applying Rear-Side Stress: A negative voltage (typically -1000V) is applied directly to the rear glass surface of the module.
- Harsh Conditions: The module is then held under accelerated stress conditions—85°C and 85% relative humidity—for 96 to 192 hours.
This setup accurately mimics the electrical potential difference between the rear-side cells and the grounded frame, providing a reliable way to measure a material’s resistance to PID-rear. This kind of in-depth analysis is a crucial part of material testing, safeguarding the long-term performance of solar modules.
From Data to Decision: Choosing PID-Resistant Materials
The goal of this specialized test isn’t just to find a problem; it’s to solve it. PID-rear is highly dependent on the choice of materials, specifically the encapsulant used to laminate the cells and the type of glass.
By running different combinations of encapsulants (like EVA or POE) and glass through the PID-rear protocol, we can generate clear, comparative data on their performance. This data empowers manufacturers to make informed decisions long before a module ever reaches the field.
The chart below shows the results of a PID-rear test on modules built with different material combinations. As you can see, one set of materials exhibits virtually no power loss, while the other shows significant degradation. This is the kind of critical insight that can prevent a costly, fleet-wide failure down the line.
This validation is essential during the prototyping stage, ensuring that new designs are not only innovative but also durable enough to last for decades.
Frequently Asked Questions (FAQ)
What exactly is Potential Induced Degradation (PID)?
PID is a loss of power in a solar module caused by a high voltage difference between the solar cells and the module frame, particularly in hot, humid weather. This voltage can cause electrical currents to leak from the cell, reducing its efficiency over time.
Why is PID a bigger concern for bifacial modules?
Many bifacial modules, especially HJT designs, have conductive layers on both sides of the cells. The rear conductive layer creates a new pathway for PID to occur from the back of the module (PID-rear), a risk that is absent in most monofacial modules.
What is the difference between PID-s and PID-p?
PID-s (shunting) is the most common type, where the electrical properties of the cell itself are damaged, often permanently. PID-p (polarization) is related to surface charges and can sometimes be temporary, reversing at night or under certain conditions. PID-rear is typically a form of PID-s.
Can PID damage be reversed?
In some cases, mild PID-p can be reversed by applying a positive voltage to the system at night. However, PID-s involves physical degradation of the cell and is generally considered irreversible. This is the type most often associated with PID-rear, so prevention through proper material selection is the best strategy.
How do I know if my materials are PID-resistant?
The only way to know for sure is through rigorous, accelerated testing that simulates real-world electrical and climate stress. For bifacial modules, this must include a specialized PID-rear test protocol to validate the glass and encapsulant combination.
Building for the Future Means Testing for It Today
Bifacial technology is a significant leap forward for the solar industry, but with this innovation comes the need for more advanced validation. PID-rear is a serious risk to the long-term performance and financial viability of bifacial projects, but it is entirely preventable.
By understanding the mechanism and employing the correct test protocols, manufacturers and developers can select the right materials to build robust, reliable, and truly bankable bifacial modules that deliver on their promise of more energy for decades to come.
If you are developing a new bifacial module and want to ensure its long-term performance, a process specialist can help you create a validation plan tailored to your technology.