Bifacial solar modules are one of the industry’s most exciting advancements. The concept is brilliant: capture sunlight from the front and the back to generate more power from the same footprint. For solar farm developers and asset owners, it promises a higher energy yield and a better return on investment.
But what if this key advantage conceals a hidden vulnerability? What if the dual-sided illumination that gives bifacial modules their edge also makes them susceptible to a unique, accelerated degradation that standard tests often miss?
Imagine seeing your state-of-the-art solar plant’s performance decline faster than any model predicted. It’s a frustrating scenario, and it points to a complex problem happening deep inside the solar cells themselves. Understanding the science behind this phenomenon is critical for the future of solar energy.
The Usual Suspect: What is LeTID?
Before we dive into the specifics of bifacial modules, let’s introduce the underlying mechanism: LeTID.
LeTID stands for Light and elevated Temperature Induced Degradation. Think of it as a slow, persistent „sunburn“ for certain types of high-efficiency solar cells, particularly PERC (Passivated Emitter and Rear Cell) technology.
In simple terms:
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The Cause: LeTID is primarily caused by an excess of hydrogen atoms within the silicon of a solar cell. During manufacturing, hydrogen is used to „heal“ tiny defects, which initially boosts the cell’s efficiency.
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The Trigger: When these cells are exposed to both light and high temperatures (typically above 50°C) for hundreds or thousands of hours, that helpful hydrogen can start to move around. It interacts with other elements and defects, creating new problems that trap electrons.
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The Effect: When electrons get trapped, they can’t contribute to the electrical current. The result is a gradual but significant drop in the solar panel’s power output, sometimes as much as 10%.
For years, the industry has worked to understand and mitigate LeTID in traditional, monofacial panels. But the dynamics shift entirely when it comes to bifacial technology.
The Bifacial Difference: A Problem Illuminated from Both Sides
A bifacial PERC module doesn’t just have a front side doing all the work. Its transparent backsheet or dual-glass design allows reflected light (albedo) from the ground to hit the rear of the solar cells and generate extra power.
While this is fantastic for energy production, it also means the entire solar cell is heated and energized by light from two directions. Standard reliability tests for LeTID were designed for monofacial modules, typically illuminating only the front side in a climate chamber.
But our applied research at PVTestLab revealed a critical flaw in that approach. We asked a simple question: What happens if you test a bifacial module the way it actually works in the real world—with light hitting both sides at once?
A New Testing Protocol for a New Reality
To investigate this, we developed a custom testing protocol. Instead of just illuminating the front of the module with 1000 W/m² (the standard), we simultaneously illuminated the rear with a low but realistic level of 100 W/m², all while maintaining an ambient temperature of 75°C to accelerate the effect.
This approach was designed to more accurately mimic the conditions a bifacial module experiences on a hot, sunny day in the field.
The Alarming Results: Accelerated Degradation
The modules exposed to simultaneous front and rear illumination degraded significantly faster than identical modules tested under the same conditions with front-side-only light.
The small amount of light on the rear acted as a powerful catalyst, speeding up the LeTID process. This means that bifacial modules in the field are likely degrading at a rate that conventional testing and energy yield models fail to predict.
The data clearly shows that power loss under bifacial conditions is both faster and deeper. This isn’t just a minor variation; it’s a fundamental change in the module’s degradation behavior.
What This Means for the Solar Industry
This discovery has significant implications for everyone involved in solar technology, from the lab to the field.
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For Module Developers: Creating a durable and reliable bifacial product requires a deep understanding of these accelerated stress factors. Simply using a PERC cell that performs well in monofacial tests is not enough. Validating new designs through comprehensive solar module prototyping is no longer a nice-to-have—it’s essential for ensuring long-term bankability and preventing costly field failures.
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For Material Suppliers: The interaction between cells, encapsulants, and backsheets becomes more critical than ever. How do different materials perform under the increased thermal and electrical stress of bifacial operation? Answering this question requires rigorous lamination and material testing under realistic bifacial conditions to ensure components don’t contribute to early degradation.
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For Solar Investors and Asset Owners: Energy yield predictions for bifacial power plants may be overly optimistic if they don’t account for this accelerated LeTID. Understanding this risk can help inform purchasing decisions, improve financial modeling, and guide operations and maintenance strategies.
The reality is that we cannot test 21st-century technology with 20th-century methods. As module designs evolve, our validation and reliability testing must evolve with them.
Frequently Asked Questions (FAQ)
Q1: What exactly is PERC technology?
PERC (Passivated Emitter and Rear Cell) is a solar cell technology that adds a special dielectric passivation layer to the rear of the cell. This layer reflects light that passes through the cell back into it, giving it a „second chance“ to generate an electron and improving the cell’s overall efficiency compared to standard cells.
Q2: Is LeTID reversible?
Partially, yes. The industry has developed „regeneration“ strategies, such as treating the modules with a controlled period of darkness at a specific temperature (dark annealing) or with a higher current injection. These processes can heal the defects caused by LeTID and recover some of the lost power. Validating that these regeneration strategies are both effective and stable is a key part of module development.
Q3: Does this accelerated LeTID affect all types of solar panels?
This specific phenomenon is most pronounced in modules using PERC cells, which are currently the dominant technology in the market. Other cell technologies, like TOPCon or HJT, have different degradation behaviors that require their own specialized testing.
Q4: How much light does the rear side of a bifacial module receive in the real world?
It varies greatly depending on the surface beneath the panels. A grassy field might reflect 20% of the light (providing 200 W/m² on a sunny day), while white sand or fresh snow can reflect over 70% (700 W/m²). Our test used a conservative 100 W/m², which shows that even low levels of rear-side illumination can have a major impact.
Moving Forward with Clearer Sight
The rise of bifacial technology is a massive leap forward for solar energy. But to harness its full potential, we must be honest about its unique challenges. The accelerated degradation caused by dual-sided illumination is a serious issue, but it’s one we can solve with better data, more realistic testing, and a collaborative, science-driven approach.
Understanding these complex degradation mechanisms is the first step toward building more reliable, efficient, and profitable solar technology for the decades to come. By asking the right questions and demanding testing protocols that reflect reality, the entire industry can innovate with confidence.