You’ve just developed a cutting-edge bifacial solar module, designed for years of superior energy production by capturing sunlight from both sides. But what if a hidden process, deep within the module’s layers, was slowly chipping away at that promise of performance?
This challenge stems from the very materials used to protect the solar cells. The choice of encapsulant—the transparent polymer bonding the glass, cells, and backsheet together—has a profound, often underestimated impact on long-term energy yield, especially for bifacial technology.
Here, we’ll examine one of the most significant degradation factors: UV-induced yellowing. Using real-world data, we’ll compare how two popular encapsulants, POE and UV-Cutoff EVA, hold up under stress and what their performance means for a module’s quantifiable bifacial gain.
The Critical Role of the Solar Module Encapsulant
A solar module is like a high-tech sandwich. The solar cells are the crucial filling, but they need protection from moisture, impact, and the elements. This is the job of the encapsulant. For decades, Ethylene Vinyl Acetate (EVA) has been the industry workhorse because it’s cost-effective and reliable.
However, as technology evolves, so do the demands on materials. Polyolefin Elastomer (POE) has emerged as a premium alternative, recognized for its superior moisture resistance and stability. The key difference we’re exploring lies in how each material reacts to its greatest adversary: relentless ultraviolet (UV) radiation.
From Yellowing to Power Loss: The Bifacial Connection
Over thousands of hours of sun exposure, certain chemical bonds within polymer encapsulants can break down. This process, known as photodegradation, often results in a distinct yellow or brownish tint.
For a traditional monofacial module, some yellowing in the front encapsulant is a problem, as it reduces the amount of light reaching the cells. For a bifacial module, however, the rear encapsulant is just as important.
When the rear encapsulant turns yellow, it acts like a filter, blocking reflected light (albedo) from reaching the back of the solar cells. This process directly sabotages the module’s ability to generate extra energy, eroding the very bifacial gain it was designed to deliver.
But just how significant is this effect? The data provides a clear answer.
Putting Materials to the Test: Accelerated Aging Insights
To simulate decades of sun exposure, materials are subjected to intense, concentrated UV radiation in a controlled environment. This process, called accelerated aging, allows us to see how they will perform over their 25+ year lifetime.
In our analysis, we compared a standard UV-Cutoff EVA with a high-stability POE after 1500 hours of UV stress.
The Yellowness Index Tells a Story
The Yellowness Index (YI) is a standardized metric quantifying the change in a material’s color, where a higher YI means more yellowing.
After 1500 hours, the POE encapsulant shows almost no change in its yellowness index, remaining crystal clear. The UV-Cutoff EVA, in contrast, shows a dramatic increase, reaching a YI of nearly 10 and becoming visibly yellowed.
Seeing is Believing: How Yellowing Blocks Light
This change in color directly impacts the material’s ability to transmit light, a property measured as Spectral Transmission. Data reveals how much light passes through the encapsulant at different wavelengths, before and after aging.
The aged EVA shows a plummeted ability to transmit light—especially in the shorter blue and violet wavelengths. The yellowed material now absorbs a significant portion of incoming light instead of letting it pass through to the cells. The POE, by contrast, remains virtually unchanged.
This kind of detailed encapsulant material testing is fundamental to predicting long-term field performance.
Quantifying the Damage: The Real Cost of Yellowing
A yellowed encapsulant blocks light, but what does this mean for power output and revenue? Let’s calculate the impact on bifacial gain for a typical module with 70% cell bifaciality.
The results are clear:
- With POE: The bifacial gain remains robust at 17.2%, suffering negligible loss due to the material’s stability.
- With UV-Cutoff EVA: The gain plummets to just 12.9%—a relative loss of over 25% of the potential bonus energy.
As PV Process Specialist Patrick Thoma notes, „This isn’t just a lab curiosity; it’s a direct hit to the lifetime energy yield and financial return of a solar project. The initial material choice made during the solar module prototyping phase has consequences that last for decades.“
Why Material Choice is Critical for Solar Innovators
This data highlights a crucial point for material manufacturers, module developers, and research institutions alike: long-term stability is not a feature but a prerequisite for high-performance bifacial technology.
Choosing an encapsulant based on cost or ease of processing without validating its UV stability can lead to a product that underperforms in the field, damaging brand reputation and the financial viability of solar projects. The interaction between materials under heat and pressure is complex, making hands-on lamination process optimization a critical step to ensure lab-proven materials perform as expected in a finished module.
The goal is to create a product that performs well on day one and continues to deliver on its promise for 25 years.
Frequently Asked Questions (FAQ)
What is a solar module encapsulant?
An encapsulant is a transparent polymer layer in solar modules that provides adhesion, electrical insulation, and physical protection for the solar cells. It bonds the glass, cells, and backsheet into a single, durable laminate.
What is the main difference between EVA and POE?
EVA (Ethylene Vinyl Acetate) is a widely used, cost-effective encapsulant. POE (Polyolefin Elastomer) is a more advanced polymer known for its excellent resistance to moisture (low water vapor transmission rate) and higher stability against UV degradation and potential-induced degradation (PID).
What does Yellowness Index (YI) mean?
The Yellowness Index is a number calculated from spectrophotometric data that measures a material’s shift in color from clear or white toward yellow. In the context of solar modules, a low and stable YI is highly desirable for encapsulants.
Why is bifacial gain so important?
Bifacial gain is the „bonus“ energy generated by the rear side of a bifacial solar module. It allows a solar power plant to produce more electricity from the same footprint, lowering the Levelized Cost of Energy (LCOE) and improving the project’s overall financial return.
Are all EVA encapsulants prone to yellowing?
While formulations have improved over the years, many EVA types are susceptible to yellowing due to their chemical structure. „UV-Cutoff“ versions include additives that block UV light to protect the cells and other layers, but the encapsulant itself can still degrade over time, as the data shows.
Charting a Path to Long-Term Reliability
Understanding material degradation is the first step toward building more reliable solar technology. The data confirms that not all encapsulants are created equal, and the choice directly impacts the long-term value of bifacial modules.
As developers create the next generation of solar technology, they must remember that the materials chosen today determine the performance customers will experience for the next quarter-century. Validating these choices through rigorous, application-focused testing isn’t just good science—it’s good business.