Imagine a brand-new solar park, its glass-glass modules gleaming under the sun, producing clean energy at peak efficiency. Now, fast forward ten years. From a distance, everything might still look fine. But up close, a subtle, yellowish tint has crept across the modules, like a slow-moving stain. This isn’t just a cosmetic issue; it’s a silent performance killer, eroding the asset owner’s revenue every single day.
This phenomenon, known as encapsulant yellowing, is one of the most critical degradation factors affecting the long-term profitability of solar projects, especially for modern bifacial modules. The choice of material used to bind the solar cell sandwich together can mean the difference between a 30-year reliable power plant and one that underperforms after its first decade.
But how can you determine which material will stand the test of time? At PVTestLab, we believe in data over assumptions. We recently conducted a head-to-head UV stress test to find the answer, and the results provide a crucial benchmark for anyone involved in designing, manufacturing, or investing in bifacial solar technology.
What is Encapsulant Yellowing and Why Does It Target Bifacial Modules?
Every solar module is a multi-layered sandwich. At its heart are the solar cells, protected on both sides by a polymer layer called an encapsulant. This material holds everything together and shields the delicate cells from moisture, mechanical stress, and the elements.
The primary enemy of these encapsulants is ultraviolet (UV) radiation from the sun. Over thousands of hours of exposure, UV rays can break down the polymer chains in some materials, causing them to discolor and turn yellow.
This is a problem for any module, but bifacial modules are uniquely vulnerable. A traditional monofacial module has an opaque backsheet, completely shielding the rear encapsulant from UV light. A bifacial module, however, uses a transparent backsheet or a second pane of glass. This means both the front and rear encapsulant layers are exposed to UV radiation: the front to direct sunlight, and the rear to light reflected off the ground (albedo). This double exposure dramatically accelerates the aging process.
To quantify this degradation, we use a metric called the Yellowness Index (YI). It’s a standardized number that measures the degree of yellowing in a material. A low, stable YI is the goal; a rising YI is a red flag for future performance loss.
Putting Encapsulants to the Test: The PVTestLab UV Stress Protocol
To simulate decades of sun exposure in a controlled environment, we designed a rigorous UV aging test. Our goal was to compare two of the most common encapsulants used in bifacial manufacturing:
- POE (Polyolefin Elastomer): A material known for its inherent resistance to UV degradation.
- UV-Cutoff EVA (Ethylene Vinyl Acetate): A common encapsulant modified with additives designed to block UV light and prevent yellowing.
We prepared several glass-encapsulant-glass laminate samples that mirror the construction of a real glass-glass bifacial module. These samples were then placed in a climate chamber and subjected to 150 kWh/m² of intense UV-A radiation at a constant temperature of 60°C.
Before and after this accelerated aging process, we measured two key performance indicators: the change in Yellowness Index (ΔYI) and the loss in optical transmission.
The Results Are In: A Clear Winner for UV Stability
After the grueling test cycle, the data painted a clear picture. The two materials, which looked identical at the start, had followed dramatically different aging pathways.
Finding #1: POE Demonstrates Exceptional Color Stability
When we analyzed the Yellowness Index, the difference was stark:
- POE: Showed virtually no change, with an average increase in YI (ΔYI) of just 0.2. The material remained crystal clear, proving its intrinsic UV stability.
- UV-Cutoff EVA: Experienced significant discoloration. The average YI increase (ΔYI) was 11.5—nearly 60 times that of POE. The additives designed to protect the material had clearly degraded, failing to prevent severe yellowing.
Finding #2: Yellowing Directly Leads to Power Loss
A change in color isn’t just about aesthetics; it directly impacts how much light reaches the solar cell. A yellowed encapsulant acts as a filter, blocking the high-energy blue and violet portions of the light spectrum—the very wavelengths solar cells are most efficient at converting into electricity.
Our optical transmission measurements confirmed this:
- POE: Maintained its transparency, with a negligible transmission loss of less than 0.1%.
- UV-Cutoff EVA: Suffered a significant drop in transparency, losing 1.5% of transmission specifically in the critical 400-500 nm wavelength range (blue light).
A 1.5% loss may not sound like much, but over the 20- to 30-year lifetime of a multi-megawatt solar farm, it represents a substantial loss in energy generation and revenue.
Beyond the Numbers: What This Means for Your Solar Project
This test highlights a critical vulnerability in bifacial module design. Using an encapsulant that relies on additives for UV protection, like UV-Cutoff EVA, introduces a significant long-term risk. Once those additives break down, degradation can accelerate rapidly, compromising the bifacial gain and the overall energy yield of the module.
Expert Commentary:
„For glass-glass bifacial modules, where the rear-side encapsulant is fully exposed to direct and reflected UV, POE’s inherent stability isn’t just a small advantage—it’s a fundamental requirement for achieving a 30-year design life. Our tests confirm that relying on UV-cutoff additives in EVA for bifacial applications is a high-risk strategy that can lead to premature power loss.“
— Patrick Thoma, PV Process Specialist, PVTestLab
Choosing an intrinsically stable material like POE provides a far more reliable foundation for long-term performance. It ensures that the module continues to capture maximum light on both its front and rear sides, preserving the bifacial advantage and protecting the project’s return on investment for decades to come.
Frequently Asked Questions (FAQ)
Is POE always better than EVA?
For bifacial glass-glass applications where both sides are UV-exposed, our data shows POE is significantly more stable. For traditional monofacial modules with opaque backsheets, high-quality EVA is often sufficient because the rear encapsulant is shielded from UV light.
Why is UV-Cutoff EVA used if it yellows?
UV-Cutoff EVA is an evolution of standard EVA, developed to improve performance. While it’s an improvement, its protection relies on additives that can degrade over time. It’s often chosen due to its lower cost and familiarity in the industry, but this can be a false economy for long-life bifacial modules.
How does this yellowing affect the module’s warranty?
Most module warranties cover catastrophic failures and power output dropping below a certain threshold (e.g., 85% of nominal power after 25 years). Slow, steady degradation from yellowing might not trigger a warranty claim for many years, but it will still systematically reduce the energy yield and financial returns of the project long before that point.
How does PVTestLab ensure these tests are accurate?
Our facility operates as a full-scale R&D production line in a 100% climate-controlled environment. We use industrial-grade equipment and standardized measurement protocols, overseen by experienced German process engineers from J.v.G. Technology. This ensures our results are not just academically sound but also industrially relevant and reproducible.
From Lab Insights to Real-World Performance
Choosing the right encapsulant is a decision with 30-year consequences. While spreadsheets might favor the cheaper initial cost of one material, real-world physics and long-term UV exposure often tell a different story. As our data shows, for a demanding application like the glass-glass bifacial module, investing in an intrinsically stable material like POE is one of the surest ways to safeguard long-term energy production.
Understanding the nuances of different materials is the first step. For a deeper dive into how these materials behave during the critical lamination phase, explore our guide on the solar module lamination process.
Validating these material choices requires a robust testing environment. Learn more about how we build and test next-generation modules in our solar module prototyping services.
Ultimately, the goal is to ensure long-term reliability. See how we verify module durability against all kinds of environmental stress in our overview of PV module reliability testing.