Bifacial solar modules are one of the most exciting advancements in photovoltaics, promising an energy gain of up to 30% by capturing sunlight from both the front and rear. It feels like a free lunch—more power from the same footprint. But what if that extra, reflected sunlight has a dark side?
For decades, we designed monofacial modules with backs completely protected from the sun. The materials used—like traditional EVA encapsulants and opaque backsheets—were never meant to see significant UV light. Now, with bifacial technology, we’ve flipped the module over, exposing its rear-side materials to a whole new environmental stressor: UV radiation reflected from the ground.
This reflected light, while boosting energy yield, introduces a critical failure mode that can silently degrade your module’s performance and longevity.
The Unseen Challenge: Reflected UV and Material Integrity
A monofacial module’s rear is a safe haven. In a bifacial module, however, that same rear side is bombarded with UV light, especially when installed over a highly reflective surface like white gravel or a cool roof.
This fundamental shift puts the encapsulants (like POE and EVA) and transparent backsheets used to protect the solar cells directly in the line of fire. These polymer-based materials—essential for insulation, structural integrity, and optical performance—were not all created equal. While newer materials like POE (Polyolefin Elastomer) were developed with better UV resistance in mind, their long-term stability under real-world bifacial conditions is still a critical area of research.
Understanding how your chosen materials will behave over a 25-year lifespan is no longer an academic exercise; it’s essential for bankability.
How Sunlight Breaks Down Your Module from the Inside Out
To understand the risk, consider what UV light actually does. The UV spectrum, particularly wavelengths between 280–400 nm, carries enough energy to act like microscopic scissors, cutting the long polymer chains that give encapsulants and backsheets their strength and clarity.
This process, known as photodegradation, initiates a cascade of problems:
- Yellowing: As polymer chains break, they can form new chemical groups called chromophores that absorb visible light, giving the material a yellow or brown tint. This directly reduces the amount of light reaching the cells and lowers power output.
- Embrittlement: The material loses its flexibility, becoming brittle and prone to cracking under mechanical stress from wind, snow, or even thermal expansion and contraction.
- Delamination: As the encapsulant degrades, its bond to the solar cells and glass or backsheet can weaken. This allows moisture to penetrate the module, leading to corrosion and rapid failure.
The core issue is that a material perfectly suitable for a monofacial module can become a liability in a bifacial design. This makes rigorous, application-specific testing non-negotiable. Validating your components is a crucial step in any successful solar module prototyping project.
Putting Materials to the Test: Measuring What Matters
You can’t manage what you don’t measure. In our lab, we quantify degradation with standardized, data-driven methods that reveal a material’s true stability.
The Yellowing Index (YI)
One of the most critical metrics is the Yellowing Index (YI), a number that quantifies the change in a material’s transparency toward the yellow end of the spectrum. Using a spectrophotometer, we measure the YI of a material sample before and after UV exposure. In the industry, an increase in the YI value of more than 3 is generally considered a significant—and often unacceptable—level of degradation.
This simple number can tell you a lot about the long-term optical stability of your encapsulant or backsheet and its potential impact on energy yield.
Beyond Optical Properties: Mechanical Strength
A material that looks clear but has become brittle is a ticking time bomb. That’s why we also test for changes in mechanical integrity. Key measurements include:
- Tensile Strength: How much force a material can withstand before snapping.
- Elongation at Break: How much a material can stretch before it breaks.
A significant loss in these properties indicates that the polymer is breaking down, even if severe yellowing isn’t yet visible. This embrittlement can lead to microcracks in cells when the module flexes. Evaluating both optical and mechanical properties is fundamental to our material testing and lamination trials.
Simulating 25 Years in a Matter of Weeks
Waiting a quarter-century to see if a new material holds up isn’t practical. This is where accelerated UV exposure testing comes in.
At PVTestLab, we use a controlled laboratory environment to simulate decades of sun exposure in a concentrated timeframe. Material coupons or even full-sized mini-modules are placed in a chamber with high-intensity UV lamps that replicate the solar spectrum.
By subjecting materials to a high-dose, continuous stream of UV radiation—often combined with heat and humidity cycles—we can trigger the same degradation mechanisms that occur over years in the field. This accelerated feedback loop allows material scientists and module designers to:
- Compare the relative stability of different encapsulants or backsheets.
- Identify failure thresholds and predict long-term performance.
- Validate new material formulations before committing to mass production.
This ability to quickly gather reliable performance data bridges the gap between a promising idea on a datasheet and a durable, bankable solar module.
FAQ: Your Questions on Bifacial Material Durability Answered
What’s the main difference between EVA and POE for bifacial modules?
Ethylene Vinyl Acetate (EVA) has been the workhorse encapsulant for decades but is more susceptible to UV degradation (yellowing) and Potential Induced Degradation (PID), especially in high-humidity environments. Polyolefin Elastomer (POE) generally offers superior UV stability, higher volume resistivity, and lower water vapor transmission rates, making it a preferred choice for bifacial and n-type cell designs. However, POE can be more challenging to process during lamination and requires fine-tuned parameters.
What exactly is a transparent backsheet?
A transparent backsheet is a multi-layer polymer composite designed to replace the rear glass in a glass-backsheet bifacial module. It offers advantages like lower weight, reduced cost, and easier handling. However, because it’s a polymer, its long-term resistance to UV degradation, moisture ingress, and mechanical stress is a critical factor that requires thorough validation.
How can you be sure accelerated test results reflect real-world performance?
While no lab test can perfectly replicate 25 years in the field, a well-designed accelerated testing protocol is the best predictor we have. The key is to create a holistic stress profile. By combining high-intensity UV exposure with damp-heat cycles and thermal cycling, we simulate the interplay of real-world stressors. This integrated approach forms the basis of our process optimization services and provides a highly reliable indication of a material’s long-term durability.
Why not just trust the supplier’s datasheet?
Datasheets provide excellent baseline information, but they typically reflect a material’s properties in isolation under ideal laboratory conditions. They don’t tell you how the material will behave after going through an industrial lamination process, interact with other module components, or perform under the combined stresses of UV, heat, and humidity. Testing the material within a complete module assembly is the only way to validate its true performance in your specific design.
From Awareness to Action
The move to bifacial technology is a leap forward for the solar industry, but it requires us to think differently about material science and module design. The hidden threat of rear-side UV degradation is real, but it’s manageable with the right knowledge and testing protocols.
By understanding the mechanisms of failure and proactively validating your encapsulants and backsheets, you can unlock the full potential of bifacial gain without sacrificing long-term reliability. The first step is awareness; the next is ensuring your materials are truly up to the challenge.