Imagine a high-performance sports car with tires that degrade in the sun. No matter how powerful the engine, its ability to transfer that power to the road is fundamentally compromised. In the world of high-efficiency solar, bifacial modules with transparent backsheets can face a similar, often overlooked, challenge.
Bifacial modules are brilliant in their design, capturing sunlight from both the front and back to boost energy production—a benefit known as „bifacial gain.“ While traditional glass-glass designs are robust, a new generation using lightweight transparent backsheets is gaining traction for its lower weight and potential cost savings.
But there’s a catch. The very transparency that enables bifacial gain also exposes the module’s internal components to a new enemy: UV radiation reflected from the ground (albedo). This rear-side UV exposure can trigger degradation mechanisms that standard testing protocols—designed for front-side exposure—completely miss.
This mismatch is a classic case of new technology outpacing old standards. The result is a hidden risk: yellowing, material breakdown, and a slow, silent drain on your module’s power output that you might not notice until years after installation.
Why Standard Tests Are Looking the Wrong Way
For decades, solar module certification has relied on tests like IEC 61215. These are the gold standard for ensuring a module can withstand years of sun, wind, and weather. However, they were designed for traditional monofacial modules, where the back is opaque and protected.
By focusing almost exclusively on front-side UV exposure, these tests create a critical blind spot for bifacial modules.
The light hitting the back of a bifacial module is different. It’s reflected off surfaces like sand, snow, or white gravel, which are often rich in UV wavelengths. This reflected light directly attacks the transparent backsheet and the encapsulants protecting the solar cells. Without a test that specifically targets this rear-side stress, manufacturers and investors are flying blind, unable to predict how these materials will truly perform over a 25-year lifespan.
This isn’t a theoretical problem—it’s a measurable risk to the long-term energy yield and financial viability of a solar project.
Putting Rear-Side UV Degradation to the Test
So, how can we see this hidden degradation in action? Bridging this knowledge gap requires a targeted experiment that simulates the harsh reality of rear-side UV exposure under controlled, industrial conditions.
The goal is to replicate the effect of years of reflected UV radiation in a compressed timeframe. A recent investigation at PVTestLab did exactly that, comparing the performance of standard glass-glass (G/G) bifacial modules against two different types of transparent backsheet (TBS) modules.
The Test Protocol: A New Standard for Bifacial Reliability
To mimic real-world aging, the modules were subjected to a rigorous test protocol:
- Baseline Measurement: Each module was first characterized using a AAA flasher and an Electroluminescence (EL) tester to get a precise baseline of its power output (Pmax) and internal condition.
- Targeted UV Exposure: The modules were then placed in a climate chamber, front-side down on a black, non-reflective surface. UV lamps were positioned to deliver a concentrated dose of UV-A and UV-B radiation only to the rear side.
- The Dose: Each module received a total UV dose of 15 kWh/m². This amount is scientifically calculated to be equivalent to several years of real-world exposure in various environments, effectively fast-forwarding the aging process.
- Final Analysis: After exposure, the modules were re-tested for power loss, and the backsheet materials were analyzed for yellowing.
This applied research provides something simulations cannot: tangible proof of how materials perform in a real solar module lamination process when faced with stresses they were never traditionally tested for.
The Results: A Clear Divide in Durability
The findings were stark, revealing a significant performance gap between the module types.
The Problem with Yellowing
One of the first visible signs of UV degradation in polymers is yellowing. This isn’t just a cosmetic issue. As the backsheet turns yellow, it blocks light from reaching the rear side of the solar cells, directly reducing bifacial gain.
This effect is quantified using the Yellowing Index (YI), where a higher number means more yellowing and less light transmission.
- Glass-Glass (G/G) Modules: As expected, the glass back provided a near-perfect barrier. The YI change was negligible, staying below 1.
- Transparent Backsheet Modules (TBS A & TBS B): Both backsheet types showed significant yellowing. Their YI increased by more than 5, a commonly accepted threshold for noticeable degradation. TBS B performed slightly worse than TBS A.
This visual evidence is the first warning sign that the material is breaking down under UV stress.
The Impact on Power
Ultimately, power output is what matters most. The power loss measurements told the same story as the yellowing.
- Glass-Glass Modules: Showed virtually no power loss, remaining stable and performing as expected.
- Transparent Backsheet Modules: Experienced a measurable power drop. Module type TBS A lost around 1.3% of its initial power, while TBS B lost nearly 1.5%.
A 1.5% loss may not sound dramatic, but when scaled across a multi-megawatt solar farm and compounded over 25 years, it translates into a substantial loss of revenue and a lower-than-expected return on investment—a loss that highlights how crucial material selection is during the initial solar module prototyping phase.
What This Means for Your Next Solar Project
The takeaway is not that transparent backsheets are inherently flawed, but that not all backsheets are created equal. Assuming a material proven on the front side will perform the same on the exposed rear side is a risky assumption.
For module developers, material suppliers, and investors, these results are a clear call to action:
- Question the Datasheet: Standard certifications don’t tell the whole story for bifacial applications. It’s essential to ask for data specific to rear-side UV stability.
- Prioritize Applied Testing: The only way to truly know how a module will perform is to test it under conditions that replicate its intended use. This means subjecting new designs and materials to specific rear-side UV testing.
- Balance Cost and Risk: While transparent backsheets may offer upfront weight and cost advantages, these benefits can be quickly erased by long-term performance degradation. A slightly higher initial investment in a more robust, UV-stable material—or a proven glass-glass design—can secure decades of reliable energy production.
By understanding this hidden threat, you can make more informed decisions, de-risk your investments, and build solar projects that deliver on their promise for the long haul.
Frequently Asked Questions (FAQ)
Q1: What is bifacial gain?
Bifacial gain is the extra electricity generated by a solar module from light captured on its rear side. This light is typically reflected from the ground (albedo). Depending on the surface (e.g., white gravel, snow, or sand), this gain can increase a module’s total energy output by anywhere from 5% to 30%.
Q2: Why is UV radiation more damaging than visible light?
Ultraviolet (UV) radiation carries more energy than visible light. This higher energy can break the chemical bonds in polymers, such as those used in backsheets and encapsulants. This process, known as photodegradation, leads to yellowing, brittleness, and loss of transparency.
Q3: Are all transparent backsheets susceptible to this kind of degradation?
Not necessarily, but their material composition is critical. Different polymers and additives offer varying levels of UV resistance. The tests show that significant performance differences exist between commercially available products, making specific testing essential. You cannot assume all transparent backsheets will perform the same.
Q4: Is a glass-glass module always the better choice?
From a durability and UV-resistance standpoint, glass is an excellent barrier. However, glass-glass modules are heavier and can be more expensive to manufacture and install. The best choice depends on the specific project’s requirements, including environmental conditions, installation type (e.g., rooftop vs. ground-mount), and budget. The key is to validate that any chosen backsheet has been tested for rear-side UV stability.
Q5: How can I test my own module or material concepts?
The most effective way is to use a dedicated R&D facility like PVTestLab, which provides access to industrial-grade equipment and controlled environmental chambers. This allows you to run customized aging sequences, like the rear-side UV test, to get reliable data on how your specific materials and designs will perform before committing to large-scale production.