You’ve invested in the latest high-efficiency solar cells—HJT or TOPCon—chasing the impressive 25-30% bifacial gain promised on the datasheet, and your models are projecting incredible energy yields. But what if the very materials meant to protect those cells are silently chipping away at that bonus energy, year after year?
The truth is, the promise of bifacial technology hinges on a factor many overlook: the long-term transparency of the materials behind the cell. Simply choosing a „clear“ backsheet and encapsulant isn’t enough. The real challenge is ensuring they stay perfectly clear for over 30 years under relentless UV radiation, heat, and humidity. This is where the gap between theoretical gain and real-world performance widens.
The Bifacial Promise Meets a Transparent Challenge
Heterojunction (HJT) and Tunnel Oxide Passivated Contact (TOPCon) cells are marvels of solar engineering. Their key advantage, besides high front-side efficiency, is their exceptional bifaciality—the ability to generate significant power from light captured on their rear side.
To unlock this potential, every layer behind the cell must act like pristine, invisible glass. This „stack-up“ typically includes:
- A rear-side encapsulant: A polymer layer that cushions the cells and bonds them to the backsheet.
- A transparent backsheet: The final protective barrier that replaces the traditional opaque white or black backsheet.
The problem? Not all transparent materials are created equal, especially when they must work together under extreme stress for decades.
Deconstructing the Bifacial Sandwich: Encapsulants and Backsheets
For bifacial modules, Polyolefin Elastomer (POE) has become the encapsulant of choice over traditional EVA (Ethylene Vinyl Acetate). POE offers superior resistance to moisture and doesn’t produce the acetic acid that can cause corrosion over time—a critical feature for sensitive HJT and TOPCon cells.
But selecting the right POE and pairing it with the right transparent backsheet is a delicate balancing act. You’re trading one set of challenges for another, navigating a complex world of polymer chemistry, UV stabilizers, and manufacturing processes. The initial datasheet might look perfect, but the real test is how these materials behave after years in the field.
The Invisible Thief: How UV Radiation Degrades Performance
The most significant threat to your bifacial gain is ultraviolet (UV) radiation. Over time, UV rays break down the polymer chains in both the encapsulant and the backsheet, leading to a process called photodegradation.
This isn’t just a theoretical concern. Our applied research at PVTestLab reveals a critical insight: while most POEs start with excellent UV transmission of over 92%, accelerated aging tests reveal significant performance decay. Some formulations can lose up to 5% of their light transmission after just 1,000 hours of damp heat testing.
This degradation manifests in two ways:
- Yellowing: The material develops a yellow or brownish tint, measured by its „Yellowing Index“ (YI). This yellowing acts like a filter, blocking the blue and UV portions of the light spectrum from reaching the cell.
- Haze & Delamination: The material can become cloudy or even begin to separate from the cell or backsheet, scattering light and creating pathways for moisture to enter.
„A datasheet only tells you the starting line. It can’t tell you how a material will finish the marathon. We’ve seen materials with nearly identical initial specs show dramatically different degradation paths when tested under realistic thermal and UV stress. This is why validating the entire material stack-up is non-negotiable for bankable bifacial performance.“ — Patrick Thoma, PV Process Specialist at PVTestLab
A 5% loss in rear-side light transmission doesn’t just mean a 5% loss in bifacial gain. The impact can be much larger, effectively undermining the primary reason for choosing premium HJT or TOPCon cells in the first place.
It’s Not Just One Material, It’s the Entire „Stack-Up“
Even if you select a highly UV-stable POE and a durable transparent backsheet, you’re not out of the woods. The next challenge is ensuring they are compatible.
Research shows that chemical interactions between certain POE additives and backsheet coatings can accelerate delamination or haze formation under thermal cycling. An adhesive promoter in the backsheet might react unexpectedly with a UV stabilizer in the POE, creating a weak point that fails years before its time.
This delicate chemical balance means that optimizing performance requires precise control over the entire system. A deep understanding of solar module lamination processes is absolutely critical to ensure these materials bond perfectly without creating long-term vulnerabilities. You aren’t just selecting components—you are engineering a composite material system.
Building Confidence Through Applied Research: A Validated Approach
How can you be certain your chosen materials will deliver on their promise for 30 years? The answer lies in moving beyond datasheets and into applied, comparative testing.
The most reliable method involves creating multiple mini-modules using different POE and transparent backsheet combinations. By manufacturing these prototypes under real industrial conditions, you can accurately replicate the stresses the materials will face.
The process is systematic:
- Prototyping: Build identical mini-modules, changing only the POE or backsheet supplier for each batch.
- Accelerated Aging: Expose these prototypes to a rigorous regimen of damp heat (DH), thermal cycling (TC), and UV exposure tests that simulate decades of fieldwork in a matter of weeks.
- Data Analysis: Systematically measure the degradation. Compare the Yellowing Index, adhesion strength (peel tests), light transmission, and electrical output (power loss) for each combination.
This data-driven approach removes the guesswork. It provides concrete evidence of which material stack-up offers the best resistance to degradation, ensuring maximum light reaches your bifacial cells for their entire lifespan. This type of solar module prototyping and development is the essential bridge between a promising concept and a truly bankable, high-performance product.
Frequently Asked Questions (FAQ)
What’s the main difference between HJT and TOPCon cells?
Both are advanced, high-efficiency n-type cell technologies. HJT (Heterojunction Technology) uses layers of amorphous silicon to passivate the crystalline silicon wafer, resulting in very high voltages. TOPCon (Tunnel Oxide Passivated Contact) uses an ultra-thin tunnel oxide layer and a layer of highly doped polysilicon to achieve passivation, which is often seen as an easier upgrade path for existing PERC production lines. Both technologies boast excellent bifaciality.
Why can’t I just use standard EVA encapsulant for bifacial modules?
While some companies use EVA, it has two main drawbacks for advanced cells. First, it is more permeable to moisture than POE, which can be a long-term reliability risk. Second, EVA releases acetic acid as it cures and ages, which can corrode the sensitive cell coatings and interconnects used in HJT and TOPCon technologies. POE is acid-free and far more resistant to moisture, making it a safer choice.
What is „Yellowing Index“ (YI) and why does it matter?
The Yellowing Index is a standardized number that measures the change in color of a plastic or polymer from clear or white toward yellow. A higher YI means more yellowing has occurred. For a bifacial module, a low and stable YI is critical because yellowing directly blocks sunlight (especially high-energy blue light) from reaching the solar cell. This reduces the amount of electricity it can generate from the rear side.
How long does it take to see if a material is unstable?
Through accelerated aging tests, significant insights can be gained in a relatively short time. For example, 1,000 to 2,000 hours in a damp heat chamber (at 85°C and 85% relative humidity) or a UV chamber can simulate many years of outdoor exposure. These tests, which can be completed in six to twelve weeks, are designed to quickly reveal weaknesses in material formulations that might otherwise take a decade to appear in the field.
Your Path from Theory to Bankable Performance
Maximizing the bifacial gain of HJT and TOPCon technology is far more than a simple item on a checklist. It’s a complex engineering challenge that demands a holistic view. Success isn’t found in picking the „best“ encapsulant or the „best“ backsheet in isolation, but in rigorously testing and validating the entire material system to ensure it performs harmoniously for decades.
By moving from reliance on datasheets to a foundation of applied research, you can eliminate uncertainty and build a product engineered for lasting, high-yield performance. The next step in this journey is to see how these principles are put into practice in a controlled, industrial-scale environment where ideas can be tested, proven, and perfected.