You’re designing the next generation of bifacial solar modules. The debate in your R&D department is a familiar one: stick with the proven, robust glass-glass (G/G) structure, or innovate with a lighter, potentially more cost-effective transparent backsheet (G/B)?

On paper, the transparent backsheet looks compelling. It promises weight savings and easier handling. But datasheets and reality often tell two different stories. How do these designs really perform when subjected to the rigors of industrial manufacturing and harsh environmental conditions?

We decided to end the debate with data. At PVTestLab, we put both designs head-to-head in an empirical test, moving beyond speculation to measure what truly matters: lamination complexity, initial performance, and long-term durability. The results might surprise you.

Setting the Stage: The Great Bifacial Debate

Bifacial modules are brilliant in their simplicity—they capture sunlight from both sides, boosting energy yield. The core challenge lies in protecting the delicate solar cells for over 25 years while allowing light to reach the rear side. This has led to two dominant design philosophies:

1. Glass-Glass (G/G): The workhorse of the bifacial world. A sheet of glass on the front and another on the back creates a symmetrical, highly durable sandwich that is exceptionally resistant to moisture, mechanical stress, and potential-induced degradation (PID). Its main drawback? Weight and rigidity.

2. Glass-Transparent Backsheet (G/B): The lightweight challenger. This design replaces the rear glass with a specialized transparent polymer backsheet. The goal is to retain bifacial performance while reducing module weight and, potentially, cost. But this design raises new questions about long-term durability and process stability.

To find a definitive answer, we set out not just to compare materials, but to build and validate new solar module concepts under real industrial conditions.

The Contenders: A Closer Look at Our Test Modules

We manufactured two distinct bifacial modules in our full-scale R&D production line, keeping all variables constant except for the rear surface and its corresponding encapsulant.

• Module 1: The Champion (Glass-Glass): A classic G/G construction using a proven POE (Polyolefin Elastomer) encapsulant, known for its excellent durability and moisture resistance.

• Module 2: The Challenger (Glass-Backsheet): A G/B module using a high-quality transparent PET-based backsheet paired with a special EVA (Ethylene Vinyl Acetate) encapsulant formulated for bifacial applications.

With our two contenders ready, the first test wasn’t under the sun, but inside the laminator.

The First Hurdle: Lamination Complexity

Anyone in module manufacturing knows that lamination isn’t just about applying heat and pressure; it’s a delicate chemical process. The goal is to achieve a specific degree of „cross-linking“ in the encapsulant, turning it from a soft film into a durable, protective gel.

This is where we hit our first critical difference.

The G/G module with POE was straightforward. Glass is thermally stable, allowing us to use the optimal, higher-temperature profile required to achieve robust cross-linking for the POE.

The G/B module, however, posed an immediate challenge. The transparent PET backsheet is sensitive to high temperatures. To avoid damaging or warping it, we were forced to adjust the lamination process parameters. This meant lowering the peak temperature and shortening the cycle time. While this protected the backsheet, it created a significant risk: Was the special EVA encapsulant being fully cured?

This is a perfect example of a hidden trade-off. The choice of a single material can force compromises across your entire production process, potentially impacting the final product’s quality and reliability. When you’re evaluating new encapsulants and backsheets, this process interplay is exactly what you need to uncover.

Round 1: Initial Performance and Bifacial Gain

After successful lamination, we took both modules to our Class AAA flasher for their first performance test. How did they stack up on day one?

• Maximum Power (Pmax): The G/G module showed a slightly higher initial power output.

• Bifaciality Factor: The G/B module edged out the G/G with a slightly higher bifaciality of 74.4% compared to 71.7%.

If our analysis stopped here, the conclusion would be murky. The G/B module’s higher bifaciality is attractive, seemingly validating the design. But initial performance is only a snapshot in time. The real test of a module’s worth is how it performs after years in the field.

The Main Event: Surviving the Torture Test

To simulate decades of harsh weather, we placed both modules into a climate chamber for a Damp Heat (DH) test. This is a brutal, industry-standard trial: 1,000 hours at a constant 85°C and 85% relative humidity. It’s designed to accelerate aging and expose any weaknesses related to moisture ingress.

After 1,000 hours, we pulled them out. The results were anything but murky.

The Glass-Glass Champion:

The G/G module was virtually unfazed.

• Power Loss: A negligible -0.5%.

• Visual Inspection: No delamination, no corrosion, no bubbles. It looked as good as new.

• Electroluminescence (EL) Test: The EL image, which reveals the health of the solar cells, was clean and uniform.

The Glass-Backsheet Challenger:

The G/B module, however, told a different story.

• Power Loss: A significant degradation of 4.7%.

• Visual Inspection: Clear signs of delamination along the edges of the cells and visible corrosion on the busbars.

• Electroluminescence (EL) Test: The EL image showed multiple dark areas, indicating inactive or heavily degraded cell sections. Moisture had clearly penetrated the module and caused significant damage.

The Verdict: Why Durability Trumps Initial Specs

The empirical evidence is clear: the glass-glass module construction demonstrated vastly superior durability and reliability.

The G/B module’s failure can be traced directly back to the lamination process. The compromised, lower-temperature cycle—forced by the heat-sensitive backsheet—likely resulted in an incomplete cure of the EVA encapsulant. This weaker bond allowed moisture to creep in during the Damp Heat test, causing corrosion and delamination that ultimately killed cell performance.

This is the critical „aha moment“ for any module developer: a component that looks good on a datasheet can introduce process limitations that undermine the entire system’s longevity. Without empirical testing, this fatal flaw would have remained hidden until modules started failing in the field.

Frequently Asked Questions (FAQ)

What is a bifacial module?

A bifacial module is a solar panel designed to capture sunlight from both its front and rear sides. This allows it to generate more electricity by collecting direct sunlight on the front and reflected light (albedo) from the ground on the back.

What is bifacial gain?

Bifacial gain is the extra energy generated by the rear side of the module compared to the front side alone. It’s often expressed as a percentage. A bifaciality factor of 75% means the rear side can produce up to 75% of the power of the front side under ideal lighting conditions.

What is a Damp Heat (DH) test and why is it important?

The Damp Heat test is an accelerated aging test used to assess the durability of solar modules against long-term exposure to high temperature and humidity. By subjecting modules to 85°C/85% RH for 1,000 hours, it simulates decades of wear and tear, effectively predicting how well a module will resist moisture-related degradation like corrosion and delamination.

What’s the difference between POE and EVA encapsulants?

POE (Polyolefin Elastomer) and EVA (Ethylene Vinyl Acetate) are both polymer films used to encapsulate solar cells. POE is known for its superior resistance to moisture and its inherent stability against potential-induced degradation (PID), but it typically requires higher lamination temperatures. EVA is a more traditional, cost-effective encapsulant, but it can be more susceptible to moisture and degradation if not processed perfectly.

Why couldn’t you just use POE with the transparent backsheet?

While technically possible, it would be extremely difficult. POE requires an even higher processing temperature than the EVA we used. The heat-sensitive transparent backsheet would likely be damaged or warped during a lamination cycle hot enough to properly cure the POE, making the combination impractical for reliable mass production.

From Test Lab to Your Production Line

This head-to-head comparison underscores a fundamental truth: module development is a system-level challenge. Every material choice has a ripple effect on your manufacturing process and the final product’s reliability. A lighter backsheet is no advantage if it leads to 5% power loss after a few years in the field.

The only way to de-risk innovation is to move from datasheets to data. By testing new material combinations in a real-world production environment, you can uncover hidden trade-offs and validate performance before committing to mass production. The next time you’re evaluating a new bill of materials, ask yourself: „Have we truly tested how these components interact and hold up under stress?“ The answer could be the difference between market leadership and costly field failures.