You’ve seen the claims splashed across industry presentations and supplier datasheets: “Our TOPCon cells have 85% bifaciality!” followed by a competitor boasting, “Our HJT cells reach 95%!”
On the surface, the numbers seem straightforward. Higher is better, right?
But what if those numbers aren’t telling the whole story? What if one was measured in a highly optimized lab module with special glass and the other was a theoretical maximum? You’re left comparing apples to oranges, making a critical technology decision based on marketing metrics instead of real-world, comparable data.
This uncertainty is a major roadblock for module developers and material suppliers. How can you confidently choose a cell technology or design a new module if you don’t know its true performance baseline?
At PVTestLab, we believe that innovation requires clarity. That’s why we developed a standardized test protocol that answers one simple question: All other things being equal, what is the intrinsic bifacial performance of PERC, TOPCon, and HJT cells?
The Challenge: Isolating the Signal from the Noise
A bifacial solar module’s final performance is a complex recipe with many ingredients: the glass, the encapsulant (like EVA or POE), the backsheet, the frame design, and, of course, the solar cell itself.
Changing any one of these variables can alter the final bifaciality factor—the ratio of the module’s rear-side power to its front-side power.
The problem is, when you’re evaluating a new cell technology, you need to isolate its contribution from the other components to establish a clear baseline. Without it, you can’t know if a performance gain came from the new cells you’re testing or from the high-transparency backsheet you used in that specific prototype.
This is where a controlled, industrial testing environment becomes essential. It’s about moving from estimates to evidence.
Our Approach: Building the „Control“ Module
To establish a true, apples-to-apples comparison, we use our full-scale R&D production line to build baseline modules where only one thing changes: the cell technology.
Here’s our standardized recipe:
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Identical Bill of Materials (BOM): Every test module is built with the exact same front glass, transparent backsheet, and high-performance POE (Polyolefin Elastomer) encapsulant. This eliminates performance variables from the materials themselves.
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Consistent Process Parameters: Each module, whether it contains PERC, TOPCon, or HJT cells, undergoes the exact same lamination and curing cycle in our industrial laminators. This ensures the manufacturing process itself doesn’t influence the outcome.
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The Single Variable: The only difference between the modules is the type of solar cell inside.
By standardizing every other component and process, we can measure the performance difference that comes exclusively from the cell’s architecture. This transforms a noisy, confusing comparison into a clean, reliable dataset.
Why Cell Architecture is the Key to Bifaciality
So, why do these cells perform so differently? It comes down to their fundamental physical design. Think of it like looking through two different types of windows—one with thick bars and one with a clear, open pane.
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PERC (Passivated Emitter and Rear Cell): PERC has been the industry workhorse for years. For bifacial applications, the rear aluminum layer is opened in a grid pattern to let light in. However, the remaining metal contacts still block a significant portion of the rear surface, which limits how much light can be converted into energy. This design means its inherent bifaciality is structurally limited.
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TOPCon (Tunnel Oxide Passivated Contact): TOPCon technology improves on PERC by adding an ultra-thin tunnel oxide layer and a layer of polysilicon to the rear. This design drastically reduces the amount of metal contacting the silicon, creating more „windows“ for light to enter the back of the cell. The result is a significant jump in bifacial potential.
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HJT (Heterojunction Technology): HJT cells have a fundamentally different and more symmetrical structure. They are built like a sandwich, with layers of amorphous silicon on both sides of a crystalline silicon wafer. This near-perfect symmetry means the front and back of the cell are almost identical in their ability to capture light, giving HJT the highest natural bifaciality of the three.
Understanding these structural differences is the „aha moment“ for many engineers. The bifaciality factor isn’t just a number on a datasheet; it’s a direct consequence of the cell’s physical engineering. Our prototyping and module development services are designed to validate these very characteristics.
Measuring What Matters: The Standardized Test in Action
Once our baseline modules are carefully constructed, they’re ready for final validation.
Using our Class AAA solar simulator (flasher), we measure the power output (Pmax) from the front side. We then flip the module and measure the Pmax from the rear side under the same controlled light conditions.
The bifaciality factor is calculated with a simple formula:
Bifaciality Factor (%) = (Rear Side Pmax / Front Side Pmax) x 100
Because we’ve eliminated all other variables, the result is a clear benchmark for each cell technology. This isn’t a theoretical simulation; it’s a physical measurement from a real module built under industrial conditions.
The Results: A Clear Baseline for Bifacial Performance
After conducting these tests across numerous cell batches, a clear hierarchy emerges. Here are the typical inherent bifaciality factors we observe under our controlled industrial protocol:
- PERC: 70% – 75%
- TOPCon: 80% – 85%
- HJT: ~90%
This data provides a powerful starting point. If you’re a module developer, you now have a reliable baseline. For example, if your new TOPCon module design achieves only 78% bifaciality, you know the limitation isn’t the cell itself—the bottleneck is likely in your other materials or design choices. This allows you to focus your R&D efforts where they’ll have the most impact, such as through targeted material testing and lamination trials.
From Baseline Data to Better Decisions
Knowing the true baseline performance of a cell empowers you to:
- Make Informed Technology Choices: Select the cell architecture that best fits your project’s cost and performance goals, based on real data.
- Optimize Module Design: Isolate and improve the components holding your module back from its full bifacial potential.
- Validate Supplier Claims: Move beyond marketing figures and verify the actual performance of the cells you procure.
- Accelerate R&D Cycles: Stop wasting time guessing. Use a data-driven approach to innovate faster and with more confidence.
Ultimately, standardized testing bridges the gap between laboratory theory and industrial reality. It provides the clear, unbiased data needed to build the next generation of high-performance solar modules. Our expertise in process optimization and training helps teams implement these findings directly on their own production lines.
Frequently Asked Questions (FAQ)
What exactly is a bifaciality factor?
The bifaciality factor is a percentage showing how much power a module’s rear side can generate compared to its front side under identical test conditions. A factor of 85% means the rear side can produce 85% of the power that the front side can.
Does the encapsulant or backsheet material affect the final bifacial gain?
Absolutely. A highly transparent backsheet and a non-yellowing, UV-stable encapsulant like POE are critical for maximizing the light that reaches the rear of the cells. This is why we standardize these materials in our baseline tests—to ensure we’re measuring the cell’s potential, not the material’s limitations.
Can I test my own proprietary cells or materials at PVTestLab?
Yes. Our entire facility is designed for this purpose. Clients can rent our R&D line to build prototypes with their own cells, encapsulants, or backsheets to gather confidential, proprietary data under controlled industrial conditions.
Is POE always better than EVA for bifacial modules?
POE generally offers higher transparency, better resistance to potential-induced degradation (PID), and lower water vapor transmission rates—all highly beneficial for long-term bifacial performance. However, the best choice depends on specific cost and application requirements. We help clients conduct direct comparison tests to make data-driven decisions.
How does this baseline bifaciality factor translate to real-world energy yield (kWh)?
The bifaciality factor is a measure of the module’s potential. The actual energy gain in the field, measured in kWh, depends on external factors like installation height, ground surface reflectivity (albedo), and the module’s tilt angle. A higher bifaciality factor gives your module a greater ability to capitalize on these real-world conditions to produce more energy.