In the relentless pursuit of solar efficiency, every fraction of a percent matters. We optimize cells, refine interconnection, and perfect front-side glass. But what if one of the most straightforward gains has been hiding in plain sight—on the back of the panel?
Bifacial solar modules, which capture light from both sides, have changed the game. Yet, we often treat the rear side as an afterthought, assuming its performance is fixed. This assumption costs developers and asset owners measurable energy yield every single day.
This article explores a simple but powerful question: What happens when we give the rear glass the same anti-reflective (AR) treatment as the front? We’ll dive into the data from a head-to-head test conducted at PVTestLab, revealing not just whether it works, but by how much, and if the investment pays for itself.
First, What Exactly Is a Bifacial Module?
Imagine a standard solar panel. It has a dark, light-absorbing front and an opaque backsheet—a one-way street for energy production.
A bifacial module is different. It replaces the opaque backsheet with a second pane of glass or a transparent backsheet, allowing sunlight that reflects off the ground (a phenomenon called „albedo“) to be captured by the rear side of the solar cells.
This design can boost a panel’s total energy output by anywhere from 5% to 30%, depending on factors like the reflectivity of the surface below it (snow and white sand are great; dark soil is not) and the module’s height off the ground. The extra energy generated by the rear side is called bifacial gain.
The Unsung Hero: Anti-Reflective (AR) Coating
You’re already familiar with AR coating, even if you don’t realize it. It’s the technology that reduces glare on your eyeglasses and camera lenses. In solar, a micro-thin layer of AR coating is applied to the front glass of most high-quality modules. Its job is simple: to help more photons pass through the glass and reach the solar cell, rather than reflecting away. This typically increases front-side power by a valuable 2-3%.
The logical question, then, is: if it works so well on the front, what could it do for the rear? The light hitting the back of a bifacial panel is less direct and more diffuse, but it’s still valuable energy. Could we capture more of it with a simple coating?
The Big Question: Does AR Coating on the Rear Glass Actually Work?
Hypotheses are great, but in solar manufacturing, data is king. To get a definitive answer, our engineers at PVTestLab designed a straightforward experiment to isolate and measure the impact of a rear-side AR coating.
Our Approach: A Head-to-Head Comparison
We started with identical bifacial PERC modules where every component—from the cells to the encapsulant—was the same. The only variable was the rear glass.
- Control Module: Built with standard 2.0 mm tempered solar glass on the rear.
- Test Module: Built with 2.0 mm tempered solar glass featuring an AR coating on the rear.
Both were assembled under identical, climate-controlled conditions to ensure the only difference was the one we wanted to measure. This level of precision is critical for effective solar module prototyping and yields trustworthy data for manufacturers.
The Results Are In: Quantifying the „Hidden“ Gain
After production, we took both modules to our Class AAA flasher for Standard Test Condition (STC) measurements. We tested the front and rear sides independently to calculate the bifaciality factor—a key metric representing the rear side’s power relative to the front.
The results were clear and immediate.
Standard Rear Glass Module:
- Front-Side Power (Pmpp): 446.5 W
- Rear-Side Power (Pmpp): 328.6 W
- Bifaciality Factor: 73.6%
AR-Coated Rear Glass Module:
- Front-Side Power (Pmpp): 446.5 W
- Rear-Side Power (Pmpp): 337.8 W
- Bifaciality Factor: 75.6%
The Improvement:
- Rear-Side Power Gain: +9.2 W (+2.8%)
- Bifaciality Factor Increase: +2.0% (absolute)
The AR-coated rear glass delivered 9.2 watts more power from the rear side under identical test conditions. This raised the module’s bifaciality factor from 73.6% to 75.6%—an absolute increase of 2.0%. While 2% might not sound like a revolution, over a 25-year lifetime, this small, consistent gain adds up to significant financial returns.
Is the Extra Power Worth the Price? A Look at the Economics
Any engineering improvement must also be financially viable. The cost of adding an AR coating to glass is approximately $1.00 per square meter. For a typical 2m² bifacial module, this translates to an added manufacturing cost of just $2.00.
Is it worth it? Let’s do the math.
Assuming the rear-side gain contributes an extra 10-15 kWh of energy per year and an average electricity price of $0.05/kWh, the initial $2.00 investment is paid back in just 1 to 2 years. For the remaining 23+ years of the module’s life, that extra energy is pure profit.
But Will It Last? Testing for Long-Term Durability
An enhancement that fades after a few years in the field is not an enhancement—it’s a liability. The rear glass of a bifacial module is exposed to the elements, so the durability of any coating applied to it is a major concern.
To simulate decades of harsh environmental exposure, we placed the AR-coated module in our facility for accelerated life testing. The industry-standard „Damp Heat“ test involves subjecting the module to 1,000 hours at 85°C and 85% relative humidity.
After the test concluded, we inspected the module for any signs of coating delamination, discoloration, or performance degradation. The results were definitive: the coating was perfectly intact, and the module’s electrical performance was unchanged.
As Patrick Thoma, PV Process Specialist at PVTestLab, notes, „Innovation is exciting, but reliability is non-negotiable. Our tests confirm that this performance gain isn’t a short-term trick; it’s a durable improvement that holds up under harsh conditions. This is crucial for validating not just the material, but the entire lamination process that integrates it.“
This rigorous climatic chamber testing gives manufacturers the confidence that the added performance will last for the entire lifespan of the solar asset.
Key Takeaways: What This Means for Module Development
This simple test offers a powerful lesson for module developers, material suppliers, and large-scale solar farm operators:
- A Measurable Gain: Rear-side AR coating is not a theoretical tweak; it provides a quantifiable 2% absolute gain in bifaciality.
- High ROI: With a payback period of just 1-2 years, it’s one of the most cost-effective performance boosters available.
- Proven Durability: Accelerated life testing confirms the coating can withstand decades of environmental stress without degrading.
- Competitive Edge: In a market driven by Levelized Cost of Energy (LCOE), this small, low-cost upgrade can provide a significant competitive advantage.
The biggest opportunities are often the ones we overlook. By paying attention to the „other side“ of the panel, we can unlock more clean energy and deliver more value with minimal extra cost.
Frequently Asked Questions (FAQ)
What is bifaciality?
Bifaciality (or the bifaciality factor) is a percentage that represents how efficient the rear side of a bifacial solar cell is compared to its front side. A bifaciality factor of 75% means the rear side can produce 75% of the power that the front side generates under the same light intensity.
What is an AR coating typically made of?
Most solar AR coatings are made from a thin film of silicon dioxide (SiO₂) with a porous nanostructure. This structure creates a gradual refractive index between the air and the glass, which minimizes reflection across a broad spectrum of light.
Does the type of ground surface still matter with a rear AR coating?
Absolutely. An AR coating helps the module capture more of the light that reaches it, but the amount of light reaching it is determined by the albedo (reflectivity) of the ground surface. A module with a rear AR coating installed over highly reflective white gravel will still produce far more bifacial gain than the same module installed over dark soil.
Is rear-side AR coating a standard feature in the industry?
Not yet. This coating is still considered a premium feature or an optional upgrade for many manufacturers. However, as the industry continues to seek out every possible efficiency gain, its adoption is growing, especially for utility-scale projects where small percentage gains translate into millions of dollars over the project’s lifetime.
Your Next Step in Solar Innovation
Understanding the potential of every component is the first step toward building a better, more efficient solar module. Whether you’re a material developer validating a new coating or a module manufacturer looking to quantify a design change, having access to an industrial-scale testing environment is critical.
Explore our resources to learn more about how applied research is bridging the gap between laboratory concepts and full-scale production. The next breakthrough in solar is waiting to be tested.