How This Bifacial Solar Panel Reaches 91% Efficiency

What if I told you there’s a way to get up to 30% more power from the same solar panel footprint? No breakthrough chemistry. No quantum physics. Just one simple change that’s finally starting to catch on.

They’re finally putting solar cells on both sides of the panel.

And now one company has achieved something remarkable: the back of their panels is 91.7% as efficient as the front. But here’s why this is actually huge news: they’ve done it with cheaper technology that was supposed to be inferior.

So what’s actually going on here? And why does the back of a solar panel matter so much?

These are Tongwei’s new and improved bifacial solar panels, and they’re shattering records with a reported efficiency of 91.7%.¹ But wait … before you think they’ve somehow broken the laws of physics, remember that the current average for commercial solar cell efficiency is around 22%.^{2 3}

Here’s the thing: solar panels can be efficient in a lot of different ways, and that can make it tricky to understand what’s actually happening. Solar cell efficiency measures how much usable energy a panel gets from the sun.⁴ But bifaciality measures how close the backside performs compared to the front.⁵ And that 91.7% bifaciality figure? That’s what’s actually revolutionary here.

So how did Tongwei pull this off?

Why It Works

Well, to understand that we also need to understand why bifacial panels work. Generally speaking, bifacial PVs (sometimes shortened to just BPVs) work exactly like the common, monofacial panels you’re used to. Each solar cell is a sandwich with a silicon semiconductor core.⁶ A common misconception is that solar panels work by just absorbing sunlight and somehow turning that directly into usable energy. In actuality, photons hitting the solar cell can knock electrons loose from silicon bonds, leaving behind positively charged “holes.” A built-in electric field inside the cell then pushes electrons one way and holes the other, steering them to different contacts before they can recombine. If they recombine first, we lose that energy as heat and get no current.⁷

When you add up all the hoops we have to jump through, that 22% photovoltaic efficiency number makes a lot more sense. But… y’know… sure would be great if we could catch more, right?

And that’s where bifacial solar panels come in. However, doesn’t that seem kinda wasteful? Why bother putting PVs on the backside of the solar panel, facing away from the sun? Won’t they always be in shade and therefore be less effective?

Nope. That’s all thanks to a little thing called albedo. This is the measure of how much light bounces off an object, and it turns out lots of stuff has a high albedo.⁸ Everyday stuff like snow, sand, and some roofs actually have really significant albedo factors. Just look at the white rooftops you see all over Arizona and the Mediterranean. Absorption of light from albedo allows for solar collection even in scattered and diffuse light, such as on cloudy days or in cities.⁹ And speaking of unexpected places for solar panels to work well, I covered some pretty surprising bifacial applications in a previous video — “Have we been doing solar wrong all along?” — including setups that are literally flipping the solar industry on its head.

Also, while solar panels are angled and tilted for maximum sun capture, the sun does, famously, move, and the panels won’t always be at the ideal capture angle. To use a baseball analogy, why not get more of the team in the outfield to catch fly balls, even if they’re not the best players? Though with Tongwei’s improvement, these BPVs are not that far behind the MVPs.

So, that brings us back to our main question: how do Tongwei’s panels actually hit those high numbers?

How It Works

Well, there’s actually three elements at play here: TOPCon solar cells, inverse-pyramid architecture, and zebra crossing passivation contact layers.¹ That’s a lot to take in.

TOPCon stands for tunnel oxide passivated contact solar cells. This is a promising type of solar cell formula that’s becoming known for its stability and high PV efficiency. The effectiveness of TOPCon cells, especially bifacial cells, is very dependent on passivation provided by the cell’s silicon oxide layer.¹⁰

Now, to understand why this is such a big deal, you need to know that different bifacial technologies have been stuck at different performance levels. The current market is dominated by PERC cells, which typically hit around 70% bifaciality.¹¹ TOPCon cells like Tongwei’s usually max out around 80-85%.¹² The real efficiency champion has been Heterojunction Solar Cells (HJT), which can reach 92-95% bifaciality.¹³ But here’s the catch: HJT costs about $70 million per gigawatt to manufacture, while TOPCon only costs $40 million.¹⁴ HJT requires entirely new manufacturing lines, while TOPCon can upgrade existing PERC facilities.¹⁵ So the industry has been stuck choosing between performance and cost…not exactly a bright spot for the market.

But what makes TOPCon technology actually work so well? Think of it as creating a protective barrier that prevents excited electrons from “getting stuck” and wasting their energy before they can contribute to electricity production. When sunlight hits a solar cell, it creates excited electrons that should flow as electricity, but surface defects can cause these electrons to recombine with holes and disappear. The ultra-thin silicon oxide layer in TOPCon cells acts as a selective barrier that allows electrons to tunnel through while blocking holes, preventing recombination and boosting efficiency.

Sunken pyramids located on the back of the cells act as an electron sinkhole. A flat surface means a photon is likely to just slam into the cell and bounce out. In contrast, the pyramids can play photon pinball, bouncing the photon around and around until it gets fully absorbed.^{1 10}

That’s where the zebra passivation contacts come in. The pyramids do make it pretty easy to tell where the bad passivation will happen, in the valleys of the pyramids. So the researchers took a laser and just sintered these problem areas. In other words, they heated them together. The peaks of the pyramids left un-sintered got one kind of passivation (Al2O3/SiNx), and the valleys of the sintered sides received another (iO2/poly-Si/Al2O3/SiNx). It’s not spelled out in the paper why they’re called zebra contacts, but my educated guess is that alternating pattern of passivation layers resulted in an unintentional (but neat) zebra-stripe pattern.^{1 10}

Then, the researchers printed tiny silver electrodes over them to allow them to efficiently collect electrons.

Cost Analysis & Comparisons

Unsurprisingly, bifacial PVs are more expensive than their monofacial cousins. They’re also more complicated and difficult to install, generally speaking, as you’ve got to account for both faces. This means that, watt-for-watt, bifacial solar panels tend to be 10 to 20% more expensive to install than standard panels.¹⁶ The high upfront cost of solar panels is already a barrier for many people, and bifacial PVs add even more expense and complexity to the mix.

Now, sure, solar panels can pay for themselves, and when it comes to the generic home, the average payback time for monofacials is around seven to 10 years.¹⁷ A more expensive upfront cost and installation is going to make that ROI wait even longer. That probably just means it’ll take few more years to pay itself off.¹⁸

Unfortunately, if you do all the math, between the complex installation and overall costs, if you’re not in a zone with a lot of albedo, it probably won’t be worth it.

Beyond the hefty price tag, dirt and the like can collect between a panel and the ground, which ordinarily isn’t much of problem, unless you’ve got a bunch of PV cells on the backside of your solar panels.¹⁹ Oops. This can lead to further maintenance costs. There’s also the retrofitting issue. Traditional PV mounting brackets often cover some of the back of the PV, because there’s no reason not to. It provides strength and simplicity. But if you’re trying to upgrade from monofacial to bifacial solar panels, you’re going to have to spring for new brackets, mounts and the associated costs. More obviously, old-school solar panels are often angled close to the roof to maximize gains, but that’s not going to fly for bifacials.²⁰ That again makes upgrading existing system something of a headache.

It all adds up to make the calculations on whether or not a bifacial PV is right for your home, or office building, or solar farm. There’s no one-size-fits-all use case here. It’s going to depend on your situation.

Real-World Impact

The big news here is that the back of Tongwei’s panels is 91.7% as efficient as the front. But what does that actually mean for your electric bill? That 91.7% is a technical measurement under lab conditions. In the real world, you’re looking at more modest but still meaningful gains.

If you’re a casual photovoltaics fan, this might not seem like that big of a deal because solar panels thrive in barren places where real estate isn’t worth much…or as much. While solar farms do well out in deserts, the deserts don’t use much energy. Cities do. Plus, power is always lost in transmission,²¹ so the closer we get panels to the energy-guzzling cities, the better. All the glass and buildings contribute to cities’ high albedo, which traditional solar panels aren’t really suited toward capturing. Because bifacial PVs maximize space and love albedo, they’re more likely to fit in among the city slickers.²²

Most installations see about 10-20% more power from bifacial panels.²³²⁴ The exact amount depends heavily on your setup. Standard grass underneath your panels? Expect around 5-10% gain.²⁵ White commercial rooftops? You could see 15-20%.²⁶ Fresh snow can push gains up to 30%.²⁷

Ground-mounted solar farms with tracking systems, the sweet spot for bifacial, typically achieve that 10-15% range.²³²⁸ That can be significant.

The real breakthrough here isn’t the power gains themselves, but achieving near-premium performance without premium costs. By hitting 91.7% bifaciality with cheaper TOPCon technology, Tongwei has essentially closed the performance gap with expensive HJT while keeping costs 40% lower.¹⁰

We’ve already mentioned the huge leap in bifaciality, but the Tongwei panels are also displaying an improved current density, or more usable juice per square centimeter than their kin. We’re talking 41.29 mA/cm2, up from the 38.03 mA/cm2 current density of a standard, polished, pyramid-free backside.¹ That’s a 8.6% boost in amps. Think of it like squeezing an extra 30 amps out of every square meter of panel. On a typical home installation, that translates to enough extra power to keep your refrigerator running … that’s pretty cool. Likewise, all these optimizations tweaks make the cell better overall. The front-facing panels have an efficiency of 25.67% and the rear ones come in at an efficiency of 24.21%. That’s well above the industry standard.¹⁰

And Tongwei isn’t the only company making progress. Other researchers and companies are developing smart reflector systems and tackling the associated recycling challenges. But we’ll save those details in our back pockets for now.

If we look at our handy-dandy NASA Technological Readiness Level (TRL), we can see that Tongwei’s bifacial panels are a textbook 7.²⁹ That’s a successful prototype demonstration in a real-world situation. There’s still a few steps before this particular tech reaches the market, but Tongwei has been producing PVs, including bifacial PVs, at scale for years now.³⁰ So, the infrastructure for scaling up production is already in place, and often that’s the hardest and most expensive hurdle that technology has to jump to reach the market. That doesn’t necessarily mean that they’re going to be on store shelves tomorrow, though, and it still remains to be seen whether or not the new-fangled zebra crossing passivation and inverse pyramid developments can feasibly be mass-produced. But it does mean that, if they can be indeed mass produced, Tongwei should be able to pretty quickly and easily get it to the market.

Even without Tongwei, bifacial PVs are slowly but steadily gaining popularity and becoming more common. The technology is maturing, costs are coming down, and more companies are entering the space.

Other Breakthroughs

For example, researchers at the National Taiwan University of Science and Technology (NTUST) just developed a smart, adjustable aluminum reflector system that boosts the performance of vertical bifacial PVs (or VBPVs).³¹ These are bifacial PVs that are — surprise! — standing vertically in an attempt to get more sunlight and maximize space.

In pursuit of this goal, NTUST is using something called the Taguchi method to help them identify key parameter interactions and performance trends of the reflector-equipped PV systems.³² Genichi Taguchi is a legendary engineer and statistician, and one of his focuses was developing systems, formulas and yes, even methods, to make for the best experiments despite a lot of uncontrollable environmental factors.³³ Taguchi’s methods, however, aren’t widely accepted outside of East Asia. He’s somewhat of a polarizing figure, but we’ll save the academic engineering and statistics drama for another video.³⁴

NTUST used other statistical analysis tools to help them optimize their reflectors. One is Analysis of Variance, or ANOVA³¹ This helped identify which variables were actually influential or statistically significant enough for the reflectors to bother to respond to. And for the hat trick, they deployed a third statistical analysis tool with a name that sounds like a failed Silicon Valley startup from 2003: TRNSYS (Transient System Simulation Tool).³¹ TRNSYS helped verify that simulations matched real-world results.

The result: the NTUST team settled on vertical bifacial modules set up in an east-west orientation, which is pretty standard. However, they dynamically adjusted their reflectors throughout the day to track the sun’s position and redirect light at the optimal angle. With this configuration, the reflected sunlight hit the solar panels straight-on, maximizing the amount of light the panels could capture.³²

Compared to standard bifacial systems, NTUST’s optimized setup was 11% more efficient. The statistical optimization itself provided a 3.19% improvement over un-optimized reflector configurations.³² But here’s the impressive part: when compared to traditional single-sided solar panels, their complete system delivered a 71.32% increase in yearly power generation.³² That huge jump comes from combining bifacial technology with optimized reflectors, essentially giving the panels much more light to work with, even though the panels themselves convert sunlight at the same efficiency.

That said, it’s not all about boosting performance of green tech. Is it really green if it doesn’t have green ends? Solar panels don’t last forever, and as we’ve covered before, they’re surprisingly tricky to recycle. After all, there’s a lot of different components to a solar panel, and they’re sandwiched into something that protects those components for over 20 years. More PVs are going up every year, and what goes up must also come down. In a 2019 study, researchers at the University of California at Santa Barbara estimated that the U.S. alone will dump 1.3 million metric tons of PV waste in 2040, and that number will grow to 5.5 million metric tons by 2050.³⁵ Naturally, bifacial recycling is just as much a necessity as monofacial. So, what are our options?

Enter SolarPanelRecycling.com (or SPR), a solar recycling specialist based in North Carolina. In January, it announced the launch of the U.S.’ first dedicated, automated bifacial solar panel recycling line.³⁶ Glass recovery in particular wasn’t very successful with traditional lines, and it required a ton of manual labor. So, SPR’s engineers and R&D teams saw a niche that wasn’t being filled, and they got to work designing a new, efficient recycling system specifically for bifacial PVs.³⁷

Traditionally, recyclers tend to shred whole panels to extract the valuables sandwiched in between the tough layers. The problem is that this causes the glass, silicon, plastics, and other components to mingle into a useless waste. SPR claims that its process cleanly separates all those components so they can go right back into the supply chain. The company also boasts that its methods enable clean glass separation of bifacial modules with recovery rates of 99% or higher.³⁷ No word on what exactly these methods are, likely for proprietary reasons.

Where does that leave us? Well, I hope you don’t think I’m being two-faced when I say that, despite all those challenges, the outlook for bifacial PV is pretty good.