New Pathway Emerges To Improve Polycrystalline Thin-Film Solar Cell Performance
Puzzling out and testing new ways to improve the efficiency of cadmium telluride (CdTe) polycrystalline thin-film photovoltaic materials is a typical day in the life of National Renewable Energy Laboratory (NREL) research scientists Matthew Reese and Craig Perkins. Like any good puzzlers, they bring curiosity and keen observation to the task. These skills led them, over time, to make an intriguing observation. In fact, their discovery may prove to be a boon for the next generation of several different types of thin-film solar cells.
When fragments of solar cell material are crystallized together, or “grown” — think of a piece of rock candy growing in layers in a cup of sugar — they create a polycrystalline solar cell. With many layers come many surfaces, where one layer ends and another begins. These surfaces can cause defects that restrict the freedom of electrons to move, reducing the cell’s efficiency. As the cells are grown, researchers can introduce specific compounds that minimize the loss of electrons at these defects, in a process called “passivation.”
Reese, Perkins, and Colorado School of Mines doctoral student Deborah McGott noticed that the three-dimensional (3D) CdTe solar cells’ surfaces appeared to be covered in a very thin, two-dimensional (2D) layer that naturally eliminated surface defects. This 2D passivation layer forms in sheets on the 3D light-absorbing layer as the cell is growing, in a standard processing technique that is used around the globe. Despite the ubiquity of this 2D passivation layer, it had not been observed or reported in the research literature. Reese, Perkins, and McGott believed 2D passivation was also occurring naturally in other thin-film solar cells, like copper indium gallium selenide (CIGS) and perovskite solar cells (PSCs). They realized that this observation could lead to the development of new methods to improve the performance of many types of polycrystalline thin-film cells.
To confirm their hypothesis, they discussed it with NREL colleagues in the CdTe, CIGS, and PSC research groups. Through many informal discussions involving coffee, hallway chats, and impromptu meetings, Reese, Perkins, and McGott arrived at an “aha” moment. Their CdTe and CIGS colleagues confirmed that, while their research communities were not generally trying to perform 2D surface passivation in the 3D light-absorbing layer, it was, in fact, occurring. The PSC researchers said that they had noticed a 3D/2D passivation effect and were beginning to intentionally include compounds in device processing to improve performance. The “aha” moment took on even more significance.
“One of the unique things about NREL is that we have large groups of experts with different pools of knowledge working on CdTe, CIGS, and PSC technologies,” Reese said. “And we talk to each other! Confirming our hypothesis about naturally occurring 3D/2D passivation with our colleagues was easy because we share the successes and setbacks of our diverse research in an ongoing, informal, and collaborative way. We learn from each other. It is not something that typically happens in academic or for-profit-based polycrystalline thin-film solar cell research, where information is closely held, and researchers tend to remain siloed in their specific technology.”
The details of Reese, Perkins, and McGott’s discovery are presented in the article “3D/2D passivation as a secret to success for polycrystalline thin-film solar cells,” published in the journal Joule.
Supporting Evidence in the Literature
To confirm their findings, McGott conducted an extensive literature search and found considerable supporting evidence. The literature confirmed the presence of passivating 2D compounds in each of the CdTe, CIGS, and PSC technologies. No mention was made, however, of the 2D compounds’ ability to improve device performance in CdTe and CIGS technologies. While many articles on PSC technologies noted the naturally occurring 3D/2D passivation effect and discussed efforts to intentionally include specific compounds in device processing, none suggested that this effect might be active in other polycrystalline thin-film photovoltaic technologies.
Polycrystalline thin-film solar cells are made by depositing thin layers, or a thin film, of photovoltaic material on a backing of glass, plastic, or metal. Thin-film solar cells are inexpensive, and many people are familiar with their more unique applications. They can be mounted on curved surfaces — to power consumer goods, for example — or laminated on window glass to produce electricity while letting light through. The largest market for thin-film solar cell applications, however, is for CdTe thin film on rigid glass to make solar modules. CdTe modules are deployed at utility scale, where they compete directly with conventional silicon solar modules. Currently, commercial thin-film modules are generally less efficient than the best single crystal silicon solar modules, making performance improvements a high priority for polycrystalline thin-film researchers.
Key Properties of 2D Materials
Reese, Perkins, and McGott’s team used surface science techniques combined with crystal growth experiments to show that the 2D layers existed at and passivated 3D absorber surfaces in the three leading polycrystalline thin-film photovoltaic technologies. They then analyzed the key properties of successful 2D materials and developed a set of principles for selecting passivating compounds.
Finally, the team outlined key design strategies that will allow 3D/2D passivation to be employed in polycrystalline thin-film photovoltaic technologies more generally. This is particularly important because each 3D material requires a specific passivation approach.
The literature results, combined with lab-based observations, show that 3D/2D passivation may be the secret to success in enabling next-generation thin-film solar cells, particularly if researchers freely share the knowledge developed for each technology. The lack of 3D/2D passivation may even shed light on the stalled performance improvements of some polycrystalline technologies such gallium arsenide. By drawing parallels between the three technologies, Reese, Perkins, and McGott hope to demonstrate how the knowledge developed in each can — and should — be leveraged by other technologies, an approach that is seldom seen in polycrystalline thin-film solar cell research.
CdTe, CIGS, and PSC thin-film research at NREL is funded by the Department of Energy’s Solar Energy Technologies Office. Additional funding for Reese and McGott’s research is provided by the Department of Defense’s Office of Naval Research.
Learn more about photovoltaic research at NREL.
Article courtesy of the NREL, The U.S. Department of Energy.
The Honda Ruckus has earned cult status thanks to its minimalist styling, exposed frame, and seemingly endless customizability. The scooter, also known in international markets as the Honda Zoomer, has spent years being seen as a blank canvas for scooter tuners, urban commuters, and anyone who just wanted something simple, small, and kind of weird to zip around town. A few years ago, Honda finally answered the call for an updated version by announcing and producing the “Zoomer e:”, which was an electric version of the Honda Ruckus. So where is it?
When Honda launched the all-electric version of the Ruckus, the Zoomer e:, back in 2023, many fans hoped it was only a matter of time before we saw it quietly glide onto U.S. streets.
But two years later, there’s still no sign of a stateside release, and no indication that Honda plans to change that anytime soon.
The Zoomer e: was first introduced in China in early 2023 alongside two other retro-inspired electrics: the Cub e: and Dax e:.
The Zoomer e: keeps the stripped-down, industrial look of the classic gas-powered Ruckus, but swaps the 49cc engine for a 400W rear hub motor and a 48V 24Ah battery (around 1.15 kWh).
It was originally given a top speed of a mere 25 km/h (15.5 mph) to keep it street legal as an electric bicycle in its first market of China, where it also came with functional but stubby pedals so riders could pretend it was actually pedalable.
The first version of the electric scooter claimed a range of up to 80–90 km (50–56 miles) from its removable lithium-ion battery, depending on conditions.
We’ve since seen the performance bumped up to 40 km/h (25 mph) top speeds when the scooter was introduced into the Philippines market, where the local L1B classification allowed for higher speeds. It’s fairly obvious that the performance can be software-tweaked by Honda depending on the market, though likely to a limit. To achieve speeds much higher than 25 mph, a motor and controller swap may be required, though neither would be complicated.
In other words, the electric Ruckus’ debut revealed an ultra-lightweight, street-legal runabout designed for countries with expansive low-speed e-bike laws. But in the U.S., these types of quasi-e-bikes that are actually scooters are few and far between. The same performance can be had from a $1,000 electric bicycle, and in fact, Class 3 e-bikes in the US can go nearly twice as fast as the original electric Ruckus.
So Honda obviously hasn’t been in a rush to bring its low-spec version of the bike to the US market, where it would be a slower and heavier competitor to the wide range of cheap imported electric bicycles. However, its iconic design and cultural legacy have kept enthusiasm up for riders who have managed to privately import their own models. One Redditor appears to have imported two Honda Zoomer e: models in parts to assemble in the US, while someone else posted a YouTube video of his completely assembled Honda Dax e: model that was launched along the Zoomer e:.
Despite clear consumer interest and a growing market for low-speed electric vehicles, as well as Honda’s own proven interest in growing its electric scooter market, the company hasn’t made any moves to release the Zoomer e: in the US. That’s not surprising since America still lacks a robust electric scooter culture (or even a gasoline scooter culture, for that matter), and anything motorcycle-shaped that doesn’t hit 30+ mph tends to get passed over by mainstream buyers.
But perhaps that could change one day. Technically, bringing the Zoomer e: to the US wouldn’t be a monumental task for Honda. The U.S. is a self-certify country, meaning Honda could design a version that meets federal vehicle safety standards, beef up the motor and controller for higher speeds, and sell it as either a Class 2/3 e-bike, or perhaps more appropriately, as a low-speed motorcycle with a top speed in the 35-45 mph range (55-70 km/h).
With the rise of micromobility, electrification, and growing frustration with car-centric cities, now might actually be the perfect time for a reborn electric Ruckus to hit US roads. But until Honda decides to take that step, American riders will have to keep dreaming – or start importing.
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BMW Motorrad’s futuristic electric scooter just got its first real refresh since beginning production in 2021. The BMW CE 04, already one of the most capable and stylish electric maxi-scooters on the market, now gets a set of upgraded trim options, new aesthetic touches, and a more robust list of features that aim to make this urban commuter even more appealing to riders looking for serious electric performance on two wheels.
The BMW CE 04 has always stood out for its sci-fi styling and high-performance drivetrain. It’s built on a mid-mounted liquid-cooled motor that puts out 31 kW (42 hp) and 62 Nm of torque. That’s enough to rocket the scooter from 0 to 50 km/h (31 mph) in just 2.6 seconds – quite fast for anything with a step-through frame.
The top speed is electronically limited to 120 km/h (75 mph), making it perfectly capable for city riding and fast enough to hold its own on highway stretches. Range is rated at 130 km (81 miles) on the WMTC cycle, thanks to the 8.9 kWh battery pack tucked low in the frame.
But while the core performance hasn’t changed, BMW’s 2025 update focuses on refining the package and giving riders more options to tailor the scooter to their taste. The new CE 04 is available in three trims: Basic, Avantgarde, and Exclusive.
The Basic trim keeps things clean and classic with a Lightwhite paint scheme and a clear windshield. It’s subtle, sleek, and very much in line with the CE 04’s clean-lined aesthetic. The Avantgarde model adds a splash of color with a Gravity Blue main body and bright São Paulo Yellow accents, along with a dark windshield and a laser-engraved rim. The top-shelf Exclusive trim is where things get fancy, with a premium Spacesilver metallic paint job, upgraded wind protection, heated grips, a luxury embroidered seat, and its own unique engraved rim treatment.
There are also a few new tech upgrades baked into the options list. Riders can now spec a 6.9 kW quick charger that reduces the 0–80% charge time to just 45 minutes (down from nearly 4 hours with the standard 2.3 kW onboard charger). Tire pressure monitoring, a center stand, and BMW’s “Headlight Pro” adaptive lighting system are also available as add-ons, along with an emergency eCall system and Dynamic Traction Control.
BMW has kept the core riding components in place: a steel-tube chassis, 15-inch wheels, Bosch ABS (with optional ABS Pro), and the impressive 10.25” TFT display with integrated navigation and smartphone connectivity. The under-seat storage still swallows a full-face helmet, and the long, low frame design means the scooter looks like something out of Blade Runner but rides like a luxury commuter.
With these updates, BMW seems to be further cementing the CE 04’s role at the high end of the electric scooter market. It’s not cheap, starting around €12,000 in Europe and around US $12,500 in the US, with prices going up from there depending on configuration. However, the maxi-scooter delivers real motorcycle-grade performance in a package that’s easier to live with for daily riders.
Electrek’s Take
I believe that the CE 04’s biggest strength has always been that it’s not trying to be a toy or a gimmick. It’s a real vehicle. Sure, it’s futuristic and funky looking, but it delivers on its promises. And in a market that’s still surprisingly sparse when it comes to premium electric scooters, BMW has had the lane mostly to itself. That may not last forever, though. LiveWire, Harley-Davidson’s electric spin-off brand, has teased plans for a maxi-scooter-style urban electric vehicle in the coming years, but as of now, it remains something of an undefined future plan.
Meanwhile, BMW is delivering not just a concept bike but a mature, well-equipped, and ready-to-ride electric scooter that keeps improving. For riders who want something faster and more capable than a Class 3 e-bike but aren’t ready to jump to a full-size electric motorcycle, the CE 04 hits a sweet spot. It delivers the performance and capability of a commuter e-motorcycle, yet with the approachability of a scooter. And with these new trims and upgrades, it’s doing it with even more style.
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