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.
Just like it says on the tin – retailers are advertising killer deals on the fun-to-drive Kia Niro EV, with one midwest auto dealer reporting more than $10,000 off the sticker price of the Niro EV Wind. That’s nearly 25% off the top line price!
SKIP THE STORY: get straight to the deals.
The Kia Niro EV gets overshadowed by its objectively excellent EV6 and EV9 stablemates – both of which are currently available with substantial lease cash and 0% APR financing, in fact – but that doesn’t mean it’s not an excellent little electric runabout in its own right.
The last time I had a Niro EV tester, my kids loved it, I liked that it was quicker and more tossable than I expected it to be, and my wife liked the fact that “it doesn’t look electric. It looks normal.” And, with well over 200 miles of real world range (EPA-rated range is 253 miles), it was more than up to the task of commuting around Chicago and making the trip up to the Great Wolf Lodge in Gurnee and back without even needing to look for a charger.
It’s not the primary family hauler I’d choose – but as a second car? As a primary car for a slightly smaller family (1-2 kids, instead of 3-4)? The Kia Niro EV Wind, with a $42,470 MSRP, seems like a solid, “can’t go wrong” sort of choice. You know?
You won’t even have to pay that much, though. Raymond Kia in Antioch, Illinois is advertising a $42,470 Niro EV for $32,431 (that’s $10,039, or about 24% off the MSRP), and several others are advertising prices in the $33,000 range.
And, while we’re at it:
Kia Niro EV gallery
SOURCE | IMAGES: CarsDirect, Edmunds, Raymond Kia.
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Many school districts who used EPA funding to help purchase Lion Electric school buses are now stuck with broken down or unsafe vehicles – but Lion’s new Canadian investors seemingly have no plans to make things right.
“All four Lion buses that we own are currently parked and not being used,” Coleen Souza, interim transportation director of Winthrop Public Schools, told Jay Traugott over at Clean Trucking. “Two of them are in need of repairs which would cost us money which we are not willing to invest in because the buses do not run for more than a month before needing more repairs.”
The story is much the same at other US school districts who deployed Lion Electric buses over the last few years – and the trouble they describe isn’t isolated to a single component or system. One district we spoke to had onboard chargers that failed almost immediately after being plugged into a L2 AC charger. Another that spoke to Traugott reported emergency door gaps, power steering failure, loss of power, and braking issues.
As bad as the revelations of safety and drivability issues and $250 million in unresolved debt have been, it’s the objectively stupid design choices that have been the most shocking.
“Lion built an auxiliary diesel heater to heat the bus, essentially writing the manual as they went,” explained a school superintendent in the midwest, who asked not to be named. “It was fascinating to watch but there were design flaws with the heater. For example, the intakes pointed downward and we’re driving across rural roads and the intake sucks in that dirt.”
“Using a diesel-powered heater to warm an electric bus also somewhat defeats the purpose of going 100% zero-emissions,” added Traugott.
Despite a new electric school bus rebate and a fresh cash injection from Vincent Chiara, president of Quebec real estate powerhouse Groupe MACH, and Lion director Pierre Wilkie, however, it seems like no help is coming.
It just gets worse and worse
Despite early speculation – some of it my own, in fact – that the new investors would take the Canadian government up on its offer to help subsidize more electric school bus production and honor the company’s outstanding warranty claims, it appears the only vehicle line the new investors are interested in reviving are the the Class 8 electric semi manufacturing operations in Saint-Jérôme, Quebec.
The US school districts who spent tens of millions of taxpayer dollars in the hopes that Lion buses would help decarbonize their fleets and reduce students’ exposure to harmful diesel emissions? Many of them are back to using diesel, while others are trying to get their deposits back so they can buy something else.
Here’s hoping any school districts on the fence for electrification recognize that their are very real, very well-engineered, and very financially sound electric school bus manufacturers out there who can deliver on their promises.
SOURCES: Chicago Tribune, Clean Trucking, Electrical Business.
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Mitsubishi is partnering with Ample and Yamoto Transports to deploy an innovative new battery swap network for electric cars in its Japanese home market — but it’s not just for electric cars. Mitsubishi Fuso commercial trucks are getting in on the action, too!
Despite a number of early EV adopters with an overdeveloped concept of ownership, battery swap technology has proven to be both extremely effective and extremely positive to the overall EV ownership experience. And when you see how simple it is to add hundreds of miles of driving in just 100 seconds — quicker, in many cases, than pumping a tank of liquid fuel into an ICE-powered car — you might come around, yourself.
That seems to be what Mitsubishi thinks, anyway, and they’re hoping they’ll be your go-to choice when it’s time to electrify your regional and last-mile commercial delivery fleet(s) by launching a multi-year pilot program to deploy more than 150 battery-swappable commercial electric vehicles and 14 modular battery swapping stations across Tokyo, where the company plans to showcase its “five minute charging” tech in full view of hundreds of commercial fleets and, crucially, the executives of the companies that own and manage them.
A truck like the Mitsubishi eCanter typically requires a full night of AC charging to top off its batteries, and at least an hour or two on DC charging in Japan, according to Fuso. This joint pilot by Mitsubishi, Mitsubishi Fuso Trucks, and Ample aims to circumvent this issue of forced downtime with its swappable batteries, supporting vehicle uptime by delivering a full charge within minutes. The move is meant to encourage the transport industry’s EV shift while creating a depository of stored energy that can be deployed to the grid in the event of a natural disaster — something Mitsubishi in Japan has been working on for years.
Trucks like the eCanter already serve a number of roles throughout the global truck market, including municipal waste collection, regional delivery support, and more.
The pilot is backed by Tokyo Metropolitan Government’s “Technology Development Support Project for Promoting New Energy,” with local delivery operator Yamato Transport testing swappable EVs for delivery operations on both its eCanter light-duty trucks and Mitsubishi Minicab kei-class electric vans.
Electrek’s Take
Electrifying the commercial truck fleet is a key part of decarbonizing city truck fleets – not just here in the US, but around the world. I called the eCanter, “a great product for moving stuff around densely packed city streets,” and eliminating the corporate fear of EV charging in the wild just makes it an even better product for that purpose.
Here’s hoping we see more “right size” electric solutions like this one (and more battery swapping tech) in small towns and tight urban environments stateside somewhat sooner than later.
SOURCES | IMAGES: Mitsubishi, Fuso.
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