Silicon dominates the solar power landscape, but it isn’t the best material for making thin, lightweight solar cells needed for satellites and drones. Atomically thin semiconducting materials such as tungsten diselenide and molybdenum disulfide, which are already being considered for next-generation electronics, hold promise for low-cost ultrathin solar cells that can also be flexible. And now, engineers have made tungsten diselenide solar cells that boast a power-per-weight ratio on par with established thin-film solar cell technologies.
The flexible solar cells reported in the journal Nature Communications have a light-to-electricity conversion efficiency of 5.1 percent, the highest reported for flexible cells of this kind. Their specific power, meanwhile, is 4.4 W/g, comparable with thin-film solar cells—such as those made of cadmium telluride, copper indium gallium selenide, amorphous silicon, and III-V semiconductors. With further engineering to reduce the substrate thickness and increase efficiency, the technology has the potential to get to 46 W/g, “way beyond what has been shown for other photovoltaic technologies,” says Koosha Nassiri Nazif, an electrical engineer at Stanford University who led the work with his colleague Alwin Daus.
It’s a thousand times thinner than silicon but with the same amount of absorption as a standard silicon wafer.
Silicon’s efficiency is hard to beat for the cost, and silicon solar panel costs have been dropping every year. But “silicon is pretty suboptimal for emerging applications,” Nassiri Nazif says. Such applications include wearable and conformable electronics, smart windows and other architectural uses, unmanned aerial vehicles, and electric vehicles. “Another important application is the Internet of Things,” he says, “where you can extend the battery life or completely remove the need for batteries to power small sensors and devices.” High specific power is critical for those uses, he says. Today’s thin-film technologies and newer perovskite solar cells all have higher specific power than silicon, with perovskites holding the record at 29 W/g.
But tungsten diselenide and molybdenum disulfide, which belong to a class of materials known as transition metal dichalcogenides (TMD), have advantages over other materials. They are more lightweight than the thin-film CdTe or CIGS cells used in aerospace now. They’re also more stable than perovskites and organic photovoltaic materials—and are more environmentally friendly than lead-containing perovskites.
Furthermore, TMD materials boast some of the highest light absorption capabilities of any photovoltaic material. “So you can have an ultrathin layer a thousand times thinner than silicon and still have the same amount of absorption with proper optical design,” Nassiri Nazif says.
Yet, the best TMD solar cells so far have had efficiencies of less than 3%, and less than 0.7% when made on a lightweight, flexible substrate. The materials’ theoretical efficiency, however, is 27%. Daus says they are simply newer on the scene and need more heavy engineering to improve efficiency. All photovoltaic materials face charge-extraction challenges. That is, once the material absorbs a photon and produces electrons and holes, those charge carriers have to be quickly extracted before they can recombine.
The trick is to find the right contact material to shuttle the charge carriers from the semiconductor to the electrodes. The researchers chose a transparent graphene sheet for that. Then they coated it with a molybdenum oxide layer, which is also transparent and enhances graphene’s ability to extract charge carriers, Daus explains.
Another key advance that lets them make high-quality flexible solar cells is the transfer method they have developed, he adds. The first deposit tungsten diselenide flakes on a silicon substrate, deposit gold electrodes on it and then coat it with a thin flexible plastic substrate. Then they put the whole ensemble in a water bath to gently peel off the flexible structure from the silicon. Finally, they flip the structure over so the tungsten diselenide is on top and coat it with graphene and molybdenum oxide. The whole device, in the end, is only 350 nm thick.
The solar cells are tiny at this point, Nassiri Nazif points out, about 100 x 100 µm. “To get to the point where it can be commercialized, we need at least 1 x 1 cm devices,” he says. “The good news is that large-area, high-quality TMD growth has already been shown.”
But most efforts have focused on making monolayer TMD materials for electronics, says Daus, whereas for solar cells you need thicker 100–200 nm films. The Stanford team has already started making 2 x 2 cm films of TMDs, but so far the thicker films haven't reached the same high quality as the smaller flakes they used in the paper
They hope that this work inspires more research in the area of TMD solar cells. “Our goal is to build a foundation for TMD photovoltaic applications,” Nassiri Nazif says. “These materials have a fundamental advantage over other technologies. If we solve the engineering issues, it could be the material of choice for next-generation photovoltaic technology.”
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