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Graphene Still Trying to Replace ITO in Organic Solar Cells

Two years after a MIT research team showed how graphene could replace ITO in photovoltaics, another MIT team does it

2 min read
Graphene Still Trying to Replace ITO in Organic Solar Cells
Illustration courtesy of the research team

Almost two years ago, researchers at MIT were heralding graphene as a possible replacement for the expensive indium-tin-oxide used in electrodes for organic solar cells. They showed a way in which the entire solar cell could be flexible—including its electrodes—and transparent.

Not long after that, research at Rice University picked up on the use of graphene for replacing ITO, but aimed their work towards creating a thin film for touch-screen displays.

Now researchers at MIT are reporting on work that, like the Rice team, combines flexible sheets of graphene with a grid of metallic nanowires. In so doing, they turned their attention back to photovoltaics. The research (“Graphene Cathode-Based ZnO Nanowire Hybrid Solar Cells”) was published in the journal Nano Letters.

While this latest research is not the first time graphene was used a replacement for ITO—even at MIT—it does have the distinction of being a graphene-nanowire solar cell with a respectable energy conversion efficiency of 4.2 percent. While this may not sound like a world-beating number, it stands up well to that of ITO-based devices with similar architectures.

“We’ve demonstrated that devices based on graphene have a comparable efficiency to ITO,” says Silvija Gradečak, one of the MIT researchers involved in the project, in a press release. “We’re the first to demonstrate graphene-nanowire solar cells without sacrificing device performance.”

The key to performing at a higher level than has been achieved by other designs was a series of polymer coatings that modified the properties of the graphene. This allowed the researchers first to bond a layer of zinc oxide nanowires to the graphene and then quantum dots that respond to light.

The prototypes developed by the MIT team remain fairly small in scale—a little over a centimeter (perhaps explaining why the Rice team felt satisfied with applying their graphene-nanowire ITO replacement to touch screens of mobile devices). But the researchers feel confident that the process they have for making the material is highly scalable.

“The size is not a limiting factor, and graphene can be transferred onto various target substrates such as glass or plastic,” says Hyesung Park, a co-author of the paper, in the release.

While Gradečak expresses a bit more caution, she does seem to think that the material could reach commercial devices in a couple of years. Yes, well, if there were efficient mechanisms for bringing research from the lab to the fab, she might be right. But until then, we may just have to wait a bit longer than that.

Illustration: Courtesy of the research team

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3D-Stacked CMOS Takes Moore’s Law to New Heights

When transistors can’t get any smaller, the only direction is up

10 min read
An image of stacked squares with yellow flat bars through them.
Emily Cooper
Green

Perhaps the most far-reaching technological achievement over the last 50 years has been the steady march toward ever smaller transistors, fitting them more tightly together, and reducing their power consumption. And yet, ever since the two of us started our careers at Intel more than 20 years ago, we’ve been hearing the alarms that the descent into the infinitesimal was about to end. Yet year after year, brilliant new innovations continue to propel the semiconductor industry further.

Along this journey, we engineers had to change the transistor’s architecture as we continued to scale down area and power consumption while boosting performance. The “planar” transistor designs that took us through the last half of the 20th century gave way to 3D fin-shaped devices by the first half of the 2010s. Now, these too have an end date in sight, with a new gate-all-around (GAA) structure rolling into production soon. But we have to look even further ahead because our ability to scale down even this new transistor architecture, which we call RibbonFET, has its limits.

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