Fullerene Device Acts as Both Solar Cell and a Current Inverter

An experimental C60 device is the first to show the photovoltaic effect and act as a spin-valve at the same time

2 min read
Schematic representation of the C60-based molecular spin-photovoltaic (MSP) device
Illustration: Science Magazine

An international team of researchers has developed a photovoltaic cell based on a combination of magnetic electrodes and C60 fullerenes— sometimes referred to as Buckyballs—that increases the photovoltaic efficiency of their device by 14 percent over photovoltaics using ordinary materials and architecture.

In research described in the journal Science, scientists from China, Germany, and Spain have taken spin valves—devices based on giant magnetoresistance and used in magnetic memory and sensors—and combined them with photovoltaic materials. The result offers a new way for solar cells to convert light into electricity.

“The device is simply a photovoltaic cell,” says Luis Hueso, research professor and leader of the Nanodevices Group at CIC nanoGUNE in Spain, in an e-mail interview with IEEE Spectrum. “However, we are using magnetic electrodes (cobalt and nickel-iron) rather than standard indium tin oxide (ITO) and aluminum as commonly used in organic photovoltaics.” The magnetic electrodes provide electrons with a certain orientation of their spin, creating what’s called a spin polarized current. Using these electrodes increased the photovoltaic efficiency by 14 percent compared to using ordinary electrodes, he says.

In order to achieve these results, the researchers needed the device to have both a photovoltaic effect and a spin transport effect. That is,  the electrons keep their spin orientation as they cross the device, according to Hueso. “These two effects have not been observed before in the same device, only separately,” Hueso adds.

One of the byproducts of these simultaneous photovoltaic and spin-polarized effects is that the device they have developed has the added functionality of serving as an inverter, which is used to convert the direct current (DC) produced by solar cells into alternating current (AC).

Hueso explains that the current inversion is created by an external magnetic field. As the magnetic field changes, the current changes direction. The reason this works is that the current inside the device has two sources: one is the current generated by the light and the other is the current coming from the magnetic electrodes.

The current generated by the light can be changed by the amount of light irradiation. The current coming from the electrodes can be changed by the magnetic field. Balancing both contributions means the flow direction of the overall current can be modified.

The key to the functioning of the device is the C60 fullerene. The C60 is both a photovoltaic material and one that can sustain the spin polarization of the electronic carriers. “Since both effects had been demonstrated in the past—the spin one by our group—we decided to use it for a proof-of-principle experiment,” says Hueso.

The actual current output in the device is fairly small, mainly due to the fact that the C60 is not an great material for photovoltaics. To address this the researchers are currently working on building a similar device using better performing materials.

While Hueso recognizes more engineering would need to be done with the device they have produced, he believes that an actual device that acts as both a photovoltaic and an inverter could indeed be possible.

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3 Ways 3D Chip Tech Is Upending Computing

AMD, Graphcore, and Intel show why the industry’s leading edge is going vertical

8 min read
Vertical
A stack of 3 images.  One of a chip, another is a group of chips and a single grey chip.
Intel; Graphcore; AMD
DarkBlue1

A crop of high-performance processors is showing that the new direction for continuing Moore’s Law is all about up. Each generation of processor needs to perform better than the last, and, at its most basic, that means integrating more logic onto the silicon. But there are two problems: One is that our ability to shrink transistors and the logic and memory blocks they make up is slowing down. The other is that chips have reached their size limits. Photolithography tools can pattern only an area of about 850 square millimeters, which is about the size of a top-of-the-line Nvidia GPU.

For a few years now, developers of systems-on-chips have begun to break up their ever-larger designs into smaller chiplets and link them together inside the same package to effectively increase the silicon area, among other advantages. In CPUs, these links have mostly been so-called 2.5D, where the chiplets are set beside each other and connected using short, dense interconnects. Momentum for this type of integration will likely only grow now that most of the major manufacturers have agreed on a 2.5D chiplet-to-chiplet communications standard.

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