Commercially available solar cells built of silicon or gallium arsenide are still too hefty to be safely transported by rocket when it comes to producing electricity for space exploration and colonization.
Solar cells constructed of a thin layer of molybdenum selenide, which fall under the broader category of 2D transition metal dichalcogenide (2D TMDC) solar cells, are one of the lightweight options being investigated to address this difficulty.
Publishing June 6 (2023) in the inaugural issue of the journal Device, researchers propose a device design that can take the efficiencies of 2D TMDC devices from 5%, as has already been demonstrated, to 12%.
“I think people are slowly coming to the realization that 2D TMDCs are excellent photovoltaic materials, though not for terrestrial applications, but for applications that are mobile more flexible, like space-based applications,” says lead author and Device advisory board member Deep Jariwala of University of Pennsylvania. “The weight of 2D TMDC solar cells is 100 times less than silicon or gallium arsenide solar cells, so suddenly these cells become a very appealing technology.”
Although 2D TMDC solar cells are less effective than silicon solar cells in terms of efficiency, they produce more electricity per unit of weight, or what is known as “specific power.”
This is due to the fact that sunlight may be absorbed by a layer that is just 3-5 nanometers thick over 1,000 times thinner than a human hair in a manner comparable to that of commercially available solar cells. They are classified as “2D” because of how thin they are; at only a few atoms thick, they are referred to as “flat.”
The number of solar cells you would have to ship up is so large that no space vehicles currently can take those kinds of materials up there in an economically viable way. So, really the solution is that you double up on lighter weight cells, which give you much more specific power.
Deep Jariwala
“High specific power is actually one of the greatest goals of any space-based light harvesting or energy harvesting technology,” says Jariwala. “This is not just important for satellites or space stations but also if you want real utility-scaled solar power in space.”
“The number of solar cells you would have to ship up is so large that no space vehicles currently can take those kinds of materials up there in an economically viable way. So, really the solution is that you double up on lighter weight cells, which give you much more specific power.”
The full potential of 2D TMDC solar cells has not yet been fully realized, so Jariwala and his team have sought to raise the efficiency of the cells even further. The performance of this kind of solar cell is often improved by making a number of test devices, but Jariwala’s team thinks it’s crucial to do so by computationally modeling it.
Additionally, the team believes that in order to accurately account for excitons, one of the device’s distinctive and difficult to model properties, it is crucial to fully push the limits of efficiency.
Excitons are created during solar cell photosynthesis, and it is because of their predominance that a 2D TMDC solar cell has such high solar absorption. The solar cell generates electricity when the positively and negatively charged parts of an exciton are directed to different electrodes.
The team was able to create a design with a design with a double the efficiency compared to what has already been achieved experimentally by modeling the solar cells in this manner.
“The unique part about this device is its superlattice structure, which essentially means there are alternating layers of 2D TMDC separated by a spacer or non-semiconductor layer,” says Jariwala. “Spacing out the layers allows you to bounce light many, many times within the cell structure, even when the cell structure is extremely thin.”
“We were not expecting cells that are so thin to see a 12% value. Given that the current efficiencies are less than 5%, my hope is that in the next 4 to 5 years people can actually demonstrate cells that are 10% and upwards in efficiency.”
Jariwala says the next step is to think about how to achieve large, wafer-scale production for the proposed design.
“Right now, we are assembling these superlattices by transferring individual materials one on top of the other, like sheets of paper. It’s as if you’re tearing them off from one book, and then pasting them together like a stack of sticky notes,” says Jariwala. “We need a way to grow these materials directly one on top of the other.”
