(Nanowerk News) In terms of supplying energy for space exploration and settlement, commonly available solar cells made of silicon or gallium arsenide are still too heavy to transport by rocket. To address this challenge, various lightweight alternatives are being explored, including solar cells made from thin layers of molybdenum selenide, which fall under the broader category of 2D transition metal dichalcogenide (2D TMDC) solar cells.
Publish in the first issue of the journal Device (“How Good Are 2D Excitonic Solar Cells?”), researchers propose a device design that can increase the efficiency of 2D TMDC devices from 5%, as has been shown, to 12%.
“I think people are slowly realizing that 2D TMDC is a very good photovoltaic material, though not for terrestrial applications, but for mobile applications—more flexible, such as space-based applications,” said lead author and advisory board member Deep Jariwala Devices from the University of Pennsylvania. “The 2D TMDC solar cell weighs 100 times less than a silicon or gallium arsenide solar cell, so it’s suddenly a very exciting technology.”
Although TMDC 2D solar cells are not as efficient as silicon solar cells, they produce more electricity per weight, a property known as “specific power”. This is because a layer only 3–5 nanometers thick—or more than a thousand times thinner than a human hair—absorbs a comparable amount of sunlight to commercially available solar cells. It is their extreme thinness that has earned them the label “2D” —they are considered “flat” because they are only a few atoms thick.
“High specific power is actually one of the biggest goals of any space-based light harvesting or energy harvesting technology,” said Jariwala. “This is not only important for satellites or space stations, but also if you want real utility-scale solar power in space.”
“The amount of solar cells you have to ship is so large that no space vehicle today can get that kind of material there in an economically viable way. So, really the solution is that you double the lighter cell, which gives you a much more specific power.
The full potential of TMDC’s 2D solar cells has yet to be fully realized, so Jariwala and his team are looking to increase cell efficiency even further. Usually, the performance of this type of solar cell is optimized through building a series of test kits, but the Jariwala team believes it is important to do this through computational modeling.
Additionally, the team thought that to really push the boundaries of efficiency, it was important to properly account for one of the defining—and challenging to model—features of a device: excitability.
Exciton is generated when the solar cell absorbs sunlight, and its dominant presence is the reason why TMDC 2D solar cells have high solar absorption. Electricity is generated by solar cells when the positively and negatively charged exciton components are supplied to separate electrodes.
By modeling the solar cell in this way, the team was able to put together a design with twice the efficiency of what had been demonstrated experimentally.
“The unique part of this device is its superlattice structure, which basically means there are alternating 2D TMDC layers separated by spacer or non-semiconductor layers,” said Jariwala. “Spacing the layers allows you to reflect light many times inside the cell structure, even when the cell structure is very thin.”
“We don’t expect very thin cells to see a value of 12%. Given that today’s efficiency is less than 5%, my hope is that in the next 4 to 5 years people can actually demonstrate cells with 10% and above efficiency.”
Jariwala said the next step is to think about how to achieve large-scale wafer production for the proposed design. “Right now, we’re assembling this super grid by moving the individual materials one on top of the other, like sheets of paper. It’s as if you ripped them out of a book, then pasted them together like a pile of sticky notes,” said Jariwala. “We needed a way to grow these materials directly on top of one another.”