Researchers at the University of Pennsylvania have developed a lightweight solar cell design that could potentially double the efficiency of cells in space-based applications.
When it comes to supplying energy for space exploration and settlements, commonly available solar cells made of silicon or gallium arsenide are still too heavy to be feasibly transported by rocket. To address this challenge, a wide variety of lightweight alternatives are being explored, including solar cells made of a thin layer of molybdenum selenide, which fall into the broader category of 2D transition metal dichalcogenide (2D TMDC) solar cells. According to the researchers, their proposed device design can improve the efficiency of 2D TMDC devices from five per cent, as has already been demonstrated, to 12 per cent.
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‘I think people are slowly coming to the realisation that 2D TMDCs are excellent photovoltaic materials, although not for terrestrial applications, but for applications that are mobile – more flexible, like space-based applications,’ said Deep Jariwala, an assistant professor in the Department of Electrical and Systems Engineering. ‘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.’
While 2D TMDC solar cells aren’t as efficient as silicon solar cells, they produce more electricity per unit of weight, a property known as ‘specific power’. This is because a layer that is just three to five nanometres thick – or more than 1,000 times thinner than a human hair – absorbs an amount of sunlight comparable to commercially available solar cells. Their extreme thinness is what earns them the label ‘2D’ – they are considered to be ‘flat’ because they are only a few atoms thick.
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‘High specific power is actually one of the greatest goals of any space-based light-harvesting or energy-harvesting technology,’ said 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 hasn’t yet been fully realised, so Jariwala and his team have sought to raise the efficiency of the cells even further. Typically, the performance of this type of solar cell is optimised through the fabrication of a series of test devices, but Jariwala’s team believes that it’s important to do so through modelling it computationally.
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Additionally, the team thinks that to truly push the limits of efficiency, it’s essential to properly account for one of the device’s defining – and challenging to model – features: excitons. Excitons are produced when the solar cell absorbs sunlight and their dominant presence is the reason why a 2D TMDC solar cell has such high solar absorption. Electricity is produced by the solar cell when the positively and negatively charged components of an exciton are funnelled off to separate electrodes.
By modelling the solar cells in this way, the team was able to devise a design with double the efficiency compared to what has already been demonstrated experimentally. ‘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,’ said 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 per cent value,’ he continued. ‘Given that the current efficiencies are less than five per cent, my hope is that in the next four to five years, people can actually demonstrate cells that are ten per cent and upwards in efficiency.’
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According to Jariwala, 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,’ he said. ‘We need a way to grow these materials directly one on top of the other.’
The research has been published in Device.