Using gossamer-like layers of flexible polymers, researchers at MIT have created the thinnest and lightest solar cells ever made. Just one-fiftieth the thickness of a human hair, and capable of producing up to 6 watts of power per gram, these cells are so thin and light that they can be supported on the surface of a soap bubble without breaking it. With such impressive credentials, the prototype cells have the potential to add solar power to everything from paper-based electronics through to all manner of mobile devices and exceptionally lightweight wearables.
Though flexible solar cells are hardly a new innovation, even being produced experimentally in everything from continuously printed rolls to spray on panels, the power-to-weight ratio is where these new cells come into their own. With a demonstrated output of 6 watts per gram, they produce an output some 400 times greater than standard glass-covered solar cells that generate about 15 watts of power per kilogram on average.
This type of light-weight/high-output performance could be crucial for such things as reducing the mass of spacecraft or electric aircraft. Or it could simply make portable devices much easier to use and carry around.
"It could be so light that you don't even know it's there, on your shirt or on your notebook," said MIT Fariborz Maseeh Professor of Emerging Technology in MIT's School of Engineering, Vladimir Bulović. "These cells could simply be an add-on to existing structures."
Leading on from previous work on ultra-thin solar cells by professor Bulović, the team sought to confirm the hypothesis that solar cells could be made even more thin and flexible, while also being practical and robust. As such, the researchers employed the common flexible polymer parylene (a material resembling cling wrap – but ten times thinner – often used to protect electronic circuit boards and biomedical devices) for both the substrate and the coating, and DBP (Dibutyl phthalate, an organic material and a commonly used plasticizer), as the main light-absorbing layer.
Unlike many other solar cell production methods, the entire process is performed at room temperature without the use of any solvents. The substrate and the cell are simply produced using standard vapor deposition methods in a vacuum chamber.
"We put our carrier in a vacuum system, then we deposit everything else
on top of it, and then peel the whole thing off," said MIT research scientist, Annie Wang.
The most important aspect in making these new solar cells, according to professor Bulović, is in producing both the supporting layer (the substrate) and the protective top coating at the same time, thereby sealing the fragile photovoltaic layer from harm early in the manufacturing process. In this way, the dangers of tearing are minimized because the cells only require handling once in fabrication, and the exposure to contaminants or foreign particulates that may reduce the cell's performance are virtually eliminated.
"The innovative step is the realization that you can grow the substrate at the same time as you grow the device," said professor Bulović.
Though the team acknowledges that the materials used in the prototype were not selected for their on-going suitability for future manufacture, but merely to validate the hypothesis, it is the proving of the one-step substrate manufacturing and coating process that is the most important factor. As a result, the researchers believe that other materials, such as quantum dots or perovskites, could easily replace the organic compounds used making the prototype.
The work is still in the early stages, and there is no guarantee of producing a commercial device yet, but the team are confident that their initial prototype shows a great deal of promise for a slew of new and innovative applications for solar power in the future."We have a proof-of-concept that works," said professor Bulović. "How many miracles does it take to make it scalable? We think it's a lot of hard work ahead, but likely no miracles needed."
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