Ultra-Thin Solar Cells Bend Around a Pencil

Ultra-thin solar cells are flexible enough to bend around small objects, such as the 1mm-thick edge of a glass slide (Source: J. Kim / APL)

Ultra-thin solar cells are flexible enough to bend around small objects, such as the 1mm-thick edge of a glass slide (Source: J. Kim / APL)

Scientists in South Korea have made ultra-thin photo­voltaics flexible enough to wrap around the average pencil. The bendy solar cells could power wearable elec­tronics like fitness trackers and smart glasses. Thin materials flex more easily than thick ones because the stress in a material while it’s being bent increases farther out from the central plane. Because thick sheets have more material farther out they are harder to bend.

“Our photo­voltaic is about 1 micrometer thick,” said Jongho Lee, an engineer at the Gwangju Institute of Science and Techno­logy in South Korea. One micro­meter is much thinner than an average human hair. Standard photo­voltaics are usually hundreds of times thicker, and even most other thin photo­voltaics are 2 to 4 times thicker.

The researchers made the ultra-thin solar cells from the semi­conductor gallium arsenide. They stamped the cells directly onto a flexible substrate without using an adhesive that would add to the material’s thickness. The cells were then cold welded to the electrode on the substrate by applying pressure at 170 degrees Celcius and melting a top layer of material called photo­resist that acted as a temporary adhesive. The photo­resist was later peeled away, leaving the direct metal to metal bond.

The metal bottom layer also served as a reflector to direct stray photons back to the solar cells. The researchers tested the efficiency of the device at converting sunlight to elec­tricity and found that it was comparable to similar thicker photo­voltaics. They performed bending tests and found the cells could wrap around a radius as small as 1.4 millimeters.

The team also performed numerical analysis of the cells, finding that they expe­rience one-fourth the amount of strain of similar cells that are 3.5 micrometers thick. “The thinner cells are less fragile under bending, but perform similarly or even slightly better,” Lee said. A few other groups have reported solar cells with thick­nesses of around 1 micrometer, but have produced the cells in different ways, for example by removing the whole substract by etching.

By transfer printing instead of etching, the new method deve­loped by Lee and his colleagues may be used to make very flexible photo­voltaics with a smaller amount of materials. The thin cells can be inte­grated onto glasses frames or fabric and might power the next wave of wearable elec­tronics, Lee said. (Source: AIP)

Reference: J. Kim et al.: Ultra-thin flexible GaAs photovoltaics in vertical forms printed on metal surfaces without interlayer adhesives, Appl. Phys. Lett. 108, 253101 (2016); DOI: 10.1063/1.4954039

Link: Research Inst. f. Solar and Sustainable Energies (RISE), Gwangju Inst. of Science and Technology (GIST), Gwangju, South Korea

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