AsianScientist (Jun. 30, 2016) – Scientists in South Korea have made ultra-thin photovoltaics flexible enough to wrap around the average pencil.
Thin materials flex more easily than thick ones—think a piece of paper versus a cardboard shipping box. The reason for the difference is that 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 one micrometer thick,” said lead author Assistant Professor Lee Jongho from the Gwangju Institute of Science and Technology in South Korea.
To put things in perspective, one micrometer is much thinner than the average strand of human hair. Standard photovoltaics are usually hundreds of times thicker, and even most other thin photovoltaics are two to four 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 °C and melting a top layer of material called photoresist that acted as a temporary adhesive.
A direct metal-to-metal bond remained after the photoresist was peeled away. 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 but thicker photovoltaics. They performed bending tests and found the cells could wrap around a radius as small as 1.4 millimeters.
“The thinner cells are less fragile under bending, but perform similarly or even slightly better,” Lee said.
The bendy solar cells, which are reported in the Applied Physics Letters, could power wearable electronics like fitness trackers and smart glasses, said Lee.
The article can be found at: Kim et al. (2016) Ultra-Thin Flexible GaAs Photovoltaics in Vertical Forms Printed on Metal Surfaces without Interlayer Adhesives.
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Source: American Institute of Physics; Photo: Kim Juho et al./Applied Physics Letters.
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