Scientists in South Korea 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.
“Our photovoltaic is about one micrometer thick,” says Jongho Lee, an engineer at the Gwangju Institute of Science and Technology, 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 two to four times thicker.
The ultra-thin solar cells are made from the semiconductor gallium arsenide. The cells were stamped directly onto a flexible substrate without using an adhesive (which would add to the material’s thickness). 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. 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. 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 mm.
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 μm thick. “The thinner cells are less fragile under bending, but perform similarly or even slightly better,” Lee says.
A few other groups have reported solar cells with thicknesses of around 1 μm, 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 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 says.
Image caption — Ultra-thin solar cells are flexible enough to bend around small objects, such as the 1 mm-thick edge of a glass slide, as shown here. Courtesy of Juho Kim, et al./APL.
