The new ultra-thin solar cells are flexible enough to bend around small objects, such as the 1mm-thick edge of a glass slide, as shown in this photo. Photo: Juho Kim, et al/APL.Scientists in South Korea have made ultra-thin photovoltaics that are flexible enough to wrap around the average pencil. The bendy solar cells could power wearable electronics like fitness trackers and smart glasses. The researchers report their work in a paper in Applied Physics Letters.
Thin materials flex more easily than thick ones – think a sheet of paper versus a piece of cardboard. The reason for the difference is that the stress in a material as 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 1µm thick," said Jongho Lee, an engineer at the Gwangju Institute of Science and Technology in South Korea. Standard photovoltaics are usually hundreds of times thicker, and even most other thin photovoltaics are two to four times thicker.
The researchers made their 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 depositing a top layer of photoresist that acted as a temporary adhesive. This 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 also performed bending tests and found the cells could wrap around a radius as small as 1.4mm.
When the researchers conducted numerical analysis of the ultra-thin solar cells, they found that the cells experience just one-fourth the amount of strain of similar cells that are 3.5µm 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 1µm, but have produced the cells in different ways, such as by removing the whole substrate by etching. By transfer printing instead of etching, the new method developed by Lee and his colleagues could be used to make very flexible photovoltaics with a smaller amount of materials.
The ultra-thin cells could be integrated onto glasses frames or fabric, and might power the next wave of wearable electronics, Lee said.
This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.