A team of scientists in South Korea have created ultra-thin photovoltaics that are flexible enough to wrap around an ordinary pencil. The bendy solar cells could have future applications in wearable electronics like fitness trackers and smart glasses.
Unsurprisingly, thin materials tend to flex more easily than thick ones-- think a piece of paper versus a cardboard shipping box-- this is because the stress in a material while that’s being bent increases farther out from the central plane.
"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 thinner than the average human hair. Typical PVs are usually hundreds of times thicker, and even most thin types are two to four times thicker.
The researchers made the ultra-thin solar cells using the semiconductor gallium arsenide. They stamped the cells directly onto a flexible substrate without using an adhesive that would add to the thickness. The cells were then "cold welded" to the electrode on the substrate, which means that they applied pressure at 170 degrees Celsius and melted a top layer of material called photoresist that acted as a temporary adhesive.
The photoresist was later peeled away, which left a direct metal-to-metal bond.
The metal bottom layer served as a reflector to direct stray photons back to the solar cells.
In order to test the device’s efficiency when it came to converting sunlight to electricity, the researchers found that it was comparable to similar, yet thicker, photovoltaics. They even performed bending tests and discovered that the cells could wrap around a radius as small as 1.4 millimeters – like a pencil.
"The thinner cells are less fragile under bending, but perform similarly or even slightly better," said Lee.
A few other groups have reported solar cells with thicknesses of around 1 micrometer, but have produced the cells in different ways. However, Lee’s method can be used to make very flexible photovoltaics with a smaller amount of materials.