Scientists in South Korea have developed ultra-thin photovoltaics flexible enough to wrap around the average pencil. The bendy solar cells could power future wearable electronics, such as fitness trackers and smart glasses.
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 a 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.
“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, with even most other thin photovoltaics being two to four times thicker.
The team also performed numerical analysis of the cells, finding that they experience a quarter 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.
Other researchers have reported solar cells with thicknesses of approximately one 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 could be used to make highly flexible photovoltaics with a smaller amount of materials. For example, the thin cells could be integrated on to glasses frames or fabric, thus powering the next wave of wearable electronics, Lee suggested.
The researchers reported their results in the journal Applied Physics Letters.