South Korean researchers have tweaked the engineering and manufacturing process of solar cells to yield ultra-thin photovoltaics, creating sheets so thin they can be wrapped around the average pencil.
To increase the possible applications for solar cells, researchers are aiming to increase their flexibility — a factor controlled by the thinness of the material. As a piece of material is bent, the stress on it increases farther out from the central plane. Thicker sheets have more material farther out, making them harder to bend.
The researchers started off by making the cells with gallium arsenide semiconductor material. They stamped the cells directly onto a flexible substrate, allowing them to eliminate the need for an adhesive, which would have added to the final cell’s thickness.
They then cold welded the cells to the electrode on the substrate by applying pressure at 170 degrees Celsius, melting a top layer of the photoresist, which acted as a temporary adhesive.
Once the photoresist was peeled away, the researchers were left with a direct metal to metal bond.
The result is a solar cell which is about one micrometer thick: much thinner than the average human hair, and two to four times thinner than most other thin photovoltaics. Due to this thinness, the cells can wrap around a radius as small as 1.4 mm. Under bending, the thinner cells are less fragile, and experience one-fourth the amount of strain of similar cells that are 3.5 micrometers thick.
Because of the way the cell was manufactured, the metal bottom layer also serves as a reflector, directing stray photons back to the solar cells. This improves its efficiency: in tests, the device was able to convert sunlight to electricity at a comparable efficiency to similar but thicker photovoltaics.
According to the researchers, other groups have successfully created solar cells with comparable thicknesses, but many of these approaches required the removal of the substrate layer by etching.
By using transfer printing instead of etching, the researchers are able to make very flexible photovoltaics with a smaller amount of materials.
The next step is to scale up the process to allow mass manufacture of these thin solar cells, then investigate the ability to integrate them onto glasses frames, fabrics and the next wave of wearable devices.
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