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The prototype solar cell can be wrapped around small objects like the edge of this glass laboratory slide Image courtesy of Juho Kim Applied Physics Letters
<p>The prototype solar cell can be wrapped around small objects like the edge of this glass laboratory slide. (Image courtesy of Juho Kim, Applied Physics Letters).</p>

Flexible Solar Cells That Can Wrap Around a Pencil

The idea of solar cells usually conjures the image of broad panels lying in the sun like scorched saltines. But in the latest probe into the physical limits of the technology, South Korean scientists have made solar cells thin and flexible enough to be wrapped around a pencil.

The new cells are more than 50 times thinner than human hairs and over 1,000 times thinner than most conventional solar cells. The cells were thinned down to the point where they could be wrapped around objects or fit the contours of unstructured materials like fabric. The researchers 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 who worked on the project at the Gwangju Institute of Science and Technology in South Korea. “The thinner cells are less fragile under bending,” experiencing only a fraction of the strain on solar cells many times thicker.

One of the problems with extremely thin solar cells is that they are more prone to energy loss than thicker versions. However, the new solar cell contains a metal electrode that reflects stray particles of sunlight back into the cell, increasing its overall efficiency.

In fact, the new solar cell, which consists of gallium arsenide semiconductor material, is actually more efficient than thicker cells. In laboratory tests, the prototype cell converted 15.2% of sunlight into electricity, while a 4-micrometer cell only achieved 14%. Normally, thicker cells inherently absorb more photons from sunlight.

The energy harvested by the prototype is meant to supplement the tiny batteries inside wearable devices, like smart watches or medical devices, extending their usually short battery lives. They could be integrated on the outside of fitness bands, for instance, enabling them to gather energy while users run on a sunny afternoon.

Other energy harvesting methods have strayed into the outlandish. Some devices attempt to generate electric current from human motions like running, walking, or tapping your fingers. Others filter out the energy from ambient wireless signals like Wi-Fi and millimeter waves.

In recent years, making new kinds of solar cells has become an admired pastime for scientists. Earlier this year, a team from the Massachusetts Institute of Technology made solar cells about two micrometers thick. The cells were actually light enough to be balanced on top of a soap bubble without popping it.

“It could be so light that you don’t even know it’s there, on your shirt or on your notebook,” said Vladimir Bulovic, an MIT professor that worked on the project. “These cells could simply be an add-on to existing structures.”

Bulovic’s project used vapor deposition to simultaneously grow all the layers of the solar cell, making it thinner and more flexible than possible with normal techniques. The South Korean scientists, on the other hand, used transfer printing to paint the solar cells directly onto a flexible substrate.

The cells were coated onto metal electrodes—a process recently outlined in the journal Applied Physics Letters. The scientists bonded the cells onto the electrodes by applying extreme pressure, using a material known as a photoresist to act as a temporary adhesive between the cell, metal, and substrate.

Once the solar cells were successfully bonded to the metal electrode, the scientists peeled away the photoresist to leave the finished device. Any permanent adhesive would have added to its thickness and weight, Gwangju’s Lee said.

Correction June 30, 2016: An earlier version of this article failed to include how efficient the flexible solar cells were. The cells built at the Gwangju Institute of Science and Technology were 15.2% efficient. The scientists found that their cells were more efficient than others made with the same material but four times thicker.

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