1-µm-Thick Solar Cell Could Advance Wearable Electronics

An ultrathin photovoltaic that is flexible enough to wrap around an object just 1-mm thick may offer the reliability and flexibility that wearable technologies require, at a performance level comparable to thicker photovoltaics.

Researchers at the Gwangju Institute of Science and Technology used transfer printing to make ultrathin, vertical type GaAs photovoltaic devices with a total thickness of 1 μm — about one quarter the size of similar devices with a lateral design.

Ultrathin solar cells are flexible enough to bend around small objects, such as the 1-mm-thick edge of a glass slide, as shown here. Courtesy of Juho Kim, et al./APL.

To achieve the desired thinness, the researchers transfer-printed their device, made from gallium arsenide, directly onto metal electrodes on flexible receiver substrates for a direct electrical interconnection, without the aid of an interlayer 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 melting a top layer of photoresist to act 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 to be recycled.

Systematic studies with four different types of solar microcells demonstrated that the vertical-type solar cells generated a level of electric power comparable to that of thicker cells. The experimental results also showed that the ultrathin vertical-type solar cells were durable under extreme bending and thus suitable for use in the manufacturing of wearable flexible electronics. In bending tests, the ultrathin cells demonstrated the ability to wrap around a radius as small as 1.4 mm.

"The thinner cells are less fragile under bending, but perform similarly or even slightly better," said researcher Jongho Lee.

The team also performed numerical analysis of the cells and found that they experienced one-fourth the amount of strain of similar cells that were 3.5 μm thick.

A few other groups have reported solar cells with thicknesses of around 1 μm, using different methods to produce them. The method developed by Lee and his colleagues, which uses transfer printing instead of etching, can be used to make flexible photovoltaics with a smaller amount of materials.

Ultrathin photovoltaic devices may benefit applications that require extreme mechanical flexibility with electrical performance capabilities similar to those of thicker devices. The thin cells can be integrated onto glasses frames or fabric and might power the next wave of wearable electronics, said Lee.

The research was published in Applied Physics Letters (doi: 10.1063/1.4954039)