“Solar glasses” provide power as well as shade

The Solar Glasses
Smart shades: solar cells instead of traditional lenses can generate power

The dark lenses of sunglasses have been replaced with organic solar cells by scientists in Germany. The cells are able to power a small mircocontroller that sends information on ambient conditions to a couple of displays in the spectacle arms, and in future might provide power for personal devices such as hearing aids.

Organic solar cells are less efficient than conventional silicon devices and not as resistant to continuous strong sunlight, making them less suited to providing power from rooftops. But, according to team member Daniel Bahro of the Karlsruhe Institute of Technology (KIT), the fact that they are light, flexible and transparent opens up a number of new, previously impractical applications.

Bahro and colleagues designed the new “lenses” to have a similar weight and transmission spectrum to those in normal sunglasses. The lenses are made from a polymer and two types of fullerene molecules sandwiched between electrodes and layers of glass. They are then inserted into a commercially available plastic frame. Once connected to a printed circuit board and liquid crystal display in each spectacle arm, they provide information on the ambient light intensity and temperature.

Can’t handle the light

The team found that in outdoor light with an intensity of 1 “sun” the device converted just 0.06% of incoming power to electricity and yielded under a milliwatt of power. Bahro says that this was in part due to the use of a single piece of solar cell for each lens. He and his colleagues could instead have joined lots of narrow cells together in order to limit the “Ohmic losses” that result when charge carriers travel through a cell’s electrodes. However, he explains, doing so would have impaired vision.

The fact that Ohmic losses are proportional to the square of the current, which rises with light intensity, meant that the glasses performed proportionally much better at lower intensities. At 0.01 suns the efficiency of each cell reached 2.4%, which yielded an output of 400 μW. As such, the researchers tailored their electronics to duller conditions such as those typical to offices and other indoor environments.

At about 0.002 suns, which is typical of indoor lighting, each lens had an efficiency of 6.7% and produced around 200 μW. That is too low for mobile phones, common light-emitting diodes or portable music players, but, says Bahro, would be enough for hearing aids, remote controls and some wrist watches. He also believes the technology could reduce the size of the battery or limit the frequency of recharging in power-hungry “smart glasses”, such as the Google Glass headset.

Showing off

Bahro acknowledges that the technology is not yet practical for many applications, given the need for cables to join the glasses to wherever the power is required (as well as the fact that the device works best in conditions that make sunglasses largely redundant). But he says that he and his colleagues were not aiming to commercialise the technology; rather they intended it as a way of showing off the benefits of organic solar cells.

Indeed, Bahro adds that many visitors to the Hannover Messe trade fair in Germany this April, where the device was on display, couldn’t tell the difference between the solar glasses and normal sunglasses. “People usually think of solar cells as bluish-coloured modules,” he says. “They don’t expect them to be transparent, or coming in different shapes and colours.”

The research is published in the journal Energy Technology.

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