Soon electricity from the T-shirt?

Researchers are developing organic solar cells that are stretchable and washable at the same time

The elastic, printed solar cell survives the washing © RIKEN
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Waterproof and flexible at the same time: A new organic solar cell could pave the way for electricity-producing clothing. Because the ultrathin, stretchable modules survive even 20 washes without breaking, as Japanese researchers report in the journal "Nature Energy". The waterproof, flexible solar cells they developed could therefore be printed on textiles in the future and thus supply the power for sensors and other smaller electronic applications.

Sensors have long since become so small that they can easily be integrated into bracelets, textiles and even plasters. For example, the mini gauges monitor our body temperature or blood sugar levels, measure our movement and heart rate, or analyze our sweat and determine the percentage in our blood. Novel fabrics can shine in the dark thanks to integrated LEDs.

Who supplies the electricity?

But all these new developments have a catch: they need electricity - and so far there is a shortage of small and flexible suppliers for it. One solution could be organic solar cells integrated into clothing. Because they are ultra-thin and elastic enough.

So far, however, it has lacked in its water resistance: If the polymer substrate of the solar cell is made so thin that it is stretchy, it is no longer tight enough to protect the sensitive electronics from water However, a solution could now Hiroaki Jinno of the Japanese RIKEN Research Center and his colleagues have found: "We have developed washable polymer solar cells that have high efficiency and ductility, " they report.

Stable even in the dipping bath

The new solar cells consist of a photovoltaically active layer that is only one micron thick and embedded between two layers of parylene - a water-repellent, transparent polymer. "These organic solar cells can be easily adapted to the wrinkles of clothing or skin and are mechanically highly flexible, " said the researchers. display

Even this "raw" solar cell is relatively stable in water: it survives two-hour immersion baths with a degradation of only about 20 percent, as reported by Jinno and his colleagues. Even a wash with detergents already keeps out this solar cell: "We demonstrated this by washing out a stain of black, water-soluble paint from these materials, " the scientists explain.

Overcomes 20 washings

To further optimize water protection, the scientists coated this three-micron thin raw solar cell on both sides with a 500 micrometer-thick elastomer. The decisive factor here: These protective layers were pre-stretched by 200 percent at the moment of joining. Once the layers were joined, this polymer contracted again and the solar cell inside corrugated.

The advantage: The organic solar cell together with protective layers could easily be stretched or compressed by around 50 percent. She lost a maximum of five percent of her performance in these procedures. At the same time she also withstood multiple mechanical stress in the water. Even after 20 "washings", the solar cell still retained about 80 percent of its initial efficiency, as the researchers report. A T-shirt printed with these solar modules would still be functional even after washing.

Performance is enough for wearables

"We hope that our washable, lightweight and ductile organic photovoltaic modules will open up new possibilities as power sources for portable sensors and other wearables, " said co-author Kenjiro Fukuda of RIKEN Research Center. After all, their new solar cell has an efficiency of 7.9 percent. At a light incidence of 100 milliwatts, one square centimeter of this waterproof, stretchable module produced 13.8 milliamps of current and 0.57 volts of voltage, the researchers report.

Although this is not much, it could be enough to supply small sensors with energy even if the surface area is a little larger. Similarly, Ning Li and Christoph Brabec from the University of Erlangen-N rnberg agree: "We believe that such washable and portable solar cells could even become a unique market opportunity for organic photovoltaics ", They write in an accompanying commentary. (Nature Energy, 2017; doi: 10.1038 / s41560-017-0001-3)

(RIKEN, 19.09.2017 - NPO)