Waterproof, wearable photovoltaics

A team of scientists from Japan-based research institute RIKEN and the University of Tokyo have developed a new type of ultra-thin organic photovoltaic (PV) cell, which can continue to generate electricity from sunlight even when soaked in water or exposed to mechanical stress, as cleaning in a washing machine. The research was led by Kenjiro Fukuda, a researcher at RIKEN, and Takao Someya, an electrical engineering professor at the University of Tokyo.

The members of the research group developed these extremely thin and flexible organic PV cells from crystalline polymer, with a chemical structure of PNTz4T. The ultra-thin device was mounted on an acrylic-based elastomer and the top side of the device was coated with an identical elastomer to prevent water infiltration. The elastomer thus allowed light to enter while preventing water and air from leaking into the cells, making it more efficient and durable.

In order to test the efficiency of the product, the researchers subjected the device to a variety of tests. The findings showed a strong energy efficiency of 7.9 per cent, producing a current of 7.86 milliwatt (mW) per square cm, when the current density was 13.8 milliampere per square cm at 0.57 V, based on simulated sunlight of 100 mW per square cm. To test its resistance to water, it was soaked in water and the efficiency decreased only by 5.4 per cent after being soaked for 120 minutes. On conducting a durability check, the PV cells remained at 80 per cent of the initial value even after 52 per cent mechanical compression for 20 cycles with 100 minutes of water exposure.

The batteries used in the device are only 3 micrometres thick and almost about two times efficient in converting solar rays into electricity than the present ultra-thin solar batteries available in the market. The efficiency of the cells also does not get affected after washing with detergent or even ironing.

Thus, this ultra-thin and stretchable material can be woven into wearable textiles or stitched on to clothes such as on the shoulder or on the back to generate solar power. This electricity could then be used to provide power to health monitors that record heartbeats or body temperature and provides early warning against medical problems.

Most of the healthcare textile-implantable devices need a steady power supply of several mW or more to continuously collect data, and the new textile-compatible PVs would be able to cater to this need. According to the researchers, these wearable sensors have opened up a new environmentally stable and robust avenue for long-term power procurement.


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