Researchers from the Japanese research institute RIKEN and the University of Tokyo have developed a new type of ultrathin, flexible organic solar cell, coated on both sides with elastomers to make it stretchable and waterproof.
After soaking the device in water for two hours, the researchers found that the efficiency had decreased by only 5.4%. After compressing it by nearly half for 20 cycles while also exposing it to water, the device still maintained 80% of its original efficiency.
Stretchable and waterproof — two important properties for wearable photovoltaics
Wearable electronics and so-called e-textiles comprise many sensors and other components that require electricity for tasks such as continuously collecting data, e.g., to monitor vital signs. Thus, Kenjiro Fukuda, scientist on the RIKEN Thin-Film Device Laboratory’s Emergent Soft System Research Team, says the development of robust and textile-compatible renewable power sources applicable to real-life situations is one of the most important topics for future self-powered wearable electronics. The joints of our arms and feet are stretched by 55% just during the basic act of walking, so any viable wearable power source will need to withstand this kind of expansion and contraction. What is more, the expert notes that such fabrics with integrated photovoltaics will need to be washable. The organic solar cells need to withstand being immersed and stirred in water.
Adding coats of elastomers
“The main breakthrough was adding a new, simple idea to our ultraflexible organic devices,” Fukuda says. “The idea is to coat them on both sides with ultrathin (3-micron-thick) elastomers that simultaneously realize stretchability and stability in water, whilst maintaining a high efficiency — 7.9% for a maximum value.”
Redesigning flexible organic solar cells
The research advance accomplished in Japan could indeed impact the development of the next generation of flexible organic photovoltaics: “In this study, we suggest a new design strategy for stretchable organic photovoltaic devices,” says Fukuda, confirming that the structure of ultrathin photovoltaic devices covered with transparent elastomers could also be used for other thin-film photovoltaic devices, such as perovskite solar cells. “This structure should be envisaged for practical wearable applications to add stretchability to the devices and prevent them from both environmental and mechanical damages obtained from scratching, shearing and squeezing.”
Fukuda and his colleagues used films made from a PNTz4T/PCBM blend as organic active layer their ultrathin solar cells. The rigid solar cell with glass substrates and the same active layer exhibits an efficiency of more than 10%. The flexible ultrathin solar cell coated with elastomers, on the other hand, performed 7.9% efficient. “The decrease might be originated from the relatively high thermal expansion rate and/or from the surface roughness of the ultrathin (1-micron-thick) polymer substrates,” the expert offers. “Although we do not really need extremely high performance, because our solar cells can cover a large area, further improvement of the energy conversion efficiency is very important for the practical devices,” says Fukuda. “From this view point, our target performance is comparable efficiency with organic solar cells fabricated on rigid glass substrates — in particular, more than 10%.”
Potential applications for this new kind of flexible organic photovoltaics are textile-compatible power sources to operate the sensors or other devices. Wearable devices that collect data continuously require a steady energy supply of several milliwatts or more. “Our solar cells can easily be attached onto the clothes and possess enough stretchability,” Fukuda assures. “As textile-compatible power sources, our washable, lightweight and stretchable OPVs will open a new avenue for use as a longterm power source system for wearables, electronic textiles and other sensors for the Internet of Things in the future.”
Moving forward with this research endeavor, the team wants to focus on improving both the energy conversion efficiency and longterm stability of the OPVs, trying to employ better active and passivation materials and structures.
The research is detailed in the paper “Stretchable and waterproof elastomer-coated organic photovoltaics for washable electronic textile applications,” co-authored by Fukuda and published in Nature Energy.
Written by Sandra Henderson, research editor Solar Novus Today