25 April 2012
Researchers have unveiled a sub-2-micron-thick organic solar cell that is the result of a collaborative research project between the Johannes Kepler University (JKU), Linz (Austria), and the University of Tokyo, Tokyo (Japan). The ultrathin-film devise is thinner than a thread of spider silk; so thin and elastic, in fact, it wraps around a human hair. Demonstrating equal power conversion efficiency to their glass-based counterparts, the novel OPV cells could pave the way to solar applications such as supplying power to medical sensors built into clothing, so-called “electronic skin” and surface conforming foils.
“The cells we made aren't the most efficient OPV solar cells, nor do they have the longest lifetimes. But they are the thinnest, lightest and most flexible solar cells ever made,” says Matthew S. White, a physics graduate of the University of Colorado Boulder, Boulder (US) who now is an assistant professor at JKU’s Linz Institute for Organic Solar Cells (LIOS). Dr. White is also a co-author of the paper “Ultrathin and lightweight organic solar cells with high flexibility,” recently published in the online journal Nature Communications.
The total thickness of the ultrathin photovoltaic devices on plastic foil substrates is approximately 1.9 micrometers — about one tenth the size of the thinnest solar cells currently available. “The solar cells are made of a conducting polymer transparent electrode, a blend of a semiconducting polymer and fullerene to absorb the light and harvest the energy, and a metal back electrode,” Dr. White explains. Both the materials used as well as the processing methods are laboratory standard. Says Dr. White: “Because OPV solar cells are so thin, the very important physical properties of weight and flexibility are entirely determined by the substrate. So we found the thinnest available plastic foil and tried to devise a successful way to construct the cells directly onto it.” The cells they made are “just over 4% efficient solar cells.” Ultimately, the researcher’s goal is to match the efficiency of current state-of-the-art OPV cells, which he puts at 9 to 11%. “We showed in this work that reducing the substrate thickness to 1.4 micrometers does not reduce the efficiency of the solar cell,” he says.
It is the devices’ record-breaking thinness in combination with its light weight and flexibility that could make a significant difference in future solar applications. “Sometimes, the available energy is not limited by area only,” Dr. White elaborates. “If someone goes into the wilderness, there may be miles and miles of open space, but there is no way they can carry miles and miles worth of conventional solar cells. A 1-mile-long, 1-meter-wide strip of this material would only weigh 6.4 kg, or 14 lbs,” he says. “It may also be desirable to harvest solar energy from the surface of something that has many moving parts. If you simply want to paste a solar cell — like wallpaper — to the outside of something like that, then the flexibility allows you to put it right down into the corners, and even at the moving joints. It shows that electronics can be applied right to your skin, even at the elbow and knee joints that bend and crease.”
The scientists estimate the technology could be put to practical use in about five years. Dr. White says, “Combining the most efficient materials, the most stable materials and the thinnest possible substrate would be the pathway to commercialize ultra-thin OPV.”
Written by Sandra Henderson, Research Editor, Solar Novus Today