Flexible Batteries That Never Need to Be Recharged

European researchers have built prototypes that combine plastic solar cells with ultrathin, flexible batteries. But don't throw away your battery recharger just yet.

Mobile phones, remote controls, and other gadgets are generally convenient--that is, until their batteries go dead. For many consumers, having to routinely recharge or replace batteries remains the weakest link in portable electronics. To solve the problem, a group of European researchers say they've found a way to combine a thin-film organic solar cell with a new type of polymer battery, giving it the capability of recharging itself when exposed to natural or indoor light.

Solar battery: European researchers have integrated thin-film organic solar cells with a flexible polymer battery to produce a lightweight and ultrathin solar battery for low-wattage electronic devices, such as smart cards and mobile phones. The battery can recharge itself when exposed to natural or indoor sunlight, meaning that some electronic gadgets would never need a separate charger. Researchers predict that such a device could be commercially available in some products next year. Credit: G. Dennler, Solar Energy

It's not only ultraslim, but also flexible enough to integrate with a wide range of low-wattage electronic devices, including flat but bendable objects like a smart card and, potentially, mobile phones with curves. The results of the research, part of the three-year, five-country European Polymer Solar Battery project, were recently published online in the journal Solar Energy.

"It's the first time that a device combining energy creation and storage shows [such] tremendous properties," says Gilles Dennler, a coauthor of the paper and a researcher at solar startup Konarka Technologies, based in Lowell, MA. Prior to joining Konarka, Dennler was a professor at the Linz Institute for Organic Solar Cells at Johannes Kepler University, in Austria. "The potential for this type of product is large, given [that] there is a growing demand for portable self-rechargeable power supplies."

Prototypes of the solar battery weigh as little as two grams and are less than one millimeter thick. "The device is meant to ensure that the battery is always charged with optimum voltage, independently of the light intensity seen by the solar cell," according to the paper. Dennler says that a single cell delivers about 0.6 volts. By shaping a module with strips connected in series, "one can add on voltages to fit the requirements of the device."

The organic solar cell used in the prototype is the same technology being developed by Konarka. (See "Solar-Cell Rollout.") It's based on a mix of electrically conducting polymers and fullerenes. The cells can be cut or produced in special shapes and can be printed on a roll-to-roll machine at low temperature, offering the potential of low-cost, high-volume production.

To preserve the life of the cells, which are vulnerable to photodegradation after only a few hours of air exposure, the researchers encapsulated them inside a flexible gas barrier. This extended their life for about 3,000 hours. Project coordinator Denis Fichou, head of the Laboratory of Organic Nanostructures and Semiconductors, near Paris, says that the second important achievement of the European project was the incorporation into the device of an extremely thin and highly flexible lithium-polymer battery developed by German company VARTA-Microbattery, a partner in the research consortium. VARTA's batteries can be as thin as 0.1 millimeter and recharged more than 1,000 times, and they have a relatively high energy density. Already on the market, the battery is being used in Apple's new iPod nano.

Dennler says that the maturity of the battery and the imminent commercial release of Konarka-style organic solar cells mean that the kind of solar-battery device designed in the project could be available as early as next year, although achieving higher performance would be an ongoing pursuit.

The paper's coauthor Toby Meyer, cofounder of Swiss-based Solaronix, says that the prototypes worked well enough under low-light conditions, such as indoor window light, to be considered as a power source for some mobile phones. Artificial light, on the other hand, may impose limitations. "Office light is probably too weak to generate enough power for the given solar-cell surface available on the phone," he says.

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Watches, toys, RFID tags, smart cards, remote controls, and a variety of sensors are among the more likely applications, although the opportunity in the area of digital cameras, PDAs, and mobile phones will likely continue to drive research. "The feasibility of a polymer solar battery has been proven," the paper concludes.

Rights to the technology are held by Konarka, though the solar company says it has no plans itself to commercial the battery.