Mobiles 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.

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.

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.

A new type of material could allow solar cells to harvest far more light. Much more efficient solar cells may soon be possible as a result of technology that more efficiently captures and uses light. StarSolar, a startup based in Cambridge, MA, aims to capture and use photons that ordinarily pass through solar cells without generating electricity. The company, which is licensing technology developed at MIT, claims that its designs could make it possible to cut the cost of solar cells in half while maintaining high efficiency. This would make solar power about as cheap as electricity from the electric grid.

The effort uses a type of material called a photonic crystal that makes it possible to "do things with light that have never been done before," says John Joannopoulos, a professor of physics at MIT who heads the lab where the new designs for solar applications were developed. Photonic crystals, which can be engineered to reflect and diffract all the photons in specific wavelengths of light, have long been attractive for optical communications, in which the materials can be used to direct and sort light-borne data. Now new manufacturing processes could make the photonic crystals practical for much-larger-scale applications such as photovoltaics.

StarSolar's approach addresses a long-standing challenge in photovoltaics. Silicon, the active material that is used in most solar cells today, has to do double duty. It both absorbs incoming light and converts it into electricity. Solar cells could be cheaper if they used less silicon. If the silicon is made thinner than it is now, it may still retain its ability to convert the photons it absorbs into electricity. But fewer photons will be absorbed, decreasing the efficiency of the cell.

MIT researchers developed sophisticated computer simulations to understand how thin layers of photonic crystal could be engineered to capture and recycle the photons that slip through thin layers of silicon. Silicon easily absorbs blue light, but not red and infrared light. The researchers found that by creating a specific pattern of microscopic spheres of glass within a precisely designed photonic crystal, and then applying this pattern in a thin layer at the back of a solar cell, they could redirect unabsorbed photons back into the silicon.

Today's solar cells already reflect some of the light that passes through the silicon. But the photonic crystal has distinct advantages. Conventional solar cells are backed with a sheet of aluminum. The photonic crystal reflects more light than the aluminum does, especially once the aluminum oxidizes. And the photonic crystal diffracts the light so that it reenters the silicon at a low angle. The low angle prevents the light from escaping the silicon. Instead, it bounces around inside; this increases the chances of the light being absorbed and converted into electricity.

As a result, the photonic crystal can increase the efficiency of solar cells by up to 37 percent, says Peter Bermel, CTO and a cofounder of StarSolar. This makes it possible to use many times less silicon, he says, cutting costs enough to compete with electricity from the grid in many markets. The savings would be especially large now, since a current shortage in refined silicon is keeping solar-cell prices high and slowing the growth of solar-cell production.

The company plans to work with existing solar-cell makers, applying its photonic crystals with a machine added to the solar-cell makers' assembly lines, Bermel says. But StarSolar needs to choose a large-scale manufacturing technique that will allow it to produce the photon crystals inexpensively. What's needed is a way to cheaply arrange two materials in an orderly three-dimensional pattern. For example, microscopic spheres of glass would be arranged in rows and columns inside silicon. Currently, techniques such as e-beam lithography can be used, but that's too slow for large-scale manufacturing.

Shawn-Yu Lin, professor of physics at Rensselaer Polytechnic Institute, has developed a method for manufacturing eight-inch disks of photonic crystal--a measurement considerably larger than what can be done with conventional techniques. The method, which employs optical lithography similar to that used in the semiconductor industry, works best for a type of solar cell that concentrates light onto a small chunk of expensive semiconductor material. Such a device would require a relatively small amount of photonic crystal compared with conventional solar cells. Lin says the technique could be applied for more-conventional solar panels, although it would be expensive.

Another potentially less-expensive method, called interference lithography, creates orderly patterns in the photonic-crystal materials. The method is fast and uses machines that are far less expensive than those used for conventional optical lithography. It also requires fewer steps than Lin's existing process, so he says it could be far cheaper. Such methods have been developed by Henry Smith, professor of electrical engineering at MIT, who was not involved with the StarSolar-related work. Smith says his interference-lithography method could be used to build templates for imprinting photonic-crystal patterns on large areas.

Another promising technique is self-assembly, in which the chemical and physical properties of material building blocks are engineered so that they arrange themselves in orderly patterns on a surface. For example, Chekesha Liddell, professor of materials science and engineering at Cornell University, has engineered building blocks in the shape of peanuts and the caps of mushrooms that line up in rows because of the way they fit together and the tug of short-range forces between them. She says this could be useful for assembling photonic crystals for solar cells.

With such approaches available, Bermel says that StarSolar hopes to have a prototype solar cell within a year and a pilot manufacturing line operating in 2008.