Plastic Solar Cells Roll Into Unlit Villages

By day, the electronic devices that Frederik Krebs rolls off his printing presses could be mistaken for old plastic overhead-projector transparencies. Nightfall reveals their ingenious purpose: Snap the metal fasteners at the corners together and the sheets glow with reading-quality light. Krebs's sheets may prove to be much more than a curiosity, for the senior scientist at Denmark's Risø National Laboratory for Sustainable Energy has found a cheap way to integrate LEDs, photovoltaic (PV) cells, and ultrathin lithium batteries into a potentially life-saving lamp. He hopes to see them on sale next year, providing an affordable alternative to kerosene lighting for the more than 1.5 billion people in developing countries who lack access to electricity.

Success would also mark an important first step to commercialization for the lamp's cheap-to-produce yet anemically inefficient organic photovoltaic technology. Most organic PVs are composed of conducting polymers and carbon nanostructures, which in the right combinations mimic the p-n junction of silicon and other inorganic photovoltaics. Efficiency is significantly lower, however, because polymers are poor charge conductors. "This is the lousiest of the solar technologies available," admits Krebs.

That's not for want of trying. Andrew W. Hannah, CEO of organic electronics materials developer Plextronics, says materials advances are bringing polymer PVs within reach of some niche applications. He says testing at the U.S. Department of Energy's National Renewable Energy Laboratory, in Golden, Colo., is confirming that polymer PV materials can survive outdoors for years if they're effectively encapsulated from air and water. Efficiency is rising too. Hannah says that Plextronics, based in Pittsburgh, will release a new set of polymer "inks" in the first quarter of 2010 capable of delivering 6 to 6.5 percent efficiency in small cells—a 1 percentage point improvement over Plextronics's prior best. These specs should, Hannah predicts, enable product development firms to begin using polymer PV materials in portable, low-energy applications such as battery charging and distributed sensing.

Large modules of organic photovoltaics like Krebs's, however, capture just 1 to 2 percent of the photon energy that hits them. And yet, Krebs says, even that measly return adds value in the price-sensitive context of rural lighting.

Access to sustainable lighting remains a tough nut for engineers to crack [see "Lighting Up the Andes," IEEE Spectrum, December 2004]. Off-grid villagers in Africa, Asia, and Latin America still rely on kerosene lamps and candles, with the average household spending US $40 to $80 annually for their dim, soot-belching light, according to Germany's development agency, the Deutsche Gesellschaft für Technische Zusammenarbeit, or GTZ. A 2009 assessment by GTZ found that the cheapest solar-LED lighting solutions marketed today cost as much as a full year's worth of kerosene.

Krebs's lamps should crash through that cost barrier. He prints their polymer solar cells and circuitry onto rolls of 25-micrometer-thick flexible plastic film by the hundreds of square meters, using standard screen and slot-die presses. Next, a circuit of copper tape is printed onto the solar cells, and the components—surface-mounted LEDs, flat batteries, and a diode—are mounted using silver epoxy. The whole thing is then encapsulated in a second sheet of film.

The total cost of the prototypes produced in Krebs's Copenhagen-area lab is 18 ($27) per lamp. Krebs says that commercial partner Mekoprint Electronics, in Støvring, aims to produce an improved version this fall for 7. He estimates that cheaper labor in China could cut Mekoprint's cost in half.

Mekoprint's version will benefit from the field testing of 196 prototypes in Zambia this summer. Danish business school students handed out the lamps to children attending the summer school the Danes run at a coffee farm outside of Lusaka, a grid-connected area plagued by daily blackouts. Excessive flexing of the 1-millimeter-thick sheets caused some to delaminate, a problem that Krebs and Mekoprint hope to fix by repositioning the solar cells within the plastic sheet. And the quality of the light was found to degrade after 10 to 20 minutes, inspiring the addition of circuitry to feed power to the LEDs at a constant current. Testing of a further 1000 to 2000 next-gen lamps in Mali and Malawi is anticipated after the fall rainy season.

Krebs expects the lamps to operate for at least a year, which he deems reasonable for a lamp that will cost less than one-quarter of the buyers' present annual lighting budget. "This is an affordable lamp, and it's better than no lamp," he says.