Harvesting solar energy from nano-thin films.

Researchers at the U.S. Army Soldier Systems Center in Natick, Mass., are developing prototypes of battery chargers and shelters that would harness solar energy through nanocomposite thin-film photovoltaics.

"It's really an exciting time right now, because there are a number of new photovoltaic technologies out there trying to drive down costs, allowing you to use them in ways that couldn't be used before," said Lynne Samuelson, a research chemist at Natick.

Photovoltaic technology has advanced from the large, heavy and expensive glass panels into smaller, lightweight and cheaper devices.

"Up until the last few years, those technologies weren't of interest to the military," said Samuelson. But now, she said, power is a number one priority for soldiers who carry an increasing amount of electronic equipment. They need something lightweight and portable to charge those devices, she said.

Her research team has produced thin-film photovoltaics, or PVs, that could eventually answer that call.

The thin-film PVs are made by coating nanoparticles of titanium dioxide with a light-harvesting dye and sandwiching them between two plastic-based electrodes, said Samuelson. The resulting device has the thickness of three sheets of paper. When light comes through the device, it hits the dye and an electron gets shuttled through the titanium dioxide to the other electrode. A redux mediator keeps the process running, she said.

The sheets of PVs can be cut to any length or width, she said. The longer the device, the more current it will produce. If you need to charge up a radio, for example, you know the watts and amps required, and you could design a PV to meet that specific requirement, she said.

The goal for these devices is to generate 30 watts per pound, said Samuelson. Soldiers currently use a battery, BA5590, that produces about 22 watts per pound. With the nanocomposite thin-film PVs, "you're providing a higher density, and it's renewable," said Samuelson.

Within the next year, these films will be incorporated into handheld battery-charger prototypes capable of recharging four AA batteries in two hours, said Samuelson. In the next two years, the team will incorporate the films into shelters as well.

"What's nice about this technology is you can do some unique things with it that you can't do with traditional materials." said Samuelson. For example, making a camouflage-pattern photovoltaic.

"Because we're using a light-harvesting dye, we can make colored patterns," she explained. "We can inkjet-print these dyes easily to make camouflage-colored PVs without having to put a mesh over it," she added.

Her team is also working on converting the thin-film PVs into fibers that could be weaved directly into textiles.

"It would make all the applications we're already going after even better," she said.