Thin-film solar cells boost efficiency.

In small villages throughout the developing world, electricity is a scarce commodity. Lacking overhead power lines, or any access to a large power source, villagers in India, Africa, South America, and Eastern Asia often rely on precarious diesel generators to light a few lamps, pump a well, or cool their perishable foods. Even the sun's boundless energy proves limited without an efficient means of capturing and storing power for use when needed.

Recent improvements in solar-power technology may make sun-derived energy more available to a host of potential users.

Rommel Noufi, a chemist at the National Renewable Energy Laboratory in Golden, Colo., reports the highest confirmed conversion efficiency - 15.9 percent - for a polycrystalline thin-film solar cell. The new film comes from many subtle improvements in the design and manufacture of a particular type of copper indium gallium diselenide material, which has been under study for nearly a decade. Noufi detailed the advance this week in Boston at a meeting of the Materials Research Society,

"We achieved such high efficiencies because we've finally acquired a deep enough fundamental understanding of this material, and the processes for making it, to make a road map of the reaction pathways," Noufi says. "What we've lacked until recently is a rich picture of the material's mechanisms. Now we have a specific chemical pathway for a high-quality material."

"Our biggest challenge now is to look at enough reaction pathways to develop a universal understanding of photovoltaic materials so that we can reach even higher efficiencies? he adds. For this type of multilayered device, the theoretical limit for efficiency is roughly 23 percent.

Other types of photovoltaic material, notably single-crystal materials, have achieved higher efficiencies, in some cases near 30 percent. But these remain expensive to make and thus aren't useful for many applications in which cost per watt is a significant factor, Noufi says. The chief advantage of the new material is that it can be mass-produced at low cost in large quantities - which could significantly increase its use in poor, remote regions.

"The fact that a polycrystalline thin-film material is getting so close in efficiency to single-crystal materials, for much less cost, is a significant achievement? says Bulent M. Basol, a chemist at International Solar Electric Technology in Inglewood, Calif. "A photovoltaic cell with 30 percent efficiency for $20 per watt may be good for a few applications but not for most. Some of the newer single-crystal photovoltaic cells cost about $4 or $5 per watt, but that is still too expensive."

"But this kind of thin film, produced in large volumes, could cost less than $1 per watt, which would make it useful to a wide range of applications that today are not cost-effective? says Basol. "So from an industrial point of view, this could be a big breakthrough."

Polycrystalline thin-film photovoltaic devices have great potential for use in remote sites, according to Basol. In space, for example, the cells could power satellites, unmanned explorers, or roving vehicles. On Earth, they could help run irrigation systems, provide additional power for trains or telephone systems, pump wells, or just keep a village lighted.

"In Africa, a village may need electrification for a few homes, or a lantern, or a refrigeration unit. Something as small as this can really improve people's lives. But what always stops these simple, immediate applications is the cost per watt. It's just been too expensive. Now, maybe, this will change," says Basol.