April 5, 2002 -- Forget about old concepts of a "power tie" or a "power suit." One day, clothing could provide electrical juice to energize a portable phone, hand-held computer, or portable music player.
Scientists at the University of California at Berkeley say their latest research work in producing cheap, plastic solar cells may lead to such stunning possibilities.
According to A. Paul Alivisatos, a chemistry professor and lead researcher on the project, the experimental solar cells use tiny rods of cadmium selenide, a material similar to those used in computer chips.
The rods measure just 7 nanometers — 7 billionths of a meter — wide and 60 nanometers long and are suspended in an organic polymer, or plastic. The mixture is then sandwiched between two electrodes, one of transparent plastic and the other of flexible aluminum.
The experimental cell works just like other commercially-available photovoltaic cell. When exposed to sunlight, the "nanorods" of cadmium selenide material yields an electron and a related "hole" or vacancy. The electrons move through the rod to the aluminum electrode while the hole moves toward the other electrode, creating "positive" and "negative" terminals, just like a battery.
And in lab tests, a prototype solar cell about 200 nanometers thick — one-thousandth the thickness of a human hair — can produce just over half the voltage of a common flashlight battery.
Cooler Production for Newer Applications
But where Berkeley's cells really differ from others is the way in which they are created.
"Today's high-efficiency solar cells require very sophisticated processing inside a clean room and complex engineering to make the semiconductor sandwiches," Alivisatos says. "And because they are baked inside a vacuum chamber, they have to be made relatively small."
But Alivisatos says the nanorods of the tiny solar cells are created in a simple laboratory flask at room temperatures. That not only cuts out the need — and cost — for complex ovens and manufacturing procedures, but could allow for unique uses.
"The beauty of this is that you could put solar cells directly on plastic, which has unlimited flexibility," says Janke J. Dittmer, a post-graduate fellow and one of Alivisatos' team members. "This opens up all sorts of new applications, like putting solar cells on clothing to power LEDs, radios or small computer processors."
Unproven for Now
Robert McConnell, a program manger for the Department of Energy's National Renewable Energy Labs in Golden, Colo., tends to agree with the exciting potential of the experimental photovoltaic cell — but tempers it with the technology's still many unknowns.
For one, McConnell says that the new cell is less than two percent efficient at converting light into electrical energy — a far cry from the 35 percent efficiency of the best quality photovoltaic cells available now. "This is a new horse in the race [for photovolatics]," says McConnell. "Right now, the new horse is pretty far behind."
In addition to efficiency, McConnell says that the Berkeley team's production methods have to be verified as a truly cost-efficient way of producing photovoltaic cells.
"Ultimately, what is important is how much electricity you can get out of it and how much you spend to make it," he says. "Right now, all we have is small lab samples that operate at very low efficiencies. We need to verify that we can make these in large quantities easily."
Alivisatos and his team admit that it may be years before the tiny power cells make it into a commercial product — especially since they are still so inefficient. "For this to really find widespread use, we will have to get up to around 10 percent efficiency," Alivisatos says. "But we think it's very doable."