Some solar devices, like calculators, only need a small panel of
solar cells to function. But supplying enough power to meet all our daily needs
would require enormous solar panels. And solar-powered energy collected by
panels made of silicon, a semiconductor material, is limited -- contemporary
panel technology can only convert approximately seven percent of optical solar
waves into electric current.
Profs. Koby Scheuer, Yael Hanin and Amir Boag of Tel Aviv
University's Department of Physical Electronics and its innovative new
Renewable Energy Center are now developing a solar panel composed of
nano-antennas instead of semiconductors. By adapting classic metallic antennas
to absorb light waves at optical frequencies, a much higher conversion rate
from light into useable energy could be achieved. Such efficiency, combined
with a lower material cost, would mean a cost-effective way to harvest and
utilize "green" energy.
The technology was recently presented at Photonics West in San
Francisco and published in the conference proceedings.
Receiving and transmitting green energy
Both radio and optical waves are electromagnetic energy, Prof.
Scheuer explains. When these waves are harvested, electrons are generated that
can be converted into electric current. Traditionally, detectors based on
semiconducting materials like silicon are used to interface with light, while
radio waves are captured by antenna.
For optimal absorption, the antenna dimensions must correspond to
the light's very short wavelength -- a challenge in optical frequencies that
plagued engineers in the past, but now we are able to fabricate antennas less
than a micron in length. To test the efficacy of their antennas, Prof. Scheuer
and his colleagues measured their ability to absorb and remit energy. "In
order to function, an antenna must form a circuit, receiving and
transmitting," says Prof. Scheuer, who points to the example of a cell
phone, whose small, hidden antenna both receives and transmits radio waves in
order to complete a call or send a message.
By illuminating the antennas, the researchers were able to measure
the antennas' ability to re-emit radiation efficiently, and determine how much
power is lost in the circuit -- a simple matter of measuring the wattage going
in and coming back out. Initial tests indicate that 95 percent of the wattage
going into the antenna comes out, meaning that only five percent is wasted.
According to Prof. Scheuer, these "old school" antennas
also have greater potential for solar energy because they can collect
wavelengths across a much broader spectrum of light. The solar spectrum is very
broad, he explains, with UV or infrared rays ranging from ten microns to less
than two hundred nanometers. No semiconductor can handle this broad a spectrum,
and they absorb only a fraction of the available energy. A group of antennas,
however, can be manufactured in different lengths with the same materials and
process, exploiting the entire available spectrum of light.
When finished, the team's new solar panels will be large sheets of
plastic which, with the use of a nano-imprinting lithography machine, will be
imprinted with varying lengths and shapes of metallic antennas.
Improving solar power's bottom line
The researchers have already constructed a model of a possible
solar panel. The next step, says Prof. Scheuer, is to focus on the conversion
process -- how electromagnetic energy becomes electric current, and how the
process can be improved.
The goal is not only to improve the efficiency of solar panels,
but also to make the technology a viable option in terms of cost. Silicon is a
relatively inexpensive semiconductor, but in order to obtain sufficient power
from antennas, you need a very large panel -- which becomes expensive. Green
energy sources need to be evaluated not only by what they can contribute
environmentally, but also the return on every dollar invested, Prof. Scheuer
notes. "Our antenna is based on metal -- aluminium and gold -- in very
small quantities. It has the potential to be more efficient and less
expensive."
