Department News

Physics Team Sees Potential for 'Perfect' Solar Cell

BC physicists (L-R) Yang Wang, Zhifeng Ren, Kris Kempa and Andrzej Herczynski have devised a microscopic antenna composed of carbon nanotubes (pictured below) that may help speed the conversion of light to electricity. (Photo by Gary Gilbert)

Tiny antenna devised by BC lab could be key to light-energy conversion

Boston College physicists have teamed on a microscopic antenna that captures visible light in much the same way that radio antennas capture radio waves.

They say the device, an array of carbon nanotubes one-70th of a human hair in length, may significantly advance the conversion of solar energy into electricity, and allow cable hookups thousands of times speedier than current broadband.

"An ultimate aim is the perfect solar cell that can convert light into electricity," at a much higher rate of efficiency than conventional solar cells, said Prof. Kris Kempa (Physics), a lead author of an article describing the invention in the Sept. 27 issue of Applied Physics Letters.

A conventional solar cell used in solar heating units is, at most, only about 30 percent efficient at converting light to electricity, and even at that rate, is quite expensive, researchers said.

They envision the antenna of carbon nanotubes, microscopic structures built of carbon atoms, converting light to energy at possibly 80-percent efficiency at a much lower cost.

"We're trying our best to make it possible, as soon as possible," said the other lead author on the article, Yang Wang, a second-year doctoral student in the lab of Prof. Zhifeng Ren (Physics), a co-author of the APL piece.

The research team currently is working on a piece needed to make the light antenna work, a diode, or electrical valve, which admits electrons but doesn't let them escape, thus allowing the energy charge to be stored.

An array of aligned, but randomly placed, carbon nanotubes which can act like a radio antenna for detecting light at visible wave- lengths. The scale bar is one micron in length.

Another use envisioned for the light antenna is in fiber optics that would allow home computer and TV cable service at speeds thousands of times faster than current broadband.

Radio and television signals are captured using antennas close to the size of the wavelength of broadcast radiation. These are often huge, thus the need for tall antennas.

In a receiver, the wave excites electrons into meaningful currents, which are amplified and tuned to carry sound and pictures. But light is carried by photons - tiny packages that have the properties of waves and particles.

No one had been able to make a device small enough to act as an antenna for light until the team fashioned one from the microscopic nanotubes built out of carbon atoms.

"Nobody had tried it in optical frequencies," Kempa said. "Nobody talks about antennas for light. Antennas are for radio."

Co-authors of the journal article include Kempa's son, Thomas '04, currently at Cambridge University on a Marshall Scholarship; Research Associate Jakub Rybczynski and Laboratory Director Lect. Andrzej Herczynski (Physics); and Wenzhi Li, a former senior research scientist in Ren's group at BC, now an assistant professor of physics at Florida International University.

Other co-authors are Brian Kimball and Joel Carlson of the US Army Natick Soldier Center (known locally as the Natick Army Labs), which designs combat clothing and equipment for American soldiers, and Glynda Benham of MegaWave Corp., a Boylston, Mass., firm that designs advanced antennas for the military. The research has been funded by the Natick Army Labs. -Material from Reuters was used in this report. •