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December 19, 2011

NREL Scientists Create Solar Energy that Could Cost Less than Fossil or Nuclear Fuels



Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) in Golden, Colo., have found a way to generate more photocurrent from a solar cell than it absorbs in radiant energy (such as sunlight).

The external quantum efficiency of this newly developed, Third- or Next-Generation Solar Cell is reportedly as high as 114 percent—representing a major breakthrough in the creation of both solar electricity and solar fuels that will be competitive with, or even less costly than, energy from fossil or nuclear fuels.

The quantum efficiency of a solar cell is a very important measure because it represents the current that a given cell will produce when illuminated by a particular wavelength. This is the first time that a cell has produced more than 100 percent quantum efficiency at any wavelength in the solar spectrum. It was accomplished at NREL when scientists photoexcited the cell with photons (or units of light) from the high-energy region of the solar spectrum.

To produce a quantum efficiency above 100 percent with solar photons, the researchers used a process called Multiple Exciton Generation (MEG), whereby a single absorbed photon of appropriately high energy can produce more than one electron-hole pair. 

NREL scientist Arthur J. Nozik first predicted 10 years ago that MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors. Quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range of 1-20 nm, where 1 nm equals one-billionth of a meter.

By confining charge carriers within their tiny volumes, quantum dots can harvest excess energy that otherwise would be lost as heat—and, therefore, greatly increase the efficiency of converting photons into usable free energy.

Furthermore, the fabrication of quantum dot solar cells also is amenable to inexpensive, high-throughput, roll-to-roll manufacturing.

The researchers achieved the 114 percent external quantum efficiency with a layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode.

A paper on the development, “Peak External Photocurrent Quantum (News - Alert) Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” appears in the Dec. 16 issue of Science Magazine. Co-authored by NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozikand , and Matthew C. Beard, the research was supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the DOE Office of Science, Office of Basic Energy Sciences. Semonin and Nozik are also affiliated with the University of Colorado at Boulder.


Cheryl Kaften is an accomplished communicator who has written for consumer and corporate audiences. She has worked extensively for MasterCard (News - Alert) Worldwide, Philip Morris USA (Altria), and KPMG, and has consulted for Estee Lauder and the Philadelphia Inquirer Newspapers. To read more of her articles, please visit her columnist page.

Edited by Carrie Schmelkin


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