proven and that it's compatible with low-cost manufacturing processes.
December 22, 2011 -- US Department of Energy (DOE) National Renewable Energy
Laboratory (NREL) researchers devised a solar cell that produces a
photocurrent with an external quantum efficiency >100% when photoexcited
with photons from the high energy region of the solar spectrum.
The external quantum efficiency for photocurrent is the number of electrons
flowing per second in the external circuit of a solar cell divided by the
number of photons per second of a specific energy (or wavelength) that enter
the solar cell. NREL reports that this is the first solar cell to exhibit
external photocurrent quantum efficiencies above 100% at any solar
wavelength.
The external quantum efficiency reached a peak value of 114%. NREL
researchers used multiple exciton generation (MEG) or carrier multiplication
(CM), where a single absorbed photon of appropriately high energy can
produce more than one electron-hole pair per absorbed photon. They created a
layered solar cell with 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. Quantum dots (QD) confine charge
carriers and therefore harvest excess energy instead of allowing it to be
wasted as heat.
NREL scientist Arthur J. Nozik first predicted in a 2001 publication that
MEG would be more efficient in semiconductor quantum dots than in bulk
semiconductors. In a 2006 publication, NREL scientists Mark Hanna and Arthur
J. Nozik showed that ideal MEG in solar cells based on quantum dots could
increase the theoretical thermodynamic power conversion efficiency of solar
cells by about 35% relative to today's conventional solar cells.
The fabrication of quantum dot solar cells could be performed on
inexpensive, high-throughput roll-to-roll manufacturing equipment and
processes.
Since MEG was demonstrated experimentally in colloidal solutions of quantum
dots in 2004 by Richard Schaller and Victor Klimov of the DOE's Los Alamos
National Laboratory, researchers have confirmed MEG in many different
semiconductor quantum dots based on ultrafast time-resolved spectroscopic
measurements of isolated quantum dots dispersed as particles in liquid
colloidal solutions. NREL's research shows MEG manifested as an external
photocurrent quantum yield greater than 100 percent measured in operating
quantum dot solar cells at low light intensity.
These cells showed significant power conversion efficiencies (defined as the
total power generated divided by the input power) as high as 4.5% with
simulated sunlight. While these solar cells are un-optimized and thus
exhibit relatively low power conversion efficiency (which is a product of
the photocurrent and photovoltage), the demonstration of MEG in the
photocurrent of a solar cell opens new and unexplored approaches to improve
solar cell efficiencies.
Another important aspect of the new results is that they agree with the
previous time-resolved spectroscopic measurements of MEG and hence validate
these earlier MEG results. Excellent agreement follows when the external
quantum efficiency is corrected for the number of photons that are actually
absorbed in the photoactive regions of the cell. In this case, the
determined quantum yield is called the internal quantum efficiency. The
internal quantum efficiency is greater than the external quantum efficiency
because a significant fraction of the incident photons are lost through
reflection and absorption in non-photocurrent producing regions of the cell.
A peak internal quantum yield of 130% was found taking these reflection and
absorption losses into account.
Results are published in the Dec. 16 issue of Science Magazine, "Peak
External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a
Quantum Dot Solar Cell," 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.
NREL is the U.S. Department of Energy's primary national laboratory for
renewable energy and energy efficiency research and development. NREL is
operated for DOE by the Alliance for Sustainable Energy, LLC. Visit NREL
online at www.nrel.gov.
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