Research Highlights Potential for
Improved Solar Cells
LOS ALAMOS, N.M., February 10, 2009 — Certain nanocrystals
shown to generate more than one electron after absorbing a single photon
A team of Los Alamos researchers led by Victor Klimov has shown that
carrier multiplication—when a photon creates multiple electrons—is a
real phenomenon in tiny semiconductor crystals and not a false observation
born of extraneous effects that mimic carrier multiplication. 
The research, explained in a recent issue of Accounts of Chemical Research,
shows the possibility of solar cells that create more than one unit
of energy per photon.
Questions about the ability to increase the energy output of solar
cells have prompted Los Alamos National Laboratory researchers to reassess
carrier multiplication in extremely small semiconductor particles.
When a conventional solar cell absorbs a photon of light, it frees
an electron to generate an electrical current. Energy in excess of the
amount needed to promote an electron into a conducting state is lost
as heat to atomic vibrations (phonons) in the material lattice. Through
carrier multiplication, excess energy can be transferred to another
electron instead of the material lattice, freeing it to generate electrical
current—thereby yielding a more efficient solar cell.
Klimov and colleagues have shown that nanocrystals of certain semiconductor
materials can generate more than one electron after absorbing a photon.
This is partly due to strengthened interactions between electrons squeezed
together within the confines of the nanoscale particles.
In 2004, Los Alamos researchers Richard Schaller and Klimov reported
the first observations of strong carrier multiplication in nanosized
crystals of lead selenide resulting in up to two electron-hole pairs
per absorbed photon. A year later, Arthur Nozik and coworkers at the
National Renewable Energy Laboratory reproduced these results. Eventually,
spectroscopic signatures of carrier multiplication were observed in
nanocrystals of various compositions, including silicon.
Recently, the claims in carrier multiplication research have become
contentious. Specifically, some recent studies described low or negligible
carrier multiplication efficiencies, which seemed to run contrary to
earlier findings. To sort out these discrepancies, Los Alamos researchers
analyzed factors that could have led to a spread in the reported carrier
multiplication results. These factors included variations between samples,
differences in detection techniques, and effects mimicking the signatures
of carrier multiplication in spectroscopic measurements.
To analyze how a particular detection technique might affect an outcome,
John McGuire, a postdoctoral researcher on Klimov’s team, investigated
carrier multiplication using two different spectroscopic techniques—transient
absorption and time-resolved photoluminescence. The results obtained
by these two methods were in remarkable agreement, indicating that the
use of different detection techniques is unlikely to explain discrepancies
highlighted by other researchers. Further, although these measurements
revealed some sample-to-sample variation in carrier multiplication yields,
these variations were much smaller than the spread in reported data.
After ruling out these two potential causes of discrepancies, the researchers
focused on effects that could mimic carrier multiplication. One such
effect is photoionization of nanocrystals.
“When a nanocrystal absorbs a high-energy photon, an electron can acquire
enough energy to escape the material,” Klimov explained. “This leaves
behind a charged nanocrystal, which contains a positive ‘hole.’ Photogeneration
of another electron by a second photon results in a two-hole, one-electron
state, reminiscent of one produced by carrier multiplication, which
can lead to false positives,” he said.
To evaluate the influence of photoionization, the Los Alamos researchers
conducted back-to-back studies of static and stirred solutions of nanocrystals.
Stirring removes charged nanocrystals from the measured region of the
sample. Therefore, when crystals are subjected to light, the stirring
eliminates the possibility that charged nanocrystals will absorb a second
photon. While stirring of some samples did not affect the results of
the measurements, other samples showed a significant difference in the
apparent carrier multiplication yields measured under static and stirred
conditions. Since most previous studies were performed on static samples,
these results suggest that discrepancies noted by other researchers
arise at least in part from uncontrolled photoionization, which stirring
seeks to eliminate.
The Los Alamos researchers re-evaluated carrier multiplication efficiencies
when photoionization was suppressed. The results are encouraging.
While the newly measured electron yields are lower than previously
reported, the efficiency of carrier multiplication is still greater
than in bulk solids. Specifically, both the energetic onset and the
energy required to generate an extra electron in nanocrystals are about
half of those in bulk solids.
These results indicate significant promise for nanosized crystals as
efficient harvesters of solar radiation.
“Researchers still have a lot of work to do,” Klimov cautioned. “One
important challenge is to figure out how to design a material in which
the energetic cost to create an extra electron can approach the limit
defined by a semiconductor band gap. Such a material could raise the
fundamental power conversion limit of a solar cell from 31 percent to
above 40 percent.”
The Los Alamos nanocrystal team’s research is funded by the U.S. Department
of Energy Office of Basic Energy Sciences and Los Alamos’ Laboratory-Directed
Research and Development (LDRD) program.
For more information on the research team, visit: http://quantumdot.lanl.gov/.
February 09
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