 circuit. And therein lies the major obstacle to building a
real-world device. "To produce current, you need to separate electrons
from holes, and that's a big problem," Klimov says. The difficulty is that
multiple excitons are extremely short-lived, lasting only tens of picoseconds,
or trillionths of a second, before the holes and electrons recombine; in
ordinary photovoltaic devices, electrons and holes remain apart for much
longer, closer to a microsecond.
that practical applications of Klimov's work are still some way off. "We
can take this as a proof of principle," says chemist Paul Alivisatos of
the University of California, Berkeley, but figuring out how
to separate and harvest the multiple charge carriers produced in a nanocrystal
remains a puzzle. "It's worth spending time on this," he says,
because if it works it is bound to yield an increase in photovoltaic
Lewis, a chemist at the California Institute of Technology in
Pasadena who led a recent US Department of Energy workshop on
research needs for solar energy, takes a similar view. The work is an
"important confirmation of theoretical predictions", he says.
"It's like knowing that there's nuclear fusion happening on the sun,"
he explains. "Doing it on Earth is another story."
step is to reliably separate the multiple electrons and holes. That requires
finding materials with electronic energy characteristics that match those of
the quantum dots. One approach uses a conductive polymer to extract the holes.
Klimov's group is collaborating with Anvar Zakhidov, a physicist at the University
of Texas at Dallas, on a prototype that blends the lead selenide
crystals with such a polymer. After a photon creates an electron-hole pair, the
holes migrate into the polymer and travel through it to an electrode; the
energised electrons, meanwhile, hop from nanocrystal to nanocrystal until they
reach the other electrode.
has encountered its share of technical difficulties, however. "We are at
the very beginning of experimental demonstration," Zakhidov says. One
issue is that the nanocrystals must be in "intimate contact" with the
polymer. Moreover, the conduction of electrons through the array of
nanocrystals is very inefficient. "There are lots of dead ends," he
alternative method for collecting the solar-induced charges has been proposed
by Peidong Yang, a chemist at Berkeley who is also an expert in nanomaterials.
Instead of requiring electrons to hop from one nanocrystal to another, Yang is
testing nanowires - highly conductive filaments with a diameter of only a few
nanometres. In principle, Yang says, the electrons and holes could zip through
an array of nanowires straight to a pair of collecting electrodes "like
cars on a freeway with no stop lights". Whether nanowires could harvest
multiple excitons in the short time they are available, however, is anyone's
area for progress is in the material used for making the quantum dots. The lead
selenide used so far is less than ideal. First, it is toxic, making its
fabrication a tricky business. Second, its band gap is large. For a photon to
produce multiple excitons, its energy must equal at least twice the band gap of
the material, and with lead selenide only photons at the high-energy end of the
spectrum are powerful enough to achieve this.
be a way around this. The smallest crystals have the largest band gaps, as the
confinement of electrons to a very tight space ratchets up the energy levels.
The way to generate the largest number of excitons would be to engineer the
crystal so that its band gap is small. One way to do this, says NREL physicist
Randy Ellingson, would simply be to grow the nanocrystals larger. That would
make it possible to use the abundant photons in the middle of the solar
spectrum to generate multiple excitons. The trade-off, Ellingson points out, is
that a lower band gap means a lower voltage across the electrodes, which may
limit the total power output of the cell.
researchers are also exploring alternative materials. Both Klimov and Nozik
have observed multiple-exciton generation in other semiconduct