 another laser beam to probe the crystal, monitoring
how much light it absorbed over the next few thousandths of a nanosecond.
Single excitons are stable, so if just one is present, absorption remains
constant during that period. If multiple excitons are created, however, that is
no longer the case: the excitons rapidly disappear, causing the crystal's
absorption properties to change in a characteristic way that can be picked up
by sensitive optical detectors. Of course, the ability of a photon to generate
multiple charge carriers has its limits. The fundamental laws of physics
dictate that the total energy of the excitons cannot exceed the energy of the
photons striking the cell. "We are still constrained by the conservation
of energy," Klimov says.
Or are they?
How the multiple excitons are produced remains a bit of a mystery. According to
Klimov, when an energetic photon strikes the material, the electron jumps to
what he calls a "virtual" state in which it has actually gained more
energy than was carried by the photon; this seeming contradiction is permitted
because the virtual state lasts for such a brief time. The hyper-excited
electron will transfer some of its energy to another, unexcited electron
essentially by bumping into it. The result: two energised electrons from a
suggests a different model. There is a "coherent superposition" of
energy states, he says - a quantum mechanical effect that defies concrete
analogy. Following the absorption of a high-energy photon, an electron will
inhabit two different energy states: one of them consistent with the formation
of a single exciton, and one consistent with multiple excitons. In effect, says
Garry Rumbles, a member of Nozik's team, "you prepare a mixture of states
- one state looks like three excitons, and another state looks like a single
exciton with very high energy". This superposition holds for a very brief
period, until the electron makes a decision, says Rumbles.
works, a solar cell does no good unless the electric charges created can be
drawn into a 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 