 here on Earth. Made from semiconducting materials, most
often silicon, solar cells convert a dismayingly small fraction of the sun's
energy into electricity. Radically improving efficiency could give solar energy
a boost at a time when it is sorely needed and funding decisions hang in the
balance. "If this could be translated into a robust system that could
generate multiple carriers, it could be revolutionary," says Eric
Rohlfing, acting director of the chemical sciences, geosciences and biosciences
division in the Office of Basic Energy Sciences at the US
Department of Energy.
results trace back to 1982, when materials scientist Alexander Efros at the Naval
Research Laboratory in Washington DC showed it was theoretically
possible for a photon to generate multiple charge carriers in certain semiconductors.
Over the next two decades, researchers learned to control the properties of
tiny semiconducting structures called nanocrystals, or quantum dots. Then in
2002, physical chemist Arthur Nozik of NREL predicted that the production of
multiple carriers should be enhanced in nanocrystals relative to bulk
semiconductors. It wasn't until 2004 that Klimov's group - interested in
developing lasers as well as photovoltaics - showed that such behaviour could
be reliably detected (Physical
Review Letters, vol 92, p 186601).
benefits of multiple carriers arise from the way photovoltaic devices interact
with the solar spectrum. When an electron in a semiconducting material becomes
free to move about and conduct current, it leaves behind a vacant site in the
crystal, called a hole; the electron-hole pair is called an exciton. The amount
of photon energy needed to create an exciton in a particular material is called
the band gap (the term refers to the difference in energy levels between a
fixed electron in the so-called "valence band" and one that is part
of the sea of freely moving electrons in the "conduction band"). Sunlight
consists of a variety of wavelengths, which we see as colours, and the photons
of each colour carry a characteristic amount of energy: lower at the infrared
and red end of the spectrum, and higher towards the blue, violet and
To make an
efficient solar cell, you need to match the photon energy to the cell
material's band gap. Silicon has a band gap that corresponds to wavelengths in
the near-infrared region of the spectrum. Incoming photons with less energy
than that will not have the quantum oomph to create even a single exciton. A
photon with exactly the band-gap energy will create one exciton and have no
energy left over, so the solar cell will make perfect use of the energy from
photons in that part of the spectrum.
Most of the
light streaming down from the sun, however, has a shorter wavelength than
infrared, so its photons have higher energy than the silicon band gap. Each of
these packets of electromagnetic energy, no matter how potent, can still
liberate only one electron. Anything left over will dribble away as heat and
contribute exactly zero to the device's electrical output. Klimov's technique
taps this otherwise wasted energy and turns it into electricity.
The key, he
says, is the small size of the quantum dots used to absorb photons. When
structures shrink to the size of a few thousand atoms, their physics takes a
turn for the weird. The multi-exciton phenomenon, which can barely be made to
occur at all in conventional silicon, becomes possible in specially fabricated
nanocrystals. In his latest series of experiments, Klimov claims to have
produced as many as 7 excitons per photon in crystals of lead selenide 4 to 8
nanometres in diameter (Nano
Letters, vol 6, p 424). "They're very cheap and only take a few
minutes to grow," says Klimov. "It's like making new atoms, to go
beyond what nature provides."
these multiple excitons, the nanocrystals' behaviour needs to be measured at
excruciatingly precise time intervals. Klimov and his colleague Richard
Schaller illuminated samples of lead selenide with laser pulses lasting only 5
Χ 10-14 seconds - that's 50 millionths of a
nanosecond. They then shone