y 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 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