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 semiconductors, including
lead sulphide, lead telluride and cadmium selenide. What's more, Klimov says
his group has identified two new materials that are less toxic and have band
gaps better matched to the solar spectrum than lead selenide, though he will
not identify the materials as he has yet to publish the work.
photon can generate multiple charge carriers, the overall power efficiency of
solar cells could be dramatically increased. The world record for a
ground-based cell is 24.7 per cent, achieved by a device made in Australia
at the University of New South Wales. Klimov predicts that the multiple-carrier
generation could one day yield a cell with double that efficiency,
approaching 50 per cent. Ellingson is slightly more conservative, but
he still projects efficiencies around 45 per cent. With more work, the chips
cranking out extra electrons in New Mexico and Colorado could one day bring a
bright solar future for us all.
is not the only useful form of energy that ultra-efficient solar cells might
generate. They could also be used to induce a chemical reaction that creates
fuel. While this approach is not strictly dependent on a cell in which each
photon yields more than one current-carrying electron, the additional charge
carriers would help by accelerating the reaction.
compelling scenario, nanocrystals could be immersed in water, and the current
flowing out of the solar cell would break down the water molecules into
hydrogen and oxygen. As with direct production of electricity, the benefit of a
multiple-exciton solar cell would come through more efficient exploitation of
the solar energy that hits it, says chemist Nathan Lewis of the California
Institute of Technology in Pasadena.
the basis for fuel-cell vehicles that would operate emissions-free and require
no gasoline. Oxygen from the air would react with hydrogen in the presence of a
catalyst to produce electricity, with water as a by-product. Calls for a move
to a hydrogen economy, however, tend to gloss over the fact that producing
hydrogen requires energy -