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Future Energy eNews
Solar power - Seriously Souped Up
New Scientist Print Edition, 31 May 2006
YOU want efficient solar power, Victor Klimov has a deal for you. Give him one
photon of sunlight, and he'll give you two electrons' worth of electricity.
impressed? You should be. In all solar cells now in use - in everything from
satellites to pocket calculators - each incoming photon contributes at most one
energised electron to the electric current it generates. Now Klimov, a
physicist at Los Alamos National Laboratory in New Mexico, has
broken through this barrier. He has shown that by shrinking the elements of
a solar cell down to a few nanometres, or millionths of a millimetre, each
captured photon can be made to generate not one, but two or even more
this multiplicity of electrons - an achievement that has been replicated by a
group at the National Renewable Energy Laboratory (NREL) in
Golden, Colorado - is a remarkable piece of physics. If the effect can be
harnessed, it could change the whole energy debate by making solar power much
more efficient and economical. While there are many ongoing efforts to improve
solar efficiency - by concentrating sunlight, for example, or by making it
easier for electrons to move around within a cell - the new approach is unique
in that it gets to the very root of the process and also complements other
decades, photovoltaics have been stranded on the effete fringe of energy
technologies - ideal for niche applications such as satellites, but not
economically competitive 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 carr