Puru Jena, Virginia Commonwealth University
incoming photons in electron–hole pairs – a limit calculated by the Nobel laureates William Shockley and
Hans Queisser back in 1961. The downside is that they
are expensive because they need highly pure, single-crystal silicon wafers. “Second generation” solar cells
aim to reduce these costs by coating a glass or ceramic
substrate with a thin film of semiconductor, such as cadmium telluride, copper indium gallium selenide, amorphous silicon or micromorphous silicon.
But where second-generation cells win on cost, they
lose on efficiency, and the main focus of research is
now on “third generation” devices. These are made
from thin semiconducting films, which makes them
cheap, but they can beat the Shockley–Queisser
threshold and have efficiencies of up to 60%. One type
of third-generation cell involves stacking multiple cells
with different semiconductor band gaps on top of one
another, which lets the device generate a current from
a much wider range of photon wavelengths than a
single-crystal cell. For example, Gavin Conibeer and
colleagues at the University of New South Wales in
Australia have been able to control the size of the band
gap by using silicon quantum dots (tiny pieces of semiconductor), the diameters of which can be adjusted by
varying the thickness of the deposited thin films from
which they are precipitated. Another option to boost
efficiency is to use carbon nanotubes or quantum dots
embedded in thin-film conductive polymers that are
High potential
Nanotechnology has
led to molecules such
as this lithium-coated
buckyball that can
store hydrogen to
power fuel cells.
