1 Solar cells – old and new
a
b
sunlight
load
current
titanium-dioxide
nanoparticles coated
with dye molecules
transparent conductor
junction
n-type
silicon
–
photons
electron flow
__
p-type
silicon
electrolyte
++
“hole” flow
+
catalytic conductor
(a) Conventional “first generation” photovoltaic cells have a single crystal of silicon containing a p-type region with an excess of holes and an n-type region with an excess
of electrons. Photons striking the cell promote electrons from the valence to the conduction band, thus creating electron–hole pairs. Any pairs formed near the
p–n junction separate, with electrons flowing in one direction and holes in the other. (b) “Dye sensitized” solar cells – also known as “Grätzel” cells – are examples of
“third generation” devices and contain semiconducting titanium-dioxide nanoparticles coated with a layer of dye. Photons are absorbed by the dye molecules, which
become excited and eject an electron. This electron can then be “injected” into the conduction band of the titanium dioxide, from where it moves to the top transparent
electrode and out to an external circuit. The dye then strips an electron from the electrolyte, which recovers its missing electron by diffusing to the bottom of the cell, where
a cathode reintroduces electrons that have flowed through the external circuit.
placed on conventional silicon cells. By varying the size
of the quantum dots, the cells can be tuned to absorb
different wavelengths, which means that efficiencies
of more than 40% may be possible.
An alternative third-generation device that also takes
advantage of nano-scale structures is the “quantum
well” solar cell, which was first developed by Keith
Barnham and colleagues at Imperial College London
in 1989. Such devices typically consist of 50 or so nano-metre-sized slices of gallium arsenide, each sandwiched
between slightly thicker layers of gallium-arsenide
phosphide – a structure that lowers the band gap of the
gallium arsenide so that it can capture a bigger fraction
of the incoming photons. Efficiencies of 27% have so
far been obtained but this figure could rise substantially
with further development. And because a bigger fraction of photons generate electron–hole pairs, fewer
photons are simply absorbed by the semiconductor and
converted into heat.
An entirely different kind of third-generation device
is the “dye sensitized” solar cell, pioneered by Michael
Grätzel and co-workers at the Swiss Federal Institute
of Technology in 1991. Also known as Grätzel cells,
these devices comprise a thin layer of chemical dye and
the wide-band-gap semiconductor titanium dioxide,
which is cheaper than silicon. Mimicking the process of
photosynthesis, sunlight enters the device through a
transparent top contact then strikes semiconducting
titanium-dioxide nanoparticles, which are roughly
20 nm in diameter, coated with a 10 µm layer of dye (see
figure 1b). Photons are absorbed by the dye molecules,
which become excited and eject an electron. This electron can then be “injected” directly into the conduction
band of the titanium dioxide, from where it moves to
the anode on top and onward into an external circuit.
But as the dye molecule has lost an electron, it will
decompose unless another electron is supplied. The
trick is to include an electrolyte – typically an iodine-based organic solvent – from which the dye can strip
an electron. The electrolyte then recovers its missing
electron by mechanically diffusing to the bottom of the
cell, where a cathode reintroduces the electrons after
flowing through the external circuit. One of the advantages of the cell is that the band gap of the semiconductor does not have to be matched to the spectrum of
light shining on the cell: the absorption spectrum of the
dye can be easily tuned to this, which is why the cheap
semiconductor titanium dioxide can be used. Last year
Grätzel, who is now at the Changchun Institute of Applied Chemistry in China, was able to obtain cell efficiencies of 8.2% using a new solvent-free electrolyte
consisting of a mixture of three different salts.
The leap from the lab
Although third-generation solar cells are currently far
less efficient than the best first-generation devices,
much of the research is still at an early stage and big
strides have been made in recent years. Indeed, those
working in the field are confident that third-generation
cells can eventually become better than conventional
silicon-based solar cells. But in order to take these
devices from the lab bench and into mass production
we will need to find ways of manufacturing them
cheaply enough. Cost is the main driver: the reason why
we do not see every building covered with solar panels
is that they are simply too expensive.
However, a number of companies are already making
progress working on a variety of Grätzel cells, especially
for applications where direct sunlight is not available –
these devices can still work if ultraviolet light penetrates
the clouds. Cardiff-based G24 Innovations, for example, has developed a series of products in which cells
have been incorporated into jackets, rucksacks and
other textiles, since the devices can be mounted on a
