Organic Photovoltaics: the Good, the Bad, and
the Inefficient
by Joe Kwiatkowski, Physicist, Imperial College London
May 19, 2008
What if making a solar cell was as easy as printing a newspaper? What if it
was flexible, light and above all, cheap? The current photovoltaic (PV)
market, dominated by expensive and fragile silicon, would be revolutionized.
These are the lofty ambitions of a growing number of scientists in companies
and universities worldwide who are developing organic photovoltaics: solar
cells that are made from carbon-based molecules.
Successful large scale commercialization of solar energy depends on three
criteria in particular: efficiency, lifetime and cost. Much of the early
excitement in organic photovoltaics arose from expectations that they could
be very cheap. First, the chemicals industry already manufactures organic
molecules by the kiloton and sells them cheap. Second, making an organic
solar cell is wonderfully slapdash when compared to the care needed in
making a silicon solar cell. However, efficiency and lifetime remain
stubborn thorns in the side of advocates of organic photovoltaics:
efficiencies waiver around 5% and ironically, although reasonably stable in
the dark, organic materials tend to degrade in the light.
Both the advantages and the shortcomings of organic photovoltaics can be
understood in terms of their material properties. Whereas the building block
of other solar cells like silicon is the atom, the building block of organic
cells is the molecule (a collection of atoms into well-defined groups). This
fundamental difference has far-reaching implications for the performance of
organic solar cells.
Because molecules are larger than atoms, they are easier to work with. For
example, by dissolving them in a solvent they can be turned into an ink that
can then be printed in much the same way as a newspaper. As evidenced the
daily press, printing is cheap and fast; the area of print produced every
day for a typical newspaper is on the same order of magnitude as the area of
all the solar cells produced every year from a large polysilicon plant.
Not only are the molecules easier to handle than atoms, it is also easy make
new designs with molecules. Whereas it is difficult to build an entirely new
material when starting from atoms, almost anything can be built when
starting from molecules. Indeed, the number of molecules that could
potentially be used in an organic cell are limited only by the imagination
of the synthetic chemist. This means that organic solar cells could be
customized for a particular application or market. Massachusetts-based
Konarka, for example, can manufacture cells with different color schemes
including cells that are camouflaged for their military customers. More
importantly, researchers hope that by careful design and with repeated
tweaks, molecules can be developed that will satisfy all three criteria
necessary for a successful solar cell: efficiency, lifetime and cost.
One benefit of the huge molecular portfolio available to organic
photovoltaics is the ability to choose molecules that absorb sunlight very
efficiently. As a result organic solar cells can be made 1000 times thinner
than silicon solar cells, thereby offering huge savings on materials.
Furthermore, because they are thin, the cells are also flexible and could be
printed on a roll-to-roll process, transported easily and simply unrolled on
the customer's roof. Konarka, amongst others, is also developing cells that
can be incorporated into tents or clothes.
Another advantage of moving from atoms to molecules, is that it opens
photovoltaics to entirely new industries. For example, powerful chemical
companies such as BASF, Merck, and Dow have recently realized that the large
scale manufacture of organic solar cells could provide an enormous market
for their products. To encourage the development of organic photovoltaics
and to ensure their place in any future markets, these companies have
devoted substantial manpower and funds to photovoltaics research.
Whilst organic photovoltaics may have cost advantages and whilst they may
open up a range of other exciting possibilities, they also have
shortcomings. To efficiently extract electricity from a solar cell,
electrical charges need to be able to travel through it quickly. If charges
move slowly they are likely to become stuck or recombine with other charges
(of opposite polarity) and disappear altogether. As a result, the number of
electrical charges available to do useful work, such as recharging a
battery, is diminished. It is not hard to get charges out of a silicon solar
cell because its atoms are neatly arranged into crystals and and so charges
can fly between them at enormous speeds. However, molecules are less
ordered, particularly when printed, and so charges move much more slowly
between them. To further compound the problem molecules hold onto a charge
very tightly and are reluctant to pass it on to their neighbors. Because
electricity can't flow easily, the efficiency of an organic solar cell is
limited.
Sadly, it's not only a problem of getting electrical charges out of an
organic solar cell: it's also a problem of generating them. When a solar
cell absorbs sunlight it gains energy but, being uncomfortable in this
state, it attempts to discharge that energy. Ideally it does so by
generating two charges but alternatively it may simply throw the energy away
as heat. Solar cells are designed to favor the former process: silicon cells
consist of two doped regions that attract positive and negative charges, and
organic cells attempt the same effect using two different types of molecule.
However, whilst the process is very efficient in silicon, it is less so in
organic cells.
Whilst poor generation and extraction of electrical charges limits the
efficiency of organic photovoltaics, a further problem is lifetime. If you
take the care to build a material from atoms, it will generally last a
fairly long time. In comparison, molecules are fickle entities that will
react with other molecules such as oxygen and water. In doing so, they
change. They might absorb less light, or generate fewer charges, or actually
trap charges and prevent them from being collected. It is an unfortunate,
but ironic, fact that molecules are more likely degrade in this way whilst
illuminated.
As with any new technology, there are many high hurdles to be cleared before
a finished product can be sold. However, the excitement growing worldwide is
testament to the potentials of organic photovoltaics; a coalition of the
German government, BASF, Bosh, and others recently announced an organic
photovoltaics research program to the tune of US $570 million. Maybe organic
photovoltaics are a long way from competing directly with silicon; however,
they would open niche markets and, with such serious backing, it would be
surprising if somebody didn't make money from molecules at some point.
Joe Kwiatkowski is a physicist at Imperial College London, where he works
on organic photovoltaics. His current interest is the development of
computational methods that can aid the design and optimization of new
photovoltaic materials.
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