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Organic Photovoltaics: the Good, the Bad, and the Inefficient

by Joe Kwiatkowski, Physicist, Imperial College London

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.

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.

Not only are the molecules easier to handle than atoms, it is also easy make new designs with molecules. 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.

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.

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.

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.

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