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

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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Volume 18, Issue 4


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