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Solar Cells Go Organic

EuroPhotonics
May 2009
Jörg Schwartz, European Correspondent, j.schwartz@europhotonics.com

An eight-member consortium that includes names such as Schott (glass), Merck (chemicals) and Varta (batteries) as well as universities and specialist start-up Konarka Technologies has started working on a joint project to develop organic solar cells, and it has been granted about €2 million per year over four years by the German government.

Buying organic is certainly the trend when it comes to food – but not really what you think of first when it comes to solar cells. Most of us associate them with silicon, the original material and one that’s still key. The other common view is that efficiency and cost are what drive the development of photovoltaic technology, so it can compete with other sources of energy.

Silicon certainly has been the fundamental material for solar technology over the past decades. Well-known semiconductor processes are used in photovoltaic production, and significant improvements have been made over the years, particularly in terms of efficiency. However, silicon wafers are fragile, which limits their potential applications and also makes processing them more difficult. In addition, silicon does not absorb light strongly in the relevant part of the spectrum; therefore, silicon wafer cells have to be fairly thick.

Unmet promises

As a result, the newer generation of solar cells uses thin-film technologies; typically, a thin layer of photoactive material – such as amorphous silicon (a-Si) or copper indium gallium diselenide (CIGS) – is deposited onto glass or a flexible substrate. Although these cells solve most of the problems mentioned above, some argue that the promise of low-cost power has not been realized yet, and questions remain about the toxic legacy of the materials, both during manufacturing and after disposal.

Solar cells based on organic materials – the third approach – use substances such as polymer plastics rather than inorganic semiconductors to convert light into electrical current. They hold potential in that they are disposable, easy to make, and even more lightweight and flexible than thin-film cells. And although the chemistry and physics of organic photovoltaic materials appear to be well understood, challenges remain in assessing these advantages and in overcoming one of the biggest shortcomings so far: inefficiency.

Improving the conversion efficiency to a level of 10 per cent from the current level of about 5 per cent is therefore one of the primary goals driving the collaborative project, together with developing cost-effective printing methods by 2012. Funding comes from the German Federal Ministry of Education and Research, which in June 2007 announced a €360 million initiative on organic photovoltaics.

Although the organic way of converting photons into electrons may still be less efficient than the processes used by today’s other methods, the organic photovoltaic cells’ advantages – that they are inexpensive and versatile – may compensate for this. The initial applications anticipated are powering portable equipment, integrating solar cells into windows (because, similar to organic LEDs, the cells can be made transparent) or making functionalised textiles that charge your laptop while it is in the briefcase.


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