From tobacco to energy

Anne L. Fischer, Contributing Editor, anne.fischer@photonics.com

Tobacco farmers, faced with fewer traditional buyers for their crops, are looking to the solar industry for income. Plants such as tobacco are sunlight collectors with many similarities to solar cells. Now, a group of scientists is trying to take advantage of that collection system to produce inexpensive and biodegradable solar cells.

In a study reported in the January 2010 issue of Nano Letters, investigators from the University of California, Berkeley, demonstrated how to program tobacco plants to enhance their sunlight collection system. They genetically engineered a virus called the tobacco mosaic virus and sprayed it over the tobacco plants. This forced the plants to artificially produce lots of chromophores, which form the basis of the collection system and convert photons from sunlight into electrons.

Scientists from the University of California, Berkeley, demonstrated how to program tobacco plants to make use of their sunlight collection system.

The trick, the scientists found, is that the chromophores must be spaced at a fairly precise distance of about 2 or 3 nm from one another. If the chromophores deviate from this spacing, the electric current is halted, or the electrons are more difficult to harvest. Fortunately, tobacco plant cells are very good at lining up chromophores at this exact distance from each other in a long spiral measuring hundreds of nanometers long. The team was able to exploit this plant structure to grow strands of chromophores.

The researchers were able to extract the chromophore structures when they harvested the plants. After dissolving the structures in a liquid solution, they sprayed the new solution on a glass or plastic substrate to create a solar cell.

The next step in their work will be to demonstrate that the resulting solar cells can turn light into electrical energy. They also demonstrated how to manipulate E. coli bacteria to produce chromophore structures. In this case, instead of using a virus, they modified the bacteria directly.

Compared with traditional solar cells, those made from plants or bacteria have several potential pluses. For instance, they don’t require the use of toxic chemicals, they’re biodegradable, and they’re inexpensive to produce. The downside is that bio-based solar cells likely have a shorter lifetime than silicon ones.