Excitons work in silicon nanowires

In all the approaches the McGehee lab from StanfordUniversity has taken, the idea has been that when sunlight strikes an absorbing material in a solar panel, it will release a pair of electrically charged particles collectively known as an “exciton. The trick is to split these excitons into their positive and negative components and to route a flow of negative ones, or electrons, out of the panel and into a circuit, where they’ll provide useful electricity.

Two things make the trick very difficult. One is finding the right materials and structures to absorb as much light as possible. The other is splitting and harvesting the excitons before their components recombine, giving off useless heat. Typically excitons move no more than 5 or 8 nanometers before that happens.

The first approach the group worked on involved building a forest of tall, thin pillars of titiania (titanium and oxygen) and filling in the forest with a conductive polymer. The idea was that the polymer would generate the excitons after absorbing the light and the titania would then split the excitons in order to harvest the electrons. But the titania pillars had to be so tall and so thin that the design became structurally unsound. Inter-molecular forces drew them together, for example, causing them to bunch up.

Since that unhappy realization, the McGehee lab has developed new ideas that are now under testing. One approach still involves combining titania with a conductive polymer, but now the titania is used to make little balls that are mixed into the polymer like peanuts in a brittle. To enhance their light absorption, the titania is coated with special dyes.

The dye’s job will be to absorb light and transfer the absorbed energy to the silicon nanowires. Inside each silicon nanowire are regions of opposite charge. At the junction between these regions, the excitons generated by the light absorption can be efficiently split off and their components conducted away.

The researchers have done some theoretical modeling and has built a proof-of-concept structure — although not yet a forest of nanowires - to test and validate the idea. The initial results with regard to light absorption and energy extraction are encouraging enough to keep going.

Stanford is the ideal top-tier research institution with ample funding to do innovative research, with some of the best scientists in the world, in the heart of Silicon Valley, with an exceptional patent office, and access to leaders in the business community.


QueltaNews from StanfordUniversity

By Vasil Sidorov on April 22, 2009



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