A promising new family of semiconductors for photovoltaic (PV) applications is fullerenes, novel forms of carbon that were discovered in 1985 and have earned their American and British discoverers the Nobel Prize in Chemistry in 1996. Fullerenes are produced by vaporizing graphite rods with a powerful laser beam, high voltage arc discharge, or concentrated solar energy. When elemental carbon is heated in an inert atmosphere with no reactive oxygen, it partially restructures itself into novel hollow spheres of carbon atoms, known as buckyballs — or tubular structures known as nanotubes. Both of these fullerene forms exhibit new mechanical and electrical properties, including photovoltaic activity. Initial studies showed that thin films of basic fullerene, buckminsterfullerene (C60), are suitable for use in heterojunction solar cells.
A molecular structure of buckministerfullerence C60.
For more than a decade, Eugene A. Katz and David Faiman at the BIDR Department of Solar Energy and Environmental Physics have been investigating the photovoltaic properties of fullerenes, as well as various approaches to improve their efficiencies and reduce costs of production .The work has been spurred on by the fact that carbon-based fullerenes are easily produced by intensive solar heating of graphite, followed by purification and crystallization using this same source of energy. In addition, fullerenes — as distinguished from many inorganic semiconductors — are environmentally benign. In fact, the BIDR group was first to show how high quality, textured polycrystalline fullerene could be rapidly deposited on silver and gold substrates. Their studies of the electrical and photovoltaic properties of these films — the world’s first “proof-of-concept” carbon PV cells — have clarified how photoelectric effects and C60 crystal structure are modified by temperature changes.
Investigators worldwide, including the BIDR group, have examined the use of fullerene thin films (which are n-type semiconductors) with various junction materials, including metals and well-known p-type materials, including silicon, gallium arsenide and gallium nitride. They have also studied various doping approaches, such as the diffusion of copper or silver into C60, which strengthened dark conductivity as well as photovoltaic efficiency. However, much work still is required to obtain fullerene-based semiconductors with high enough efficiencies and electricity production to compete with commercial fossil fuel energy.
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