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Carbon Nanotubes to Improve Solar Cells

Researchers from Cambridge University's engineering departmenthave developed photovoltaic devices that, when doped with single-wall carbon nanotubes (SWNTs), perform better than undoped devices.

The nanotube diodes were made by depositing organic filmscontaining SWNTs on to glass substrates coated with indium-tin oxide(ITO). Aluminium electrodes were then thermally evaporated under a vacuum to form a sandwich configuration.

The interaction of the carbon nanotubes with the polymer poly(3-octylthiophene) (P3OT) allows excitons generated by light in thepolymer to dissociate into their separate charges and travel more easily.

Emmanouil Kymakis, co-author of the papers in Applied Physics Letters that describe the work, said: "The operating principle of this device is that the interaction of the carbon nanotubes with thepolymer allows charge separation of the photogenerated excitons in the polymer and efficient electron transport to the electrode throughthe nanotubes.

"The electrons travel through the nanotube length and then hop or tunnel to the next nanotube."

This results in an increase in the electron mobility and balancesthe charge carrier transport to the electrodes. In addition, the researchers found that the composite's conductivity is increased by a factor of 10, indicating percolation paths within the material.

This doping of P3OT polymer diodes with SWNTs also improves the device's photovoltaic performance, increasing the photocurrent bymore than two orders of magnitude and doubling the open-circuitvoltage.

The team believes that further improvements in device performance will occur with more controlled film preparation and polymer doping.

Kymakis said: "The next stage of our research is the optimisation of the device so it can be compared with other photovoltaic cellswhich use different electron acceptors, [such as C60 `buckyballs'].

"Furthermore, we will try to increase the absorption in thenanotube-polymer junction by incorporating an organic dye."

As far as an industrial scale process goes, Kymakis said: "The fabrication of such composites is easy and cheap. A practical advantage of these composites is that it makes the preparation of products with complex shapes and patterns, using simple processing technology, easy and so reduces the manufacturing cost."

(C) 2002 EE Times.