Nanotubes May Revolutionize Solar Cells

The power output from solar cells may increase from a breakthrough in the use of carbon nanotubes.

CAMBRIDGE, England (UK) 2002-01-30 [SolarAccess.com] The ‘super microscopic’ material is 100 times stronger than steel and only one-fifth the weight, and is a superconductor. The field of nanotechnology is an emerging one that combines chemistry and engineering to develop methods of building at the level of atoms, and scientists are exploring ways to blend nanotubes with plastics and ceramics to produce new composites with unprecedented strength-to-weight ratios and high conductivity. Researchers from Britain’s Cambridge University engineering department have developed photovoltaic devices that, when doped with single-wall carbon nanotubes (SWNT), perform better than undoped devices. Nanotube diodes were made by depositing organic films containing SWNT on glass substrates coated with indium-tin oxide (ITO). Aluminium electrodes were then thermally evaporated under a vacuum to form a composite sandwich. The interaction of the carbon nanotubes with the polymer poly-3-octylthiophene (P3OT) allows excitons generated by light in the polymer to dissociate into their separate charges and travel more easily. “The operating principle of this device is that the interaction of the carbon nanotubes with the polymer allows charge separation of the photogenerated excitons in the polymer and efficient electron transport to the electrode through the nanotubes,” explains Emmanouil Kymakis, co-author of papers in Applied Physics Letters that describe the work. “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 balances the charge carrier transport to the electrodes. In addition, the composite’s conductivity is increased by a factor of 10, indicating percolation paths within the material. The doping of P3OT polymer diodes with SWNT improves the device’s photovoltaic performance, increasing the photocurrent by more than two orders of magnitude and doubling the open-circuit voltage. The scientists believe that further improvements in device performance will occur with more controlled film preparation and polymer doping. “The next stage of our research is the optimization of the device so it can be compared with other photovoltaic cells which use different electron acceptors,” says Kymakis. “Furthermore, wewill try to increase the absorption in the nanotube-polymer junction by incorporating an organic dye.” “The fabrication of such composites is easy and cheap” on an industrial scale, he adds. “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.” In Japan, the Mitsubishi and Mitsui trading houses are already competing in the field of the mass production of nanotubes at a price they claim is potentially less than $80 a kilogram, or 1 percent of current prices for the experimental material. Global demand for carbon nanotubes is estimated at four trillion yen (US$8 billion) by 2020. Mitsui says its subsidiary, Carbon Nanotech Research Institute, will start manufacturing carbon nanotubes at an annual rate of 120 tons next September, to be used in such products as cars, flat televisions and fuel cells.