BY JOANNE ASLETT
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.
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