Page 407 - Organic Electronics in Sensors and Biotechnology

384 Chapter Ten

Nanostructured conducting polymers can be obtained through
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the EPD process. Tada and Onoda have reported the preparation of
nanostructured conjugated polymer films through EPD from their
colloidal suspension. The morphology of the films observed by an
atomic-force microscope has indicated that the films consist of nano-
particles. It is suggested that the deposition as well as the drying of the
film in the nonsolvent atmosphere is a key to obtain nanostructured
film. A device fabricated from the nanostructured conjugated poly-
mer film emitted light, indicating that pinhole-free nanostructured
films can be obtained. On the other hand, this technique is another
way to prepare dense films as used in ordinary polymer LEDs, if the
optimal postdeposition treatment has been carried out. Tada and
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Onoda have shown that a simple method like injection of a small
amount of toluene solution of target material into a large amount of
acetonitrile (nonsolvent for the relevant materials) can generate rela-
tively stable colloidal suspensions of various materials such as C ,
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poly(3-alkylthiophene), polyfluorene derivative, and MEH-PPV. The
colloidal suspension of the mixtures of the materials can be similarly
obtained. The colloidal particles can be easily collected through EPD
to make nanostructured films, which are pinhole-free and uniform
enough to make light-emitting devices as demonstrated. In compari-
son with the traditional approaches such as the spin-coating method,
EPD requires less solubility and thus is applicable to a wider range of
materials and composites. The key for this feature is the separation of
solidification and film formation. Since the polymer films obtained
through this process work as emission layers in a light emitting
device, they are expected to work also in photocells as the active
layer.

10.3.3 Organic Photocells
It is known that the use of donor-acceptor composite improves the
photon/electron conversion performance of photocells with conju-
gated polymers. 97–98 If both components are incorporated at more
than the percolation threshold concentration in the film, to make
interpenetrating networks, both electrons and holes can be trans-
ported to and collected by the electrodes. Since most conjugated
polymers have p-type nature, much effort has been devoted to syn-
thesize acceptor materials with high solubility. Tada and Onoda 10
have proposed a novel route to obtain donor-acceptor nanocomposite
by using EPD. The suspension required for the EPD consists of MEH-
PPV as donor and C as acceptor. The film obtained by EPD shows
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notable quenching of photoluminescence indicating photo-induced
charge transfer between the MEH-PPV donor and the C acceptor.
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This result suggests that the donor-acceptor nanocomposite obtained
through the electrophoretic deposition in the mixture of suspensions
is a promising material for organic photocells.