sensitive electronic components, and
electromagnetic shielding for electromagnetic equipment.
For these applications, polymers
are dissolved or dispersed in various liquids and applied to metallic and plastic
surfaces by dip coating, roller coating,
or spraying. After a short drying step in
an infrared oven, the applied polymer
coatings reach their optimum conductivity values and become functional.
Several companies have begun to market specially developed formulations
for such applications.
Passive components like resistors,
capacitors, and fuses have also been
developed with conducting polymers.
Miniature trimmer resistors are now
available that employ plastic resistive
elements contacted by a metallic
wiper. These components show a
superior temperature coefficient of
resistance and lower noise characteristics than their carbon and cermet
counterparts. Similarly, capacitors
with one electrode made from a
conducting plastic, such as polythiophene, exhibit longer life and reduced
Equivalent Series Resistance (ESR).
Such capacitors are proving to be
a superior alternative to conventional
aluminium electrolytic types. When
conducting polymers are heated to
temperatures above 250° C, their electrical properties degrade rapidly and
they stop conducting. This fact has
been made use of in making slow blow
fuses with conducting forms of polyaniline. These are ideal for protecting
devices like electric motors and high-brightness solid-state lighting units.
Polymers have also found their
way in batteries, with lithium polymer
cells being their prime example.
Replacing metallic electrodes with
conducting polymers results in
batteries that are lighter, have higher
capacity, and are more resistant to
harsh environments. For the same
reasons, these materials are also being
investigated for use in fuel cells.
Passives are, however, not the
only components that could make use
of conducting polymers in novel ways.
The conductivity of these materials
could be tailored to lie almost
anywhere from being metallic through
semiconducting behavior to insulating.
This capability opens up many possible applications where polymers, such
as polyaniline, could be usefully
employed. Indeed, researchers around
the world have come up with many
devices made from conducting polymers. These include diodes, transistors,
light-emitting diodes, photodiodes,
solar cells, chemical sensors, batteries,
and dot-matrix displays. Whereas poly-mer-based electronics will not replace
silicon as the semiconductor of choice
because of the low speed of electrons
and holes in these organic materials,
some polymer devices have unique
functionalities and these devices are
emerging as the main applications of
conducting polymers in electronics.
Polymer light emitters — also called
Organic Light-Emitting Diodes (OLEDs)
— are, perhaps, the most prominent of
all such devices. These are made by
coating glass or plastic sheets with
various charge conducting and light-emitting material layers, such as
polyaniline and poly paravinylene. The
manufacturing process is much simpler
and far less labor- and capital-intensive
than that for ordinary LEDs. As a result,
OLEDs are a potentially cheaper
alternative to their more established
cousins. Unfortunately, their electricity-to-light conversion efficiency is still
quite low and their operating lifetimes
also leave much to be desired.
Researchers are working on optimizing these devices and they should
appear in several products by the beginning of 2007. One of these will be full
color, large format flat panel displays,
currently dominated by plasma and LCD
panels. OLED-based flat panel displays
— now under intensive development by
companies like Philips — promise to be
cheaper, lighter, less power-hungry, and
capable of producing all-angle viewable
images with highly vibrant colors.
The first such products are going
to be OLED graphic displays for
handheld gadgets like cell phones,
◆ FIGURE 1. A segment of a polyacetylene chain showing alternating single and
double bonds between carbon atoms (red
spheres), lighter hydrogen atoms (green
spheres), and a grey cloud indicating the
presence of free electrons on the polymer
personal digital assistants, and
electronic games. A close-up screen
shot from a display recently developed at Philips is shown in Figure 2.
Apart from their uses in monochrome and full-color displays, OLEDs
have also been investigated for making
large area light emitters for space lighting. This is an area where conventional
LEDs have not succeeded so far due to
their directional emission characteristics
and relatively high cost per unit.
The fabrication of OLEDs relies on
depositing polymer materials on rigid or
flexible substrates like glass or plastic.
Various techniques have been
developed to coat substrates with active
polymer materials and deposit contact
metals for making functional devices.
One approach to building light-emitting
diodes from organic materials is to sandwich them between a glass substrate
coated with a transparent conducting
◆ FIGURE 2. A close-up screen shot of
an OLED flat panel display.
June 2006 55