Journal of Scientific & Industrial Research
Vol. 63, September 2004, pp 715-728
Electrically conducting polymers: Materials of the twentyfirst century
A K Bakhshi* and Geetika Bhalla
Department of Chemistry, University of Delhi, Delhi 110 007
The paper critically reviews some recent developments in the field of electrically conducting polymers which have
grown very rapidly since the discovery that there is a very sharp increase in conductivity when intrinsically insulating
organic conjugated polymers such as, polyacetylene are doped with oxidizing or reducing agents. These polymers, also
called synthetic metals, combine the electrical properties of polymers with the advantages of polymers and have such a vast
scope of diverse applications that these are being perceived as the materials of the twentyfirst century. Doped organic
conducting polymers, though conducting, suffer from two disadvantages of chemical instability and poor processibility. One
of the fundamental challenges in the field of conducting polymers, therefore, is to design low band gap intrinsically
conducting polymers so that there is no need to dope them. Various strategies presently used for designing polymers with
tailor-made conduction properties and some recent results obtained using these strategies are discussed. Lastly, some of the
important applications of electrically conducting polymers are also discussed with a view to highlight the great potential of
these materials.
Keywords: Electrically conducting polymers, Synthetic metals, Band-gap, Electronic structure, Donor-acceptor polymers,
Copolymers, Band structure engineering
IPC Code: Int. Cl.7: C 08 G 18/76, C 08 G 18/83
Introduction
Polymers have always been considered as insulators of electricity. No one would have believed 30 y ago that
polymers could conduct as good as metals. But now such feats have been achieved and that through simple
modification of ordinary organic conjugated polymers. Called electrically conducting polymers or synthetic
metals, these materials combine the electrical properties of metals with the advantages of polymers such as,
lighter weight, greater workability, resistance to corrosion and chemical attack and the lower cost and have
infiltrated our day-to-day life with a wide range of products, extending from most common consumer goods to
highly specialized applications in space, aeronautics, electronics, and non-linear optics. It is, therefore, no
wonder that these polymers are called the Materials of the twentyfirst century.
The first major breakthrough in the field of electrically conducting polymers took place around 1978 when it
was demonstrated by Shirakawa et al.1,2 that polyacetylene (PA), an intrinsically insulating organic conjugated
polymer, exhibits dramatic increase in electrical conductivity3 on treatment with oxidizing (electron-accepting)
or reducing (electron-donating) agents. These oxidation and reduction reactions, which induce high conductivity
in PA are termed as p-doping and n-doping, respectively.
The discovery of highly conducting PA led to a sudden spurt in research activity directed towards the study of
new conducting polymeric systems. The instability of PA in air4 further intensified this research (on exposure to
air, covalent bonds are formed between oxygen and carbon atoms and these bonds lower the conductivity of PA
because of their interruption of conjugated double bonds). The result is that at present many novel conducting
systems are known and these include polypyrrole (PPY), poly (phenylacetylene) (PPA), poly (p-phenylene
sulphide) (PPS), poly (p-phenylene) (PPP), polythiophene (PTP), polyfuran (PFU), polyaniline (PAN),
polyisothianaphthene (PIN) and, their derivatives. These polymers, though they share many structural features
such as a conjugated backbone, planarity and large anisotropy ratio (i.e. the intrachain conductivity is much
larger than the inter-chain conductivity), however have a wide range of conductivity depending upon; (i) The
doping per cent, (ii) The alignment of polymer chains, (iii) The conjugation length, and (iv) The purity of the
sample.
Some Special Features of Conducting Polymers
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J SCI IND RES VOL 63 SEPTEMBER 2004
Molecularity and Disorder
Electrically conducting polymers unlike inorganic semiconductors are molecular in character and lack long
range order. The molecular character of polymers makes electronic motion along the individual macromolecules
one-dimensional. This reduced dimensionality implies that, even if polymeric materials were perfectly
crystalline solids, their electronic properties would be generated by certain types of collective ground states
called Fermi surface instabilities which characteristically occur in one- and sometimes two-dimensional systems.
For example, in PA, as a direct consequence of the well known Peierls instability5 of 1-D coupled electronphonon systems, the distortion of the backbone lattice which produces the bond alternation, creates a gap exactly
at the Fermi surface and thus changes a would be metal into a semiconductor.
The occurrence of disorder in polymers leads to the concept that even the intrinsic electronic states in these
materials may be localized. In such a case, intrinsic activated charge carrier mobilities should be observed, in
contrast to the traditional energy band semiconductors, for which intrinsic carrier mobilities decrease, with
increasing temperature, as T-n, n > 0. In addition, the consequences of disorder are enhanced as the
dimensionality of the system is reduced.
Therefore, although organic polymers seem to exhibit transport and optical properties analogous to those of a
crystalline network of semiconductors, the interpretation of these properties and the design of materials involve
different physical phenomena.
Nature of Doping Processes
The nature of processes inducing high conductivity are different for polymers and inorganic semiconductors.
In the doping of inorganic semiconductors the dopant species occupies positions within the lattice of the host
material thereby resulting in the presence of either electron-rich or electron-deficient sites with no charge
transfer occurring between the two sites. The doping reaction in organic conjugated polymers, on the other hand,
is a charge transfer reaction, resulting in the partial oxidation or reduction of the polymer, rather than the
creation of holes, etc. It is now well established6,7 that the exposure of PA to an oxidizing agent X (or reducing
agent M) leads to the formation of positively (or negatively) charged polymeric complex and of a counter ion
which is the reduced X- (or the oxidized M+) form of the oxidant or reductant.
The “doping process” in the case of conducting polymers may be, therefore, more correctly classified as redox
processes of the following general scheme:
Polymer + X
( Polymer) n + + X n −
in the case of an oxidation (p-doping) process and
Polymer + M
( Polymer) n − + M n +
for a reduction (n-doping) process
X = I2, Br2, AsF5,..….. and M= Na, Li……
The above reactions most likely occur in the case of unsaturated polymers with π-electrons as they can be
easily removed or added to the polymeric chains to form polyions and, therefore, these are the types of polymers
which assume high conductivity on doping.
Solitons, Polarons and Bipolarons as Charge Carriers
The increase in conductivity observed upon doping organic conjugated polymers was initially thought to
result from the formation of unfilled electronic bands. This assumption was, however, quickly challenged by the
discovery that polyacetylene (PA)8 and poly-paraphenylene (PPP)9 display conductivity which does not seem to
be associated with unpaired electrons but rather with spinless charge carriers. It has been found that high
conductivities obtained upon doping in these polymers are associated with formation of self-localized
excitations10 such as solitons, polarons and bipolarons. These quasi-particles which arise from a strong
BAKHSHI & BHALLA: ELECTRICALLY CONDUCTING POLYMERS
717
interaction between the charge on the chain (electron or hole) acquired as a result of doping and the molecular
structure are the direct consequence of the strong electron-phonon interaction present in these quasi-onedimensional polymers. Thus, charge-carrying species in doped organic conjugated polymers are not free
electrons or holes as in the case with inorganic semiconductors but quasi-particles such as, solitons, polarons and
bipolarons which may move relatively freely through the material. It is now very well established that in
polymers with degenerate ground state such as, trans-polyacetylene, charged solitons (positively charged or the
negatively charged depending upon whether p-doping or n-doping) are the charge carriers, whereas in polymers
with non-degenerate ground state such as, cis – polyacetylene, polypyrrole, polythiophene or poly (pphenylene), initially polarons (positively charged or negatively charged) are formed on doping. These polarons
then combine to form spinless bipolarons which act as the charge carriers. The formation of bipolaron is also
supported by calculations which show that the formation of one bipolaron is thermodynamically more stable
than that of two separated polarons despite the Coulomb repulsion between two similar charges.
Band Structure Engineering of Low Band-Gap Conducting Polymers
The increase in the electrical conductivity of various organic conjugated polymers on doping with oxidizing
or reducing agents is not without accompanying problems. The process of doping, though enhancing the
conductivity of organic conjugated polymer, is often the source of chemical instability and poor processibility in
them. The possible elimination of doping in preparing conducting polymers while still achieving high
conductivity is one of the original motivations for need of small band-gap polymers11-17. Such polymers are
expected to be intrinsic conductors of electricity and hence there will be no need to dope them. The efforts to the
design of conjugated organic polymers with a small band-gap go under the name of Band Structure Engineering
of novel polymers. The structures of some of the low band-gap polymers are shown in (Fig. 1).
Strategies Used for Band Structure Engineering of Polymers
Various routes are presently used for designing novel conjugated polymers with tailor-made conduction
properties. These include:
(a)
(b)
(c)
(d)
(e)
Substitution/Fusion,
Ladder Polymerization,
Topological methods,
Copolymerization, and
Donor-Acceptor polymerization
Substitution / Fusion
In this method, one starts with small band-gap polymers and tries to modify their electronic properties by
substitution provided their chemical nature and experimental conditions allow these substitution reactions. The
following two guidelines are of great help in the strategy of substitution:
(a) In polymers with degenerate ground state such as, trans-PA, it is now well established that the band-gap
decreases as a function of decreasing bond length alternation along the chain. Thus, if a substituent
decreases the bond length alternation along the backbone the band-gap of the resulting polymer shall
decrease and vice-versa.
(b) On the other hand, in polymers with non-degenerate ground state such as, poly (p-phenylene) (PPP),
polypyrrole (PPY), etc., it has been found that the band-gap decreases as a function of increasing quinoid
character of the polymer backbone.
Using the above guidelines, the effect of substituents on the band structure of PA has been investigated in few
cases like, fluorinated polyacetylenes18, halogen and cyano substituted polyacetylenes19 and alkoxy-substituted
poly (p-phenylenevinylene)s20. Polymers having azobenzene substituents in the main chain have been studied by
Izumi et al.21. The azobenzene units in the conjugated polymer backbone make the polymers thermally stable
polymer.
Recently, a new low band-gap polymer (1.16 eV), namely poly (5,6-dithiooctyl isothianaphthene) has been
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J SCI IND RES VOL 63 SEPTEMBER 2004
synthesized22. This polymer has been found as a useful active material in construction of solar cells in
combination with PCBM (6,6 phenyl C61-butyric acid). Highly conductive new aniline copolymers containing
butylthio substituent have also been successfully prepared23 with conductivity of the
order of 1 S cm-1. All these new butylthioaniline copolymers are highly soluble in common organic solvents
despite the presence of large amount of bulky butythio substituent. Although in some cases, substitution may
decrease the conductivity of the polymer but the resulting polymer has higher electron affinity and can,
therefore, be used in LEDs. A series of cyano-substituted distyryl benzenes24 have also been synthesized. It has
been observed that by properly adjusting copolymer compositions, a combined high electron affinity and
transport was achieved in a statistic copolymer, namely poly (fluorenebenzothiadiazsolecyanophenylenevinylene) (PFB-CNPV)24.
Thus, all these various kinds of substituents are in use for improving solubility, decreasing band-gaps,
increasing polarizabilities and conductivity, and finally optimizing luminescence efficiencies.
BAKHSHI & BHALLA: ELECTRICALLY CONDUCTING POLYMERS
Fig. 1  Some low band-gap conjugated polymers
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J SCI IND RES VOL 63 SEPTEMBER 2004
Fig. 2  Various members of the one-dimensional graphite family
Ladder Polymerization
Ladder polymers are formed by joining simple polymers into symmetrical polymeric rings. The small energy
gap in ladder polymers is a consequence of the direct interplay of electron-lattice and electron-lattice interactions
in them. Among ladder polymers, hydrocarbon polymers with fused aromatic rings have been the focus of
enormous interest. This new class of polymers, frequently referred to as one-dimensional graphite family,
includes members such as polyacene (PAc), polyacenacene (PAcA), polyphenanthrene (PPh), polyphenanthrophenanthrene (PPhP) and polyperinaphthalene (PPN). The electronic structure and conduction properties of the
members of 1-D graphite family (Fig. 2) have been the subject of many theoretical investigations25,26. The
ladder-type poly-p-phenylenes (LPPP) offer the opportunity to study large, rod-like chains of planarised
phenylene units. The ground-state properties and excited states of ladder-type paraphenylene oligomers have
been calculated by applying semi-empirical methods for up to eleven-phenylene rings27. A scheme to interpret
the excited states has been developed which reveals the excitonic nature of the excited state. Ladder thiophene
polymers have also been synthesized with decreased band-gap values28. It has been observed that ladder
polymers with higher molecular weight showed better thermal resistance. Ladder polymers with band-gaps as
small as 0.2 eV have already been synthesized by Kerterz and Hughbanks29. Various other ladder polymers with
improved
properties
have
also
been synthesized. These include poly(aroylene benzimidazoles), polyepoxysiloxanes30, and ladder polymers with
thienylene units31. Recently, photoconduction study on a ladder type poly (paraphenylene) has also been done32.
The energy spectra of one-dimensional stacks consisting of large π-π interacting polycyclic aromatic
hydrocarbons have been investigated theoretically33, taking into account electron correlation. The band-gap of
these stacks is about 0.8 eV. These polymers are candidates for new materials with unique electronic properties
such as, electroconductivity, photoconductivity or magnetic properties. Simultaneously, they are models for
nanometers scaled graphites (nanographites).
Topological Methods
In the case of fused ring polymers the electronic properties are found to depend strongly on the particular way
the rings are fused and the recognition of this has led to the employment of topological methods based on the
concept of topomers for designing novel polymers. It means that one has to construct the corresponding
BAKHSHI & BHALLA: ELECTRICALLY CONDUCTING POLYMERS
721
oligomers of a pair of topomers as S- and T- topomers. In the S- topomer, the two bonds connect pairwise
topologically equivalent atoms. While in the T- topomer, the end points of the two bonds are interchanged in one
subunit. If, e.g., A is the T- and B the S- topomer of the same subunit, then the following relations which are the
consequences of the interlacing theorem are valid for these pairs:
(i) The ionization potential (IP) of A is smaller than the IP of B.
(ii) The electron affinity (EA) of A is smaller than the EA of B.
(iii) The fundamental band-gap (Eg) of A is smaller than the gap of B.
Point (iii) follows immediately from (i) and (ii) because Eg = I P – E A. The gap of A is, therefore, located
inside the gap of B on energy scale. The above topological arguments have been used to rationalize the large
differences in the electronic properties of fused ring polymers such as, polyacene, polyphenanthrene and
polybenzanthracene34and in search for novel low band-gap conjugated polymers35. Polyisophenanthrene, a new
hypothetical polymer is predicted to have a band-gap between polyacene and polybenzanthracene.
Unique new ladder polymer (polyindenoindenes) consisting of condensed succession of six- and fivemembered conjugated carbon rings have been synthesized36. Seven topological isomers of these
polyindenoindenes are considered theoretically. The results are analysed in terms of topological band-gaps and
geometrical relaxation. Three isomers are expected to have a band-gap smaller than 0.2 eV
(ref. 29).
Copolymerization
The strategy of growing co-polymers is highly exciting and promising. Copolymers can have tailor-made
properties depending upon the choice of two semiconducting components, their relative amounts and their
arrangement in the polymer chain. The electronic properties of a copolymer (AmBn)x (where m= block size of
component A and n= block size of component B), though generally intermediate between those of its
components (A)x and (B)x, can be tuned by varying the molecular composition of the copolymer and by varying
the arrangement of components (periodic or aperiodic) in the copolymer chain.
The electronic DOS of the various periodic and aperiodic quasi-one-dimensional model and real copolymers
of the type (AmBn)x belonging to the class of Type-I and Type-II staggered have been studied in both tight
binding approximation and by considering multineighbour interaction37-40. For each of these types of
copolymers, the trends in their electronic structure and
conduction properties as a function of : (i) Composition
(m/n), (ii) Block sizes m and n, and (iii) Arrangement of
blocks in the copolymer chain have been investigated.
The results of these studies are summarized in Table 1.
In the case of copolymers of both Type-I and
Type-I
Type-II staggered, it has been found that increasing the
proportion of low band-gap component (B)x in the
copolymer chain increases the electron affinity (EA) and
hence improves the n-dopantphilicity of the copolymer
chain. Increasing the percentage of large band-gap
component (A)x in the copolymer chain decrease the
ionization potential (IP) and hence improves the pdopantphilicity of the copolymer chain in the case of
Type-I copolymers while in case of Type-II staggered
Type-II staggered
copolymers, this has the opposite effect of increasing the
IP
and
hence
making
it
less
p-dopantphilic. To have a copolymer with prospects for both p- and n- doping, as well as better intrinsic
J SCI IND RES VOL 63 SEPTEMBER 2004
722
Table 1Trends in the electronic properties of copolymers belonging to the class of type-I (trends in the
parenthesis correspond to those for type-II staggered)
System
Ionization potential
Electron affinity
Band-gap
(ABn)x
increase in n
(AmB)x
increase in m
(AmBn)x increase in m
and n for constant m/n
Decreases
(Increases)
Increases
(Decreases)
Decreases
(Decreases)
Increases
(Increases)
Decreases
(Decreases)
Increases
(Increases)
Decreases
(Decreases)
Increases
(Decreases)
Decreases
(Decreases)
conductivity, increasing the block sizes m and in of the two components A and B for a given composition is the
best solution for both Type-I and Type-II staggered copolymers.
Further, it has also been found that the electronic properties of periodic copolymers cover a wider range than
those of aperiodic copolymers. It, therefore, means that the tuning the electronic properties to a particular value
is easier by synthesizing periodic copolymers. In the case of aperiodic copolymers, on the other hand the
saturation in electronic properties is reached much faster. Aperiodic copolymers are also predicted to be better
intrinsic and extrinsic conductors of electricity than the corresponding periodic copolymers. The results obtained
here are important guidelines for designing copolymers with tailor-made conduction properties.
There have also been quite interesting investigations of the various types of other copolymers recently. These
include systems such as, cyclodiborazane−dithiafulvene copolymers41, copolymers of fluorine− and alkoxy−
substituted poly (p-phenylene vinylene)20, carbazole-quinoline, and phenothiazine-quinoline copolymers42.
Copolymers of aniline with o- or m- toluidine and o-ethyl aniline have also been reported43,44. It has been found
that these copolymers of aniline with substituted anilines show fairly good conductivity. The electronic
properties of the hypothetical thiophene copolymers: poly (thienylenecyclopentadienylene) (PThS), poly
(thienylene-oxocyclopentadienylene) (PThOPD) and poly (thienylenethiocyclypenta-dienylene) (PThTPD) have
also been theoretically investigated45.
Copolymers of aniline and pyrrole46 and copolymers having S-S links47 have recently been studied. These
copolymers with S-S links in the backbone have better solubility and are expected to find application in Li
batteries. Novel carbazole-based copolymers48 with different comonomers have been synthesized. The emission
colour can be tuned in entirely visible region by careful choice of narrow band comonomers.
Donor–Acceptor Polymerization
Another very exciting possibility and successful route in designing of low band–gap electrically conducting
polymers is provided by the donor-acceptor polymers. The principal idea behind donor-acceptor polymers is that
a regular alternation of donor- and acceptor- like moieties in a conjugated chain will induce a low band-gap.
Various novel donor-acceptor polymers differing in their electron-donating and electron-accepting moieties have
been theoretically designed and investigated49. Recently, we have studied some donor-acceptor polymers based
on polysilole50 and poly (diflurosilane), respectively51, (Fig. 3) on the basis of ab-initio Hartree Fock crystal
orbital method52. These polymers have also been studied53-55 on the basis of the one-dimensional tight binding
SCF-CO method at the MNDO-AM1 level of approximation. The calculated ab-initio electronic properties of
these donor-acceptor polymers are given in Table 2. In both the classes of polymers (Table 2), the polymers with
Y= > C=C(CN)2 have the smallest band-gap while those with Y= > C=O have the largest band-gap, implying
hereby that in these donor-acceptor polymers, > C=C(CN)2 is the strongest electron-withdrawing group and >
C=O the weakest.
BAKHSHI & BHALLA: ELECTRICALLY CONDUCTING POLYMERS
723
Table 2Calculated electronic properties (in eV) of polysilole based donor-acceptor polymers. Values
in parenthesis are the corresponding values for donor-acceptor polymers based on poly (diflurosilane)
Y
IP
EA
Eg
> C=CH2
> C=O
> C=CF2
> C=C (CN)2
8.084 (8.682)
2.419 (3.017)
5.665 (5.665)
8.734 (9.381)
2.931 (3.618)
5.803 (5.763)
8.278 (8.861)
2.601 (3.241)
5.677 (5.620)
8.997 (9.560)
3.804 (4.415)
5.193 (5.145)
Further, since the polymer with X=SiF2 and
Y=>C=C(CN)2 has smaller band-gap than the
corresponding polymer with X= SiH2, it means that in
these polymers SiF2 acts as a stronger electron-donating
group than SiH2. The calculated band-gap values of all
the polymers are quite large, i.e. between 5.0 to 6.0 eV.
This is the well-known overestimation of the band-gap by
a
factor
of
three
to
four
of
the
ab-initio Hartree-Fock crystal orbital method and is due
to the use of Clementi’s minimal basis set and the neglect
of electron correlation effect. On scaling down the actual
band-gap values of these polymers are expected to lie
between 1 and 2 eV. An examination of the calculated πbond order values of the polymers studied shows that all Fig. 3  Alternate arrangement of donor and acceptor units
the polymers have the benzenoid-like electronic
structures. The orbital patterns of both the HOCO and
LUCO for all the polymers are found to be similar (Fig.
4).
It is interesting to note that the contribution of the
electron-accepting group Y to the HOCO is negligibly Fig. 4  Orbital patterns of the HOCO (top of the valence band)
and LUCO (bottom of the conduction band) of PSICH (Y= >
small, while it makes a significant contribution to the C=CH ), PSICO (Y= > C=O), PSICF (Y= > C=CF ) and PSICN
2
2
LUCO of these polymers. The electron-donating group (Y= > C=C(CN)2). White (black) circle indicates positive
(SiH2 or SiF2), on the other hand, makes significant (negative) LCAO (linear combination of atomic orbitals)
contributions to both the HOCO and the LUCO. It, coefficients. The pseudo-orbitals of X = SiH2 or SiF2 are omitted
therefore, means that when these polymers are doped
with oxidising agents (p-doping), they attract π-electrons of the entire skeleton (except those of the group Y),
while when these are doped with reducing agents (n-doping), the electrons are donated to the entire π-electron
system including the group Y. From the patterns of the orbitals, it also follows that for a given geometry and the
group SiH2 or SiF2, IP should not change significantly with the group Y. The observed differences in the values
of IP (Table 2) are primarily due to the changes in the geometric structures resulting from change of Y.
In the light of the above results, one can rationalize the observed band-gap values of these polymers by
visualizing their formation through the interaction of the conjugated skeleton containing X=SiH2 or SiF2 with an
electron - accepting group Y terminated by H-atoms (Fig. 5). It can be seen that the band-gaps of these polymers
are primarily determined by the strength of the bonding interaction between the LUCO of the conjugated
skeleton containing X=SiH2 or SiF2 and the LUMO of the electron-withdrawing group Y.
Applications of Conducting Polymers
The discovery of electrically conducting polymers has attracted a lot of attention mainly because of their great
potential for diverse applications. Some of these important applications of conducting polymers are discussed
subsequently:
Light Weight and Rechargeable Batteries
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J SCI IND RES VOL 63 SEPTEMBER 2004
This is one of the most publicized and promising
application. In polymers, where both p- and n-doping
processes are feasible the possibility exists of their use as
both positive and negative electrodes in the same battery
system. Some prototype cells are comparable to, or better
than nickel-cadmium cells now available in the market.
The polymer battery, such as polypyrrole-lithium cell
operates by the oxidation and reduction of the polymer
backbone. During charging the polymer oxidizes anions
in the electrolyte that enter the porous polymer to balance
the charge created. Simultaneously, lithium ions in
electrolyte are electrodeposited at the lithium surface.
During discharging, electrons are removed from the
lithium, causing lithium ions to re-enter the electrolyte.
The positive sites on the polymer are reduced, releasing
the charge-balancing anions back to the electrolyte. This
process can be repeated about as often as in a typical
secondary cell56. The above mentioned principle has also
been used57,58 to make PA batteries with the following
configuration in its fully discharged state,
Fig. 5  Schematic energy levels depicting the formation of
polymers from the interactions of the conjugated skeleton (CH) / LiClO – PC/(CH)
x
4
x
containing X= SiH2 or SiF2 and the electron - accepting group Y
terminated by H atoms (i.e. CH2=C(CN)2). The pseudo-orbitals of
PA battery has higher energy and power densities as
X= SiH2 ; SiF2 are omitted. White (black) circle indicates positive
compared
to ordinary batteries. The polymer electrode
(negative) LCAO (linear combination of atomic orbitals)
batteries have a longer shelf-life than the conventional
coefficients
ones. Another advantage of polymer electrode batteries is
the absence of toxic materials in them and therefore
disposal problems are minimized. These batteries could be a potential breakthrough in the making of an electric
car. The Bridgestone Corporation of Japan have developed coin type rechargeable polymer lithium batteries with
a conducting polymer polyaniline and the higher capacity lithium aluminium alloy as the two electrodes. One of
the unique features of this rechargeable polymer lithium battery is that it can be used as a power source in
combination with solar cells.
Solid State Batteries
The application of intrinsically conducting polymers in solid-state lithium ion polymer batteries has generated
a lot of interest during the past few years. Batteries with high energy density and with full solid state
configuration for both electrodes and electrolyte (crystalline, glassy, and polymeric) using electrically
conducting polymers have been studied, both experimentally and theoretically59. An iodine-doped PA film is
placed in direct contact with a lithium disk in a Li/I2-PA solid state battery. Contact between lithium and iodine
doped PA brings about immediately a reaction with the formation of lithium iodide.
2x Li + CH (I2) yx → 2x (1–n) Li + CH (I2)y–nx + 2xn LiI
These types of batteries have high durability and reliability. Using thin films of conducting polymers, these
solid state batteries may provide plasticity – a feature which would be welcome in various applications. Study of
polypyrrole (PPy) / polyimides (PI) composite has also shown its promising properties and potential for use in
polymer lithium ion batteries and in supercapacitors. PPy film was found to be switchable between the anion,
and cation-exchange states and PI was chosen as a matrix for polymer filled conducting composites because it
possesses electroactivity and excellent mechanical properties. Thus, PPy/PI conducting composite
is studied for application as a solid polymer electrolyte for lithium ion batteries. Lithium manganate / manganese
composite oxides and lithium ion secondary batteries have also been synthesized60.
BAKHSHI & BHALLA: ELECTRICALLY CONDUCTING POLYMERS
725
Electromagnetic Shielding
The dissipative abilities of polymers also make them ideal for electromagnetic shielding. By coating the inside
of the plastic casing with a conductive surface, this radiation can be absorbed. This can best be achieved by
using conducting plastics, which have good adhesion and thus give a good coverage and good thickness56.
Incorporated into computer cases, conducting polymers can block out electromagnetic interference in the
megahertz range.
Molecular Electronics
Molecular electronics concerns itself with the electronic structures assembled atom by atom. One proposal for
this method involves conducting polymers. A possible example is a modified PA with an electron-accepting
group at one end and an electron-withdrawing group at the other. A short section of the chain is saturated in
order to decouple the functional groups. This section is known as a spacer or a modulator barrier. This can be
used to create a logic device. There are two inputs, one light pulse which excites one end and another which
excites the modulator barrier. There is one output, a light pulse to see if the other end has become excited. To
use this, there must be lot of redundancy to compensate for switching errors61. Depending on the conducting
polymer chosen the doped and undoped states can be either colourless or intensely coloured.
Chemical, Biochemical and Thermal Sensors
The chemical properties of conducting polymers make them very useful in sensors62. This utilizes the ability
of such materials to change their electrical properties during reaction with various redox agents (dopants) or via
their instability to moisture and heat. An example of this is the development of gas sensors63. It has been shown
that polypyrrole behaves as a quasi ‘p’ type material. Its resistance increases in the presence of a reducing gas
such as ammonia and decreases in the presence of an oxidizing gas such as NO2. The gases cause a change in the
near surface charge carrier (here electrons or holes) density by reacting with surface absorbed oxygen ions64. An
ideal chemical sensor should exhibit high sensitivity, selectivity, high operation speed, reversibility and stability
under operating conditions and conducting polymers meet the above requirements. Conducting polymers such as
polyfulvenes (PFV) and polythiophene (PTP) are expected to have profound uses in humidity sensors and
radiation detectors. Another type of sensor developed is biosensors65. This utilizes the ability of triiodide to
oxidize PA as a means to measure glucose concentration. Glucose is oxidized with oxygen with the help of
glucose oxidase. This produces hydrogen peroxide which oxidizes iodide ions to form triiodide ions. Hence,
conductivity is proportional to the peroxide concentration which is further proportional to the glucose
concentration56. Recently, phenylene vinylene and aromatic amine segments based alternating copolymers66
have been synthesized. In these copolymers, phenylene vinylene part plays the emission role and the aromatic
amine portions impart the hole transporting mobility and increase the thermal stability.
Electromechanical Actuators
Polymer based actuators are a new technology. Actuators67 can function by using changes in a dimension of a
conducting polymer, changes in the relative dimensions of a conducting polymer and a counter electrode and
changes in total volume of a conducting polymer electrode, electrolyte and counter electrodes. The method of
doping and dedoping is very similar as that used in rechargeable batteries discussed earlier. What is required are
the anodic strip and the cathodic strip, changing size at different rates during charging and discharging. The
applications
of
this
include
microtweezers,
microvalves,
micropositioners for microscopic optical elements, and actuators for micromechanical sorting68. These types of
actuators are seen as a promising technology both for existing applications and future applications in the area of
process and manufacturing automation.
Welding Plastics with Conducting Polymers
The development of intrinsically conductive polymers, especially polyanilines, provides an opportunity for
use of conductive polymers in welding (joining) of thermoplastics and thermosets69. Either a pure, intrinsically
conducting polymer film or a gasket prepared from a compression-molded blend of the intrinsically conductive
726
J SCI IND RES VOL 63 SEPTEMBER 2004
polymer and a powder of the thermoplastic or thermoset to be blended in placed at the interface between two
plastic pieces to be joined. The resulting joint may be as strong as that of the pure compression molded
thermoplastic or thermoset. Depending upon the chemical composition of the conducting polymer and the
dopant used, the resulting joint may be permanent.
Light Emitting Diodes
Conducting polymers can also be used as the light sources in displays70,71. Burroughes et al.72 first reported a
light- emitting diode (LED) based on poly (p-phenylenevinylene) (PPV) in 1990. Today, PPV and its derivatives
and polythiophenes (PTPs)73, and polyfluorenes (PFs)74 are the most frequently used conjugated polymers in
light-emitting diodes (LEDs). Substantial research has been dedicated to improving light output, efficiency and
lifetime of polymer light-emitting diodes as these are promising candidates for cheap, bright, and even flexible
large area displays. To achieve highly efficient LED devices, charge (holes and electrons) injection and transport
from both the anode and the cathode should be balanced at the junction of the emitting layer to yield the
maximum exciton fomation75.
In LEDs the conjugated polymer is sandwitched between electrode layers. The first electrode (cathode) is
fabricated from aluminium, magnesium or calcium-metals with low ionization potential which upon application
of an electric field inject electrons into the CB of the polymer. The second electrode (anode) is fabricated from
Indium/tin oxide, which injects holes into the VB of the conjugated polymer. The radiative recombination of the
injected electron and hole leads to emitted light. During the past few years, many new applications using
conjugated polymers as the active substances have emerged. Among these are light-emitting electrochemical
cells (LECs)76, photovoltaic diodes, and micro cavity laser devices77. A novel series of efficient thiophene based
light-emitting conjugated polymers78 and p-n diblock light-emitting copolymers based on oligothiophenes and
1,4-bis (oxadiazolyl) – 2,5 – dialkyoxy benzene79 and on poly (p-phenylene vinylene) with oligo (ethylene
oxide)80 have been synthesized and their applications in polymer light-emitting diodes have been proposed. New
conjugated polymers containing cyano-substituents and quinoline-based copolymers81 for light-emitting diode
have also been synthesized. Yang et al.82 have recently synthesized a series of novel soluble conjugated
copolymers
derived
from
9,9-dioctylfluorene (DOF) and pyridine (Py). These copolymers emit blue light in the region of 438-446 nm.
Highest electroluminescence quantum efficiency (0.72 per cent) is observed for device with 30 per cent Py unit
in the copolymer. These copolymers could be a promising blue-light emitting materials. Novel PPV-based
copolymers consisting of siloxane linkage have been synthesized by Sun et al.83 The rigid PPV segments act as
chromosphere and allow fine tuning of band-gap for blue light emission while the flexible siloxane units lead to
the effective interruption of conjugation and the enhancement of solubility.
Other Applications
Some of the other applications of conducting polymers that have been proposed are:
Use of conducting polymers as conductive paints, tones for reprographics, printing and as components for
aircraft.
Commercially available applications utilizing conductive polymers also include antistatic coatings for
electronic packaging and electrochromic windows.
These polymers are presently being investigated as possible candidates for molecular wires and in supercapacitors84,85.
They may be used as artificial muscles86 where simple tweezers made from strips of polymers with different
conductivities work together to form a muscle.
Conclusions
In this paper, we have given an overview of the emerging field of electrically conducting polymers with
special reference to the their special features such as molecularity and disorder, nature of doping processes and
nature of charge carriers produced on doping. One of the fundamental challenges in the field of conducting
polymers is the designing of low band-gap organic conjugated polymers. This problem has become all the more
BAKHSHI & BHALLA: ELECTRICALLY CONDUCTING POLYMERS
727
important in view of the disadvantages of poor processibility and instability associated with doped organic
conjugated polymers. Low band-gap polymers are expected to be (i) Intrinsically good electrical conductors or
semiconductors without the need of any doping; (ii) Transparent in either the intrinsic or doped state; and (iii) Of
great interest as new polymeric materials for non-linear optics (because of fewer contact problems) and other
properties. Various strategies viz., substitution/fusion, ladder polymerization, topological methods,
copolymerization and donor-acceptor polymerization currently used for designing novel low band-gap
conducting polymers and the electronic structures and conduction properties of some novel low band-gap
polymers designed using these strategies have been discussed. Lastly, we have discussed some of the
applications of these electrically conducting polymers. Significant among them are the applications in light
weight and rechargeable batteries, solid state batteries, light emitting diodes, electrochromic devices, sensors,
molecular electronics etc. Much research will be needed before many of the above applications become a reality.
The stability and processibility both need to be substantially improved if these polymers are to be used in the
market place. Regardless of the practical applications that are eventually developed for electrically conducting
polymers, they will certainly continue to challenge researchers in the years to come with new and unexpected
phenomena.
Acknowledgements
One of the authors (A K Bakshi) is thankful to DST (Department of Science and Technology) for the financial
support. Geetika Bhalla is grateful to CSIR (Council of Scientific and Industrial Research), New Delhi for the
award of fellowship.
References
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Chiang C K, Drug M A, Gan S C, Heeger A J, Lewis E J, Mac Diarmid A G, Park Y W & Shirakawa H, Synthesis of highly
conducting films of derivatives of polyacetylene, (CH)x, J Am Chem Soc, 100 (1978) 1013.
Chiang C K, Finchee C R, Park Y W, Heeger A J, Lewis E J, Gan S C & Mac Diarmid A G, Electrical conductivity in doped
polyacetylene, Phys Rev lett, 39 (1977) 1098.
Shirakawa H, Lewis E J, Mac Diarmid A G, Chiang C K & Heeger A J, Synthesis of electrically conducting organic polymers:
halogen derivatives of polyacetylene, (CH)x , J Chem Soc Chem Commun, (1977) 578.
Aldissi M, Review of the synthesis of polyacetylene and its stabilization to ambient atmosphere, Synth Met, 9 (1984) 131.
Peierls R E, Quantum theory of solids (Clarendon Press, Oxford) 1955.
Ferroro J R & Williams J M, Introduction to synthetic electrical conductors (Academic Press, New York) 1987, 126.
Scrosati B, Electrochemical properties of conducting polymers, Prog Solid State Chem, 18 (1988) 1.
Ikehata S, Kaufer J, Woerner T, Pron A, Druy M A, Sivak A, Heeger A J & Mac Diarmid A G, Solitons in polyacetylene – Magnetic
susceptibility, Phys Rev Lett, 45 (1980) 423.
Peo M, Roth S, Dransfeld K, Tieke B, Hocker J, Gross H, Grupp A & Sixl H, Apparent absence of Pauli paramagnetism in metallic
polyparaphenylene, Solid State Commun, 35 (1980) 119.
Su W P, Schrieffer J R & Heeger A J, Solitons in polyacetylene, Phys Rev Lett, 42 (1979) 1698
Bakhshi A K, Electronic structures of conducting polymers, Annu Rep Royal Soc Chem, Sec C, 89 (1993) 147.
Chen
W
C
&
Jenekhe
S
A,
Small-band
gap
conducting polymers based on conjugated poly (heteroarylene methines). 1. Precursor poly (heteroarylene methylenes), Macromolecules,
28 (1995) 454.
Goris L, Loi M A, Cravino A, Neugebauer H, Sariciftci N S, Polec I, Lutsen L, Andries E, Manca J, De Schepper L & Vanderzande
D, Poly (5,6-dithiooctylisothianaphtene), a new low band-gap polymer: spectroscopy and solar cell construction, Synth Met, 138
(2003) 249.
Kyunghoon Lee & Gregory A. Sotzing, Poly (thieno [3,4-b] thiophene)- A new stable low band-gap conducting polymer,
Macromolecules, 34 (2001) 5746.
Martin Pomerantz, Xiaomin Gu & Simon Xinnong Zhang, Poly (2-decylthieno [3,4-b] thiophene-4,6-diyl]) - A new low band-gap
conducting polymer, Macromolecules, 34 (2001) 1817; Jonforsen M, Johansson T, Inganas O & Andersson R M, Macromolecules,
35 (2002) 1638.
Meng H & Wudl F, A robust low band-gap processable n-type conducting polymer based on poly (isothianaphthene),
Macromolecules, 34 (2001) 1810; Kim Tae & Ronald L Elsenbaumer, Synthesis, characterization & electrical properties of poly (1alkyl-2,5-pyrrylene vinylenes): New low band-gap conducting polymer, Macromolecules, 33 (2000) 6407; Carlito G Bangcuyo, Una
Evans, Myrick M L & Bunz U H F, Synthesis and characterization of a 2,1,3-benzothiadiazole-b-alkyne-b-1,4-bis (2ethylehexyloxy) benzene terpolymer, a stable low band-gap poly (heteroaryleneethynylene), Macromolecules, 34 (2001) 7592.
Aouchiche H A, Djennane S & Boucekkine A, DFT study of conjugated bihetero-cyclic oligomers exhibiting a very low HOMOLUMO energy gap, Synth Met, 140 (2004) 127; D Miihlbacher, Neugebauer H, Cravino A & Sariciftci N S, Comparison of the
electrochemical and optical band-gap of low band-gap polymers, Synth Met, 137 (2003) 1361; Francois J & Cote M, Electronic,
structural and optical properties of conjugated polymers based on carbazole, fluorine and borafluorene, J Phys Chem B, 108 (2004)
728
J SCI IND RES VOL 63 SEPTEMBER 2004
3123.
18 Yang C, Liang W A & Bo C, Calculation of band structure of doped polyacetylene, Int J Quant Chem, 37 (1990) 679.
19 Springborg
M,
Theoretical
study
of
substituted
polyacetylenes, Synth Met, 135 (2003) 347.
20 Riehn R, Morgado J, Iqbal R, Moratti S C, Holmes A B, Votta S & Cacialli F, Electrochemical and electroluminescent properties of random copolymers of fluorine and alkoxy- substituted poly (p-phenylene vinylene)s, Macromolecules, 33
(2000) 3337.
21 Atsushi Izumi, Masahiro Teraguchi, Ryoji Nomura & Toshio Masuda, Synthesis of Poly (p-phenylene)-based photoresponsive conjugated polymers having azobenzene units in the main chain, Macromolecules, 33 (2000) 5347.
22 Goris L, Loi M A, Cravino A, Neugebauer H, Sariciftci N S, Polec I, Lutsen L, Andries E, Manca J, De Schepper L & Vanderzande
D, Poly (5,6-dithiooctyl-isothianaphtene), a new low band gap polymer: spectroscopy and solar cell construction, Synth Met, 138
(2003) 249.
23 Chien-Chung Han, Hong S P, Yang K F, Bai M Y, Lu C H & Huang C S, Highly conductive new aniline copolymers containing
butylthio substituent, Macromolecules, 34 (2001) 587.
24 Liu M S, Jiang X, Sen Liu, Herguth P & Jen A K Y, Effect of cyano substituents on electron affinity and electron-transporting
properties of conjugated polymers, Macromolecules, 35 (2002) 3532.
25 Bakhshi A K, Designing of one-dimensional graphite: Electronic structure of polyacene and polyphenanthrene, Superlatt
Microstruct, 11 (1992) 465; Bakhshi A K & Ladik J, Electronic structure and conduction properties of polyacenaacene and
polyphenan-throphenanthrene : Design of one-dimensional graphite, Indian J Chem, 33 (1994) 494; Bakhshi A K, Electronic
structure of polyacene: one dimensional graphite, Frontiers of polymer research, edited by P N Prasad & J K Nigam (Plenum Press,
New York and London) 1991, p 425.
26 Bozovic I, Violation of the Peierls theorem in graphite chains, Phys Rev B, 32 (1985) 8136.
27 Rissler J, Bassler H, Gebhard F & Schwerdtfeger P, Excited states of ladder-type poly-p-phenylene oligomers, Phys Rev B, 64
(2001) 045122.
28 Tsuchide H, Oyaizu K, Iwasaki T & Yonemarie H, Ladder thiophene polymers and preparation thereof, Jpn Kokai Tokkyo Koho JP,
261 (2001) 794.
29 Kerterz M, Hughbanks T R, Low band-gap ladder polymers, Synth Met, 69 (1995) 699.
30 Lin Y, Kumari P, Chen W, Chung T S & Zhang R, Ladder like polyepoxysiloxanes, Polym Prepr, 40 (1999) 944.
31 Michael Forster, Annam K O & Scherf U, Conjugated ladder polymers containing thienylene units, Macromolecules, 32 (1999)
3159.
32 Pan J, Scherf U, Shreiber A, Bilke R & Haarer D, Photoconduction study on a ladder-type poly (para-phenylene), Synth Met, 115
(2000) 79.
33 Tyutyulkov N, Ivanov N, Mullen K, Staykov A & Dietz F, Energy spectra of one-dimensional stacks of polycyclic aromatic
hydrocarbons without defects, J Phys Chem, 108 (2004) 4275.
34 Liegener C M, Bakhshi A K & Ladik J, Effects of topology on the electronic properties of fused-ring polymers, Chem Phys Lett, 199
(1992) 62.
35 Bakhshi A K, Liegener C M & Ladik J, Theoretical design of polymers from topological arguments: electronic properties of
polyisophenanthrene, Chem Phys, 173 (1993) 65.
36 Sherf U & Milier K, Polyarylenes and poly (arylenevinylene)s : 8. The first soluble ladder polymer with 1,4-benzoquinonebismethide subunits, Polymer, 33 (1992) 2443.
37 Parul Bhargava & Bakhshi A K, On the electronic structures of quasi-one-dimensional model superlattices of type-I, Indian J Chem,
Sec A, 41 (2002) 2450; Parul Bhargava & Bakhshi A K, On the electronic structures of quasi-one-dimensional model superlattices of
type-II, Indian J Chem, Sec A, 42 (2003) 1822.
38 Geetika Gandhi & Bakhshi A K, Theoretical designing of copolymers of donor-acceptor polymers based on polysilole, Indian J
Chem, Sec A, 42 (2003) 240.
39 Geetika Gandhi & Bakhshi A K, On the electronic structures and conduction properties of quasi-one-dimensional copolymers (type
– II staggered) of donor-accepter polymers, Indian J Chem, Sec A, 42 (2003) 1575.
40 Geetika Gandhi & Bakhshi A K, Electronic structure and conduction properties of copolymers of silole based donor-accepter
polymers: Effect of multi-neighbour interaction, Solid State Commun, 128 (2003) 467.
41 Naka K, Umeyama T & Yoshiki Chujo, Synthesis and properties of alternating acceptor-donor -conjugated copolymers of
cyclodiborazane with dithiafulvene, Macromolecules, 33 (2000) 7467.
42 Samson A Jenekhe, Lu L & Alam M M, New conjugated polymers with donor-acceptor architectures: synthesis and photophysics of
carbazole-quinoline and phenothiazine-quinoline copolymers and oligomers exhibiting large intramolecular charge transfer,
Macromolecules, 34 (2001) 7315.
43 Conklin J A, Huang S C, Huang S M, Wen T & Kaner R B, Thermal properties of polyaniline and poly (aniline-co-o-ethylaniline),
Macromolecules, 28 (1995) 6522.
44 Dhawan S K & Trivedi D C, Influence of polymerization conditions on the properties of poly (2-methylaniline) and its copolymer
with aniline, Synth Met, 60 (1993) 63.
45 Hong S Y, Si J Kwon & Kim S C, Theoretical study of geometrical and electronic structures of new -conjugated thiophene
BAKHSHI & BHALLA: ELECTRICALLY CONDUCTING POLYMERS
729
copolymers, J Chem Phys, 104 (1996) 1140.
46 Kim J W, Cho C H, Liu F, Choi H J & Joo J, Physical characteristics of aniline / pyrrole copolymer, Synth Met, 135 (2003) 17.
47 Chen S, Wen T C & Gopalan A, Electrosynthesis and characterization of a conducting copolymer having S–S links, Synth Met, 132
(2003) 133.
48 Huang J, Niu Y, Xu Y, Hou Q, Wang W, Mo Y, Yuan M & Cao Y, Novel electroluminescent polymers derived from carbazole,
Synth Met, 135 (2003) 181.
49 Bakhshi
A
K,
Deepika
&
Ladik
J,
Ab-initio
study
of
the electronic structures of polycyclopentadithiophene-4-one and polydicyanomethylenecyclopentadithioph-ene: two conjugated polymers with small band-gaps, Solid State Commun, 101 (1997) 347.
50 Bakhshi A K & Geetika Gandhi, Ab-initio study of the electronic structures and conduction properties of some novel low band-gap
donor-acceptor polymers, Solid State Commun, 129 (2004) 335.
51 Bakhshi A K & Bhargava Parul, Ab-initio study of the electronic structures and conduction properties of some novel donor-acceptor
polymers and their copolymers, J Chem Phys, 119 (2003) 13159.
52 Del Re G, Ladik J & Biczo G, Self–consistent field tight binding treatment of polymers- I- Infinite three dimensional case, Phys Rev,
155 (1967) 997.
53 Bakhshi A K, Yamaguchi Y, Ago H & Yamabe T, Design of novel donor-acceptor polymers with low band gaps, Synth Met, 79
(1996) 115.
54 Yamabe T, Bakhshi A K, Yamaguchi Y & Ago H, Electronic structures of some novel functional polymers, Macromol Symp, 118
(1997) 513..
55 Bakhshi A K, Yamaguchi Y, Ago H & Yamabe T, Theoretical design of donor-acceptor polymers with low band gaps, J Mol Struct
(Theochem), 427 (1998) 211.
56 James Margolis, Conductive polymers and plastics (Chapman and Hall) 1989, p 33.
57 Nigrey P J, Mac Diarmid A G & Heeger A J, Electrochemistry of polyacetylene, (CH)x: electrochemical doping of (CH)x films to the
metallic state, J Chem Soc Chem Commun, (1979) 594.
58 Mac Innes D, Druy M A, Nigrey P J, Nairns D P, Mac Diarmid A G & Heeger A J, Organic batteries: reversible n- and p- type
electrochemical doping of polyacetylene, (CH)x , J Chem Soc Chem Commun, (1981) 317.
59 Scrosati B & Owens B B, Solid-state lithium-polyacetylene batteries, Solid State Ionics, 23 (1987) 275
60 Yamawaki T & Hideki S, Thermally stable soluble silicon-containing step ladder polymers and their preparation, Jpn Kokai Tokkyo
Koho JP, 68 (2002) 746.
61 Salaneck W R, Clark D T, Samuelsen E J, Sci Applicat Conduct Polym, IOP, Publishing, (1991) 135.
62 Kunugi Y, Nigorikawa K, Harima Y & Yamashita K, A selective organic vapour sensor based on simultaneous measurements of
changes of mass and resistance of a poly (pyrrole) thin film, J Chem Soc Chem Commun, (1994) 873.
63 Sotzing G A, Birglin S, Grubbs R H & Lewis N S, Preparation and properties of vapour detector arrays formed from poly (3,4ethylenedioxy) thiophene-poly (styrene sulfonate) / Insulating polymer composites, Anal Chem, 72 (2000) 3181.
64 Alacacer L, Conducting polymers special applications (D Reidel Publishing Company) 1987, 192.
65 Sharma S K, Sehgal N & Kumar A, Biomolecules for development of biosensors and their applications, Curr Appl Phys, 3 (2003)
307.
66 Jin S H, Park H J, Park D K, Jeon B C, Gal Y S & Park W W, Poly (p-phenylene-vinylene)-based alternating copolymers with hole
transport moiety, Synth Met, 137 (2003) 1067.
67 Smela E, Inganas O & Lundstrom I, Controlled folding of micrometer-size structures, Science, 268 (1995) 1735.
68 Salaneck W R, Clark D T & Samuelsen E J, Sci Applicat Conduct Polym, IOP Publishing, (1991) 52.
69 Epstein A J, Joo J, Wu C Y, Benatar A, Faisst C F, Zegarski J & Mac Diarmid A G, Proc NATO Adv Res Workshop Applicat
Intrinsic Conduct Polym, (1993) 165.
70 Karasz F E, Proc Int Symp Polym Beyond AD 2000, IIT, Delhi, Polymer lasers and light emitting diodes, (1999) 28.
71 Yu G & Heeger A J, High efficiency photonic devices made with semiconducting polymers, Synth Met, 85 (1997) 1183.
72 Burroughes J H, Bradley D D C, Brown A R, Marks R N, Mackay K, Friend R H, Burns P L & Holmes A B, Light-emitting diodes
based on conjugated polymers, Nature, 347 (1990) 539.
73 Anderson M R, Thomas O, Mammo W, Svensson M, Theandes M & Inganas O, Substituted polythiophenes designed for
optoelectronic devices and conductors, J Mater Chem, 9 (1999) 1933.
74 Pei Q & Yang Y, Efficient photoluminescence and electroluminescence from a soluble polyfluorene, J Am Chem Soc, 118 (1996)
7416; Johansson D M, Theander M, Gramlind T, Inganas O & Mats R Andersson, Synthesis and characterization of polyfluorenes
with light-emitting segments, Macromolecules, 34 (2001) 1981.
75 Kambili A & Walker A B, Transport properties of highly aligned polymer light-emitting diodes, Phys Rev B, 63 (2001) 012201; Ye
A, Shuai Z & Bredas J L, Coupled-cluster approach for studying the singlet and triplet exciton formation rates in conjugated polymer
LED's, Phys Rev B, 65 (2002) 045208.
76 Pei Q, Yang Y, Yu G, Zhang C & Heeger A J, Polymer light-emitting electrochemical cells: In situ formation of a light-emitting p-n
junction, J Am Chem Soc, 118 (1996) 3922.
77 Granlund T, Theander M, Berggren M, Andersson M, Ruzeckas A, Sundstrom V, Bjork G, Granstrom M & Inganas O, A
polythiophene microcavity laser, Chem Phys Lett, 288 (1998) 879.
78 Pei J, Yu W L, Huang W & Heeger A J, A novel series of efficient thiophene-based light-emitting conjugated polymers and
J SCI IND RES VOL 63 SEPTEMBER 2004
730
79
80
81
82
83
84
85
86
application in polymer light-emitting diodes, Macromolecules, 33 (2000) 2462; Pei J, Yu W L, Ni J, Lai Y H & Heeger A J,
Thiophene based conjugated polymers for light-emitting diodes: Effect of aryl groups on photoluminescence efficiency and redox
behavior, Macromolecules, 34 (2001) 7241.
Huang W, Meng H, Yu W L, Pei J, Chen J K & Lai Y H, A novel series of p-n diblock light-emitting copolymers based on
oligothiophenes and 1,4-bis (oxadiazolyl)-2,5-dialkyloxybenzene, Macromolecules, 32 (1999) 118.
Morgado J, Cacialli F, Friend R H, Rost H & Holmes A B, Light-emitting devices based on a poly (p-phenylenevinylene) statistical
copolymer with oligo (ethylene oxide) side groups, Macromolecules, 34 (2001) 3094; Sang Ho Lee, Bin Jang B & Tsutsiu T,
Sterically hindered fluorenyl-substituted poly (p-phenylenevinylenes) for light-emitting diodes, Macromolecules, 35 (2002) 1356.
Liu Y, Ma H & Jen A K Y, Synthesis and characterization of quinoline-based copolymers for light-emitting diodes, J Mater Chem,
11 (2001) 1800.
Yang W, Liu C, Niu Y, Hou Q, Huang J, Yang R, Zeng X & Cao Y, Highly electroluminescent fluorene-based copolymer, Synth
Met, 135 (2003) 191.
Sun H H, Tong H, Hu Y F, Su G P, Cheng Y X, Ma D G, Wang L X, Jing X B & Wang F S, Novel PPV-based light-emitting
polymers containing siloxane moiety, Synth Met, 137 (2003) 1121.
Soudan P, Lucas P, Ang Ho H, Jobin D, Breau L & Belanger D, Synthesis, chemical polymerization and electrochemical properties
of low band gap conducting polymers for use in supercapacitors, J Mater Chem, 11 (2001) 773.
Krings L H M, Havinga E E, Donkees J J T M & Vork F T A, The application of polypyrrole as counterelectrode in electrolytic
capacitors, Synth Met, 54 (1993) 453.
Baughman R H, Conducting polymer artificial muscles, Synth Met, 78 (1996) 339.
____________
*Author for correspondence