electrically conducting polymers: materials of the...

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

J SCI IND RES VOL 63 SEPTEMBER 2004

716

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 electron-

phonon 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:

XPolymer + −+

+nn

XPolymer)(

in the case of an oxidation (p-doping) process and

MPolymer + +−

+nn

MPolymer)(

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-one-

dimensional 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 (p-

phenylene), 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) Substitution/Fusion,

(b) Ladder Polymerization,

(c) Topological methods,

(d) Copolymerization, and

(e) 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


718

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 (fluorenebenzothiadiazsole-

cyanophenylenevinylene) (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.


719

Fig. 1 Some low band-gap conjugated polymers


720

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), polyphenanthro-

phenanthrene (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

Fig. 2 Various members of the one-dimensional graphite family


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

polybenzanthracene34

and 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 five-

membered 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 multi-

neighbour 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-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 p-

dopantphilicity of the copolymer chain in the case of

Type-I copolymers while in case of 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

Type-I

Type-II staggered


722

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.

Table 1Trends 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

Decreases

(Increases)

Increases

(Increases)

Decreases

(Decreases)

(AmB)x

increase in m

Increases

(Decreases)

Decreases

(Decreases)

Increases

(Decreases)

(AmBn)x increase in m

and n for constant m/n

Decreases

(Decreases)

Increases

(Increases)

Decreases

(Decreases)


723

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 π-

bond order values of the polymers studied shows that all

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

small, while it makes a significant contribution to the

LUCO of these polymers. The electron-donating group

(SiH2 or SiF2), on the other hand, makes significant

contributions to both the HOCO and the LUCO. It,

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

Fig. 3 Alternate arrangement of donor and acceptor units

Table 2Calculated 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 > C=CH2

> C=O

> C=CF2 > C=C (CN)2

IP 8.084 (8.682) 8.734 (9.381) 8.278 (8.861) 8.997 (9.560)

EA 2.419 (3.017) 2.931 (3.618) 2.601 (3.241) 3.804 (4.415)

Eg 5.665 (5.665) 5.803 (5.763) 5.677 (5.620) 5.193 (5.145)

Fig. 4 Orbital patterns of the HOCO (top of the valence band)

and LUCO (bottom of the conduction band) of PSICH (Y= >

C=CH2), PSICO (Y= > C=O), PSICF (Y= > C=CF2) and PSICN

(Y= > C=C(CN)2). White (black) circle indicates positive

(negative) LCAO (linear combination of atomic orbitals)

coefficients. The pseudo-orbitals of X = SiH2 or SiF2 are omitted


724

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,

(CH)x / LiClO4 – PC/(CH)x

PA battery has higher energy and power densities as

compared to ordinary batteries. The polymer electrode

batteries have a longer shelf-life than the conventional

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

.

Fig. 5 Schematic energy levels depicting the formation of

polymers from the interactions of the conjugated skeleton

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

X= SiH2 ; SiF2 are omitted. White (black) circle indicates positive

(negative) LCAO (linear combination of atomic orbitals)

coefficients


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, micro-

positioners 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

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 devices

77. 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 super-

capacitors84,85

.

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


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.

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