Preface

The versatility of polymer materials has expanded as electroactive behavior has been included in the characteristics of some of the polymers. The most exciting development in this area is related to the discovery of intrinsically conductive polymers or conjugated polymers. Some examples are polyacetylene, polyaniline, polypyrrole, polythiophene, etc., as well as their various derivatives. The conjugated polymers which are a field of interest for researchers are also well known as synmet or synthetic metal due to the incorporation of some metallic characteristics, i.e., conductivity. Interest in this field is increasing day by day after the awarding of Nobel Prize for the discovery and development of electrically conducting conjugated polymers in the year 2000 by three scientists: Prof. Alan J. Heeger, Prof. Alan G. MacDiarmid and Prof. Hideki Shirakawa. Generally, the conductivity of these undoped conjugated polymers is 10-7-10-11 S cm-1. But for the application of conjugated polymers instead of inorganic or traditional semiconductors some higher conductivity is required. The conductivity of conjugated polymers, which are either weak semiconductors or insulators, increases by several folds due to doping. These conjugated polymers convert to a conductor or semiconductor from the insulator or low semiconductor by doping. Although the conductivity of doped conjugated polymers is higher than that of saturated insulating polymers, it is much less than that of conducting metals, e.g., Cu, Ag, Au, etc., and most of the doped conjugated polymers show conductivity in the semiconducting region. However, it is universally agreed that the doping process is an effective method to produce conducting polymers. As doping makes a semiconducting polymer from an insulting or low conducting one, it is of very much importance for the real applications of the conjugated polymers as semiconducting material.

The performance of doped conjugated polymers is greatly influenced by the nature of dopants and their level of distribution within the polymer. Therefore, the electrochemical, mechanical, and optical properties of the doped conjugated polymers can be tailored by controlling the size and mobility of the dopants counter ions. The essential idea about the unusual nature of the species bearing charges, i.e., excited doped states of the conjugated systems, has been intensively discussed in the last twenty years. In this context the understanding of the nature of interaction by dopant with the π-conjugated systems is of foremost importance from the real application point of view. This rapid growth of interest in conjugated polymer-dopant interaction has been stimulated due to its fundamental importance to a cross-disciplinary section of investigators, chemists, electrochemists, biochemists, experimental and theoretical physicists, and electronic and electrical engineers. Finally, I wish to extend my sincere thanks and gratitude to all who helped me complete this project.

Pradip Kar

Chapter 1

Introduction to Doping in Conjugated Polymer

1.1 Introduction

Recently, polymers have become the most widely used, versatile material on earth. This is due to some of the advantages they have over other materials such as flexibility, tailorability, processability, environmental stability, low cost, light weight, etc. [1]. Polymers are macromolecules which are formed by the repetitive union (mer unit or repeating unit) of a large number of reactive small molecules in a regular sequence. The simplest example is polyethylene, where ethylene moiety is the mer or repeating unit (Scheme 1.1). A major percentage of polymers are generally made up of carbon and hydrogen atoms with a minor percentage of some heteroatoms such as nitrogen, oxygen, sulfur, phosphorous, halogens, etc. In general, polymer is more than a million times bigger with respect to its size and molecular weight than that of small molecular compounds. The properties of polymers depend on their chemical composition, molecular structure, molecular weight, molecular weight distribution, molecular forces and morphology. Even in the fifth decade of the last century polymers were well known as electrically insulating materials. In modern civilization, polymers have been used as insulating cover on electrical wire, insulating gloves, insulating switches, insulating coatings on electronic circuit boards, low dielectric coatings, etc. [1]. The so called insulating polymers generally have a surface resistivity higher than 10¹² ohm-cm. The polymers are insulating in nature due to the saturated covalent long-chain carbon framework structure or saturated covalent long-chain framework of carbon and some heteroatoms such as nitrogen, oxygen, sulfur, phosphorous, halogens, etc. In these polymers, the nonavailability of free electrons is responsible for their insulating behavior [2].

Scheme 1.1 Monomer and repeating unit for polyethylene.

The versatility of polymer materials has expanded as electrochemical behavior has been included in the characteristics of some of the polymers. The electrochemical behavior means the mode of charge propagation, which is linked to the chemical structure of the polymer. In short, the chemical change within the polymer can help charge propagation, or the polymer can carry the charge through its chemical structure. The composites of conducting particles (carbon, graphite, metal, metal salt, etc.) with insulating polymers also show electrochemical behavior [3], e.g., composites of polyethylene oxide [4], polyethylene adipate [5] or polyethylene succinate [6] with Li salts. However, these materials are electrochemically active due to the electron transport within the conducting filler. In a true sense, the polymers themselves are not electrochemically active in these conducting composites. Based on the mode of charge propagation linked to the chemical structure of the polymer, the electrochemically active polymers can be classified into two main categories:

1. ion (proton)-conducting polymers, and

2. electron-conducting polymers.

In ion (proton)-conducting polymers the conduction of electricity is due to the transfer of proton which is present in its structure. Examples of ion-conducting polymers are sulfonated polystyrene, polyaryl sulfone, polyaryl ketone, etc. Similarly, the electron-conducting polymers can conduct electricity due to the transfer of electron through the polymer structure. The electron-conducting polymers can be further distinguished on the basis of mode of electron transport. In one type, the electrons can transport by an electron exchange reaction, i.e., reversible oxidation or reduction between neighboring electrostatically and spatially localized redox sites. Some examples are poly(tetracyanoquinodimethane) (PTCNQ), poly(viologens), some organometallic redox polymers, etc. The other type, which is the most exciting development in this area, is related to the discovery of intrinsically conductive polymers or conjugated polymer. Some examples are polyacetylene, polyaniline, polypyrrole, polythiophene, etc. [3] and their derivatives as well. The polymers having conjugated double bonds are intrinsically conducting, and these polymers can be oxidized or reduced using charge transfer agents (dopant) more easily for better electrochemical activity. The conjugated polymers which are a field of interest for researchers are also well known as synmet or synthetic metal due to the incorporation of some metallic characteristics, i.e., conductivity [7]. The interest in this field is increasing day by day after the awarding of the Nobel Prize for the discovery and development of electrically conducting conjugated polymers in the year 2000 by three scientists: Prof. Alan J. Heeger, Prof. Alan G. MacDiarmid and Prof. Hideki Shirakawa. This is because these conjugated conducting polymers have attracted attention of scientific and technical community for their use in various fields.

1.2 Molecular Orbital Structure of Conjugated Polymer

The molecular structure of the organic molecule is well explained in molecular orbital theory. In the organic molecule, the carbon 2s and all three of the 2p orbitals are combined first to make four new orbitals. This process of mixing is called hybridization and it generally also occurs in all the heteroatoms present within an organic molecule. The sp³, sp², Sp hybridized carbon atom contains four single bonds, three single bonds with one double bond and two single bonds with two double bonds respectively. The single bond in the organic compound is constructed by a sigma bond, while the double bond is constructed by both a sigma (σ) and pi (π) bond. The σ-bond results from strong axial overlapping of hybridized electron cloud and π-bond results due to weak lateral overlapping of unhybridized p-orbital electron cloud. An example of such a type of simple organic molecule with double bond is ethylene. Here both the carbon atoms have three single bonds with one double bond. So, both the carbon atoms are Sp² hybridized and the double bond is formed due to lateral overlapping of perpendicular unhybridized p-orbital of both the carbon atoms. According to molecular orbital theory, the two π-electrons of the double bond in atomic orbital (AO) are placed into the bonding π-molecular orbital (MO), and anti-bonding π-MO remains vacant. The last filled bonding MO is known as highest occupied MO (HOMO) and the vacant lower anti-bonding MO is known as lowest unoccupied MO (LUMO). In the case of the simplest molecule with conjugated double bond, such as 1,3-butadiene, the picture is somewhat different. All the carbon atoms are also sp² hybridized here. In each atom one unhybridized p-orbital (generally pz) remains perpendicular to the plane of carbon chain, and all those p-orbitals in each atom remain parallel to each other. So one p-orbital can laterally overlap to form π-orbital with either of the two nearest p-orbital, and ultimately the p-orbitals are delocalized (Figure 1.1). In light of molecular orbital theory, first the two bonding π-MOs and anti-bonding π-MOs in 1,3-butadiene are formed. Then, the two bonding π-MO with comparable energies and two anti-bonding π-MO with comparable energies are rearranged to two bonding and two anti-bonding π-MO due to delocalization (Figure 1.2). The HOMO and LUMO is now distinctly separated and the energy of HOMO is further enhanced, while the energy of LUMO is further decreased for conjugated 1,3-butadiene compared to that of nonconjugated form.

Figure 1.1 Delocalization of p-orbital in a conjugated carbon chain.

Figure 1.2 Comparison of π-MO energies for localized and delocalized systems in conjugated butadiene.

The molecular structure of conjugated polymer is comparable to that of conjugated organic molecule. A π-electron conjugated polymer contains alternative single and double bond throughout the chain. Some examples for such types of conjugated polymers are: polyacetylene, polyaniline, polypyrrole, polythiophene, polyfuran, poly(p-phenylene), polycarbazole, etc. The molecular structures of some of the conjugated polymers are shown in Scheme 1.2. Like other polymers, the conjugated polymer chain is also made of carbon atoms or carbon atoms with some heteroatoms such as nitrogen, oxygen, sulfur, etc., and all the atoms throughout the main polymer backbone are Sp² hybridized. Here, one p-orbital (unhybridized) in each atom also remains perpendicular to the plane of the polymer chain, and all those p-orbitals in each atom remain parallel to each other. So, ultimately the p-orbitals are delocalized throughout the polymer chain due to lateral overlapping of p-orbitals on either side. An example of such energy splitting resulting from the linking together of conjugated polythiophene is shown in Figure 1.3 [8]. Due to so many carbon atoms in the long conjugated polymer chain, the final HOMO and LUMO energy distribution is very complicated. When the number of connected repeat units is very high, like in a long-chain conjugated polymer, splitting results in densely packed HOMO and LUMO energy states. The conjugated organic molecule shows different property due to this type of delocalization, which reduces the ground state energy of the molecule.

Figure 1.3 π-MO energy splitting due to delocalization in conjugated polythiophene.

Reproduced with permission from ref. [8], Copyright © 1998 Elsevier Science Ltd.