ABSTRACT
The story regarding the discovery of conductive polymers, specifically halogen-doped polyacetylene film, is well known [1]. Its progress, however, did not end with the award of the year 2000 chemistry Nobel Prize to Alan G. MacDiarmid, Alan J. Heeger, and Hideki Shirakawa [2-9]. The discovery of traditional plastics occurred in the early twentieth century, and plastics have a distinct place from other materials of choice (such as metals, ceramic, etc.) due to their cost effectiveness, excellent processability, and dielectric properties. The contribution of plastics to the development of the modern society can best be represented by Lord Todd (Nobel laureate in chemistry 1957), then president of the Royal Society of London (1980), in answering the question “What do you think has been chemistry’s biggest contribution to science, to society?” His answer was “I am inclined to think that the development of polymerization is, perhaps, the biggest thing chemistry has done, where it has had the biggest effect on everyday life. The world would be a totally different place without artificial fibers, plastics, elastomers, etc. Even in the field of electronics, what would you do without insulation? And there you come back to polymers again?” [10]. It was quite inconceivable at that time that plastic could one day replace metals such as copper, silver, and gold for its conductivity. The π-conjugated polymers, including polyacetylene, initially investigated by MacDiarmid, Heeger, and Shirakawa, however, are not truly thermoplastics [11]. These polymers
20.1 Introduction ..........................................................................................................................669 20.1.1 Development of Conductive Polymers ......................................................................669 20.1.2 Organic Semiconductors ........................................................................................... 671
20.2 Main-Chain Conductive Polymers (π-Conjugated Polymers) .............................................. 672 20.2.1 Solubilization of π-Conjugated Polymers ................................................................. 672
20.3 Soluble Main-Chain Conductive Polymers .......................................................................... 674 20.3.1 Polyethylene Dioxythiophene:Polystyrene Sulfonate Complexes ............................ 674 20.3.2 Polyfluorenes and Their Copolymers ....................................................................... 674 20.3.3 Substituted Polythiophene Oligomers....................................................................... 675
20.4 Conductive Vinyl Polymers .................................................................................................. 676 20.4.1 Mobility Measurements ............................................................................................ 677 20.4.2 Hole-Transporting Vinyl Polymers ........................................................................... 678 20.4.3 Electron-Transporting Vinyl Polymers and Their Copolymers ................................680 20.4.4 Conductive Vinyl Polymers as Emitters ................................................................... 682 20.4.5 π-π Stacking Poly(Dibenzofulvene) .........................................................................684
20.5 Conclusions ...........................................................................................................................686 Acknowledgments ..........................................................................................................................686 References ......................................................................................................................................687
are neither soluble nor temperature processable as their processing temperatures are well above their degradation temperatures. Methods to render some of these π-conjugated conductive polymers such as polyacetylene, poly(p-phenylene) vinylene (PPV), or polythiophene soluble will be further discussed in the following sections. The pristine (undoped) conductive polymers, however, are only semiconductive in nature. The distinction of insulator, semiconductor, and conductor can best be described by the intrinsic value of volume conductivity (σ), which has the unit siemens per meter (S/m) or siemens per centimeter (S/cm). The volume conductivity (σ) is inversely proportional to volume resistivity (ρ) (i.e., σ = 1/ρ), which has the unit ohm meter (Ω m) or ohm centimeter (Ω cm). Another relative unit, Ω/sq, is also commonly used in the industry for the comparison of surface (sheet) resistivity only. Figure 20.1 shows the relative S/m value for different metals, semiconductors, and insulators in comparison to conjugated polymers before and after doping. Although the absolute volume conductivity of most doped conductive polymers is lower than that of metals, their ratio of volume conductivity to density, however, can be one to two orders of magnitude higher due to the fact that plastics usually have lower density or specific gravity than metals. The volume resistivity (ρ) can be measured typically by employing a gold-plated four-pointed probe to measure the current (J) and voltage (V) across a bulk conductive polymer with known dimensions (width W, length L, and thickness t), in which ρ = R (A/L) = (V/J)(W t/L), R (= V/J) is the resistance defined by Ohm’s law, and A (= Wt) is the cross-section area of the sample.
