While polymers and organic materials have been known and utilized broadly for many decades, finding such materials with the intrinsic properties of semiconductors and metals is a relatively recent phenomenon. The report in 1977 about doping polyacetylene to achieve relatively high conductivity opened up important new vistas for chemistry and physics, and for technology in general. This report recognized that a key feature of the electronic polymers and organic materials was a backbone consisting of alternating single and double bonds resulting in a “π-conjugated network.” This in turn led to a relatively small energy gap, enabling the appearance of both semiconducting and metallic properties.

Initially these polymers were unstable in air and not readily processed. Over the past decade, major advances have occurred in the synthesis of new forms of conducting and semiconducting polymers that enable processing under a broad range of conditions including organic solvents, inorganic solvents, and aqueous media. There are even meltprocessable versions of some of the electronic polymers.

A prime focus of the field has been the determination of the mechanisms for charge conduction and the intrinsic conductivity of these fascinating materials, especially doped polymers. In the past decade, interest has increased in the semiconducting (generally undoped) forms of these polymers and organic materials, including their photophysics and their use in a wide variety of devices. The reports of light-emitting devices fabricated from molecular and oligomeric constituents in the mid-1980s and from polymeric constituents in 1990 stimulated interest in this area of research.

For the past 50 years, conventional insulating-polymer systems have increasingly been used as substitutes for structural materials such as wood, ceramics, and metals because of their high strength, light weight, ease of chemical modification/customization, and processability at low temperatures. In 1977 the first intrinsic electrically conducting organic polymer—doped polyacetylene—was reported, spurring interest in “conducting polymers.” Intrinsically conducting polymers are completely different from conducting polymers that are merely a physical mixture of a nonconductive polymer with a conducting material such as metal or carbon powder. Although initially these intrinsically conducting polymers were neither processable nor air-stable, new generations of these materials now are processable into powders, films, and fibers from a wide variety of solvents, and also are airstable. Some forms of these intrinsically conducting polymers can be blended into traditional polymers to form electrically conductive blends. The electrical conductivities of the intrinsically conductingpolymer systems now range from those typical of insulators (<10−10 S/cm (10−10 Ω−1 cm1)) to those typical of semiconductors such as silicon (~10 5 S/cm) to those greater than 10+4 S/cm (nearly that of a good metal such as copper, 5 × 105 S/cm). Applications of these polymers, especially polyanilines, have begun to emerge. These include coatings and blends for electrostatic dissipation and electromagnetic-interference (EMI) shielding, electromagnetic-radiation absorbers for welding (joining) of plastics, conductive layers for light-emitting polymer devices, and anticorrosion coatings for iron and steel.

The common electronic feature of pris tine (undoped) conducting polymers is the π-conjugated system, which is formed by the overlap of carbon pz orbitals and alternating carbon-carbon bond lengths.

The discovery in 1977–78 that trans-polyacetylene — (CH)x, the prototype conducting polymer (Figure 1)—could be chemically p-doped (partly oxidized) or n-doped (partly reduced) with a concomitant increase of its conductivity through the semiconducting to the metallic regime introduced new concepts of considerable theoretical and possible technological importance to condensed matter science. In 1979 it was discovered that p- or n-doping of trans-(CH)x could be accomplished electrochemically and that these processes were electrochemically reversible. Polyacetylene is the simplest example of a conjugated polymer, a polymer in which the “backbone” atoms are joined alternately by single and double bonds. All conducting polymers, “synthetic metals,” are conjugated polymers, at least in their doped forms. Other conducting polymers, including for example, poly(paraphenylene), polypyrrole, polythiophene, and polyaniline, have since been examined as electrochemically active materials. These findings have stimulated much industrial and academic interest in the electro-chemistry of conducting polymers and their possible technological applications in for example, energy storage, electrochromic displays, electrochemical drug-delivery systems, electromechanical devices, and light-emitting devices.

This article will show the relationship between the doping of a conjugated polymer, the reduction potential of the polymer, and the role of “dopant” ions. These interrelationships have frequently caused considerable confusion in understanding electrochemical doping. Electrochemical synthesis of conjugated polymers and the role of cyclic voltammetry in elucidating the mechanism of electrochemical redox processes involving conjugated organic polymers will also be discussed. This article will also summarize a few selected applications involving electro-chemical properties of conjugated polymers. The coverage is intended to be

exemplary rather than exhaustive. Furthermore since the electrochemistry of (CH), the “prototype” conducting polymer, has been extensively studied and comprises a relatively simple, reversible electrochemical system, it will be used to exemplify the basic concepts involved. These basic concepts can then be applied with appropriate modification as necessary to the electrochemistry of other conjugated polymers. Polyaniline will then be used to illustrate a more complex conjugated polymer electrochemical system.

Electroluminescence (EL) is the emission of light generated from the radiative recombination of electrons and holes electrically injected into a luminescent semiconductor. Conventional EL devices are made of inorganic direct-bandgap semiconductors, such as GaAs and InGaAs. Recently EL devices based on conjugated organic small molecules and polymers have attracted increasing attention due to easy fabrication of large areas, unlimited choice of colors, and mechanical flexibility. Potential applications of these organic/polymeric EL devices include backlights for displays, alphanumeric displays, and high-density information displays.

Electroluminescence from an organic material was first demonstrated in the 1960s on anthracene crystals by Pope et al. at New York University. Subsequently several other groups also observed this phenomenon in organic crystals and thin films. These organic EL devices had high operating voltages and low quantum efficiency. Consequently they did not attract much attention. In 1987 a breakthrough was made by Tang and VanSlyke at Eastman Kodak who found that by using multilayers of sublimated organic molecules, the operating voltage of the organic EL devices was dramatically reduced and the quantum efficiency was significantly enhanced. This discovery touched off a flurry of research activity, especially in Japan. The Japanese researchers, as welt as the group at Kodak, have since improved the device efficiency and lifetime to meet commercial requirements. This progress is reviewed by Tsutsui in this issue.

Organic electroluminescence (EL) is moving from a simple curiosity in laboratories to the reality of commercial use. The day will soon arrive when high-quality green EL displays find practical usage. Charge-injection-type EL involves the combination of positive and negative charge carriers injected from electrodes in contact with an organic thin film. The occurrence of EL through bipolar charge injection into organic solids was clarified for single crystals of anthracene and related compounds in the 1960s. Few new developments in physics exist today for organic EL. However worldwide enthusiasm for charge-injection-type EL, which started in the mid-1980s, has been increasing rapidly. The motivation for this renewed interest is straightforward. High-efficiency surface emission across the whole visible spectral range can be obtained easily, and prospects now exist for full-color flat-panel-display technology.

Since the demonstration of high-performance EL devices made of multilayers of vacuum-sublimed dye films by Tang and VanSlyke, much progress has occurred in the research and development of EL devices made from molecular materials. A variety of molecular materials such as vacuum-sublimed dye films, fully π-conjugated polymers, polymers with chromophores orr skeletal chains, or side chains, and polymer-dispersed dye films can be used for EL devices.

Among a variety of EL devices, multilayer-structure versions made of vacuum-sublimed dye films exhibit the best performance. Application-oriented research in the development of high-quality flat-panel displays has been performed during the past 10 years, mainly in Japan's private-sector laboratories.

Since the discovery of high electrical conductivity in doped polyacetylene in 1977, π-conjugated polymers have emerged as viable semiconducting electronic materials for numerous applications. In the context of polymer electronic devices, one must understand the nature of the polymer surface's electronic structure and the interface with metals. For conjugated polymers, photoelectron spectroscopy—especially in connection with quantum-chemical modeling—provides a maximum amount of both chemical and electronic structural information in one (type of) measurement. Some details of the early stages of interface formation with metals on the surfaces of conjugated polymers and model molecular solids in connection with polymer-based light-emitting devices (LEDs) are outlined. Then a chosen set of issues is summarized in a band structure diagram for a polymer LED, based upon a “clean calcium electrode” on the clean surface of a thin film of poly(p-phenylene vinylene) (PPV). This diagram helps to point out the complexity of the systems involved in polymer LEDs. No such thing as “an ideal metal-on-polymer contact” exists. There is always some chemistry occurring at the interface.

Organic semiconductors have been studied since the early 1950s, and the large amount of work devoted to them has allowed a better understanding of their charge-transport properties. However owing to their very poor semiconducting characteristics, they were merely considered as exotic materials with little potential interest for applications until the late 1980s, when two significant steps simultaneously appeared in the literature. Richard Friend's group showed that light-emitting diodes could be made from a conjugated semiconducting polymer, and our laboratory showed that efficient field-effect transistors (FETs) could be realized from short-conjugated oligomers. These two results launched intensive research on these two types of organic-based devices, and the extensive work accomplished since has largely confirmed the technological pertinence of organic semiconductors, showing the promise for applications in flexible and large-area electronics. Two categories of organic semiconductors are actually under development: (1) conjugated polymer-based ones whose amorphous state is favorable to strong luminescence and (2) conjugated oligomer-based ones, in which charge-transport efficiency is directly related to the long-range packing of molecules in the semiconducting film. In fact conjugated oligomers can be said to be forming molecular polycrystals whose electrical properties are essentially controlled by molecular order. Thus performance of sexithiophene-based FETs has been improved by a factor of nearly 50 by controlling the molecular ordering in the evaporated film, from a disordered three-dimensional structure to a well-ordered two-dimensional organization where all the molecules stack along a packing axis nearly parallel to the substrate surface.