Introduction

We all know from daily experience that metal wire conducts electricity whereas plastic wire or glass fiber behaves as an insulator. But what is the physical source of the difference between conducting and insulating materials? Electric conductivity of solids is one of the most important characteristics differentiating between materials, and it depends on the concentration of free electrons within the material. These electrons are called conduction electrons. They are not attached to individual atoms, nor do they participate in chemical bonds between the atoms, but rather they create a sort of "free electron gas," which moves with ease through the material under the influence of electric voltage. The higher the concentration of these electrons, the better the material conducts electricity.

Metals such as copper, silver and iron have a large number of conduction electrons (between one and four electrons per atom) and are excellent conductors (Figure 1). On the other hand, in insulating materials such as wood, glass or ceramic nearly all the electrons are bound (there are no free electrons), and their electric conductivity is poor, about 10^-25 times poorer than metal conductivity! Among the insulators (Figure 1) are many types of non-metallic materials such as molecular or ionic crystals (quartz, glass and table salt) as well as ceramics and plastic materials.

An additional type of materials belonging to the middle area between metals and insulators are the semi-conductors such as silicon. In these materials the conductivity can be changed by many orders of magnitude by a change of temperature or controlled insertion of dopants, as is done in the electronics industry.

Figure 1. The specific electrical conductivity of metals, semi-conductors, insulators and a variety of conducting polymers spans over a large number of orders of magnitude. It is measured in one over Ohm-meter.

In the late 1970s', a number of research groups succeeded in manufacturing plastic materials capable of conducting electric current. These materials were called “synthetic metals” and gave rise to much scientific activity and various applications which had not existed before. In this article we will explain what conducting plastic materials are, and present a wide variety of surprising and interesting applications for these materials, from a small electronic component up to entire display screens, and many other applications.

Conductors, Insulators and Semi-conductors

Before we show how plastic is made to conduct electricity, we will need to clarify the term “energy bands,” which explains the difference between conductors, semi-conductors and insulators. The explanation given here is based on one of the great breakthroughs of quantum theory in the 20^th century ─ a success which led to the birth of the transistor in 1948, integrated circuits and the entire contemporary microelectronics industry. According to quantum theory electrons can occupy only specific energy levels. The energy levels in solids create “bands” which can be occupied by a specific number of electrons; “forbidden” energy states between the bands are called energy gaps (Figure 2). Electric conductivity in materials is related to the motion of electrons under the influence of electric voltage, and is very different for conductors, insulators and semi-conductors.

When the top band (called conduction band) is partially filled with electrons, the electrons, under the influence of an electric field, can continuously increase their energy within the band by rising to available higher energy levels. This state characterizes metals which conduct electricity. Electrons flow in response to an electric voltage as is depicted on Figure 2a. However, if the highest occupied band is completely full with electrons, the electrons cannot move freely within it under the influence of an external field, and, therefore, there is no electric conductivity and the material will be an insulator (Figure 2c).

Figure 2. The electronic band structure of solids: (a) If there is no energy gap between conduction and valence bands, the material is a conductor. (c) If the energy gap is large, the material is an insulator. (b) For intermediate cases, where the gap is small, the material is a semi-conductor.

If the conduction band is empty and the band below it, called the valence band, is full, there is a possibility for electrons to “jump” from the full valence band to the empty conduction band. This form of conduction by electrons jumping into the conduction band is unique to semi-conductors (Figure 2b). In order for the electron to “jump” from the valence band to the conduction band two conditions must occur: (i) the energy gap between the bands must be sufficiently small and, (ii) the temperature must be sufficiently high for the electrons to overcome the energy gap. Semi-conductors such as silicon fulfill these conditions, and are the basis of all existing computer chips. At very low temperatures they are insulators because their valence band is full of electrons, whereas at room temperature (and even more so at high temperatures of hundreds of degrees Celsius) an increasing number of electrons “jump” into the empty conduction band and take part in electric conduction. However, as shown in Figure 1, their conductivity is not as good as that of metals and this is what lies behind their name: “semi-conductors."

Doping of Semi-conductors

In order for semi-conductors to be useful in electronic components, their electric conductivity at room temperature must be increased by many orders (see Figure 1). This is done in a process called doping, in which a small quantity of special dopants (impurity atoms) is added. The dopants have a special characteristic: they easily donate electrons into the conduction band, or in other cases, they "trap" some of the electrons from the nearly full valence band leaving “holes” behind (see Figure 3a). These holes in the valence band behave like positively charged particles. In both cases, the electric current from the donated electrons or the holes can be substantial. It is carried either by flow of electrons inside the conduction band, or by movement of holes in the valence band. In order to illustrate how a hole conducts electricity as if it were a real particle with positive charge (opposite to electron charge), we compare it to a hole created in the 15-puzzle game: the numbered tiles cannot move unless there is at least one empty “hole” (see Figure 3b).

Figure 3 (a) Process of hole creation: the doping atom traps an electron from the nearly full valence band, leaving in its place an empty space (a hole). The hole moves in the valence band as if it were a particle with positive charge.

Figure 3 (b) The 15-puzzle game: the game simulates the nearly full valence band – the number tiles cannot move until we create at least one empty “hole”. The movement of the hole is opposite to the movement of the tiles.

What is a Plastic Polymer?

Plastic materials are artificial materials, most of which are manufactured from by-products of the petrochemical industry (the chemical industry based on oil as its raw material). Plastic is made of macro-molecules called polymers combined with other components and additives depending on the type of plastic. Each polymer is a long chain of basic units (monomers), which repeat themselves hundreds and even thousands of times (Figure 4). In the polymerization process, the adjacent monomers are linked together along the chain via covalent chemical bonds, created when their electrons are shared (see Box I below). Polymers such as polyethylene, polystyrene, nylon and PVC are based on artificial polymerization processes, but there is also a variety of natural polymers from animal and vegetable sources. The best known natural polymer is the DNA molecule; polysaccharides (such as starch) and proteins are also types of natural bio-polymers. Spider webs, silk threads, linen, cotton fibers, wool and hair are also made of natural polymers.

The most useful characteristics of plastic materials are strength, low density, chemical inertness and thermal and electrical insulation. Indeed, one of the important industrial revolutions of the 20^th century was the “plastic revolution." Thanks to the ease and flexibility with which it is possible to manufacture and process plastic into any possible form, by injecting it into a mold at high temperature, weaving fibers and manufacturing thin sheets, countless applications were found for plastic in daily life and in industry. One of its common applications is as insulating material ensuring that electric networks and electronic circuits work properly with no short circuits.

Figure 4. Three examples of long polymer molecules. The different colors represent atoms of different elements; the repeating monomer is marked in parentheses. The middle polymer is a DNA molecule.

How can plastic be made to conduct electricity?

The perception that plastic materials were insulators and qualitatively different in their electrical characteristics from metals was the common dogma until the surprising discovery, about 30 years ago, of plastic materials capable of conducting electricity as well as metals. In order for a polymer to become a conductor, it must have characteristics resembling metal. Namely, it must contain a large quantity of free electrons not attached to atoms. And indeed in 1977 American researchers Alan Heeger, Alan MacDiarmid and the Japanese researcher Hideki Shirakawa generated a true revolution when they succeeded in manufacturing a special type of conjugated polymers which are conducting using a doping process. For this important discovery Heeger, MacDiarmid and Shirakawa received the Nobel Prize for chemistry in 2000.

A metallic wire conducts electric current because its conducting electrons are free to move. In order to explain how organic polymers, which have undergone a doping process, can conduct electricity, we must explain in more detail the origin of chemical bonds between carbon atoms. These are covalent bonds created by pairs of electrons between two neighboring carbon atoms (for a more detailed explanation see Box I below).

When we describe the chemical bonds between carbon atoms along the polymer chain we distinguish between two types of bonds: sigma (σ) bonds and pi (π) bonds. These bonds are also the basis for understanding the conduction mechanism in organic and polymeric materials based on carbon (see Box II below).

The conducting polymers are characterized by a series of alternating single and double bonds called conjugate bonds, which repeat themselves along the polymer chain. For example, in the polyacetylene molecule, one of the common conducting polymers, the conjugate bond is between carbon atoms which form the skeleton of the chain (Figure 7a). In other polymers the conjugate bonds are mostly within closed rings along the chain (Figure 7b).

It is the σ bonds which hold the atoms of the chain together, but they do not enable electrical conduction due to the proximity of the cloud of electrons to the carbon atoms themselves. The π bonds, on the other hand, enable movement of electrons (and electrical conduction) along the chain and even "hopping" of electrons between two nearby chains. The conduction mechanism of π electrons is complicated. We note here that the populated and empty energy bands resemble those of the metals, insulators and inorganic semi-conductors (Figure 2). The difference in relation to metals or silicon is that here the electronic characteristics are related to molecular building blocks (the polymer chains), as opposed to the atomic building blocks of metal or silicon atoms that are arranged in crystalline order.

But how is it possible for π electrons taking part in the double bond to move? After all in a pure polymer material, without impurities, these electrons have no empty states into which they could jump. The principle in doping semi-conductors is the same here: injection of dopants decreases the amount of electrons in the valence band. Thus, "holes" with positive charge are created in this band and participate in the electrical conduction.

Figure 7 (a) The polyacetylene polymer is created by polymerization of acetylene monomers and creation of a long chain of carbon atoms (C), connected together by conjugate bonds. The carbon atoms are also connected to different hydrogen atoms (H).

Figure 7 (b) Additional examples of conducting polymers with conjugate bonds. At the junction points between the bonds there are carbon atoms (C), which by convention are not explicitly indicated, or else as indicated: nitrogen (N), sulfur (S), etc.

Electrical conduction of conjugate polymers such as polyacetylene varies in the widest area. While natural state of polyacetylene is a semi-conductor, with low electrical conductivity at room temperature, its conductivity may be increased by many orders of magnitude (in a process similar to doping of semi-conductors) to reach conductivities equal to those of excellent conductors such as copper and silver. There is also another family of polymers which are good conductors even in a pure state, without impurities: these polymers are called Inherently Conducting Polymers (ICP). These are polymers whose conductivity stems from the polymer's molecular structure, and their conductivity is in the conducting region, between the semi-conductors and the metal conductors with conductivities in the region between 10^-4 and 10⁴ in units of one over Ohm-meter.

General Applications

Perhaps plastic-based electronics will never win over silicon chips, common today in the microelectronics industry, in terms of calculation speed and miniaturization capacity. However due to its low cost, simplicity of manufacture and processing and inherent flexibility, it can find its way to places that silicon will never reach. The various applications of conducting plastic may be divided into two main groups. The first group includes conducting polymers (which simulate conduction characteristics of the ordinary metals); while the other group of semi-conducting polymers (which will be discussed here) is used in applications such as light emitting diodes (LED), transistors, display screens and integrated circuits.

We would like to distinguish between polymers which enable electronic conduction (as discussed here) and other polymeric systems where the conductivity is ionic. The latter are used extensively in batteries and fuel cells. For example, lithium ion polymers are systems where the polymer itself is not conducting, but it contains many lithium ions which conduct. These materials are found in small batteries in digital cameras, MP3 players and mobile phones. They constitute a very different family of polymer materials and will not be further discussed here.

As possible applications for conducting and semi-conducting polymers we include cheap identification tags containing plastic chips and working by means of radio waves; simple means of storage for a large volume of data; inexpensive displays and bulletin boards (even disposable units, or the kind which can be easily bent and hung as advertisements on round pillars); and perhaps even articles of clothing containing flexible electronic components. In addition, many polymers such as polyacetylene appear in a wide variety of colors, due to the different levels of oxidation of the constituent elements. These materials are called photo-chromic (or thermo-chromic), because a change in their color can be induced following a change in lighting (or temperature) conditions.

The ability of plastic to conduct electricity enables production of integrated circuits of diodes and transistors which include the advantages of cheap and flexible plastic materials. In the electronics industry conducting polymers can be used as conducting glue in electronic printed circuits and as packing materials which prevent the creation of static electricity, which may damage delicate electric circuits. There are also many applications not related to electronic circuits, such as emulsion coatings of photographic film to prevent creation of static electricity, concealing objects from radar detectors (by means of a coating which absorbs and does not reflect the micrometric waves transmitted by radar), manufacture of artificial nerves and skin, and more.

Applications in the Electro-Optics Industry

One of the unique characteristics of polymers as semi-conducting material is their ability to emit light in reaction to electric voltage. This characteristic, first discovered in 1989, enables manufacture of a wide variety of flexible electrical displays and various optical instruments based on polymer materials. These displays have many advantages relative to the commonly used displays, as we shall see next.

Polymeric light emitting diode - PLED

A great deal of research and development have been devoted in the past 15 years to a new type of light emitting diode (LED) made of polymeric material. A light emitting diode is an electronic component which acts as a unidirectional valve, enabling passage of electric current in only one direction. When the current flows in this direction, the device emits light. The new polymeric LED can be used to build very thin and flexible display screens. They can be used as electronic paper, as screens for mobile phones and as semi-transparent display screens in cockpits.

It is well known that if we pass a sufficiently strong electric current through a filament, the filament will greatly heat up and radiate light and heat – that is how an ordinary incandescent light works. In a LED, on the other hand, the light is emitted from a thin layer made of semiconducting material under a weak electric field, without heating. This principle is called electroluminescence. LED lamps have many advantages over incandescent and fluorescent lamps: the emitted light is monochromatic, consisting of a single wavelength, and there is no need to introduce additional color filters to obtain light in a particular color. Since the lamp is based on a semiconducting device, low voltage is sufficient for its operation (less than 4 volts). Heating is minimal and the switching time is very short (of the order of microseconds, as opposed to a tenth of a second for incandescent lamps and a half second for fluorescents). LEDs have a very long lifetime, and they are mechanically robust and withstand blows and jolts. These lamps have a variety of applications (Figure 8). They serve as indicator lights in electronic equipment (such as the indicator light in a television remote control), as traffic and automotive lights, motion detectors (as in an optical computer mouse) and even as sources of illumination and the creation of spectacular visual effects.

Figure 8. Examples of use of LED lamps: boards displaying bus arrival times, indicator lights in television remote controls, car lamps, traffic lights and small flashlights.

Figure 9. Structure of the PLED: a thin layer of polymeric semiconducting material is packed as a sandwich between two electrodes: a transparent anode and a metal cathode. When electric voltage is applied, the polymer glows and the light is emitted through the transparent anode.

Figure 10. An example of colored light emitted from polymers in response to being illuminated by white light, with no source of electricity. Each polymer has its own characteristic color.

Figure 11. A small control board integrated in an electric circuit and containing PLED lamps shown from different viewing angles.

Today many PLED displays exist in industry (as well as other organic LEDs) and are used for varied display purposes, such as for example a mobile phone (Figure 12a) or a television screen of the size of a watch (Figure 12b). Even though most of the existing PLED displays have small screens, larger PLED screens (regular television screens size) already exist.

Figure 12. (a) Mobile phone whose cover serves as a display screen made of PLED.

Figure 12. (b) Small television screen made of PLED on a watch.

Organic transistors

The transistor is the key component in the entire modern electronics industry, and serves a very wide variety of purposes. It can be described as a kind of ultra-fast and sophisticated electronic switch. The most common transistor is made of silicon, but in recent years organic transistors have been manufactured. Some are made of polymeric materials.

The outstanding advantage of organic transistors over those made of silicon is their ease of manufacture. In order to build a sophisticated silicon chip great effort must be invested in complicated and expensive processes carried out at high temperatures, in high vacuum conditions and in rooms that are particularly clean of all pollutants. In contrast, organic transistors may be manufactured in quicker and cheaper processes which do not require such stringent conditions.

Recently organic transistors have been manufactured and work well even when they are bent. These technologies enable manufacture of flexible electronic components such as electronic paper (Figure 13), and sensors for packages and for flexible products. It is expected that in future it will be possible to manufacture even more complicated electronic instruments, such as flexible laptops which can be folded up!

Figure 13. A flexible electronic book made of organic transistors constructed from polymeric material. Adapted from the homepage of Plastic Logic co.

Printed computer screens

Using a printer similar to the home inkjet printer it is possible to “print” polymer based electric circuits within a few hours. In the summer of 2004 a group from the R&D center of Du Pont Company reported the first such printing on large surfaces, by means of a technology called thermal printing. In this technology a laser beam fuses the polymer and fixes it on a platform, similar to the process of ironing. It is expected that using such fusing of electro-active polymers (whose physical characteristics change under the influence of an electric field) it will be possible to manufacture entire television screens, including all of their electronic circuits and pixels.

Artificial skin for robots

In November 2003 Takayo Someya and colleagues from Tokyo University announced the development of a flexible sheet which included transistors made of relatively small pentacene molecules, in order to manufacture a layered material sensitive to pressure. This layer mimics skin and would enable to manufacture robots with a sense of touch. The component of the sheet sensitive to touch is a layer of compound material made of rubber and carbon (rubber is a plastic material of enhanced elasticity in which polymer chains are connected at several intersections), whose electrical resistance depends on its degree of compression. The robot’s sense of touch is based on a change in the resistance of the sheet at the place where it is touched, where the change in resistance operates an organic transistor. The team manufactured a system of 16x16 sensors, 3 square millimeters in size for each sensor, and transistors which transmitted the signals received from the rows and columns of the sensors. In order to cover a wider surface the overlapping electrodes were tied by attaching their ends and gluing them with tape (Figure 14a). The entire structure is made of polymers and of a layer of pentacene, except for the gold electrodes and the copper coating attached to the compound rubber-carbon layer. The sheet may be rolled up to a diameter of one centimeter, a diameter small enough to coat a thin finger. They managed to wrap such a sheet round the hand of a robot (Figure 14b) and to sample the signals it received when local pressure was exerted.

The robots most common today are unsophisticated robots operating as lawn mowers and vacuum cleaners, or in industrial mass production lines. The expectation is that along with these robots, robots and devices with artificial intelligence will come into use, which will include a score of displays and sensors made out of plastic electronics.

Figure 14. (a) A flexible sheet containing pentacene transistors, intended to create a layer of “skin” sensitive to pressure.

Figure 14. (b) The flexible sheet is wrapped around the robot’s hand and gives it ability to touch and sense pressure.

Conclusions

The development of devices and instruments based on conducting plastic materials is only at infancy stage. The expectation is that these materials will gradually replace silicon and metals in certain applications. Conducting plastic includes characteristics of inorganic materials (metals and semi-conductors) as well as organic materials (such as polymers and biological materials). Perhaps in the future such materials will have many uses in hybrid systems, which will connect the living world, from the molecular level up to the level of an entire organism, with the world of technology at all levels, from the single molecule up to the computer chip. Perhaps, the conducting polymers will offer a more efficient interface between living tissues and artificial replacements.

Acknowledgments

The authors would like to thank Michal Andelman, Roy Beck-Barkai, Raphael Blumenfeld, Yoram Burak, Michael Cogan, Yoram Dagan, Dan Davidov, Eli Eisenberg, Moshe Gottlieb, Or Graur, Gilad Haran, Judy Kupferman, Olga Machtey, Abraham Nitzan, Yosi Paltiel, Val Parsegian, Yitzhak Rabin, Ron Rosensweig, Romi Shamai, Reshef Tenne, and Anuk Yosebashvili for their helpful comments.

Internet Links

• http://nobelprize.org/nobel_prizes/chemistry/laureates/2000
Nobel Prize in Chemistry for the year 2000: for the discovery and applications of conducting polymers.

• http://www.cdtltd.co.uk/
Website of the Cambridge Display Technology Company, which specializes in PLED applications.

• http://www.plasticlogic.com/
Website of Plastic Logic Company which specializes in manufacture of flexible display screens.

• http://www.polymervision.com/index.html
Website of Polymer Vision Company, which manufactures flexible display screens from plastic materials.

Additional Reading

1. S. R. Forrest, “The Path to Ubiquitous and Low-cost Organic Electronic Appliances on Plastic”, Nature 428, 911–918 (2004).
2. G. Malliaras and R. Friend, “An Organic Electronics Primer”, Physics Today 58, May 2005, p. 53–58.
3. C. Pratt, “Applications of Conducting Polymers”, http://homepage.ntlworld.com/colin.pratt/applcp.htm
4. A. G. MacDiarmid, “Nobel Lecture: Synthetic Metals - A Novel Role for Organic Polymers”, Rev. Mod. Phys. 73, 701-712 (2001).
5. T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi and T. Sakurai, “A Large-area, Flexible Pressure Sensor Matrix with Organic Field-effect Transistors for Artificial Skin Applications”, Proc. Natl. Acad. Sci. (USA) 10, 9966-9970 (2004).
6. F. Hide, M. A. DÍaz-GarcÍa, B. J. Schwartz and A. J. Heeger, “New Developments in the Photonic Applications of Conjugated Polymers”, Acc. Chem. Res. 30, 430-436 (1997).
7. R. H. Friend, “Polymers Show They’re Metal”, Nature 441, 37 (2006).
8. R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Brédas, M. Lögdlund and W. R. Salaneck, “Electroluminescence in Conjugated Polymers“, Nature 397, 122-128 (1999)