Conductive polymer

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Conductive polymers or more precisely intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity.[1] Such compounds may have metallic conductivity or be semiconductors. The biggest advantage of conductive polymers is their processability. Conductive polymers are also plastics, which are organic polymers. Therefore, they can can combine the mechanical properties (flexibility, toughness, malleability, elasticity, etc.) of plastics with high electrical conductivity. These properties can be fine-tuned using the methods of organic synthesis.[2]

Correlation of chemical structure and electrical conductivity

In traditional polymers such as polyethylenes, the valence electrons are bound in sp3 hybridized covalent bonds. Such "sigma-bonding electrons" have low mobility and do not contribute to the electrical conductivity of the material. The situation is completely different in conjugated materials. Conducting polymers have backbones of contiguous sp2 hybridized carbon centers. One valence electron on each center resides in a pz orbital, which is orthogonal to the other three sigma-bonds. The electrons in these delocalized orbitals have high mobility when the material is "doped" by oxidation, which removes some of these delocalized electrons. Thus the p-orbitals form a band, and the electrons within this band become mobile when it is partially emptied. In principle, these same materials can be doped by reduction, which adds electrons to an otherwise unfilled band. In practice, most organic conductors are doped oxidatively to give p-type materials. The redox doping of organic conductors is analogous to the doping of silicon semiconductors, whereby a small fraction silicon atoms are replaced by electron-rich (e.g., phosphorus) or electron-poor (e.g. boron) atoms to create n-type and p-type semiconductors, respectively.

Although typically "doping" conductive polymers involves oxidizing or reducing the material, conductive organic polymers associated with a protic solvent may also be "self-doped."

The most notable difference between conductive polymers and inorganic semiconductors is the mobility, which until very recently was dramatically lower in conductive polymers than their inorganic counterparts. This difference is diminishing with the invention of new polymers and the development of new processing techniques. Low charge carrier mobility is related to structural disorder. In fact, as with inorganic amorphous semiconductors, conduction in such relatively disordered materials is mostly a function of "mobility gaps"[3] with phonon-assisted hopping, polaron-assisted tunneling, etc., between localized states.

The conjugated polymers in their undoped, pristine state are semiconductors or insulators. As such, the energy gap can be > 2 eV, which is too great for thermally activated conduction. Therefore, undoped conjugated polymers, such as polythiophenes, polyacetylenes only have a low electrical conductivity of around 10−10 to 10−8 S/cm. Even at a very low level of doping (< 1 %), electrical conductivity of increases several orders of magnitude up to values of around 0.1 S/cm. Subsequent doping of the conducting polymers will result in a saturation of the conductivity at values around 0.1–10 kS/cm for different polymers. Highest values reported up to now are for the conductivity of stretch oriented polyacetylene with confirmed values of about 80 kS/cm.[4][5][6][7][8][9] Although the pi-electrons in polyactetylene are delocalized along the chain, pristine polyacetylene is not a metal. Polyacetylene has alternating single and double bonds which have lengths of 1.44 and 1.36 Å, respectively.[10] Upon doping, the bond alteration is diminished in conductivity increases. Non-doping increases in conductivity can also be accomplished in a field effect transistor (organic FET or OFET) and by irradiation. Some materials also exhibit negative differential resistance and voltage-controlled "switching" analogous to that seen in inorganic amorphous semiconductors.

Classes of materials

File:ConductivePoly.png
Structures of various conductive organic polymers. Clockwise; polyacetylene, polyphenylenevinylene, polypyrrole (X = NH), and polythiophene (X = S), polyaniline (X = N, NH) and polyphenylene sulfide (X = S).

The following table presents some organic conductive polymers according to their composition. The well-studied classes are written in bold and the less well studied ones are in italic.

The main chain contains No heteroatom NH or NR S
Aromatic cycles Poly(fluorene)s, polypyrenes, polyazulenes, polynaphthalenes The N is in the aromatic cycle: poly(pyrrole)s (PPY), polycarbazoles, polyindoles, polyazepines... The N is outside the aromatic cycle: polyanilines (PANI) The S is in the aromatic cycle: poly(thiophene)s (PT)... The S is outside the aromatic cycle: poly(p-phenylene sulfide) (PPS)
Double bonds Poly(acetylene)s (PAC)
Aromatic cycles and double bonds Poly(p-phenylene vinylene) (PPV)

PPV and its soluble derivatives have emerged as the prototypical electroluminescent semiconducting polymers. Today, poly(3-alkylthiophenes) are the archetypical materials for solar cells and transistors.[2]

Synthesis of conductive polymers

Many methods for the synthesis of conductive polymers have been developed. Most conductive polymers are prepared by oxidative coupling of monocyclic precursors. Such reactions entail dehydrogenation:

n H–[X]–H → H–[X]n–H + 2(n–1) H+ + 2(n–1) e–

One challenge is usually the low solubility of the polymer. This has been addressed by some researchers through the formation of nanostructures and surfactant stabilized conducting polymer dispersions in water. These include polyaniline nanofibers and PEDOT-PSS. These materials have lower molecular weights than that of some materials previously explored in the literature. However, in some cases, the molecular weight need not be high to achieve the desired properties.

Properties and applications

Conductive polymers enjoy few large-scale applications due to their poor processability. They have been known to have promise in antistatic materials[2] and they have been incorporated into commercial displays and batteries, but there have had limitations due to the manufacturing costs, material inconsistencies, toxicity, poor solubility in solvents, and inability to directly melt process. Literature suggests they're also promising in organic solar cells, printing electronic circuits, organic light-emitting diodes, actuators, electrochromism, supercapacitors, biosensors, flexible transparent displays, electromagnetic shielding and possibly replacement for the popular transparent conductor indium tin oxide.[11] Conducting polymers are rapidly gaining traction in new applications with increasingly processable materials with better electrical and physical properties and lower costs. The new nanostructured forms of conducting polymers particularly, provide fresh air to this field with their higher surface area and better dispersability.

Electroluminescence

Electroluminescence is light emission stimulated by electrical current. In organic compounds, electroluminescence has been known since the early 1950s, when Bernanose and coworkers first produced electroluminescence in crystalline thin films of acridine orange and quinacrine. In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doping. In some cases, similar light emission is observed when a voltage is applied to a thin layer of a conductive organic polymer film. While electroluminescence was originally mostly of academic interest, the increased conductivity of modern conductive polymers means enough power can be put through the device at low voltages to generate practical amounts of light. This property has led to the development of flat panel displays using Organic LEDs, solar panels, and optical amplifiers.

Barriers to applications

Since most conductive polymers require oxidative doping, the properties of the resulting state are crucial. Such materials are often salt-like, which diminishes their solubility in organic solvents and hence their processability. Furthermore, the charged organic backbone is often unstable towards atmospheric moisture. Compared to metals, organic conductors can be expensive requiring multi-step synthesis. The poor processability for many polymers requires the introduction of solubilizing substituents, which can further complicate the synthesis.

History

File:Gadget128.jpg
voltage-controlled switch, an organic polymer electronic device from 1974. Now in the Smithsonian Chip collection.

The history of the field has been recounted from several perspectives.[1][12] The first report on polyaniline goes back to the discovery of aniline. In the mid 1800s, Letheby reported the electrochemical and chemical oxidation products of aniline in acidic media, noting that reduced form was colourless but the oxidized forms were deep blue. In the early 1900s, German chemists named several compounds "aniline black" and "pyrrole black" and used them industrially. Classically, such polymer "blacks", their parent compound polyacetylene, and their co-polymers were called "Melanins".[13]

The first highly-conductive polymers were organosilicon plastics. For example, page 52 of the December 26, 1949, issue of LIFE magazine shows a light bulb in a circuit without wires. The conductor was Markite, a clear organosilicon plastic.[14] The conductivity of Markite ranges from as low as distilled water to as high as mercury.[15][16]

In the 1950s, researchers reported that polycyclic aromatic compounds formed semi-conducting charge-transfer complex salts with halogens.[2] This indicated that organic compounds could carry current. While organic conductors were previously intermittently discussed, the field was particularly energized by the prediction of superconductivity [17] following the discovery of BCS theory.

As for "pure" conductive organic polymers—in 1963, Bolto and co-workers reported conductivity in iodine-doped polypyrroles.[18][19][20] This Australian group eventually claimed to reach resistivities as low as 0.03 ohm·cm with other conductive organic polymers. This resistivity is roughly equivalent to present-day efforts.

Subsequently, DeSurville and coworkers reported high conductivity in a polyaniline.[21] Similarly, in 1980, Diaz and Logan reported films of polyaniline that could serve as electrodes.[22]

Similarly, because of its medical relevance, much early work on the physics and chemistry of conductive polymers was done under the melanin rubrick. For example, in the 1960s Blois et al. showed semiconduction in melanins, as well as further defining their physical structures and properties[23] Nicolaus et al. further defined the conductive polymer structures.[24] Classically, all polyacetylenes, polypyrroles and polyanilines are melanins, "The most simple melanin can be considered the acetylene-black from which it is possible to derive all the others .. Substitution does not qualitatively influence the physical properties like conductivity, colour, EPR, which remain unaltered."[25]

In 1974, McGinness and coworkers described[26] an "active" organic-polymer electronic device, a voltage-controlled bistable switch. This device used DOPA-melanin, a well-characterized self-doping copolymer of polyaniline, polypyrrole, and polyacetylene. The "ON" state of this device exhibited low conductivity with switching, with as much as five orders of magnitude shifts in current. Their material also exhibited classic negative differential resistance.

In 1977, Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa reported similar high conductivity in oxidized iodine-doped polyacetylene. This research earned them the 2000 Nobel prize in Chemistry "For the discovery and development of conductive polymers."[27] Some authors have expressed doubts about the Nobel citation's discovery assignment. Thus, Inzelt notes that,[1] while the Nobelists deserve credit for publicising and popularizing the field, conductive polymers were " ..produced, studied and even applied " well before their work.[28]

Trends

Most recent emphasis is on organic light emitting diodes and organic polymer solar cells.[29] The Organic Electronics Association is an international platform to promote applications of organic semiconductors.[30] Conductive polymer products with embedded and improved electromagnetic interference (EMI) and electrostatic discharge (ESD) protection has led to prototypes and products. Polymer Electronics Research Center at University of Auckland is developing a range of novel DNA sensor technologies based on conducting polymers, photoluminescent polymers and inorganic nanocrystals (quantum dots) for simple, rapid and sensitive gene detection. Typical conductive polymers must be "doped" to product high conductivity. To date, there remains to be discovered an organic polymer that is intrinsically electrically conducting.[31]

References

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External links

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  1. ↑ 1.0 1.1 1.2 György Inzelt (2008). Conducting Polymers A New Era in Electrochemistry. Springer. pp. 265–269. doi:10.1007/978-3-540-75930-0_8. ISBN 978-3-540-75930-0.  line feed character in |title= at position 20 (help)
  2. ↑ 2.0 2.1 2.2 2.3 Herbert Naarmann “Polymers, Electrically Conducting” in Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH, Weinheim. doi:10.1002/14356007.a21_429
  3. ↑ McGinness, John E. (1972). "Mobility Gaps: A Mechanism for Band Gaps in Melanins". Science. 177 (52): 896–897. doi:10.1126/science.177.4052.896. PMID 5054646. 
  4. ↑ Heeger, A. J.; Schrieffer, J. R.; Su, W. -P.; Su, W. (1988). "Solitons in conducting polymers". Reviews of Modern Physics. 60: 781. doi:10.1103/RevModPhys.60.781. 
  5. ↑ Heeger, A. J., Nature of the primary photo-excitations in poly(arylene-vinylenes): Bound neutral excitons or charged polaron pairs, in Primary photoexcitations in conjugated polymers: Molecular excitons versus semiconductor band model, Sariciftci, N. S., Ed., World Scientific, Singapore, 1997.Handbook of Organic Conductive Molecules and Polymers; Vol. 1–4, edited by H.S. Nalwa (John Wiley & Sons Ltd., Chichester, 1997).
  6. ↑ Handbook of Conducting Polymers; Vol.1,2, edited by T.A. Skotheim, R.L. Elsenbaumer, and J.R. Reynolds (Marcel Dekker, Inc., New York, 1998). Semiconducting Polymers; Vol., edited by G. Hadziioannou and P.F.v. Hutten (Wiley-VCH, Weinheim, 2007)
  7. ↑ Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; MacKay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. (1990). "Light-emitting diodes based on conjugated polymers". Nature. 347: 539. doi:10.1038/347539a0. 
  8. ↑ Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. (1992). "Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene". Science. 258 (5087): 1474. doi:10.1126/science.258.5087.1474. PMID 17755110. 
  9. ↑ Sirringhaus, H. (2005). "Device Physics of Solution-Processed Organic Field-Effect Transistors". Advanced Materials. 17: 2411. doi:10.1002/adma.200501152. 
  10. ↑ Yannoni, C. S.; Clarke, T. C. (1983). "Molecular Geometry of cis- and trans-Polyacetylene by Nutation NMR Spectroscopy". Physical Review Letters. 51: 1191. doi:10.1103/PhysRevLett.51.1191. 
  11. ↑ The Future of ITO: Transparent Conductor and ITO Replacement Markets
  12. ↑ Hush, Noel S. (2003). "An Overview of the First Half-Century of Molecular Electronics". Annals of the New York Academy of Sciences. 1006: 1. doi:10.1196/annals.1292.016. 
  13. ↑ The Nobel Prize in Chemistry, 2000: Conductive polymers
  14. ↑ Advanced Hardened Antenna Window Materials Study, Army Materials and Mechanics Research Center, 1972
  15. ↑ "Electricity Without Wires: Tough, light plastics can carry a current better than many metals" LIFE Dec. 26, 1949.[1]
  16. ↑ "New Plastics Conduct Current" Popular Science Sept., 1949
  17. ↑ Little, W. A. (1964). "Possibility of Synthesizing an Organic Superconductor". Physical Review. 134: A1416. doi:10.1103/PhysRev.134.A1416. 
  18. ↑ B A Bolto, R McNeill and DE Weiss "Electronic Conduction in Polymers. III. Electronic Properties of Polypyrrole" Australian Journal of Chemistry 16(6) 1090, 1963.
  19. ↑ R McNeill, D E Weiss and D Willis "Electronic conduction in polymers. IV. Polymers from imidazole and pyridine" Australian Journal of Chemistry 18 (1965) 477
  20. ↑ BA Bolto, D E Weiss and D Willis "Electronic conduction in polymers. V. Aromatic semiconducting polymers" Australian Journal of Chemistry 18 (1965) 487
  21. ↑ De Surville, R.; Jozefowicz, M.; Yu, L.T.; Pepichon, J.; Buvet, R. (1968). "Electrochemical chains using protolytic organic semiconductors". Electrochimica Acta. 13: 1451. doi:10.1016/0013-4686(68)80071-4. 
  22. ↑ Diaz, A; Logan, J (1980). "Electroactive polyaniline films". Journal of Electroanalytical Chemistry. 111: 111. doi:10.1016/S0022-0728(80)80081-7. 
  23. ↑ Blois, M; Zahlan, AB; Maling, JE (1964). "Electron Spin Resonance Studies on Melanin". Biophysical Journal. 4: 471. doi:10.1016/S0006-3495(64)86797-7. PMC 1367600 Freely accessible. PMID 14232133. 
  24. ↑ Nicolaus, R; Piattelli, M; Fattorusso, E (1964). "The structure of melanins and melanogenesis—IV , On some natural melanins". Tetrahedron. 20 (5): 1163. doi:10.1016/S0040-4020(01)98983-5. PMID 5879158. 
  25. ↑ Nicolaus, R.A., Parisi, G., The Nature of Animal Blacks. Atti Accademia Pontaniana XLIX, 197-233 (2000). ("acetylene-black" = polyacetylene).
  26. ↑ McGinness, J; Corry, P; Proctor, P (1974). "Amorphous semiconductor switching in melanins". Science (New York, N.Y.). 183 (127): 853–5. PMID 4359339. 
  27. ↑ Chemistry 2000
  28. ↑ "Book Rreview: György Inzelt, Conducting Polymers - A New Era in Electrochemistry". 2009-12-03. Retrieved 2010-07-12. 
  29. ↑ Overview on Organic Electronics
  30. ↑ Organic Electronics Association
  31. ↑ Conjugated Polymers: Electronic Conductors (April 2001)
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