Organic semiconductor

From Wikipedia, the free encyclopedia

Semiconductors are compounds whose electrical conductivity is midway between that of typical metals and that of insulating compounds. Both short chain oligomers and long chain (polymers) organic semiconductors are known. Typical examples for semiconducting oligomers are: pentacene, anthracene and rubrene. Some semiconducting polymers are: poly(3-hexylthiophene), poly(p-phenylene vinylene), F8BT, as well as polyaectylene and its derivatives.

With significant overlap, there are roughly two major classes of organic semiconductors. These are 1) the organic charge-transfer complexes and 2) various derivatives of polyacetylene. The latter "linear backbone" polymers include polyacetylene itself, polypyrrole, and polyaniline. At least locally, the charge-transfer complexes often exhibit similar conduction mechanisms to inorganic semiconductors. This includes the presence of a hole and electron conduction layer and a band gap. As with inorganic amorphous semiconductors, tunneling, localized states, mobility gaps, and phonon-assisted hopping also contribute to conduction, particularly in the polyacetylenes. Like inorganic semiconductors, organic semiconductors can be doped. Highly doped organic semiconductors, for example Polyaniline (Ormecon) and PEDOT:PSS, are also known as organic metals.

Several kinds of carriers mediate conductivity in organic semiconductors. These includeπ-electrons and unpaired electrons. Almost all organic solids are insulators. However, when their constituent molecules have π-conjugate systems, electrons can move via π-electron cloud overlaps. Polycyclic aromatic hydrocarbons and phthalocyanine salt crystals are examples of this type of organic semiconductor.

In some organic molecules, even unpaired electrons can stay stable for a long time. In such cases, unpaired electrons will be the carriers. This type of semiconductor is also obtained by pairing an electron donor molecule and an electron acceptor molecule and is called a charge transfer complex.

[edit] History

Voltage-controlled switch, an "active" organic polymer electronic device from 1974. Now in the Smithsonian.

For a history of the field, see: "An Overview of the First Half-Century of Molecular Electronics" by Noel S. Hush, Ann. N.Y. Acad. Sci. 1006: 1–20 (2003). Some key events:

The study of conductive charge-transfer complexes began with the discovery of the strikingly high conductivity of perylene-iodine complex (8 Ωcm) in 1954. In 1972, researchers reported metallic conductivity in a TTF-TCNQ complex. In 1980, superconductivity was observed in TMTSF-PF₆ complex.

In 1963, Weiss et all reported [1] passive high conductivity in iodine-"doped" oxidized polypyrrole. This was the first report of modern highly-conductive polyacetylenes and related linear-backbone polymer "Blacks" or Melanins. They achieved a resistance of 1 ohm/cm. These authors also described the effects of iodine doping on conductivity, the conductivity type (n or p), and electron spin resonance studies on polypyrrole. In later papers, they achieved resistances as low as .03 ohm/cm [2][3], on the order of present-day efforts. Likewise, these authors noted an Australia patent application (5246/61, June 5, 1961) for conducting polypyrrole. Interestingly, highly-conductive Polypyrrole is often reported as being discovered in 1979 by Diaz et al. J. Chem. Soc., Chem Comm, 1979: 635-6.[4].

In 1977, Shirakawa et al reported [5] equivalent high conductivity in similarly oxidized and iodine-doped polyacetylene. The latter researchers received the 2000 Noble prize in Chemistry for " The discovery and development of conductive polymers " [6]. The Nobel committee made no reference to the Australian's earlier work. See Nobel Prize controversies.

Likewise, an organic electronic device was reported in a 1974 paper in Science [7]. Here, John McGinness and his coworkers reported a high conductivity "ON" state and hallmark negative differential resistance in DOPA Melanin, an oxidized copolymer of polyacetylene, polypyrrole, and polyaniline. This device was a "proof of concept" for an earlier paper in Science [8] outlining what is now the classic mechanism for electrical conduction in such materials, long considered part of the "development" cited in the 2000 Nobel award. In a typical "active" device, a voltage or current controls electron flow. This gadget is now in the Smithsonian's collection.

Analogous rigid-backbone organic semiconductors are now-used as active elements in optoelectronic devices such as organic light-emitting diodes (OLED), organic solar cells, organic field effect transistors (OFET), electrochemical transistors and recently also in biosensing applications. There are many strong points of organic semiconductors, such as easy fabrication, mechanical flexibility, and low cost. Melanin is a semiconducting polymer currently of high interest to researchers in the field of organic electronics in both its organic and synthesized forms.