Organic semiconductor

Encyclopedia : O : OR : ORG : Organic semiconductor

Semiconductors in general are compounds whose electrical conduction 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) and F8BT.

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 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 , 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.

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 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.

Similarly, McGinness, Corry, and Proctor first reported a high conductivity "ON" state and hallmark negative differential resistance in an oxidized polyacetylene (melanin). They also reported the first "active" organic semiconductor electronic device, a voltage-controlled switch. In a typical "active" device, a voltage or current controls electron flow. See [Science, vol 183, 853-855 (1974}]. This gadget is now in the Smithsonian's collection.

Analogous polyacetylene-derived 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.

Organic semiconductor

See also