An organic semiconductor is any organic material that has semiconductor properties. A semiconductor is any compound whose electrical conductivity is between that of typical metals and that of insulating compounds. Single molecules, short chain (oligomers) and long chain (polymers) organic semiconductors are known. Examples of semiconducting small molecules (aromatic hydrocarbons) are : pentacene, anthracene and rubrene. Examples of polymers are: poly(3-hexylthiophene), poly(p-phenylene vinylene), F8BT, as well as polyacetylene and its derivatives.
There are two major classes of organic semiconductors, which overlap significantly: organic charge-transfer complexes, and various "linear backbone" polymers derived from polyacetylene, such as polyacetylene itself, polypyrrole, and polyaniline. Charge-transfer complexes often exhibit similar conduction mechanisms to inorganic semiconductors, at least locally. 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 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 include π-electrons and unpaired electrons. Almost all organic solids are insulators. But 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 charge transfer complexes, even unpaired electrons can stay stable for a long time, and are the carriers. This type of semiconductor is also obtained by pairing an electron donor molecule and an electron acceptor molecule.
The study of conductive charge-transfer complexes began with the discovery of the strikingly high conductivity of perylene-iodine complex (8 Ω·cm) 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 al reported  passive high conductivity in iodine-"doped" oxidized polypyrrole. While not generally acknowledged, this is the first report of modern highly-conductive polyacetylenes and related linear-backbone polymer "Blacks" or Melanins. They achieved a resistance of 1 Ω/cm. The 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 0.03 Ω/cm,  on the order of present-day efforts. They noted an Australia patent application (5246/61, June 5, 1961) for conducting polypyrrole. Highly-conductive polypyrrole is often incorrectly reported as being discovered in 1979 by Diaz et al. J. Chem. Soc., Chem Comm, 1979: 635-6..
In a similar 1977 paper, Shirakawa et al reported  equivalent high conductivity in similarly oxidized and iodine-doped polyacetylene. They received the 2000 Noble prize in Chemistry for "The discovery and development of conductive polymers". The Nobel committee made no reference to the Australian's earlier reports, which also were never cited by the Nobel winners. See Nobel Prize controversies.
Likewise, an organic electronic device was reported in a 1974 paper in Science . 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  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 in biosensing applications.
Organic semiconductors have many advantages, 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 natural and synthesized forms.
"An Overview of the First Half-Century of Molecular Electronics" by Noel S. Hush, Ann. N.Y. Acad. Sci. 1006: 1–20 (2003).
- Semi-Conducting Polymers and Optoelectronics - Richard Friend, Cavendish Professor, Cambridge Freeview video by the Vega Science Trust.
- Electronic Conduction in Polymers. III. Electronic Properties of Polypyrrole BA Bolto, R McNeill and DE Weiss , Australian Journal of Chemistry 16(6) 1090 - 1103.