Organic semiconductor
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include STM image of organic semiconductor
[[Image:Selfassembly OrganicSemiconductor Trixler LMU.jpg|250px|thumb|STM image of self-assembled [[supramolecular]] chains of the [[organic semiconductor ]] [[Quinacridone]] on [[Graphite]]. ]]
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 [[molecule]]s, short chain ([[oligomer]]s) and long chain ([[polymer]]s) 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 complex|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 gap]]s, and [[phonon]]-assisted hopping also contribute to conduction, particularly in polyacetylenes. Like inorganic semiconductors, organic semiconductors can be [[Doping (semiconductors)|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 [[Pi_bond#Pi_Bonds |π-electrons]] and unpaired electrons. Almost all organic solids are [[Electrical insulation |insulator]]s. But when their constituent molecules have [[conjugated system| π-conjugate systems]], electrons can move via [[aromaticity|π-electron cloud]] overlaps. Polycyclic [[aromatic hydrocarbon]]s and [[phthalocyanine]] salt crystals are examples of this type of organic semiconductor.
In [[charge transfer complex]]es, 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.
==History==
[[Image:Gadget128.JPG|frame|right|Voltage-controlled switch, an "active" organic polymer electronic device from 1974. Now in the Smithsonian.]]
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 [[Charge transfer complex|TTF-TCNQ complex]]. In [[1980]], [[superconductivity]] was observed in TMTSF-PF<sub>6</sub> complex.
In 1963, Weiss ''et al'' reported [http://www.drproctor.com/os/weiss.htm] 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 [[Melanin]]s. They achieved a resistance of 1 [[ohm|Ω]]/[[centimeter|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, [http://www.publish.csiro.au/nid/51/paper/CH9650477.htm][http://www.publish.csiro.au/nid/51/paper/CH9650487.htm] 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.[http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=C39790000635&JournalCode=C3].
In a similar 1977 paper, Shirakawa ''et al'' reported [http://www.rsc.org/Publishing/Journals/C3/article.asp?doi=C39770000578] 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".[http://nobelprize.org/nobel_prizes/chemistry/laureates/2000/index.html] 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'' [http://www.drproctor.com/os/amorphous.htm ]. 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'' [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=5054646&query_hl=1&itool=pubmed_docsum] 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 [[Optoelectronics |optoelectronic]] devices such as [[organic light-emitting diode]]s (OLED), [[organic solar cell]]s, [[organic field effect transistor]]s ([[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.
== References ==
''"An Overview of the First Half-Century of Molecular Electronics"'' by Noel S. Hush, ''Ann. N.Y. Acad. Sci. 1006:'' 1–20 (2003).
==External links==
* [http://www.organicsemiconductors.com www.organicsemiconductors.com]
* [http://www.vega.org.uk/video/programme/91 ''Semi-Conducting Polymers and Optoelectronics''] - Richard Friend, Cavendish Professor, Cambridge Freeview video by the Vega Science Trust.
* [http://www.drproctor.com/os/weisspaper.pdf Electronic Conduction in Polymers. III. Electronic Properties of Polypyrrole] BA Bolto, R McNeill and DE Weiss , ''Australian Journal of Chemistry 16(6) 1090'' - 1103.
==See also==
* [[Conductive polymers]]
* [[Melanin]]
* [[Molecular electronics]]
[[Category:Organic semiconductors]]
[[Category:Conductive polymers]]
[[Category:Molecular electronics]]
[[fr:Semi-conducteur organique]]
[[ja:有機半導体]]
[[es:polímero semiconductor]]