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There are two major classes of organic semiconductors, which overlap significantly: organic , 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 Insulator s. But when their constituent molecules have π-conjugate Systems , electrons can move via 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 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 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 Melanin s. They achieved a resistance of 1 Ohm / 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 .03 ohm/cm, [http://www.publish.csiro.au/nid/51/paper/CH9650477.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 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'' . Here, 's collection. Analogous rigid-backbone organic semiconductors are now-used as active elements in 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
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