An organic semiconductor is an organic substance with low electrical resistance. As the origin of conductivity, two kinds of carriers are well known. One is a π-electrons and the other is unpaired electrons.

Generally speaking, electronically neutral molecules gathered by Van der Waals force are composed of organic crystals. Therefore, almost all organic solids are insulators, which do not conduct electricity easily. However, in crystals that consist of molecules that have the π-conjugate system, electrons can move via π-electron cloud overlaps. That is why these crystals can conduct electricity. 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.

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

Organic semiconductors are attractive as active elements in optoelectronic devices such as field effect transistors (FET), light emitting diodes (LED), and photovoltaic and solar cells. There are many strong points of organic semiconductors, such as easy fabrication, mechanical flexibility, and low cost. Melanin, for example, is an organic semiconductor. It is also a neuropeptide, a polymeric substance that influences neural activity and mediates the conduction of light, heat and kinetic energy. As such, it is currently a subject of intense interest in biotech research and development, most notably in plastic electronics and nanotechnology. Research indicates that melanin, in both its organic and synthesized forms, may one day routinely supplant conventional inorganic materials like gallium arsenide and silicon in high-tech devices such as microchips.

See also

• Conductive polymers
• Melanin