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Organic semiconductor
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An organic semiconductor is an organic material that has semiconductor properties. A semiconductor is compound whose electrical conductivity is inversely proportional to resistivity (temperature). Semiconductivity is exhibited by single molecules, short chain (oligomers) and organic polymers. Semiconducting small molecules (aromatic hydrocarbons) include the polycycic aromatic compounds pentacene, anthracene, and rubrene. Examples of polymeric semiconductors 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.
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An organic semiconductor is an organic material that has semiconductor properties. A semiconductor is compound whose electrical conductivity is inversely proportional to resistivity (temperature). Semiconductivity is exhibited by single molecules, short chain (oligomers) and organic polymers. Semiconducting small molecules (aromatic hydrocarbons) include the polycycic aromatic compounds pentacene, anthracene, and rubrene. Examples of polymeric semiconductors 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. Such mechanisms arise from the presence of a hole and electron conduction layer separated by 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. Organic semiconductors susceptible to doping polyaniline (Ormecon) and PEDOT:PSS, are also known as "organic metals."
Typica carriers in organic semiconductors are holes and electrons in π-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.
History
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 resistivity of 1 O 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 resistivities as low as 0.03 O 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.
Processing of organic semiconductors
One of the differences between small molecules and polymers is their processing techniques. Thin films of soluble conjugated polymers can be prepared by solution processing methods, while small molecules are quite often insoluble and typically deposited via vacuum sublimation. Both approaches yield amorphous or polycrystalline films with variable degree of disorder. “Wet” coating techniques require polymers to be dissolved in a volatile solvent, filtered and deposited onto a substrate. Common examples of solvent-based coating techniques are drop casting, spin-coating, doctor-blading, inkjet printing and screen printing. Spin-coating is a widely used technique for small area thin film production that results in a high material loss. The doctor-blade technique has a minimal material loss and was primarily developed for large area thin film production.
Vacuum based thermal deposition of small molecules requires evaporation of molecules from a hot source. The molecules are then transported through vacuum onto a substrate. Condensation of these molecules on the substrate surface results in thin film formation. Wet coating techniques can be applied to small molecules but to a lesser extent depending on material solubility.
Characterisation of organic semiconductors
Organic semiconductors differ from inorganic counterparts in many ways including optical, electronic, chemical and structural properties.
In order to design and model the organic semiconductors, their optical properties like absorption and photoluminescence are required to be characterized. .
Optical characterization for this class of materials can be done using UV-VIS absorption spectrophotometers and photoluminescence spectrometers. Semiconductor film appearance and morphology can be studied with Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Electronic properties such as ionisation potential can be characterized by probing the electronic band structure with Ultraviolet Photoelectron Spectroscopy (UPS).
Charge carrier transport properties of organic semiconductors can be studied by a number of techniques. For example, Time-Of-Flight (TOF) and space charge limited current techniques are used to characterise “bulk” conduction properties of organic films. Organic Field Effect Transistor (OFET) characterization technique is probing “interfacial” properties of semiconductor films and allows to study the charge carrier mobility, transistor threshold voltage and other FET parameters. OFETs development can directly lead to novel device applications such as organic-based flexible circuits, printable Radio Frequency Identification tags (RFID) and active matrix backplanes for displays.
Chemical composition and structure of organic semiconductors can be characterized by Infra-Red Spectroscopy, Secondary Ion mass Spectroscopy and X-ray photoelectron spectroscopy (XPS).
Charge transport in disordered organic semiconductors (Hopping transport)
Charge transport in organic semiconductors is dependent on p-bonding orbitals and quantum mechanical wave-function overlap. In disordered organic semiconductors there is limited p-bonding overlapping between molecules and conduction of charge carriers (electrons or holes) is described by quantum mechanical tunnelling. Charge transport depends on the ability of the charge carriers to pass from one molecule to another. Due to the quantum mechanical tunnelling nature of the charge transport, and its subsequent dependence on a probability function, this transport process is commonly referred to as hopping transport.
The charge carriers hopping from molecule to molecule are dependent upon the energy gap between HOMO and LUMO levels. Carrier mobility is reliant upon the abundance of similar energy levels for the electrons or holes to move to and hence will experience regions of faster and slower hopping. This can be affected by the temperature and also electric field across the system. A theoretical study has shown that in a low electric field the conductivity of organic semiconductor is proportional to T-1/4 and in a high electric field is proportional to e-(E/aT) , where a is a constant of the material.
See also
External links
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- - Richard Friend, Cavendish Professor, Cambridge Freeview video by the Vega Science Trust.
- BA Bolto, R McNeill and DE Weiss , Australian Journal of Chemistry 16(6) 1090 - 1103.
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