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An organic semiconductor is an organic material with semiconductor properties, that is, with an electrical conductivity between that of insulators and that of metals. Single molecules, oligomers, and organic polymers can be semiconductive. Semiconducting small molecules (aromatic hydrocarbons) include the polycyclic aromatic compounds pentacene, anthracene, and rubrene.[not verified in body] Polymeric organic semiconductors include poly(3-hexylthiophene), poly(p-phenylene vinylene), as well as polyacetylene and its derivatives.[not verified in body]
There are two major overlapping classes of organic semiconductors. These are organic charge-transfer complexes and various linear-backbone conductive polymers derived from polyacetylene. Linear backbone organic semiconductors include polyacetylene itself and its derivatives polypyrrole, and polyaniline.[not verified in body] At least locally, charge-transfer complexes often exhibit similar conduction mechanisms to inorganic semiconductors. Such mechanisms arise from the presence of hole and electron conduction layers separated by a band gap.[not verified in body] Although such classic mechanisms are important locally, as with inorganic amorphous semiconductors, tunnelling, localized states, mobility gaps, and phonon-assisted hopping also significantly contribute to conduction, particularly in polyacetylenes.[not verified in body] Like inorganic semiconductors, organic semiconductors can be doped. Organic semiconductors susceptible to doping such as polyaniline (Ormecon) and PEDOT:PSS are also known as organic metals.[not verified in body]
Typical current carriers in organic semiconductors are holes and electrons in π-bonds. Almost all organic solids are insulators. But when their constituent molecules have π-conjugate systems, electrons can move via π-electron cloud overlaps, especially by hopping, tunnelling and related mechanisms.[not verified in body] Polycyclic aromatic hydrocarbons and phthalocyanine salt crystals are examples of this type of organic semiconductor.[not verified in body]
Mainly due to low mobility, even unpaired electrons may be stable in charge-transfer complexes.[not verified in body] Such unpaired electrons can function as current carriers.[not verified in body] This type of semiconductor is also obtained by pairing an electron donor molecule with an electron acceptor molecule.[not verified in body]
In 1862, Henry Letheby obtained a partly conductive material by anodic oxidation of aniline in sulfuric acid. The material was probably polyaniline. In the 1950s, researchers discovered that polycyclic aromatic compounds formed semi-conducting charge-transfer complex salts with halogens. In particular, high conductivity of 0.12 S/cm was reported in perylene-iodine complex in 1954. This finding indicated that organic compounds could carry current. In 1972, researchers found metallic conductivity in the charge-transfer complex TTF-TCNQ. Superconductivity in charge-transfer complexes was first reported in the Bechgaard salt (TMTSF)2PF6 in 1980.
In 1977, Shirakawa et al. reported high conductivity in oxidized and iodine-doped polyacetylene. They received the 2000 Nobel prize in Chemistry for "The discovery and development of conductive polymers". Similarly, highly-conductive polypyrrole was rediscovered in 1979.
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.
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There are significant differences between the processing of small molecule organic semiconductors and semiconducting polymers. Thin films of soluble conjugated polymers can be prepared by solution processing methods. On the other hand, small molecules are quite often insoluble and typically require deposition 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 include drop casting, spin-coating, doctor-blading, inkjet printing and screen printing. Spin-coating is a widely used technique for small area thin film production. It may result 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.
Organic semiconductors differ from inorganic counterparts in many ways. These include optical, electronic, chemical and structural properties. In order to design and model the organic semiconductors, such optical properties as absorption and photoluminescence need to be characterized. Optical characterization for this class of materials can be done using UV-visible 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).
The charge-carrier transport properties of organic semiconductors are examined by a number of techniques. For example, time-of-flight (TOF) and space charge limited current techniques are used to characterize “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 infrared spectroscopy, secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS).
Charge transport in disordered organic semiconductors
Charge transport in organic semiconductors is dependent on π-bonding orbitals and quantum mechanical wave-function overlap. In disordered organic semiconductors, there is limited π-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. Because of 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. Hopping of charge carriers from molecule to molecule depends 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 both the temperature and the 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. Another study shows that the AC conductivity of the organic semiconductor pentacene is frequency-dependent and provided evidence that this behavior is due to its polycrystalline structure and hopping conduction.
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