domingo, 21 de marzo de 2010

Organic semiconductor

STM image of self-assembled supramolecular chains of the organic semiconductor Quinacridone on Graphite.

An organic semiconductor is an
organic material that has semiconductor properties. Semiconductivity is exhibited by single molecules, short chain (oligomers) and organic polymers. Semiconducting small molecules (aromatic hydrocarbons) include the polycyclic 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 overlapping classes of organic semiconductors: organic
charge-transfer complexes, and various linear-backbone conductive 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 hole and electron conduction layers 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.
Typical 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 may be stable for a long time, and they function as the carriers. This type of semiconductor is also obtained by pairing an electron donor molecule and an electron acceptor molecule.

Voltage-controlled switch, an "active" organic polymer electronic device from 1974. Now in the Smithsonian Chip collection.

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 the charge-transfer complex TTF-TCNQ. In 1980, superconductivity was observed in TMTSF-PF6 complex.
In 1963, Weiss et al. reported passive high conductivity in iodine-"doped" oxidized polypyrrole.[2] This was the first report of modern highly-conductive polyacetylenes and related linear-backbone polymer "Blacks" or Melanins. They achieved a resistivity of 1 Ω 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 Ω cm,[3][4] on the order of present-day efforts. They noted an Australia patent application (5246/61, June 5, 1961) for conducting polypyrrole.[citation needed] Highly-conductive polypyrrole was reported as being discovered in 1979 by Diaz et al.
In a similar 1977 paper, Shirakawa et al. reported equivalent high conductivity in similarly oxidized and iodine-doped polyacetylene. They received the 2000 Nobel prize in Chemistry for "The discovery and development of conductive polymers". The Nobel citation made no reference to Weiss et al.'s similar earlier work (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. In a typical "active" device, a voltage or current controls electron flow. This gadget is now in the Smithsonian's collection (see figure).
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.

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.

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 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 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 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. 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.

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