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. Polymeric
organic semiconductors include poly(3-hexylthiophene), poly(p-phenylene vinylene), as well as polyacetylene
and its derivatives.
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. 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. 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.
History
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
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.
Processing
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.
Characterization
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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