Organic semiconductors are solids whose building blocks are pi bonded molecules or polymers made up by carbon and hydrogen atoms and – at times – heteroatoms such nitrogen, sulfur and oxygen. They exist in form of molecular crystals or amorphous thin films. In general, they are insulators but become semiconducting when charges are either injected from appropriate electrodes, upon doping or by photoexcitation.
In molecular crystals the energetic separation between the top of the valence band and the bottom conduction band, i.e. the band gap, is typically 2.5 – 4 eV while in inorganic semiconductors the band gaps are typically 1 – 2 eV. This implies that they are, in fact, insulators rather than semiconductors in the conventional sense. They become semiconducting only when charge carriers are either injected from the electrodes or generated via intentional or unintentional doping. Charge carriers can also be generated in the course of optical excitation. It is important to realize, however, that the primary optical excitations are neutral excitons with a Coulomb-binding energy of typically 0.5 – 1.0 eV. The reason is that in organic semiconductors their dielectric constants are as low as 3-4. This impedes efficient photogeneration of charge carriers in neat systems in the bulk. Efficient photogeneration can only occur in binary systems due to charge transfer between donor and acceptor moieties. Otherwise neutral excitons decay radiatively to the ground state - thereby emitting photoluminescence – or non-radiatively. The optical absorption edge of organic semiconductors is typically 1.7 – 3 eV, equivalent to a spectral range from 700 to 400 nm (which also corresponds with the visible spectrum).
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.
The fact that organic semiconductors are, in principle, insulators but become semiconducting when charge carriers are injected from the electrode(s) was discovered by Kallmann and Pope. They found that a hole current can flow through an anthracene crystal contacted with a positively biased electrolyte containing iodine that can act as a hole injector. This work was stimulated by the earlier discovery by Akamatu et al. that aromatic hydrocarbons become conductive when blended with molecular iodine because a charge transfer complex is formed. Since it was readily realized that the crucial parameter that controls injection is the work-function of the electrode, it was straightforward to replace the electrolyte by a solid metallic or semiconducting contact with an appropriate work-function. When both electrons and holes are injected from opposite contacts they can recombine radiatively and emit electroluminescence. It was observed in organic crystals in 1965 by Sano et al.
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.
The discovery by Kallman and Pope paved the way for applying organic solids as active elements in semiconducting electronic devices, such as organic light emitting diodes (OLEDs) that rely on the recombination of electrons and hole injected from "ohmic"electrodes, i.e. electrodes with unlimited supply of charge carriers. The next major step towards the technological exploitation of the phenomenon of electron and hole injection into a non-crystalline organic semiconductor was the work by Tang and Van Slyke. They showed that efficient electroluminescence can be generated in a vapor-deposited thin amorphous bilayer of an aromatic diamine (TAPC) and Alq3 sandwiched between an indium-tin-oxide (ITO) anode and an Mg:Ag cathode. Another milestone towards the development or organic light emitting diodes (OLEDs) was the recognition that also conjugated polymers can be used as active materials. The efficiency of OLEDs was greatly improved when realizing that phosphorescent states (triplet excitons) may be used for emission when doping an organic semiconductor matrix with a phosphorescent dye, such as complexes of iridium with strong spin-orbit coupling.
Work on conductivity of anthracene crystals contacted with an electrolyte showed that optically excited dye molecules adsorbed at the surface of the crystal inject a charge carriers. The underlying phenomenon is called sensitized photoconductivity. It occurs when photo-exciting a dye molecule with appropriate oxidation/reduction potential adsorbed at the surface or incorporated in the bulk. This effect revolutionized electrophotography which is the technological basis of today’s office copying machines. It is also the basis of organic solar cells (OSCs) in which the active element is an electron donor and an electron acceptor material is combined in a bilayer or a bulk hetero junction.
Doping with strong electron donor or acceptors can render organic solids conductive even in the absence of light. Examples are doped polyacetylene and doped light emitting diodes.
Today organic semiconductors are used as active elements in organic light emitting diodes (OLEDs), organic solar cells (OSCs) and organic field-effect transistors (OFETs).
Amorphous molecular films
Amorphous molecular films are produced by evaporation or spin-coating. They have been investigated for device applications such as OLEDs, OFETs, and OSCs. Illustrative materials are tris(8-hydroxyquinolinato)aluminium, C60, phenyl-C61-butyric acid methyl ester (PCBM), pentacene, [carbazole]]s, and phthalocyanine.
Molecularly doped polymers
Molecularly doped polymers are prepared by spreading a film of an electrically inert polymer, e.g. polycarbonate, doped with typically 30% of charge transporting molecules, on a base electrode. Typical materials are the triphenylenes. They have been investigated for use as photoreceptors in electrophotography. This requires films have a thickness of several micrometers that can be prepared using the doctor-blade technique.
In the early days of fundamental research into organic semiconductors the prototypical materials were free-standing single crystals of the acene family, e.g. anthracene and tetracene. The advantage of employing molecular crystals instead of amorphous film is that their charge carrier mobilities are much larger. This is of particular advantage for OFET applications. Examples are thin films of crystalline rubrene prepared by hot wall epitaxy.
Neat polymer films
They are usually processed from solution employing variable deposition techniques including simple spin-coating, ink-jet deposition or industrial reel-to-reel coating which allows preparing thin films on a flexible substrate. The materials of choice are conjugated polymers such as poly-thiophene, poly-phenylenevinylene, and copolymers of alternating donor and acceptor units such as members of the poly(carbazole-dithiophene-benzothiadiazole (PCDTBT) family. For solar cell applications they can be blended with C60 or PCBM are electron acceptors.
To design and characterize organic semiconductors used for optoelectronic applications one should first measure the absorption and photoluminescence spectra using commercial instrumentation. However, in order to find out if a material acts as an electron donor or acceptor one has to determine the energy levels for hole and electron transport. The easiest way of doing this, is to employ cyclic voltammetry. However, one has to take into account that using this technique the experimental deter oxidation and reduction potential are lower bounds because in voltammetry the radical cations and anions are in a polar fluid solution and are, thus, solvated. Such a solvation effect is absent in a solid specimen. The relevant technique to energetically locate the hole transporting states in a solid sample is UV-photoemission spectroscopy. The equivalent technique for electron states is inverse photoemission.
There are several techniques to measure the mobility of charge carriers. The traditional technique is the so-called time of flight (TOF) method. Since this technique requires relatively thick samples it is not applicable to thin films. Alternatively, one can extract the charge carrier mobility from the current flowing in a field effect transistor as a function of both the source-drain and the gate voltage. One should be aware, though, that the FET-mobility is significantly larger than the TOF mobility because of the charge carrier concentration in the transport channel of a FET (see below). Other ways to determine the charge carrier mobility involves measuring space charge limited current (SCLC) flow and "carrier extraction by linearly increasing voltage (CELIV).
In contrast to organic crystals investigated in the 1960-70’s, organic semiconductors that are nowadays used as active media in optoelectronic devices are usually more or less disordered. Combined with the fact that the structural building blocks are held together by comparatively weak van der Waals forces this precludes charge transport in delocalized valence and conduction bands. Instead, charge carriers are localized at molecular entities, e.g. oligomers or segments of a conjugated polymer chain and move by incoherent hopping among adjacent sites with statistically variable energies. Quite often the site energies feature a Gaussian distribution. Also the hopping distances can vary statistically (positional disorder). A consequence of the energetic broadening of the density of states (DOS) distribution is that charge motion is both temperature and field dependent and the charge carrier mobility can be several orders of magnitude lower than in an equivalent crystalline system. This disorder effect on charge carrier motion is diminished in organic field-effect transistors because current flow is confined in a thin layer. Therefore, the tail states of the DOS distribution are already filled so that the activation energy for charge carrier hopping is diminished. For this reason the charge carrier mobility inferred from FET experiments is always higher than that determined from TOF experiments.
In organic semiconductors charge carriers couple to vibrational modes and are referred to as polarons. Therefore, the activation energy for hopping motion contains an additional term due to structural site relaxation upon charging a molecular entity. It turns out, however, that usually the disorder contribution to the temperature dependence of the mobility dominates over the polaronic contribution.
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