Organic semiconductor
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 as nitrogen, sulfur and oxygen.
The optical absorption edge of organic semiconductors is typically 1.7–3 eV, equivalent to a spectral range from 700 to 400 nm (which corresponds to the visible spectrum).
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.
This work was stimulated by the earlier discovery by Akamatu et al.[6] that aromatic hydrocarbons become conductive when blended with molecular iodine because a charge-transfer complex is formed.
It was observed in organic crystals in 1965 by Sano et al.[7] In 1972, researchers found metallic conductivity in the charge-transfer complex TTF-TCNQ.
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 holes injected from "ohmic" electrodes, i.e. electrodes with unlimited supply of charge carriers.
[13] 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.
[14] 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.
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 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.
[25] The highly ordered and directional intermolecular π-π interactions and hydrogen-bonding network allow the formation of quantum confined structures within the peptide self-assemblies, thus decreasing the band gaps of the superstructures into semiconductor regions.
[26] As a result of the diverse architectures and ease of modification of peptide self-assemblies, their semiconductivity can be readily tuned, doped, and functionalized.
Alternatively, one can extract the charge carrier mobility from the current in a field effect transistor as a function of both the source-drain and the gate voltage.
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.
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 activation energy for hopping motion contains an additional term due to structural site relaxation upon charging a molecular entity.
Understanding and regulating the yield point of organic semiconductors is essential to designing devices that can endure operational stress without permanent deformation.
The viscoelastic properties help the materials absorb energy during these processes, enhancing durability and ensuring long-term functionality under continuous physical stress.
