Organic semiconductor

Organic semiconductors are semiconductors are based on organic materials, and are used in particular electronic components. The application is called organic electronics and in addition to general electronic circuits (also called plastic electronics ) and specialty applications such as organic light-emitting diode ( OLED) and organic solar cell.

Electronic Properties

After hybridization model, the electrical conductivity of organic carbon compounds and graphite can be attributed to the sp2 hybridization of the carbon: According formed when atoms bound by the hybridization of the 2s atomic orbital and two 2p atomic orbitals ( 2px and 2py ) three equivalent sp2 hybrid orbitals which lie in a plane and form an angle of 120 °. The bonds between the atoms via σ - bonds, which is due to the overlap of two hybrid orbitals between neighboring atoms about. The third p- orbital ( 2p ), which does not form a hybrid orbital itself is perpendicular to the plane of hybrid orbitals. Between 2pz orbital atom adjacent carbon atoms there is a π - bond, which consists of a lateral overlap of - p orbitals occurs - as described in the model as a dumbbell shape. Thus between two neighboring atoms can be both a σ - and a π - bond to occur - a case that is referred to as " double bond ".

Intramolecular conductivity

To change these double bonds with single bonds in a regular sequence from several ways of representing this sequence in the interconnect structure are open: Which of the three hybrid orbitals of carbon is superimposed on a π - bond and thus defines a double bond position is not fixed and can therefore only by three different structural formulas ( resonance structures, resonance structures) are described. However, since the energy content of each of these limiting structures is larger than the actual energy content of a compound and experimentally determine any differences between the bindings (cf. ), it must be assumed that none of these limiting structures is realized alone, but must be assumed by a superposition of all the resonance structures. Such a superposition of single and double bonds is referred to as " conjugated structure "; within these are no longer localized π - bonds, so that a so-called " delocalized π -electron system " is present. In such conjugated structures with delocalized π - electrons is based molecular conductivity of organic compounds.

Also the color of pigments based on intramolecular conductivity. Delocalized π - electrons are easy to transition from the highest occupied molecular orbital ( HOMO) to the lowest unoccupied molecular orbital ( LUMO) encourage, as the energy difference between the bonding and antibonding π molecular orbitals, which in an sp2 -hybridized carbon system and at the same time the HOMO or LUMO represent, is located in a size range corresponding to the energy of light having a wavelength near or within the visible Lichtspektralbereiches. The energy difference is smaller, the greater is the delocalization. This results in addition to the semi-conductive properties to the π - conjugated structure may act as a part of chromophoric system, that can appear in color by light absorption and fluorescence in the visible spectral range molecules, and thus act as organic color pigments.

Intermolecular charge transport

Intermolecular conductivity organic semiconductor is a function of various factors. They include both structural parameters (mutual arrangement of the molecules, type of intermolecular interactions, degree of order, density of structural defects ) as well as influences from the environment ( eg temperature).

In highly ordered supramolecular associations (pure crystals ), there is an electronic coupling between the π - systems via hydrogen bonds or van der Waals interactions. In undisturbed crystalline Association interact all, by the individual π - molecular orbitals represented (HOMO and LUMO ) levels and split into corresponding valence and conduction bands. On this basis, the carrier transport for many crystalline organic semiconductor can be described by a belt -like conveyor. However, a prerequisite for the dominance of this mechanism is that a sufficiently low temperature is given ( in the size range of about 30 K); the thermally activated polaron hopping - - the temperature rises, on the other hand, another transport mechanism is always more effective and eventually dominates. The charge transport in disordered semiconductors may also be described by the so-called hopping (English, thermal excitation of electrons over the potential barrier ).

For conducting polymers hopping plays a role in so far as thus given away a possibility of charge transfer between different polymer chains. In the polymer chains themselves π - bonds over the entire length of the chain can delocalize, so that a quasi- one-dimensional electronic system exists. The band gap between the filled valence band and the empty conduction bands can be eliminated by doping, so that the conductivity comparable to metals arises. In such samples, this results in a total of a highly anisotropic conductivity, which is based in the tape transport with metallic conductivity along the polymer chains on the one hand and a hopping transport at a much lower conductivity between chains on the other.

Classification

Organic semiconductors may be classified on the criterion of molecular mass into two classes:

Conjugated molecules

• Linearly fused ring systems (e.g. oligoacenes such as anthracene, pentacene and derivatives thereof (for example, quinacridone ), or also, for example Benzenthiolate )
• Two-dimensional condensed ring systems (for example, perylene, PTCDA and its derivatives, naphthalene derivatives, hexabenzocoronene )
• Metal complexes ( eg, phthalocyanines, or Alq3, Beq2 )
• Dendritic molecules, starburst molecules ( for example, 4,4 ', 4 " -tris (N, N -diphenyl- amino) triphenylamines ( TDATA ) )
• Heterocyclic oligomers (eg, oligothiophenes, Oligophenylenevinylene )

Conjugated polymers

• Heterocyclic polymers (e.g. polythiophenes, polyparaphenylene, polypyrrole, polyaniline )
• Hydrocarbon chains (eg, polyacetylene, Polysulfurnitride )

Such a classification turns out in terms of the suitability of the substance classes for various research and application areas of electronics to be favorable for an overview, because while mono- and oligomers with a low molar mass only for use in the plastic electronics due to their small size as functional elements for molecular nanoelectronics are (molecular electronics), conjugated polymers are essentially limited to the plastic electronics.

Uses

The uses of the substance mentioned groups can be assigned to the following areas mainly:

• Molecular electronics
• Organic Electronics (plastic electronics)
• Organic pigments: in particular, pentacene and perylene derivatives and phthalocyanines act by their chromophoric systems as intense colorant. Due to the widespread industrial applications mainly as printing inks, car paints or for coloring plastics, they are produced by the paint industry commercially in large quantities and are available as artist colors in retail.