Organic semiconductors

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Organic semiconductors are semiconductors that are based on organic materials and are used in special electronic components. The application is called organic electronics and, in addition to general electronic circuits (also called plastic electronics ), also includes special applications such as organic light-emitting diodes (OLED) and organic solar cells .

Scanning tunnel microscope image of self-assembled molecular chains of the organic semiconductor quinacridone

Electronic properties

According to the hybridization model , the electrical conductivity of organic carbon compounds and graphite can be traced back to the sp2 hybridization of carbon : According to this, three equivalent sp2 hybrid orbitals are formed in bonded atoms through the hybridization of one 2s atomic orbitals and two 2p atomic orbitals (2px and 2py) that lie in one plane and form an angle of 120 °. The bonds between the atoms take place via σ-bonds , which are created by the overlapping of two hybrid orbitals between neighboring atoms. The third p orbital (2pz), which itself does not form a hybrid orbital, is perpendicular to the plane of the hybrid orbitals. Between these 2pz atomic orbitals of neighboring carbon atoms there is a π bond , which results from a lateral overlap of the p orbitals - described as dumbbell-shaped in the model. Thus, both a σ and a π bond can occur between two neighboring atoms - a case known as a “ double bond ”.

Intramolecular conductivity

If these double bonds alternate with single bonds in regular succession, there are several options for representing this sequence in the connection structure: with which of the three hybrid orbitals of carbon a π bond is superimposed and thus a double bond position is defined is not fixed and can therefore can only be described by three different structural formulas ( mesomeric boundary structures , resonance structures). However, since the energy content of each of these boundary structures is greater than the actual energy content of a connection and no differences between the bonds can be determined experimentally (cf.), it must be assumed that none of these boundary structures is realized alone, but that all boundary structures are superimposed . Such a superposition of single and double bonds is called a “ conjugate structure”; within these, π bonds can no longer be localized, so that a so-called “delocalized π electron system” is present. The molecular conductivity of organic compounds is based on such conjugated structures with delocalized π electrons .

The color of pigments is also based on intramolecular conductivity. Delocalized π electrons can easily be excited to transition from the highest occupied molecular orbital ( HOMO ) to the lowest unoccupied molecular orbital ( LUMO ) because the energy difference between the bonding and antibonding π molecular orbitals, which in an sp2 -hybridized carbon system also forms the HOMO or represent LUMO, lies in a size range which corresponds to the energy of light with a wavelength close to or within the visible light spectral range. The greater the delocalization, the smaller the energy difference. In addition to the semiconducting properties, this means that the π-conjugated structure can act as part of a chromophoric system which, through light absorption and fluorescence, makes molecules appear colored in the visible spectral range and thus allows them to act as organic color pigments.

Intermolecular Charge Transport

The intermolecular conductivity of organic semiconductors depends on various factors. These include both structural parameters (mutual arrangement of molecules, type of intermolecular interactions, degree of order, density of structural defects) and influences from the environment (e.g. temperature).

In highly ordered supramolecular assemblies (pure single crystals ) there is an electronic coupling of the π systems via hydrogen bridges or van der Waals interactions . In the undisturbed crystalline association, all of the (HOMO and LUMO) levels represented by the individual π molecular orbitals interact and split into corresponding valence and conduction bands. On this basis, the charge carrier transport for many crystalline organic semiconductors can be described by a band-like transport. A prerequisite for the dominance of this mechanism is, however, that the temperature is sufficiently low (in the order of magnitude of approx. 30 K); the temperature rises, however, is another transport mechanism - the thermally activated Polaron - hopping ( "hopping") - more effectively and eventually dominated. The charge transport in disordered semiconductors can also be described by so-called hopping (English, thermal excitation of the electrons over the potential barrier).

For conductive polymers , hopping plays a role insofar as it gives the possibility of charge transfer between different polymer chains. In the polymer chains themselves, π bonds can delocalize over the entire length of the chain, so that a quasi-one-dimensional electronic system is present. The band gap between the filled valence and the empty conduction bands can be eliminated by doping, so that a conductivity comparable to that of metals is created. In such samples, the result is a highly anisotropic conductivity, which is due to the belt transport with metallic conductivity along the polymer chains on the one hand and hopping transport with significantly lower conductivity between the chains on the other.

Classification

Organic semiconductors can be divided into two classes using the molar mass criterion : conjugated molecules and conjugated polymers.

Examples of conjugated molecules:

  • linearly condensed ring systems (e.g. oligoacenes such as anthracene , pentacene and their derivatives (e.g. quinacridone ), or e.g. benzothiolates)
  • two-dimensional fused ring systems (e.g. perylene , PTCDA and its derivatives, naphthalene derivatives, hexabenzocorones)
  • Metal complexes (for. Example, phthalocyanines , or Alq3 , Beq2)
  • dendritic molecules , starburst molecules [e.g. B. 4,4 ', 4 "-tris (N, N-diphenyl-amino) triphenylamine (TDATA)]
  • heterocyclic oligomers (e.g. oligothiophenes, oligophenylenevinylenes)

Examples of conjugated polymers:

Such a classification proves to be favorable for an overview under the aspect of the suitability of the substance classes for various research and application areas of electronics , because while mono- and oligomers with a low molar mass are not only used in plastic electronics due to their small size, but also as functional elements for molecular nanoelectronics ( molecular electronics are) conjugated polymers in substantially the plastic electronics are limited.

Uses

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

  • Molecular electronics
  • Organic electronics (plastic electronics)
  • Organic pigments : In particular, pentacene and perylene derivatives and phthalocyanines act as intensive colorants due to their chromophoric systems . Due to their widespread industrial applications, primarily as printing inks, car paints or for coloring plastics, they are commercially produced in large quantities by the paint industry and are also available in retail outlets as artists' colors.

Individual evidence

  1. ^ E. Riedel: General and Inorganic Chemistry. 6th edition, de Gruyter, Berlin, New York 1994, p. 104
  2. ^ I. Ledoux et al .: Properties of novel azodyes containing powerful acceptor groups and thiophene moiety . In: Synthetic Metals 115, 2000, pp. 213-217.
  3. ↑ JL Brédas, JP Calbert, DA da Silva Filho, J. Cornil: Organic semiconductors: A theoretical characterization of the basic parameters governing charge transport . In: PNAS 99, 2002, pp. 5804-5809.
  4. ↑ J. Kalinowski, W. Stampor, P. Di Marco, V. Fattori: Electro absorption study of excited states in hydrogen-bonding solids: epindolidiones and linear trans-quinacridone . In: Chem. Phys. 182, 1994, 341-352.
  5. ↑ PJF de Rege, SA Williams, MJ Therien: Direct evaluation of Electronic Coupling Mediated by Hydrogen Bonds: Implications for Biological Electron Transfer . In: Science 269, 1995, pp. 1409-1412).
  6. ^ A b N. Karl: Charge carrier transport in organic semiconductors . In: Synthetic Metals 133-134, 2003, 649-657.
  7. ↑ a b C. D. Dimitrakopoulos, PRL Malenfant: Organic Thin Film Transistors for Large Area Electronics . In: Advanced Materials 14, 2002, pp. 99-118.
  8. ↑ Lexicon of Physics: Hopping Mechanism (Spektrum.de, accessed on March 17, 2018)
  9. ↑ CK Chiang, CR Fincher, YW Park, AJ Heeger, H. Shirakawa, EJ Louis, SC Gau, Alan G. MacDiarmid: Electrical Conductivity in Doped Polyacetylene . In: Physical Review Letters . tape 39 , no. 17 , 1977, pp. 1098-1101 , doi : 10.1103 / PhysRevLett.39.1098 .
  10. ^ AB Kaiser: Electronic transport properties of conducting polymers and carbon nanotubes . In: Reports on Progress in Physics 64, 2001, pp. 1-49.
  11. ^ HE Katz, Z. Bao, S. Gilat: Synthetic chemistry for Ultrapure, Processable, and High-Mobility Organic Transistor Semiconductors . In: Accounts of Chemical Research 34, 2001, pp. 359-369.
  12. ^ Y. Shirota: Organic materials for electronic and optoelectronic devices . In: Journal of Materials Chemistry. 10, 2000, pp. 1-25.
  13. ↑ W. Herbst, K. Hunger: Industrial organic pigments. Production, properties, application . 2nd edition, Wiley-VCH, Weinheim 1995, ISBN 3-527-28744-2 , p. 8 f.