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Organic semiconductor

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Organic semiconductor

STM image of self-assembled Quinacridone on a Gold (111) surface.

An organic semiconductor is an 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 organic metals.

Contents

  • History 1
  • Processing 2
  • Characterization 3
  • Charge transport in disordered organic semiconductors 4
  • See also 5
  • References 6
  • Further reading 7

History

In 1862, Henry Letheby obtained a partly conductive material by anodic oxidation of aniline in sulfuric acid. The material was probably polyaniline.[1] 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.[2] This finding indicated that organic compounds could carry current. In 1972, researchers found metallic conductivity in the charge-transfer complex TTF-TCNQ.

An organic polymer voltage-controlled switch from 1974. Now in the Smithsonian Chip collection[3]
Superconductivity

in charge-transfer complexes was first reported in the Bechgaard salt (TMTSF)2PF6 in 1980.[4]

In 1973 Dr. Smithsonian Institution.

In 1977, Shirakawa et al. reported high conductivity in oxidized and iodine-doped polyacetylene.[5] They received the 2000 Nobel prize in Chemistry for "The discovery and development of conductive polymers".[6] Similarly, highly-conductive polypyrrole was rediscovered in 1979.[7]

Rigid-backbone organic semiconductors are now-used as active elements in 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.[8] 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.[9][10] 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).[11]

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.[9][12] 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 quantum mechanical tunnelling.[13] 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.[14] 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[15] 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 pentacene is frequency-dependent and provided evidence that this behavior is due to its polycrystalline structure and hopping conduction.[16]

See also

References

  1. ^ The Nobel Prize in Chemistry, 2000: Conductive polymers, nobelprize.org
  2. ^ Herbert Naarmann "Polymers, Electrically Conducting" in Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH, Weinheim. doi:10.1002/14356007.a21_429
  3. ^ Smithsonian Chip collection
  4. ^ Jérome, D.; Mazaud, A.; Ribault, M.; Bechgaard, K. (1980). "Superconductivity in a synthetic organic conductor (TMTSF)2PF 6". Journal de Physique Lettres 41 (4): 95.  
  5. ^ Shirakawa, Hideki; Louis, Edwin J.; MacDiarmid, Alan G.; Chiang, Chwan K.; Heeger, Alan J. (1977). "Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH) x". Journal of the Chemical Society, Chemical Communications (16): 578.  
  6. ^ "Chemistry 2000". Nobelprize.org. Retrieved 2010-03-20. 
  7. ^ Diaz, A. F.; Kanazawa, K. Keiji; Gardini, Gian Piero (1979). "Electrochemical polymerization of pyrrole". Journal of the Chemical Society, Chemical Communications (14): 635.  
  8. ^ Sirringhaus, H.; Sele, C. W.; von Werne, Timothy; Ramsdale, C. (2007). "Manufacturing of Organic Transistor Circuits by Solution-based printing". In Hadziioannou, Georges; Malliaras, George G. Semiconducting polymers: Chemistry, Physics and Engineering 2 (2nd ed.). Wiley-VCH. pp. 667–694.  
  9. ^ a b Brütting, Wolfgang (2005). Physics of organic semiconductors. Wiley-VCH.  
  10. ^ Masenelli, B.; S. Callard; A. Gagnaire; J. Joseph (2000). "Fabrication and characterization of organic semiconductor-based microcavities". Thin Solid Films 364 (1-2): 264–268.  
  11. ^ Salaneck, W. R.; Antoine Kahn (2002). Conjugated polymer and molecular interfaces. CRC Press.  
  12. ^ Dost, René; Das, Arindam; Grell, Martin (2007). "A novel characterization scheme for organic field-effect transistors". Journal of Physics D: Applied Physics 40 (12): 3563–3566.  
  13. ^ Nabok, Alexei (March 2005). Organic And Inorganic Nanostructures (2 ed.). Artech House Publishers.  
  14. ^ Hirsch, J. (1979). "Hopping transport in disordered aromatic solids: a re-interpretation of mobility measurements on PKV and TNF". Journal of Physics C: Solid State Physics 12 (2): 321.  
  15. ^ Li, L.; Meller, G.; Kosina, H. (2007). "Temperature and field-dependence of hopping conduction in organic semiconductors". Microelectronics Journal 38 (1): 47–51.  
  16. ^ Lenski, Daniel R.; Adrian Southard; Michael S. Fuhrer (2009). "Frequency-dependent complex conductivity of an organic thin-film transistor". Applied Physics Letters 94 (23): 232103–3.  

Further reading

  • An Overview of the First Half-Century of Molecular Electronics by Noel S. Hush, Ann. N.Y. Acad. Sci. 1006: 1–20 (2003).
  • Organic Semiconductors by Yoshikuko Okamoto and Walter Brenner, Reinhold (1964). Chapt.7, Polymers—multiple reports of oxidized polyacetylenes with conductivities less-than 1 ohm/cm.
  • Electronic Processes in Organic Crystals and Polymers, 2 ed. by Martin Pope and Charles E. Swenberg, Oxford University Press (1999), ISBN 0-19-512963-6
  • Handbook of Organic Electronics and Photonics (3-Volume Set) by Hari Singh Nalwa, American Scientific Publishers. (2008), ISBN 1-58883-095-0
  • Semi-Conducting Polymers and Optoelectronics - Richard Friend, Cavendish Professor, Cambridge Freeview video by the Vega Science Trust.
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