Organic electronics

Melanin voltage-controlled switch, an "active" organic polymer electronic device from 1974.

Organic electronics, plastic electronics or polymer electronics, is a branch of electronics that deals with conductive polymers, plastics, or small molecules. It is called 'organic' electronics because the polymers and small molecules are carbon-based, like the molecules of living things. This is as opposed to traditional electronics (or metal electronics) which relies on inorganic conductors such as copper or silicon.

Polymer electronics are laminar electronics, that also includes transparent electronic package and paper based electronics.

In addition to organic charge transfer complexes, technically, electrically conductive polymers are mostly derivatives of polyacetylene black (the "simplest melanin"). Examples include polyacetylene (PA; more specificially iodine-doped trans-polyacetylene); polyaniline (PANI), when doped with a protonic acid; and poly(dioctyl-bithiophene) (PDOT).

History

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, it was discovered that polycyclic aromatic compounds formed semi-conducting charge-transfer complex salts with halogens. This finding indicated that organic compounds could carry current. High conductivity of 1 S/cm in linear backbone polymers (in a iodine-"doped" and oxidized polypyrrole black) was reported in 1963. Likewise, an actual organic-polymer electronic device was reported in the journal Science in 1974. This device is now in the "Smithsonian Chips" collection of the American Museum of History (see figure).

However, Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa are often credited for the "discovery and development" of conductive polymers and were jointly awarded the Nobel Prize in Chemistry in 2000 for their 1977 report of similarly-oxidized and iodine-doped polyacetylene.

Conduction mechanisms in such materials involve resonance stabilization and delocalization of pi electrons along entire polymer backbones, as well as mobility gaps, tunneling, and phonon-assisted hopping.

Technology for plastic electronics on thin and flexible plastic substrates was developed at Cambridge University’s Cavendish Laboratory in the 1990s. In 2000, Plastic Logic was spun out of Cavendish Laboratory to develop a broad range of products using the plastic electronics technology.

Features

Main article: Printed electronics

Conductive polymers are lighter, more flexible, and less expensive than inorganic conductors. This makes them a desirable alternative in many applications. It also creates the possibility of new applications that would be impossible using copper or silicon.

Organic electronics not only includes organic semiconductors, but also organic dielectrics, conductors and light emitters.

New applications include smart windows and electronic paper. Conductive polymers are expected to play an important role in the emerging science of molecular computers.

In general organic conductive polymers have a higher resistance and therefore conduct electricity poorly and inefficiently, as compared to inorganic conductors. Researchers currently are exploring ways of "doping" organic semiconductors, like melanin, with relatively small amounts of conductive metals to boost conductivity. However, for many applications, inorganic conductors will remain the only viable option.

Organic electronics can be printed.

Organic electronic devices

Organics-based flexible display

A 1972 paper in the journal Science proposed a model for electronic conduction in the melanins. Historically, melanin is another name for the various oxidized polyacetylene, polyaniline, and Polypyrrole "blacks" and their mixed copolymers, all commonly-used in present day organic electronic devices. For example, some fungal melanins are pure polyacetylene. This model drew upon the theories of Neville Mott and others on conduction in disordered materials. Subsequently, in 1974, the same workers at the Physics Department of The University of Texas M. D. Anderson Cancer Center reported an organic electronic device, a voltage-controlled switch.

Their material also incidentally demonstrated "negative differential resistance", now a hall-mark of such materials. A contemporary news article in the journal Nature noted this materials "strikingly high conductivity". These researchers further patented batteries, etc. using organic semiconductive materials. Their original "gadget" is now in the Smithsonian's collection of early electronic devices.

This work, like that the decade-earlier report of high-conductivity in a polypyrrole, was "too early" and went unrecognized outside of pigment cell research until recently. At the time, few except cancer research institutes were interested in the electronic properties of such polymers, which are applicable to the treatment of melanoma.

Plastic solar cells

Main articles: Organic photovoltaics, Photovoltaics, and Low cost solar cell

Organic solar cells could cut the cost of solar power by making use of inexpensive organic polymers rather than the expensive crystalline silicon used in most solar cells. What's more, the polymers can be processed using low-cost equipment such as ink-jet printers or coating equipment employed to make photographic film, which reduces both capital and manufacturing costs compared with conventional solar-cell manufacturing..

Silicon thin film solar cells on flexible substrates allow a significant cost reduction of large-area photovoltaics for several reasons :

1. The so-called 'roll-to-roll'-deposition on flexible sheets is much easier to realize in terms of technological effort than deposition on fragile and heavy glass sheets.
2. Transport and installation of lightweight flexible solar cells also saves cost as compared to cells on glass.

Inexpensive polymeric substrates like polyethylene terephtalate (PET) or polycarbonate (PC) would be a way out towards further cost reduction in photovoltaics. Protomorphous solar cells prove to be a promising concept for efficient and low-cost photovoltaics on cheap and flexible substrates for large-area production as well as small and mobile applications.

One beauty of printed electronics is that the different electrical and electronic components can be printed on top of each other, saving space and increasing reliability and sometimes they are all transparent. One ink must not damage another, and low temperature annealing is vital if low-cost flexible materials such as paper and plastic film are to be used. There is much sophisticated engineering and chemistry involved here, with iTi, Pixdro, Asahi Kasei, Merck, BASF, HC Starck, Hitachi Chemical and Frontier Carbon Corporation among the leaders.

See also

• Annealing
• Bioplastic
• Carbon nanotube
• Circuit deposition
• Conductive ink
• Disposable electronics
• Flexible display
• Laminar
• Melanin
• Nokia Morph
• Organic field-effect transistor
• Organic semiconductor
• Organic LED
• Photodetector
• Printed electronics
• Radio frequency identification
• Radio tag
• Spin coating

External links

• oeindex - an index for subfields of Organic Electronics

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).
• Hari Singh Nalwa (2008), Handbook of Organic Electronics and Photonics (3-Volume Set), American Scientific Publishers. ISBN 1-58883-095-0