Conducting polymers

Details Conducting polymers Conducting polymers

2001-11-06

2004-04-06

Kopec, Mark (Department: 1751)

Compositions

Electrically conductive or emissive compositions

C252S06230Q, C428S620000, C257S040000

Reexamination Certificate

active

06716371

ABSTRACT:

This invention concerns new compounds and the use of such compounds in organic compositions as electrically conducting and electronically active materials. In particular it relates to the use of sidechain polymer systems comprising covalently linked polymer chains bearing electrically active organic substitution.
It is recognised that a variety of organic compounds can be made to conduct electricity. The mechanism of electrical conduction may be either ionic conduction or electronic conduction. Ionic conduction is most important in blends of polymer materials with ionic compounds which may be regarded as solutes in the polymer. Movement of the ions under an applied electric field results in a flow of electric current. Typical materials in which ionic conductivity is important include blends of lithium salts in polymers such as poly(ethylene oxide).
Electronic conduction occurs in organic compounds even in the absence of addition of ionic salts. Various classes of organic compounds show this class of conduction, including conjugated polymers such as poly(acetylene), poly(phenylene vinylene) and poly(thiophene). This class of conduction is also found in molecular solids of low molecular mass such as N,N′-diphenyl-N,N′-ditolylbenzidine and aluminium tris 8-hydroxyquinolinate. Polymers which do not posses a conjugated main chain structure can also show electronic conductivity. Such polymers include poly(vinyl carbazole) as a known example.
Electronic conduction in these organic compounds relies on the insertion of electrical charge into the materials. Such electrical charge may be introduced in a variety of known ways, including doping with an oxidising or reducing agent, by chemical doping with a charge transfer reagent, or by direct insertion of positive or negative electric charge from a conducting electrode. Known examples of such introduction of charge include the chemical doping of poly(acetylene) with the oxidising agent iodine, doping of poly(vinyl carbazole) with the charge transfer reagent trinitrofluorenone, and the injection of charge into evaporated thin films of 3-biphenyl-5-(4-t-butylphenyl)oxadiazole from a low work function electrode such as a magnesium metal electrode by application of a negative potential to the electrode. This injection of charge may also be regarded as an electrochemical reduction of the conducting material, or as electrochemical doping of the material. Other electronically conducting organic compounds may be subjected to charge injection by a positive potential, which is commonly applied via an electrode composed of a high work function metal such as gold. Such charge injection is commonly described as injection of positive charges denoted holes, and this description is understood to be equivalent to the extraction of electrons from the conducting material. A further route to generation of charge in organic solids is the ionisation of molecules of the solid under the influence of an electric field or of incident light, or both. In this case, an electron is removed from a molecule of the material, and captured by another molecule, or by a different part of the molecule. In this way, a pair of separated positive and negative charges is produced, movement of either or both of which may contribute to conduction in the material.
Conduction within electronically conducting organic solids involves, in practical materials, the transfer of electrical charge from one molecule to another. Such transfer of charge is described by a charge hopping or charge tunnelling mechanism which may allow an electron to overcome the energetic barrier between different molecules or molecular sub-units. In systems such as conjugated polymers including poly(acetylene) and poly(phenylenevinylene) charge can also flow along the conjugated chain by movement of charged discontinuities in the regular bonding sequence of the polymer. In such cases, charge will normally be transferred through the bulk sample by a large number of individual molecules, and hopping or tunnelling remains an important mechanism.
The use of organic electrically conducting materials as semiconductors in the fabrication of electronic devices has been explored by many investigators. Field effect transistors have been fabricated from organic compounds such as poly(acetylene), pentacene, and poly(phenylenevinylene). Light emitting diodes have been demonstrated using a wide range of organic and molecular solids, including N,N′-diphenyl-N,N′-ditolylbenzidine, aluminium tris 8-hydroxyquinolinate, 3-biphenyl-5-(4-t-butylphenyl)oxadiazole and poly(phenylene vinylene). Preferably, such light emitting diodes are fabricated using at least two organic semiconductors which transport charge respectively by transport of holes and electrons. Such multilayer diodes attain higher efficiency than single layer devices, by trapping the charge carriers in the device until carrier recombination and light emission can take place. Photovoltaic devices have been fabricated using compounds such as copper phthalocyanine and perylene bisbenzimidazole. Photoconductive materials have been prepared and are recommended for use as sensitive layers in electrophotography, photocopying and printing applications. Such photoconductive compositions include compositions based on poly(vinylcarbazole) doped with trinitrofluorenone.
Conducting polymers may be used as components of optical storage and switching equipment by using the photorefractive effect. Photorefractive layers combine the capacity for photogeneration of charge, transport of charge carriers by diffusion or under an applied field and a linear electro-optic coefficient. Such properties may be obtained by addition of selected dopants to a conducting polymer. Suitable dopants for inducing the capability for photogeneration of charge include C
60
fullerene. Suitable dopants to provide a linear electro-optic coefficient include dimethylamino nitrostilbene.
Many shortcomings have been identified in organic conducting materials described in the prior art. Among these shortcomings, chemical stability is an important parameter. Poly(acetylene), which shows a high electrical conductivity in the doped state, is converted to a non-conducting product on exposure to air, and must be handled and used in an inert atmosphere. Poly(phenylenevinylene) is believed to undergo oxidation of the conjugated double bonds in the main chain, yielding non-luminescent oxidation products. Further degradation mechanisms have been identified or proposed for poly(phenylenevinylene) when it is incorporated in devices, including sensitivity to ambient ultraviolet radiation. Molecular solids such as N,N′-diphenyl-N,N′-ditolylbenzidine may undergo crystallisation, changes in morphology, or melting in operating devices. These effects may cause premature failure or loss in efficiency of the device.
A further important shortcoming which is common in many organic conducting materials rests in the difficulty and high cost of processing the materials. Both poly(acetylene) and poly(phenylenevinylene) are insoluble and infusible materials which are prepared for use in devices by use of a precursor polymer route. Typically a soluble precursor polymer is first synthesised and deposited onto a prepared substrate. A combination of heat and vacuum is then used to chemically convert the precursor polymer into the target product, usually with elimination of smaller volatile molecules of one or more by-products. Processing of the target polymer to a dense film in bulk quantities requires critical control of this step which moreover entails the use of costly and time consuming vacuum processing stages. The final polymer is difficult to further process, and steps such as patterning and lithography are difficult to accomplish. Many attempts have been made to provide solution processable organic conductors. Substitution of alkyl, alkoxy and other flexible organic radical onto the polymers is understood to improve their solubility and processibility. Such substitution, however, may also commonly red

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