Compositions – Electrically conductive or emissive compositions
Reexamination Certificate
2002-06-10
2004-02-17
Kopec, Mark (Department: 3637)
Compositions
Electrically conductive or emissive compositions
C528S373000, C528S377000, C252S301320
Reexamination Certificate
active
06692663
ABSTRACT:
TECHNICAL FIELD
The present disclosure generally relates to conductive polymer compositions produced by a solvent exchange method, and more specifically to conductive polymer compositions produced by a method that involves the exchange of the water in a polythiophene dispersion with a specific mixture of organic solvents. The present disclosure also relates to methods for producing such compositions. This disclosure further pertains to the application of these compositions to fabricate a variety of articles, such as coatings, and to making and using the same in the fabrication of electronic and opto-electronic devices.
DESCRIPTION OF THE RELATED ART
Conductive polymers (CPs) have received considerable attention in recent years due to their potential applications in a variety of electronic devices. The realization that organic polymeric materials could be made to exhibit electrical conductivity by doping was first discovered in 1977 [H. Shirakawa, E. J. Louis, A. G. MacDarimid, C. K. Chang and A. J. Heeger, J. Chem. Soc. Chem. Comm. 579 (1977)]. This discovery was considered such a breakthrough that the Nobel Prize in Chemistry was awarded to these researchers (McDiarmid, Heeger and Shirakawa) in 2000 for this work. CPs are presently used in commercial products as anti-static coatings on plastics such as photographic film and electronic packaging materials. Other applications include solid electrode capacitors, through-hole plating of printed circuit boards, coatings for cathode ray tubes (to prevent dust attraction), hole injecting layers on indium tin oxide (ITO) substrates for electroluminescent devices, and sensors. Future applications such as an ITO replacement leading to completely flexible, organic electronic devices will require improvement in conductivity without sacrificing other properties such as optical transparency.
A variety of conductive polymers have been prepared and characterized, and several are commercially available such as Baytron® P from Bayer and Panipol® from Uniax. Of the different CP families, [i.e. polyacetylenes, polyphenylenes, poly(p-phenylenevinylene)s, polypyrroles, polyanilines, and polythiophenes] polythiophenes are arguably the most stable-thermally and electronically [(“Handbook of Oligio- and Polythiophenes”, D. Fichou, Editor, Wiley-VCH, New York (1999), J. Roncali,
Chem. Rev.,
97, 173 (1997), A. Kraft, A. C. Grimsdale and A. B. Holmes,
Angew. Chem.,
110, 416 (1998), J. Roncali,
J. Mater. Chem.,
9, 1875 (1999), J. Roncali,
Annu. Rep. Prog. Chem. Sec. C.,
95, 47 (1999), A. J. Heeger,
Synth. Met.,
55-57, 3471 (1993) and G. Kobmehl and G. Schopf,
Adv. Polym. Sci.,
129, 1 (1996)]. The Baytron® P product is a poly 3,4-ethylenedioxythiophene/polystyrene sulfonate (PEDOT/PSS) composition available as an aqueous dispersion containing ~1.3% solids. This aqueous dispersion is typically used to prepare coatings on various substrates. Baytron® P coatings exhibit no change in conductivity after 1000 hours in air at 100° C. and can survive intact at temperatures as high as 200° C., albeit for shorter exposure periods. It is prepared from 3,4-ethylenedioxythiophene (EDT) in aqueous or predominately aqueous media in the presence of polystyrenesulfonic acid (PSS, dopant) using an oxidant such as iron trichloride [L. B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik and J. R. Reynolds,
Adv. Mater.,
12(7), 481 (2000)]. Coatings of Baytron® P have been reported to exhibit a wide range of surface resistance, depending upon thickness. It is well known for Baytron® P, as well as other CP based coatings, that the surface conductivity will increase with increasing coating thickness while the optical transmission will decrease. In most coating applications, the coatings must exhibit a specific combination of electrical conductivity, optical transparency and environmental stability (i.e. stability to moisture and oxygen) to be useful. The coating must exhibit good adhesion to the substrate as well. The appropriate balance or combination of these properties is of critical importance; thus, a means for improving this combination of properties would represent a significant advancement and enable new applications for these materials.
One approach to improve the electrical conductivity of polythiophenes is by the use of organic additives. It has been shown that certain additives, when mixed with Baytron® P aqueous dispersion and subsequently used to make a coatings, can produce an increase in the electrical conductivity (i.e. decrease in surface resistivity), however a high temperature treatment (~200° C.) is also required [Jonas et al, U.S. Pat. No. 5,766,515, (1998) to Bayer AG]. The high temperature treatment is a major disadvantage since certain-substrates cannot tolerate this step. No explanation of the mechanism associated with conductivity enhancement is offered; thus, it is impossible to elucidate what additives may bring about this increase in electrical conductivity.
Another method has involved a solvent exchange process in which most or all of the water present in a Baytron®P aqueous dispersion is exchanged with an organic solvent (see U.S. Ser. No. 09/999,171; 60/298,174 and 60/269,606). Employing the solvent exchange method also brings about a fundamental change to the material that results in significant improvement in the combination of electrical conductivity, optical transparency environmental stability and adhesion characteristics to a variety of substrates. Consequently, this method enables the solvent exchanged product to meet specifications for a variety of applications that the aqueous based precursor cannot meet.
Surface resistance of CP based coatings is typically measured using a four-point probe device. Certain other measurements must also be performed, such as coating thickness, in order to calculate volume resistivity. The volume resistivity is calculated using the following equation:
Volume resistivity=(&pgr;/ln2)(
k
)(
t
)(surface resistance in ohm/square)
Wherein “t” is the coating thickness, measured in centimeters (cm), “k” is the geometrical correction factor, and “ln2” is the natural log of 2. The constant k is related to the coating thickness, probe spacing and sample size. Due to the variables associated with these measurements, quantitative comparison between measurements of the volume resistivity of coatings performed using different devices and different operators can be problematic.
Organic polymers that are intrinsically conductive typically contain sp
2
hybridized carbon atoms that have (or can be adapted to have) delocalized electrons for storing and communicating electronic charge. Some polymers are thought to have conductivities neighboring those traditional silicon-based and metallic conductors. These and other performance characteristics make such conductive polymers desirable for a wide range of applications. See Burroughes, J. H. et al. (1986)
Nature
335:137; Sirringhaus, H. et al. (2000)
Science,
290, 2123; Sirringhaus, H. et al. (1999)
Nature
401: 2; and references cited therein, for example.
There is recognition that many conductive polymers can be used to coat a wide range of synthetic or natural articles such as those made from glass, plastic, wood and fibers to provide an electrostatic or anti-static coating. Typical coatings can be applied as sprays, powders and the like using recognized coating or printing processes.
However, there is increasing understanding that many prior conductive polymers are not useful for all intended applications. For example, many of such polymers are not sufficiently conductive or transparent for many applications. In particular, many suffer from unacceptable conductivity, poor stability, and difficult processing requirements. Other shortcomings have been reported. See e.g, the U.S. Pat. Nos. 6,084,040 and 6,083,635. Efforts have focused on improving properties of conductive polymers such as solubility or conductivity. However, for many applications, having an improvement in only one property, such as elec
Freitag Dieter
Mulford Karen
Ryu Jae
Zhou Qingye
Elecon, Inc.
Kopec Mark
Miller Raymond A.
Pepper Hamilton LLP
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