Intrinsically conducting polymers (ICPs) have attracted much attention because of their potential applications in organic light emitting diodes (OLEDs), printed circuits, chemical sensors, electronic switches, rechargeable batteries, electrolytic capacitors, smart windows, EMI shielding and electrostatic charge dissipation (ESD) coatings. In spite of the thousands of papers published and patents filed in this field, the number of commercial applications of ICPs is still small. Poor long term stability and lack of reasonable processing methods have been the major showstoppers to the commercialization of ICPs. TDA's research on ICPs has focussed on improving the solvent processability of conducting polymers with proven stability, including poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole.
Materials that combine electronic conductivity with optical clarity are sought for the fabrication of flat panel displays and other electronic devices. PEDOT has excellent transparency in the visible region, good electrical conductivity, and environmental stability. Unfortunately PEDOT, like most conducting polymers, is infusible and insoluble and therefore difficult to process in a thin-film form or in other shapes. Lack of processability has been a major impediment to the commercial acceptance of this polymer. A water dispersion of PEDOT doped with poly(styrenesulfonate) (PSS) is available from H.C. Starck under the trade name of Baytron® P (see figure below right for structure).
The PEDOT/PSS dispersion is today the most promising and most widely used hole injecting material in OLED research and development. It has been observed that the deposition of a thin layer of PEDOT/PSS on the ITO surface increases the maximum luminance of OLED devices up to three orders of magnitude, reduces the threshold voltage by more than 50%, and increases the lifetime by a factor of 10 (Pichler 1997, Moliton 2001). Furthermore, the PEDOT /PSS layer acts as a physical barrier against the many defect sites known to be present in ITO.
PEDOT/PSS is commercially available in a number of grades from H.C. Starck (a Bayer subsidiary) as a dispersion in water (typically at 1-3% wt. solids) under the trade name of Baytron® P. Baytron® P solutions can be spin coated or inkjet printed, and devices that contain Baytron® P as the HIL have shown dramatically improved brightness, lower driving voltages, and fewer malfunctions. Thus, most of the current state-of-the-art OLEDs that are near commercialization contain a layer of Baytron® P. However, Baytron® P has several drawbacks. The dispersion is very acidic because of the high loading of PSS in its acidic form, which can cause problems during application, and does not wet organic substrates without a high loading of binders or complex formulation. Unfortunately, binders decrease the conductivity of the resulting film. Moreover, many electronic applications cannot tolerate the presence of water.
During the past three years TDA has successfully developed a new class of polymers that have the electronic and optical properties of PEDOT but can be processed from non-acidic organic solutions. TDA’s materials have good mechanical properties, low acidity, and wet organic substrates without the use of binders or additives. The new materials are block copolymers of PEDOT and a flexible polymer such as polyethers, polysiloxanes, polyesters, or polyacrylates. The blocks of PEDOT and the flexible polymer are covalently bonded in a variety of ways that lead to the formation of different geometries among the blocks (see figure at right). TDA has successfully prepared block copolymers of PEDOT with poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG), polydimethylsiloxane (PDMS), and acrylic derivatives.
To date, our most successful copolymers are our AedotronTM polymers, and we have recently developed a line of organic processable oligomers of PEDOT, called OligotronTM materials, which have reactive methacrylate endgroups. AedotronTM polymers are dispersible in organic solvents such as nitromethane and propylene carbonate. Films cast from these solutions are smooth and scratch-resistant and have good adhesion properties on many substrates. TDA’s polymers have optical properties similar to PEDOT, and thin films appear transparent blue with an optical transmittance up to 75%. The figure at right shows a photograph of spin-cast thin films on polyester and glass; the samples were placed over TDA’s logo to show their transparency.
Depending on what anion is used to dope these copolymers, bulk conductivity is measured in the 0.1-1.0 S/cm range (AedotronTM-C), or 0.0001-0.001 S/cm (AedotronTM-P). Cast films can exhibit a wide range of sheet resistance depending on film thickness, preparation method and material choice. The graph at right shows a plot of the typical visible transmittance of cast thin films of AedotronTM-C that were dip coated on polycarbonate from a propylene carbonate dispersion and dried at 80 deg. C. The sheet resistance of each film is also shown on the graph (click to enlarge).
TDA's block copolymer design allows great flexibility in tailoring bulk material properties. By choosing the proper ratio and length of the flexible polymer segments and PEDOT segments, the conductivity of our material can be tuned in a wide range from 10-5 to 10 S/cm (see graph below right, click to enlarge). At low PEDOT/PEG ratio the bulk conductivity of our copolymers is ideal for OLED applications. TDA’s polymerization method produces reproducible materials with consistent electronic and solution properties within each batch and among batches. The data cluster at ca. 58 wt.% EDOT demonstrates TDA's batch to batch reproducibility, which is quite good.
Our initial goal in this project was to make a transparent conducting polymer that was as highly conducting as possible. In communicating with our many new partners working on various applications, it is apparent that high conductivity is actually undesirable in many cases. For example, electronic grade Baytron® P is commonly employed as a hole-injection layer in OLED and PLED displays. Many display companies are using ink-jet printing to pattern the electrode and pixel layout of each layer in a display. As these patterns and pixels become smaller (features on the order of 10 microns), cross-talk between pixels through the hole injection layer becomes a problem with higher conductivity materials. Because TDA can directly control the conductivity of our copolymers simply by varying the ratio of block segments, we can readily prepare samples to suit a variety of needs for a variety of applications. We are just beginning to step back and look at our lower conductivity compositions, but based on the excitement of our newest partners in displays and ESD, these materials hold a great deal of promise. We are also continuing to improve our dispersability in more coatings friendly solvents like methylethylketone (MEK), and polypyrrole-containing analogs of these materials are showing promise as processable and biocompatible materials.
Download a copy of Shawn Sapp's presentation from the Xth International Conference on Intrinsically Conducting Polymers in Boston, MA (Summer 2003):
"TDA-Boston-2003.pdf" (Adobe Acrobat File, 226 KB) - Download Now