Highly Conductive Coatings

Generally speaking, polymers are insulators. However, there is a special class of polymers - the intrinsically conductive polymers - that have conductivity levels between those of semiconductors and metals (from 10^-4 to 10³ S/cm). The combination of metal and polymer properties opens up new opportunities in many applications, particularly in the electronics industry.

With PEDOT (poly(3,4-ethylenedioxythiophene)) - available under the trade name Clevios™ - Heraeus has developed the latest generation of conductive polymers which are characterized by outstanding properties:

For high conductive coatings Clevios™ PH 1000 or its ready to use formulation, Clevios™ FE-T can be used. These materials offer not only high conductivities but also exceptional levels of transparency.

A conductivity of 900-1000 S/cm (approx. 200 Ohm /sq) can be reached by using Clevios™ PH 1000 together with a conductivity enhancement agent such as DMSO or ethylene glycol. The ready to use formulation CLEVIOS™ FE-T is water-based and contains a polyester dispersion for force dry applications.

Coating formulations have been optimized for individual substrates, such as A-PET, PET, polycarbonate, glass for different wet film thicknesses and surface resistivities.

Coating can be achieved by standard printing processes, such as slit die, flexographic, screen or gravure methods. Also brushing, spraying, spin-coating or roller coating can be used.

Touchscreens are becoming more and more popular as HMIs (human-machine interfaces) not only for mobile phones and computers but everywhere mechanical switches are used today. To construct a touchscreen a transparent electrode is necessary. The read-out electronics can be based on resistive or capacitive effects. Today the transparent electrode is mostly made of ITO (indium tin oxide), vacuum sputter coated glass or plastic film. However, indium is a rare metal which is in short supply and its price has been rising steadily. As ITO is a glass-like brittle material the most common failure mechanism of touchscreens is the formation of micro cracks.

Clevios™ PEDOT:PSS is an inherently conductive polymer and offers several advantages over ITO:

The highly conductive material Clevios™ PH1000 or its formulation Clevios™ FE-T can be used for touchscreens as an alternative to ITO for the coating of films. The formulation shows high stability in environmental tests at 85 °C / 85% rH:

The coated films can be patterned by a simple etching process with Clevios™ Etch. The resulting patterns are nearly invisible. See also Clevios™ Etch.

Coatings of high conductive PEDOT:PSS dispersions based on Clevios™ PH 500, PH510, PH 1000 or formulations such as Clevios™ FE-T are all cost-efficient alternatives to ITO for transparent conductive electrode layers in OLEDs and Organic Solar Cells. To compensate for ohmic losses in large area devices conductive polymers can be combined with printed metal-ink bus-bar lines.

The thickness of the Clevios™ coatings after spin-coating onto a substrate is determined by the following parameters:

• spin speed
• acceleration
• spin time
• design of the spin-coater
• substrate size
• quality of the pre-conditioning of the substrate surface.

The pre-conditioning of the substrate strongly affects the uniform dispersion of Clevios™ onto the substrate surface. Therefore, no general rule can be given to predict which specific coating thickness will result at a given spin speed. The spin-curves were obtained on cleaned and ozonized glass substrates of size 5.0 x 5.0 cm² . For spin-coating a Carl Süss Spin-Coater RC 8 with a 3” gyrset-lid was used. About 1-2 ml of Clevios™ dispersion was deposited onto the substrate by using a Pasteur pipette (Hilgenberg). The polymer dispersion was distributed manually across the entire substrate surface prior to spin-coating. In general, the layer-thickness was found to be homogeneous across the substrate, with a somewhat increased thickness at the edges of the substrate. After spin-coating, the coatings of Clevios™ including 5% DMSO are dried for 15 min on a hot plate set to 130 °C.