УДК 541.64:537.3
PEDOT - PROPERTIES AND APPLICATIONS
© 2014 г. Wilfried Lövenich
Heraeus Precious Metals GmbH & Co. KG, Chempark Leverkusen, Building B202, 51368 Leverkusen, Germany e-mail: wilfried.loevenich@heraeus.com
Abstract — Poly(3,4-ethylenedioxythiophene) (PEDOT) is an intrinsically conductive polymer that is used in a wide range of applications such as antistatics, capacitors, touch panels, organic light emitting diodes, organic solar cells and printed electronics. This paper describes the characteristic parameters of 31 different commercially available PEDOT dispersions. It shows how the PEDOT dispersions have been tailored to each of those applications and thereby the opportunities and limitations of this material.
DOI: 10.7868/S2308114714010063
INTRODUCTION
The first thoroughly investigated intrinsically conductive polymer (ICP), polyacetylene, was discovered in 1977 [1, 2]. Due to the availability of free charge carriers in ICPs, they can conduct current in contrast to traditional polymeric insulators such as polyethylene or polypropylene [3]. The free charge carriers are generated in ICPs redox reactions, most often by oxidation . As a result, the electrical behavior of these materials changes from a typical semiconductor to a conductor. For example, when neutral polyacetylene is oxidized by AsF5, the conductivity increases from 10~5 to 200 S/cm [4]. Similar to inorganic semiconductors this process is often referred to as doping, even though it is actually a chemical oxidation. The conductivity of the doped polymer is due to the mobility of the free holes or electrons that are a result of the oxidation or the reduction, respectively.
Although the conductivity of ICPs is much lower than that of metals, they combine electrical conductivity with the advantages of polymers such as easy processing, flexibility and drying at low temperatures. Because of these features they have found a wide range of industrial application in the last 30 years.
Polyacetylene has never been fully commercialized, since the material is unstable under ambient conditions. However, other ICPs are very stable. This article describes the highly stable ICP poly(3,4-ethylene-dioxythiophene) (PEDOT).
Chemically polymerized PEDOT was first discovered in 1988. Due to its ability to build transparent and conductive films its first commercial application was the use as an antistatic layer in photographic films [5, 6]. Since then, PEDOT has penetrated into other markets, including solid electrolyte capacitors, printed wiring boards, packaging films, touch screens and as hole transport layers in organic light-emitting diodes (OLEDs) and organic photovoltaics (OPV) [7].
PEDOT
Synthesis
PEDOT can be prepared by oxidative polymerization of the monomer 3,4- ethylenedioxythiophene (EDOT) [8]. During the oxidation each monomer loses two electrons and a conjugate polythiophene chain is formed (see Scheme 1). Under typical oxidation conditions the neutral PEDOT is not present, but the oxidation proceeds directly to a multiply positively charged polymer PEDOT chain. These positive charges need to be stabilized by the presence of a suitable counterion. It has been found that sulfonic acids are best suited to achieve a stable PEDOT /counterion complex [9]. It should be noted that the oxidation is not achieved by the counterion but indeed by the presence of the oxidation agent. The counter ion merely stabilized the obtained product.
O
+ «Na2S2O8
Fe2(SO4)3 PSS '
O
n
O O
O O
OO
+ 2kSO|- + 2nNa+ + 2«II' + "/6Na2S2Os ■
OO
OO
OO
42—
+ n/3SO4 + n/3Na+
O_O O_^O O_O
Scheme 1
In case the counter ion is a small molecule such as ^-toluene sulfonic acid the resulting complex is obtained as a deep blue powder which is intractable to any further processing. Therefore the chemical polymerization needs to be performed on the surface to be covered with the PEDOT film. This can be accomplished using the so called in-situ process where a solution of the monomer EDOT and a solution of an oxidation such as Fe(III) tosylate are mixed, the reactive solution is coated and the polymer is produced on the substrate surface [10]. Alternatively, the EDOT monomer can also be evaporated onto a substrate where the PEDOT polymer is formed. This process can be done with simultaneous evaporation of the oxidation agent such as FeCl3 or deposition of the oxidation agent such as Fe(III) tosylate, beforehand [11, 12].
In case the sulfonic acid is present in the form of a polymer and in particular if this polymeric sulfonic acid has a surplus of free sulfonic acid groups, then dis-persable polyelectrolyte complex consisting of positively charged PEDOT and negatively charged polya-nion can be formed. For example if EDOT is polymerized with persulfate in the presence of poly(styrene sulfonic acid) (PSS), a polyelectrolyte complex is prepared, that forms a stable dispersion in water [13, 14]. This dispersion consists of PEDOT/PS S polyelectrolyte complex gel particles in water. Depending on type and concentration it is stable for months up to years and can be used in many deposition techniques such as spin-coating, doctor blading, screen printing, dip-coating, slot-die coating, curtain coating, ink jet printing and many others [15].
The gel particles of the PEDOT/PSS polyelectrolyte complex have typically a size of 20-500 nm [7]. The dispersions typically have solids contents in the range of1-5% and even though the gel particles might have a diameter of 30 nm it is possible to prepare very smooth films with a surface roughness of 1 nm or less since the gel particles collapse upon drying. The preparation of dispersions with higher solids content beyond 5% PEDOT/PSS content is limited by the ag-
glomeration of the polyelectrolyte complex at higher concentrations. Viscosities of PEDOT/PSS dispersions are typically in the range of 10-500 mPa s.
Due to the excess of the PSS the dispersions have a low pH — typically in the range of 1-2. However, the dispersions can be neutralized by the addition of base. If the neutralization proceeds beyond pH 7 the positively charged PEDOT is not stable. As a consequence PEDOT dispersions are observed to change color and films significantly lose conductivity.
Upon drying the PEDOT/PSS gel particles form a homogeneous transparent film which conducts the electrical current. Alteration of the gel particle size, the ratio of PEDOT to PSS and the solvent composition during drying are expected to have an impact on the resulting morphology and indeed the precise composition of the coating formulation and processing conditions have a significant effect on the conductivity of the film which can be adjusted over many orders of magnitude.
PEDOT Conductivity
Since the conductivity is obviously such a critical value for a conductive polymer it is worthwhile to explain how it can be measured and how it relates to the values of resistivity and sheet resistance.
In order to measure the conductivity a of a conductive polymer, it is advisable to form a thin and uniform layer on a flat substrate. This can be done using a deposition technique such as spin-coating or doctor-blading. The thickness of the layer d is typically in the range of 50-500 nm in order to be conductive and transparent. Thicker layers are also possible but are less transparent. The layer thickness d and the sheet resistance Rsq need to be determined. The conductivity a and the resistivity p are calculated according to:
a = p = I
RSqd a
Table 1. Parameters, symbols and units for conductivity and related parameters
Measured Variable Symbol Unit
Conductivity a S(iemens)/cm
Resistivity P Q cm
Resistance R Q
Sheet resistance Rsq, R Q, Qsq, Q
Film thickness D cm
Length of contacts A cm
Width of contacts B cm
The layer thickness can be determined using a stylus surface profilometer. The measurement of the sheet resistance can be performed using a 4-point probe measurement, i.e. by pressing the 4-point sensor directly onto the film. Alternatively 2-point probe measurements can be made by depositing two parallel metallic contacts onto the Clevios™ layer, i.e. by on painting silver ink or by evaporating metals through a shadow mask. The film resistance R between the contacts can be measured with a simple Ohm-meter.
The sheet resistance is calculated according to:
Table 1 summarized parameters, symbols and units for conductivity and related parameters
PROPERTIES AND APPLICATIONS OF PEDOT DISPERSIONS
This section describes the variety of a large number of PEDOT/PSS dispersions commercially available and highlights their potential applications. In particular dispersions are explained with respect to their composition, their solids content, viscosities, their pH and the conductivity of the resulting film. The aim of this overview is to make the reader understand the scope and limitations of PEDOT/PSS dispersions and applications served. It will allow the reader to select specific PEDOT/PSS dispersions for specific application and possibly even inspire new applications. All PEDOT/PSS dispersions described in this article
are manufactured by Heraeus Precious Metals GmbH & Co. KG, Leverkusen, Germany and sold under the trade name CLEVIOS™ [16]. It has to be noted that the development of PEDOT dispersions is a continuous process and the list of materials can only be today's snapshot. However this article aims to highlight generic properties of PEDOT/PSS dispersions and take into account the findings of the last 20 years of PEDOT research.
PEDOT Dispersions for Antistatic Layers
Historically, the first industrial application of PEDOT was its use as antistatic agent in photographic films [17]. The motivation of this development was the need for a highly efficient, transparent and easily coatable antistatic agent. When a polymer film e.g. media from polyethylene, polyacetate or polyethylene-terephthalate passes through a machine such as a roll to roll coating machine, cha
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