Synchronous Vapor-Phase Coating of 

Conducting Polymers for Flexible 

Optoelectronic Applications 

Keon-Soo Jang and Jae-Do Nam 

Sungkyunkwan University 

Republic of Korea

1. Introduction 

Since conducting polymers (CP) were first reported, poly(3,4-ethylenedioxythiophene) 

(PEDOT) is arguably one of the most commercially useful and most studied CPs in the last 

20 years (Shirakawa et all., 1977; Chiang et al., 1977; Winther-Jensen et al., 2007; Truong et 

al., 2007) . PEDOT has been studied extensively on account of its many advantageous 

properties, such as high electrical conductivity, good transmittance and thermal stability 

with a low optical bandgap and thermal stability (Winther-Jensen & West, 2004; Jonas et al., 

1991). These properties make PEDOT very attractive for applications, such as electrochromic 

windows (Welsh et al., 1999), organic electrodes for organic photovoltaic cells (OPVs) 

(Admassie et al., 2006; Gadisa et al., 2006) and hole injection layers (HIL) in organic light 

emitting devices (OLEDs) (Wakizaka et al., 2004; Hatton et al., 2009) and dye-sensitized 

solar cells (Saito et al., 2005). In particular, PEDOT is commonly used as a hole extraction 

layer in OPVs (Colladet et al., 2007; Kim et al., 2005). In most optoelectronic applications as a 

buffer or electrode layer, the bandgap of the layer plays an important role in determining 

the operating characteristics, quantum efficiency and electron/hole transport. Therefore, the 

main issues for electronic device applications include both the electrical conductivity and 

bandgap. 

Oxidized PEDOT can be produced in a variety of forms using different polymerization 

techniques. Solution processing is used most commonly in synthesizing PEDOT in the form 

of spin-coating, solvent-casting or ink-jet printing. However, these PEDOT systems are 

relatively insoluble in most solvents, making it necessary to attach soluble functional groups 

to the polymer or dope it with stabilizing polyelectrolytes (Terje & Skotheim, 1998). An 

aqueous dispersion of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-

PSS), commercially available as Baytron P, is a stable polymer system with a high 

transparency up to 80% (Groenendaal et al., 2000). However, the PEDOT-PSS film exhibits 

relatively low electrical conductivity, 10-500 S/cm (Groenendaal et al., 2000), which does not 

often meet the high conductivity required for most applications. In addition, scanning-

tunneling microscopy, neutron reflectivity measurements, and x-ray photoelectron 

spectroscopy have revealed a PSS rich layer on the top of the spin-coated PEDOT-PSS films 

due to the phase separation (Lee & Chung, 2008; Kemerink et al., 2004; Higgins et al., 2003). 

Since PSS is an electrical insulator, excessive PSS can limit the film conductivity (Kemerink 

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152 Optoelectronic Devices and Properties 

et al., 2004), and an acidic PEDOT-PSS dispersion can etch indium tin oxide (ITO) during the 

polymer spin-coating process. Moreover, the hydrolysis of the deposited PEDOT-PSS by 

moisture absorption can also etch ITO to cause indium and tin incorporation into the 

polymer (Lee et al., 2007). 

For the PEDOT systems without using polyelectrolytes, PEDOT can be deposited directly 

on the substrate surface by several in-situ polymerization techniques. One of the options 

is electrochemical polymerization, which has been reported to have higher electrical 

conductivity (Groenendaal et al., 2001). However, electrochemical polymerization results 

in a poor transparency and requires conducting substrates, which limits its practical 

applications. As an alternative, oxidative chemical polymerization either in the liquid or 

vapor phase is more versatile because it is not restricted by the substrates. In particular, 

one way of achieving a clear thin film with a smooth surface is to apply the oxidant using 

solvent coating processes and expose the coated surface to a reactive monomer vapor. 

This process is often referred to as vapor phase polymerization (referred to herein as VPP) 

(Kim et al., 2003; Fabretto et al., 2008; Cho et al., 2008; Ha et al., 2004; Winther-Jensen et 

al., 2004). The PEDOT films by VPP have been reported to have conductivities of 

approximately 15 S/cm at a thickness of 300 nm without any additives (Kim et al., 2003). 

Recently, a PEDOT film with high conductivity, exceeding 1000 S/cm, was reported using 

a base-inhibited VPP (Winther-Jensen et al., 2007). However, it should be noted that VPP 

PEDOT has a high bandgap and relatively low transmittance (Fabretto et al., 2008; Cho et 

al., 2008). 

As another thiophene-based conducting polymer, poly(3-hexylthiophene) (P3HT) is also one 

of the most indispensable materials in OLEDs, OPVs, field effect transistors (FETs) and thin 

film transistors (TFTs) (Hatton et al., 2009; Kim et al., 2005; Grecu et al., 2006; Bartic et al., 

2002). Over the last decade, blending or copolymerization techniques of conducting 

polymers have been investigated not only to overcome the drawbacks of a pristine 

conductive polymer, such as inadequate bandgap, rough surface, low conductivity and poor 

transmittance, but also to tailor the properties for various applications (Kim et al., 2009; Xu 

et al., 2006; Sarac et al., 2003). This study investigated P3HT thin films deposited using a 

vapor-phase polymerization (VPP) technique (Ha et al., 2004; Winther-Jensen et al., 2004), 

which desirably ensures thin film formation in various substrate materials without the 

additional processes to liquefy polymers (Jang et al., 2009). It is believed that the VPP 

technique for P3HT will allow the fabrication of thin coatings over a large surface area of 

various substrate materials. We used a similar route to VPP of PEDOT choosing appropriate 

catalyst and solvent systems. Using the VPP technique, therefore, PEDOT and P3HT may be 

copolymerized in the state of vaporized monomers and subsequently polymerized to form, 

most probably, a PEDOT/P3HT copolymer structure. However, it should be mentioned that 

the traditional VPP route, where the monomer is maintained in the state of thermodynamic 

equilibrium, may not simply be applied because the vapor pressure and polymerization rate 

of the EDOT and 3HT are different and, thus, the relative composition of the VPP copolymer 

is not controllable (Jang et al., 2010). 

Therefore, the PEDOT to P3HT ratio was kinetically controlled in this study by adjusting the 

relative feed amount of the evaporating monomers to the reaction chamber to fabricate the 

PEDOT/P3HT films containing different ratios of PEDOT to P3HT. The developed 

synchronous VPP technique successfully provided PEDOT/P3HT copolymer thin coatings 

with tunable bandgap and optoelectronic properties.

2. Conducting polymers 

2.1 Summary of conducting polymers 

Conducting polymers–plastics that conduct electricity–continue to find market niches in 

consumer electronics and antistatic textiles, some of which have military applications. 

Among the most exciting applications is the use of conducting polymers in light-emitting 

devices (LEDs), replacing silicon as the traditional substrate material for clock radios, audio 

equipment, televisions, cellular telephones, automotive dashboard displays, and aircraft 

cockpit displays. Conducting polymers provide benefits to industries such as electronics by 

shielding against electromagnetic interference (EMI). Conductive polymers are also already 

used in devices that detect environmentally hazardous chemicals, factory emissions, and 

flavors or aromas in food products. Currently, their conductivity is being explored in 

electrostatic materials, conducting adhesives, electromagnetic shielding, artificial nerves, 

aircraft structures, diodes, and transistors. 

2.2 Correlation of chemical structure and electrical conductivity 

In traditional polymers such as polyethylenes, the valence electrons are bound in sp3

hybridized covalent bonds. Such "sigma-bonding electrons" have low mobility and do not 

contribute to the electrical conductivity of the material. The situation is completely different 

in conjugated materials. Conducting polymers have backbones of contiguous sp2 hybridized 

carbon centers. One valence electron on each center resides in a pz orbital, which is 

orthogonal to the other three sigma-bonds. The electrons in these delocalized orbitals have 

high mobility when the material is "doped" by oxidation, which removes some of these 

delocalized electrons. Thus the p-orbitals form a band, and the electrons within this band 

become mobile when it is partially emptied. In principle, these same materials can be doped 

by reduction, which adds electrons to an otherwise unfilled band. In practice, most organic 

conductors are doped oxidatively to give p-type materials. The redox doping of organic 

conductors is analogous to the doping of silicon semiconductors, whereby a small fraction 

silicon atoms are replaced by electron-rich (e.g., phosphorus) or electron-poor (e.g. boron) 

atoms to create n-type and p-type semiconductors, respectively. 

Although typically "doping" conductive polymers involves oxidizing or reducing the 

material, conductive organic polymers associated with a protic solvent may also be "self-

doped." 

The most notable difference between conductive polymers and inorganic semiconductors is 

the mobility, which until very recently was dramatically lower in conductive polymers than 

their inorganic counterparts. This difference is diminishing with the invention of new 

polymers and the development of new processing techniques. Low charge carrier mobility is 

related to structural disorder. In fact, as with inorganic amorphous semiconductors, 

conduction in such relatively disordered materials is mostly a function of "mobility gaps" 

with phonon-assisted hopping, polaron-assisted tunneling, etc., between localized states. 

The conjugated polymers in their undoped, pristine state are semiconductors or insulators. 

As such, the energy gap can be > 2 eV, which is too great for thermally activated conduction. 

Therefore, undoped conjugated polymers, such as polythiophenes, polyacetylenes only have 

a low electrical conductivity of around 10−10 to 10−8 S/cm. Even at a very low level of doping 

(< 1 %), electrical conductivity of increases several orders of magnitude up to values of 

around 0.1 S/cm. Subsequent doping of the conducting polymers will result in a saturation 

of the conductivity at values around 0.1–10 kS/cm for different polymers. Highest values 

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154 Optoelectronic Devices and Properties 

reported up to now are for the conductivity of stretch oriented polyacetylene with confirmed 

values of about 80 kS/cm. Although the pi-electrons in polyactetylene are delocalized along 

the chain, pristine polyacetylene is not a metal. Polyacetylene has alternating single and 

double bonds which have lengths of 1.44 and 1.36 Å, respectively. Upon doping, the bond 

alteration is diminished in conductivity increases. Non-doping increases in conductivity can 

also be accomplished in a field effect transistor (organic FET or OFET) and by irradiation. 

Some materials also exhibit negative differential resistance and voltage-controlled "switching" 

analogous to that seen in inorganic amorphous semiconductors. 

2.3 Synthesis of conducting polymer 

Chemical, oxidative coupling, electrochemical, chemical oxidative, Grignard poly-

condensation, and quasi-living polymerization are commonly used. In chemical oxidative 

polymerization, a monomer, oxidizing agent, and a dopant are reacted on the dielectric 

surfaces to form the conductive polymer. In electochemical polymerization, conducting 

polymers such as Polypyrrole and Polyaniline films of uniform morphology and good 

electrical conductivity are formed on Indium Tin Oxide glass electrodes in supercritical CO2

where the only added ingredient is electrolyte dopant. Furthermore, conducting polymers 

are chemically polymerized in an aqueous medium to which an oxidant is added. 

Sometimes, in addition to the ingredients, a protonic acid is added to the aqueous 

polymerization mixture that renders the final polymers conductive. The oxidative 

polymerization of monomers to obtain conductive polymer is affected by the oxidation 

potential level of monomer as well as the oxidizing capability of oxidizing agent. In the 

example of Fe (III) oxidizing agent, the Fe (III) ion would form a complex with a specific 

compound having lone pair electrons, which alters the oxidizing strength of Fe (III) ion. The 

stronger the complex bonding is, the lower the oxidizing capability of the oxidizing agent is. 

3. Fabrication of the VPP conducting polymers 

3.1 VPP-P3HT 

The substrates were washed and rinsed with DI-water and acetone while being sonicated for 

10 minutes to remove any organic contaminants. The glass substrates plasma treated (KSC 

Korea switching, Korea) for 10 minutes (10 kHz & 10 V & 7 A at a speed of 50 cc 

Helium/min), and the ITO glass substrates were UV-treated for 20 min. The catalyst was a 

mixture of MeOH and EtOH at 1:1 ratio with FeCl3ï6H2O. After sonicating the catalyst 

solution for 2 minutes at 40oC, it was spin-coated onto the substrates at a speed of 500 rpm 

for 5 seconds and then at 1400 rpm for 5 seconds. Subsequently, the catalyzed substrates 

were placed in the vapor-phase-polymerization (VPP) chamber containing 3-hexylthiophene 

(3HT) monomer to evaporate and fill therein under a nitrogen purge, which is similar to that 

reported elsewhere (Truong et al., 2008). The 3HT monomers in the VPP chamber were 

polymerized for one hour at 50oC. The sample was soaked and washed sequentially with 

MeOH to eliminate the monomers remaining on the substrate. The washed P3HT film was 

further dried to remove the residual solvents for 10 minutes in ambient atmosphere. 

3.2 VPP-P3HT/PEDOT copolymers 

The substrates were washed and rinsed with DI-water and acetone and sonicated for 10 

minutes to remove any organic contaminants. The glass substrates were plasma-treated 

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Synchronous Vapor-Phase Coating of Conducting Polymers 

for Flexible Optoelectronic Applications 155 

(PLAMAX, SPS Co., Ltd., Korea) for 30 seconds, and the ITO glass substrates were UV-

treated for 20 minutes. The oxidant was a mixture of MeOH and EtOH at a 1:1 ratio with 

iron (III) chloride hexahydrate. The wt% ratio between the alcohol mixture and FeCl3·6H2O 

was 10:1, 5:1, and 3:1. A ratio of 10:1 was chosen to fabricate most thin films in this study. 

After sonicating the oxidant solution for 2 minutes at 40oC, it was spin-coated onto the 

substrates at a speed of 500 rpm for 5 seconds and then at 1400 rpm for 5 seconds. The 

oxidized substrates were placed in the vapor-phase-polymerization (VPP) chamber, where 

the containers of 3HT and EDOT were also placed. The vaporized monomers were supplied 

into the chamber through the feed inlets, which were made at the center of the container 

ceiling with different inlet diameters of D3HT and DEDOT (Scheme 1). The flow rate of flowing 

nitrogen was kept constant during polymerization, and the feed ratio of 3HT and EDOT 

monomer vapors was adjusted by the inlet diameters of D3HT (0, 2, 4, 6, and 8 mm) and 

DEDOT (fixed at 20 mm). The EDOT and 3HT monomers in the VPP chamber were 

polymerized for 20 min, 30 min, or 1 hour at 60oC. After polymerization, the sample was 

soaked and washed sequentially with MeOH to eliminate the monomers and Fe(III) solution 

remaining on the substrate. The washed PEDOT/P3HT copolymer film was dried further 

using a hot-air gun for 1 minute in an ambient atmosphere to remove the residual MeOH . 

Fig. 1. Schematic of experimental setup for synchronous polymerization of EDOT and 3HT 

monomers, where the monomer concentrations are controlled by the inlet sizes of 

monomers (DEDOT and D3HT) to the reaction chamber under the flowing inert gas 

4. Measurement 

The thickness of the VPP PEDOT/P3HT was measured using an alpha step IQ (KLA Tencor 

corporate, the Yield Management company, San Jose CA, U.S.A.) and FE-SEM (Field 

emission scanning electron microscope, 1.0nm guaranteed at 15kV, JSM6700F, JEOL, 

Japan). The bandgap was determined by UV-vis-spectrophotometry (UV-3600, SHIMADZU,