Vapor phase polymerization of poly (3,4-ethylenedioxythiophene) on flexible substrates for enhanced transparent electrodes

Synthetic Metals 161 (2011) 1159–1165

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Vapor phase polymerization of poly (3,4-ethylenedioxythiophene) on flexible substrates for enhanced transparent electrodes Christopher M. Madl a , Peter N. Kariuki a , Jessica Gendron a , Louis F.J. Piper b , Wayne E. Jones Jr. a,∗ a b

Department of Chemistry and Materials Science Program, Binghamton University, Binghamton, NY 13902, USA Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, NY 13902, USA

a r t i c l e

i n f o

Article history: Received 25 January 2011 Received in revised form 18 March 2011 Accepted 24 March 2011 Available online 7 May 2011 Keywords: PEDOT Conducting thin film Flexible electrode Vapor phase polymerization

a b s t r a c t Recently, conducting polymer thin films have been investigated as transparent electrodes in photovoltaic devices and organic light emitting diodes. Due to its relatively high conductivity and excellent transmission in the visible region, poly (3, 4-ethyelenedioxythiophene) (PEDOT) has been shown to be a viable option for such applications. Herein described is a method for the vapor phase polymerization (VPP) of transparent PEDOT thin film electrodes on flexible polyethylene naphthalate (PEN) substrates and the comparison of this VPP method with two current approaches to PEDOT deposition: solution-based in situ polymerization and spin coating a dispersion of PEDOT:PSS. Electrical conductivities and UV–vis transmittances were measured for films produced by each of these methods, with VPP PEDOT showing both the highest conductivity (approx. 600 S/cm) and transmittance (>94% at 550 nm). The surface morphologies of the films were compared using AFM and SEM imaging. The stability of these PEDOT films, stored under ambient conditions, was investigated by monitoring the conductivity and transmittance of the thin films over time. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, electrically conducting organic polymers have gained a great deal of attention due to their potential applications in flexible electronic devices. Among these conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) has been the focus of much research. This polythiophene derivative was first synthesized in the 1980 by scientists at the German based Bayer AG [1] (Fig. 1). The addition of two heteroatoms to the thiophene monomer was suggested to stabilize the positive charge centered on the sulfur atom, while connecting these two atoms to form a second ring was intended to reduce steric hindrance and allow the polymer to adopt the most favorable conformation to result in high conductivity. Among potential monomers tested by Jonas and coworker, 3,4-ethylenedioxythiophene (EDOT), was shown to have the highest conductivity and to remain reasonably stable [2]. More recently, as the advantages of PEDOT have been recognized, it has become the conducting polymer of choice in many prototype organic and hybrid electronic devices. Because of its relatively high conductivity and high transmittance in the visible region of the electromagnetic spectrum, PEDOT has been suggested as a potential replacement for the more com-

∗ Corresponding author. Tel.: +1 607 777 2421; fax: +1 607 777 4478. E-mail address: [email protected] (W.E. Jones Jr.). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.03.024

monly used transparent anode, indium tin oxide (ITO) [3]. Aside from the high cost of indium, the environmental concerns with tin toxicity, and the predicted future shortage of indium, ITO has also been shown to have some less desirable chemical and mechanical properties [3,4]. The presence of ITO in proximity to organic layers in photoelectric devices can result in the diffusion of indium and oxygen into the organic layers, which contributes to the degradation of these devices over time [5,6]. ITO exhibits poor transmittance in the blue region of the spectrum as well [7]. As more research is focused on adapting these photoelectric devices to flexible substrates, the mechanical disadvantages of ITO become more apparent. As opposed to conducting polymers which are inherently flexible, ITO is quite brittle and has been shown to crack in bending tests [8]. These cracks in the ITO anodes increase the sheet resistance of the thin film, decreasing device performance. Due to the above limitations with ITO, PEDOT is beginning to be investigated as a transparent anode for photovoltaic devices and organic light emitting diodes (OLEDs). PEDOT electrodes have been used in bulk heterojunction photovoltaic devices on both glass and flexible substrates [3,9–11]. Devices prepared on flexible substrates have exhibited efficiencies as high as 2.8%, which are comparable to devices prepared using bare ITO as the electrode instead of PEDOT. Furthermore, the PEDOT devices performed significantly better than the ITO devices after both were exposed to mechanical bending tests [3]. PEDOT thin films have also been successfully used as transparent electrodes in flexible OLEDs [12]. Quantum efficiencies of 3.5% were reported, which were

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S S O

O

sonicated in acetone and dried under a stream of nitrogen gas. PEDOT:PSS (Clevios S V3) was purchased from H.C. Starck (Newton, MA). All other reagents were purchased from Sigma–Aldrich and used as received. 2.2. Experimental methods

Thiophene

3,4-Ethylenedioxythiophene (EDOT)

Fig. 1. Structure of thiophene and EDOT monomers.

almost twice those of devices utilizing ITO as the transparent electrode. The primary industrial scale method for depositing conducting PEDOT thin films involves coating a dispersion of PEDOT and poly(styrenesulfonate) (PEDOT:PSS) in aqueous or alcohol solution onto the desired substrate. Aside from coating a dispersion of PEDOT:PSS, two other means of depositing the conducting polymer on the substrate are vapor phase polymerization and solutionbased in situ polymerization (Fig. 2). The VPP method involves coating the desired substrate with a thin layer of oxidant, typically an iron(III) salt, and exposing the substrate to EDOT vapors. A recent study has shown that the highest conductivities of VPP polypyrrole and polythiophenes were attained when using iron(III) p-toluenesulfonate as the oxidant [13]. Conductivity enhancement in PEDOT was attained by introducing pyridine along with the iron (III) p-toluenesulfonate oxidant to act as a basic inhibitor, suppressing an acid-initiated polymerization reaction which results in thin films with significantly lower conductivities [14]. The solution-based in situ method involves immersing the substrate in a reaction mixture in which the polymerization reaction occurs [15]. Two solutions are prepared, one containing the EDOT monomer and the other containing the oxidant, iron(III) chloride. These solutions are combined and the polymerization reaction occurs in solution and on the surface of the substrate. The reaction produces thin films of PEDOT, doped with iron chloride salts. Because VPP and in situ methods result in the synthesis of the PEDOT thin film on the surface of the substrate, it is possible to fine tune polymer properties through various chemical methods. This is a distinct advantage over pre-manufactured dispersions of PEDOT:PSS. Precise control over humidity during the VPP process has been shown to influence the conductivity of PEDOT thin films produced [16], and using copper(II) chloride in place of iron(III) salts as the oxidant has proven to yield nanoporous, as opposed to smooth PEDOT films [17]. Previous work in our lab has shown that conducting polymers prepared by the in situ method exhibited significantly higher conductivity when grown on surfaces modified with transition metal complexes than on surfaces lacking these complexes [18]. As technologies incorporating PEDOT as electrodes move toward industrial scale production, it is increasingly important to establish a reproducible method to produce highly conducting, transparent, and stable PEDOT thin films. This paper compares the electrical and optical properties of PEDOT thin film electrodes prepared by vapor phase polymerization, solution-based in situ polymerization, and spin-coating a dispersion of PEDOT:PSS; and investigates the stability of these polymers over time under ambient conditions. 2. Experimental 2.1. Materials PEN substrates (Teonex Q65FA) were purchased from Dupont. Prior to deposition of PEDOT thin films, PEN substrates were

UV–vis absorption measurements were taken with a Perkin Elmer Lambda 2S UV–vis spectrometer blanked with a clean sample of the PEN substrate. Transmittance measurements were taken with a Hewlett-Packard 8452A diode array spectrophotometer blanked with a clean sample of the PEN substrate. X-ray powder diffraction data was collected on a Scintag XDS2000 0-0 powder diffractometer equipped with a Ge(Li) solid-state detector and ˚ AFM analysis was performed CuK␣ sealed tube ( = 1.54178 A). using a Digital Instruments NanoScope IIIa atomic force microscope set to tapping mode, equipped with a single crystal silicon probe tip. SEM images were obtained using a Zeiss Supra 55 VP field emission scanning electron microscope, equipped with energy dispersive spectroscopy (EDS). Conductivity measurements were made using a custom fabricated four-probe cell equipped with four spring loaded copper contacts spaced 1 mm apart. The tips were engaged with the substrate using a pair of threaded contact screws to deliver a steady contact pressure. Conductivity data were generated using a Keithley 182 voltmeter interfaced to a Keithley programmable current source set to provide 1.0 ␮A of current. Chemical composition from relative areas of the principle core-levels of X-ray photoemission spectroscopy (XPS) was obtained using the Phi VersaProbe 5000 system, with a monochromated Al K␣ source with a 16-channel detector. The survey scans use a pass energy of 111.7 eV (200 ␮m spot and 46.1 W), with high-resolution core-level scans employing a pass energy of 23.5 eV. The PEDOT samples for XPS were prepared and mounted in a dry N2 filled glove box, and then transferred to the XPS chamber in a sealed vessel. Atmospheric exposure occurred only during the initial pumping stages. X-ray beam exposure was found to slightly increase the S and C ratios, while reduce the O signal. This was attributed to a reduction of the adventitious O contribution. No severe beam damage effects (i.e. significant changes in spectral lineshape or concentration) were observed beyond those aforementioned. In order to minimize any charging effects, an electron neutralizer gun combined with lowenergy Ar ion compensation was employed. Various regions were measured to ensure that the films were uniform in composition; we found this to be the case within ±0.5% for each species measured. 2.2.1. VPP PEDOT A solution of iron(III) p-toluenesulfonate hexahydrate (1.00 g, 1.48 mmol) and pyridine (59.5 ␮L, 0.74 mmol) in 1-butanol (10 mL) was spin coated onto clean PEN substrates. The oxidant coated substrates were heated at 75 ◦ C for 3 min in order to evaporate excess butanol. The substrates were then suspended above liquid EDOT monomer in a polymerization chamber held at a constant temperature of 50 ◦ C. The polymerization reaction was allowed to proceed for variable amounts of time to obtain PEDOT films of various thicknesses. Polymerized films were rinsed with ethanol and dried under a stream of nitrogen gas. 2.2.2. In situ PEDOT A monomer solution of EDOT (1.07 mL, 10.0 mmol) in acetonitrile (100 mL) and an oxidant solution of iron(III) chloride hexahydrate (5.40 g, 20.0 mmol) in acetonitrile (100 mL) were prepared. One surface of the clean PEN substrates was masked with tape, and the substrates were suspended in the monomer solution. The oxidant solution was added to the stirring monomer solution, completely submerging the substrates. The reaction mixture was allowed to stir for variable amounts of time in order to obtain thin

C.M. Madl et al. / Synthetic Metals 161 (2011) 1159–1165

a O

O

O

H

O

O O

*

OO

S

S

O

O O

OO

O

+ Fe(OTs)2

Fe(OTs)3 *

*

S

2 Fe(OTs)2 + 2 HOTs

*

S

O +

*

+

H

S

O

2 Fe(OTs)3

+

1161

S

S

*

S

OTs

b O

O

O +

H

O

O O

OO

*

*

S

S

O O

OO

O

+ FeCl2

FeCl3 *

*

S

*

S

O

O +

2 FeCl2 + 2 HCl

+

H

S

O

2 FeCl3

S

S

S

*

Cl

c O

O

O

O S

S

S S

O

O

O

O n

SO3

SO3H

SO3H

SO3

m Fig. 2. (a) Vapor phase polymerization of EDOT using Fe(OTs)3 as the oxidant and pyridine as a basic inhibitor. (b) In situ oxidative polymerization of EDOT in acetonitrile using FeCl3 as the oxidant. (c) Chemical structure of PEDOT:PSS.

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Fig. 3. AFM section analysis was used to measure PEDOT film thickness.

films of various thicknesses. The polymerized films were rinsed with ethanol and dried under a stream of nitrogen gas.

Fig. 3. The four-probe voltage and film thickness were then related by the following equation to give resistivity:

2.2.3. PEDOT:PSS A dispersion of commercially available PEDOT:PSS in a 1:1 mixture of 2-propanol and water was spin coated onto clean PEN substrates. The polymer coated substrates were heated at 70 ◦ C for 30 min to remove excess solvent. While this heating may not remove all of the water associated with the PEDOT:PSS, further increases in temperature or heating duration proved to yield films with significantly lower conductivities. This observation is likely due to a more disordered polymer network favored at higher temperatures. The possible adverse effects on the performance of these thin films caused by this residual water is thus insignificant compared to the loss in conductivity observed upon heating to temperatures capable of driving off all of the excess water.

=

3. Results and discussion 3.1. PEDOT thin film preparation PEDOT thin films were prepared on PEN substrates by three methods: (1) vapor phase polymerization utilizing iron(III) ptoluenesulfonate hexahydrate as the oxidant and pyridine as a basic inhibitor, (2) in situ polymerization in which EDOT and the oxidant, iron(III) chloride hexahydrate, were reacted in solution to afford PEDOT, and (3) spin coating a dispersion of PEDOT:PSS in a 2-propanol/water mixture directly onto the substrate. Reaction times for the VPP and in situ methods were varied in order to yield PEDOT films of varying thicknesses so that trends in conductivity versus film thickness could be observed.

(t)V ln(2)I

where t is the film thickness, V is the measured voltage, and I is the applied current. The conductivity of the films was obtained by taking the inverse of the resistivity. The conductivity and transmittance were compared for each film deposited (Table 1). VPP PEDOT films exhibited the highest conductivity, followed by PEDOT:PSS and then in situ PEDOT. A correlation between film thickness and conductivity is observed for both VPP and in situ films. In the case of VPP PEDOT, conductivity decreases as film thickness increases. This is likely due to less ordered polymer growth and increased incidents of cross-linking between polymer chains as polymer deposition is allowed to proceed for a longer period of time. A decrease in the doping level for thicker VPP PEDOT films may also account for the decrease in conductivity observed. Since the p-toluenesulfonate responsible for doping the polymer is supplied only in a thin layer on top of the substrate, as polymerization proceeds and layers are grown further outward from the oxidant layer, less p-toluenesulfonate is likely to be incorporated into the outer polymer layers, decreasing their conductivity.

Table 1 Film thickness, conductivity, and transmittance were measured for PEDOT films synthesized by VPP and in situ methods and compared to the respective values for a typical preparation of a PEDOT:PSS thin film. Method of deposition

Film thickness (nm)

VPP PEDOT

24 45 90 122 37 71 105 170 195 80

3.2. Conductivity Conductivity measurements were conducted using the standard four-probe technique. Film thickness was obtained by masking part of the substrates with tape prior to deposition of PEDOT and removing the tape to leave an interface between the polymer film and the PEN substrate [15,19]. The difference in height between the PEN and PEDOT was measured by AFM section analysis as shown in

In situ PEDOT

PEDOT:PSS

± ± ± ± ± ± ± ± ± ±

4 9 10 12 5 7 15 18 15 15

Conductivity (S/cm) 575 420 237 155 21 52 126 81 67 188

± ± ± ± ± ± ± ± ± ±

83 70 23 14 3 5 16 8 5 30

Transmittance at 550 nm (%) 94 93 93 91 94 89 84 77 73 91

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Fig. 4. UV–vis absorption spectra of VPP PEDOT, in situ PEDOT, and PEDOT:PSS, normalized for film thickness.

A different trend is observed for in situ PEDOT. Initially, film thickness and conductivity increase together, until thickness reaches approximately 100 nm. After this point, conductivity begins to decrease as thickness continues to increase. The initial positive correlation between thickness and conductivity is likely a consequence of the mechanism of polymer growth. In the case of VPP PEDOT, the substrate is completely coated with a layer of oxidant, allowing for thin, uniform layers of PEDOT having high conductivities to be formed. However, for in situ PEDOT, polymer growth on the substrate occurs from the surrounding reaction mixture. The substrate is not able to be uniformly covered with PEDOT for the short deposition times required to produce thin layers. The resulting patchy films will have lower conductivities. As deposition time and film thickness increase, the substrate becomes completely covered and polymer growth continues. This continued growth results in a less ordered polymer network, leading to the same trend of decreasing conductivity with increasing film thickness observed for VPP PEDOT. In situ PEDOT is less prone to suffer from lower doping levels in thicker films since the chloride dopant is present in the polymerization solution and can be easily incorporated into the growing polymer network as evidenced by the XPS analysis in supplementary information. 3.3. UV–vis spectroscopy and powder X-ray diffraction UV–vis absorption spectroscopy and powder X-ray diffraction were employed to investigate the reason for the significantly higher conductivity of VPP PEDOT. The UV–vis spectra of all three films show a broad absorbance band at higher wavelengths as seen in Fig. 4. This absorption feature, known as a “free carrier tail,” correlates with the conductivity of the polymers [15]. The presence of this band has been shown to correspond to the polymer having a longer conjugation length and greater order, which allows for greater mobility of charge carriers [20]. The spectra were normalized for film thickness, which revealed that a stronger absorption is seen in the case of the VPP PEDOT, which is consistent with the VPP PEDOT having a greater conductivity than the in situ PEDOT or PEDOT:PSS. X-ray diffraction measurements show a more highly ordered polymer network in VPP PEDOT films than in in situ PEDOT and PEDOT:PSS films. The higher degree of order observed in VPP PEDOT by these characterization methods can be used to explain the increased conductivity of VPP PEDOT. 3.4. Transmittance UV–vis transmission spectra were taken for each of the PEDOT films. Among the highest conducting PEDOT film prepared by each of the three methods discussed, those films prepared by the in situ method proved to have lower transmittance across the spectrum than either VPP PEDOT or PEDOT:PSS, while still having lower con-

Fig. 5. SEM images of (a) VPP PEDOT, (b) in situ PEDOT, and (c) PEDOT:PSS.

ductivity than either of these methods. As wavelength increases beyond 500 nm, the transmittance of all PEDOT thin films decreases due to the presence of the free carrier tail. 3.5. Surface morphology The surface morphology of the films was analyzed using SEM (Fig. 5) and AFM (Table 2). Based on SEM images, vapor phase polymerization produces a very smooth and uniform thin film that is almost non-porous. In contrast, in situ deposition yields a less uniform and more porous surface. This difference is likely due to the different mechanisms by which polymer growth on the surface of the substrate occurs. In vapor phase polymerization, the substrate is coated with a thin, generally uniform layer of oxidant, allowing for polymerization to proceed simultaneously and in a more directed

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Table 2 Surface roughness of PEDOT thin films obtained by AFM. Deposition method

Surface roughness (nm)

VPP PEDOT In situ PEDOT PEDOT:PSS

5.6 24.5 41.3

a

700 VPP PEDOT In−situ PEDOT PEDOT:PSS

600

manner across the entire surface of the substrate. In the case of in situ deposition, the polymer growth occurs from the surrounding reaction mixture, leading to more irregular polymerization on the substrate. SEM images of PEDOT:PSS show a generally smooth polymer network, with small interspersed pores. This is consistent with depositing PEDOT:PSS as a dispersion of nano-sized polymer gel particles. AFM surface roughness analysis confirms these observations that VPP PEDOT is quite smooth, with in situ PEDOT being significantly rougher and PEDOT:PSS having an intermediate roughness.

Conductivity (S/cm)

1164

500

400

300

200

100

0 0

The chemical compositions of thin (∼20 nm) VPP PEDOT and (∼50 nm) in situ PEDOT samples were determined using survey XPS, with the results summarized in Table 3 below. Residual contributions from the oxidant were observed for the in situ PEDOT, but were absent in the VPP sample. Higher resolution sweeps focused on the Fe 2p region indicated that the level of Fe in the VPP case must be below the detection limit of the XPS system. Curve-fitting of the O 1s region revealed two distinct species in the VPP PEDOT, which were attributed to the PEDOT oxygen (binding energy (BE) = 532.1 eV) and adventitious oxygen (BE = 529.7 eV), in agreement with previous XPS studies of chemical vapor deposited PEDOT [24]. After removing the adventitious oxygen contribution, the VPP PEDOT composition was found to be 68.0% carbon, 20.5% oxygen, and 11.5% sulfur, in better agreement with that expected from the EDOT molecule. From considering both the Fe 2p (kinetic energy ∼780 eV) mean elastic escape depth () as 1.275 nm from the NIST database standard reference 82 (http://www.nist.gov/srd/nist82.cfm) and the attenuation profile of the photoemission signal (i.e. 98% of signal within 3), a residual oxidant dopant (i.e. Fe or S) gradient could exist for the VPP PEDOT, but it would be restricted to within ∼15 nm of the interface with the substrate. Considering the residual (i.e. dopant) concentrations from XPS combined with the aforementioned better surface morphology of the VPP samples from AFM, a clear correlation between morphology and conductivity is deduced. As a result, the thickness dependence of the VPP PEDOT is more likely associated with increased incidents of cross-linking between polymer chains as the thickness increased rather than associated with a well-defined dopant gradient.

80

VPP PEDOT

In situ PEDOT

C (1s) O (1s) S (2p) Fe (2p) Cl (2p)

66.7% 22.2% 11.1% – –

65.2% 26.1% 8.6% – –

58.5% 30.4% 4.4% 3.6% 3.1%

Transmittance (%)

90

EDOT molecular composition (expected)

30

40

50

60

100

3.6. X-ray photoemission spectra analysis

Species (principle core-level)

20

Time (days)

b

Table 3 XPS determined compositions from the VPP and in situ PEDOT samples. The core-level areas of the indicated principle core-lines were corrected to their corresponding sensitivity factors for the VersaProbe 5000.

10

70 60 VPP PEDOT In−situ PEDOT PEDOT:PSS

50 40 30 20 10 0

0

10

20

30

40

50

60

Time (days) Fig. 6. (a) Conductivity of PEDOT thin films exposed to ambient conditions over time. Uncertainty is higher for the VPP PEDOT measurements due to the high sensitivity of conductivity calculations for slight variations in very thin films. (b) Transmittance of PEDOT thin films at 550 nm over time. Error bars are 1 SD, n = 3.

3.7. Stability of PEDOT thin films over time The stability of the PEDOT thin films was assessed by monitoring changes in the conductivity and transmittance of films exposed to ambient conditions over time (Fig. 6). Previous studies have suggested that exposure to water vapor in the atmosphere leads to an increase in PEDOT sheet resistance as the hygroscopic polymer absorbs moisture over time [21–23]. For the VPP and in situ methods, the conductivity decreases significantly within the first week after film preparation and then becomes more stable, decreasing only gradually for the remainder of the study. In the case of the PEDOT:PSS films, a more gradual and consistent decrease in conductivity was observed over the first month of the study and then stabilized. EDS analysis of the three different PEDOT films revealed the presence of trace amounts of iron in films prepared by the in situ method, but not in films produced by the VPP method or in PEDOT:PSS films. The residual iron salts in the in situ PEDOT are strongly hygroscopic, making the films even more susceptible to absorbing moisture from the atmosphere. The error bars for the VPP PEDOT conductivity are larger than those for either in situ PEDOT or PEDOT:PSS due to the fact that the highest conducting VPP films are

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much thinner than the in situ PEDOT or PEDOT:PSS films. For such very thin films, small variations in thickness among films results in large differences in the calculated conductivity. The transmittance of the films proved to be more stable. Over the course of the experiment, the data tended to drift slightly, which can be attributed to natural variations in the PEDOT films.

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The stability of PEDOT thin films highlights their potential for use in organic electronic devices employed in everyday life, and the high processability of VPP PEDOT shows promise for the industrial scalability and widespread use of such devices utilizing PEDOT thin film electrodes. Acknowledgements

3.8. Thin film adherence To test if the PEDOT polymer strongly adhered to the PEN substrate, we carried out the scotch tape test using freshly prepared PEDOT VPP and in situ films. We taped the scotch tape onto the side of the substrate with the PEDOT polymer and then ripped it off. No polymer was detected on the sticky side of the tape and the film remained highly conducting. All of the films were able to pass the scotch tape test. 3.9. Potential for large-scale production One additional pertinent point of comparison in discussing these three deposition methods is their potential for industrial scalability. PEDOT:PSS dispersions are already used in industrial processes, such as depositing anti-static coatings for films. These dispersions are highly amenable to high-throughput procedures, such as screen printing and slot die coating, since the polymer is coated directly onto the substrate. The VPP and in situ methods in which the polymer is synthesized on the surface of the substrate by oxidative polymerization are not as simple. In situ deposition would be far less practical to use in mass production of PEDOT thin films. The substrates must be immersed in a reaction mixture, which will generate a large amount of waste compared to the amount of product produced. However, vapor phase polymerization is a much better candidate for use in industrial scale production. The oxidant can easily be coated onto the substrate by screen printing or slot die coating. An entire roll of substrate can be exposed to EDOT vapor, allowing the polymerization reaction to occur directly on the surface of the substrate.

The authors would like to thank the Center for Advanced Microelectronics Manufacturing (CAMM) at Binghamton University, DARPA HR0011101002, NASA through the Center for Autonomous Solar Power (CASP) at Binghamton University and the Advanced Diagnostic Laboratory at Binghamton University for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.synthmet.2011.03.024. References [1] [2] [3] [4] [5] [6] [7]

[8] [9] [10] [11] [12] [13]

4. Conclusion PEDOT thin films were deposited onto flexible PEN substrates by vapor phase polymerization. Comparing the conductivity and UV–vis transmittance of films produced by the VPP method with films produced by two current methods of PEDOT thin film deposition (solution-based in situ polymerization and spin-coating a dispersion of PEDOT:PSS) revealed that VPP PEDOT had both the highest conductivity and highest transmittance. These observations are a result of a more highly ordered polymer network in the case of VPP PEDOT compared to the other two methods, as confirmed by powder X-ray diffraction, SEM, XPS and AFM analyses. PEDOT films prepared by vapor phase polymerization and exposed to ambient conditions for 60 days exhibited an initial decrease in conductivity, but were shown to be more stable in the long run, with their transmittance remaining essentially unchanged over time.

[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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