Polymeric Material with Metal-Like Conductivity for Next Generation

Oct 1, 2012 - ITO-free electrochromic device with a flexible electrode. a, The change in transmission for the electrochromic device made with the high...
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Polymeric Material with Metal-Like Conductivity for Next Generation Organic Electronic Devices Manrico V. Fabretto,† Drew R. Evans,*,† Michael Mueller,† Kamil Zuber,† Pejman Hojati-Talemi,† Rob D. Short,† Gordon G. Wallace,‡ and Peter J. Murphy† †

Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Australia ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, NSW 2522, Australia

‡

S Supporting Information *

ABSTRACT: The reduced pressure synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) with sheet-like morphology has been achieved with the introduction of an amphiphilic triblock copolymer into the oxidant thin film. Addition of the copolymer not only results in an oxidant thin film which remains liquid-like under reduced pressure but also induces structured growth during film formation. PEDOT films were polymerized using the vacuum vapor phase polymerization (VPP) technique, in which we show that maintaining a liquid-like state for the oxidant is essential. The resulting conductivity is equivalent to commercially available indium tin oxide (ITO) with concomitant optical transmission values. PEDOT films can be produced with a variety of thicknesses across a range of substrate materials from plastics to metals to ceramics, with sheet resistances down to 45 Ω/□ (ca. 3400 S·cm−1), and transparency in the visible spectrum of >80% at 65 nm thickness. This compares favorably to ITO and its currently touted replacements. KEYWORDS: vapor phase polymerization, PEDOT, high conductivity, organic electronics

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INTRODUCTION Organic electronic materials have generated widespread interest as a pathway to low-cost flexible devices due to their ease of manufacture.1,2 Advances in this area can be attributed to the discovery of inherently conducting polymers (ICPs)3 which allow for the replacement of traditional inorganic materials.4 For example, organic electrochemical transistors developed by Malliaras et al.5,6 are used in the development of biosensors. In devices such as organic photovoltaics,7 electrochromic devices,8 and organic light emitting diodes,9 the active materials are coupled to the outside world through use of a transparent electrode. The commonly used material is indium tin oxide (ITO). The use of ITO is favored due to its optical transparency and high conductivity/low sheet resistance.10 However, the brittleness of ITO is not well-suited to flexible devices, and the sourcing of rare earth metals such as indium may make future costs prohibitive and long-term prospects for its continued use doubtful. For these reasons organic materials are being presented as a viable alternative to directly replace ITO in organic electronic devices. Organic materials of interest for replacing ITO are graphene,11 carbon nanotube sheets,12 and ICPs.13−15 The active materials used in organic electronic devices generally consist of various forms of ICPs,16 for example, the ptype (hole) conductor poly(3,4-ethylenedioxythiophene) © 2012 American Chemical Society

(PEDOT). Of the ICPs reported over the last 30 years, PEDOT is regarded as one of the polymers having current technological and commercial potential.17 This is due to its facile synthesis, high conductivity, dynamic electrochromic activity, biocompatibility, and long-term air stability.18 Upon the basis of the pioneering work on the vapor phase polymerization (VPP) technique by Winther-Jensen and West,19 recent developments have seen the conductivity of PEDOT raised to ca. 1500 S·cm−1, typically achieved using the vacuum variant of the VPP technique.20 VPP typically employs an oxidant solution, iron tosylate (Fe(Tos)3) in n-butanol, with addition of a weak base such as pyridine,19 or a glycol-based surfactant such as 2900 Da PEG-PPG-PEG.21 The conductivity of PEDOT compares favorably to ICPs such as polyaniline13 ca. 1900 S·cm−1 and polypyrrole14 ca. 1600 S·cm−1 and to other organic electromaterials such as graphene11 ca. 72 S·cm−1 but currently falls short of the commonly used inorganic material ITO10 ca. 3300 S·cm−1. When the device architecture requires that PEDOT be deposited onto polymeric substrates, the reported high conductivities cannot be achieved via harsh post polymerization processing steps such as exposure to high Received: September 10, 2012 Revised: September 27, 2012 Published: October 1, 2012 3998

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temperatures (>100 °C) or aggressive solvents (concentrated acids or bases).22 In applications where an ICP forms the functionally active material in an organic electronic device, it needs to be deposited as part of the device architecture using methods such as spray-, spin-, or dip-coating. Alternatively the polymer can be polymerized in situ by means of electropolymerization, or VPP which can take place under atmospheric or vacuum conditions. Despite the use of various additives in the oxidant solutions used in VPP as a means of achieving enhanced conductivity PEDOT, the polymer growth mechanism has been the subject of some conjecture.23,24 A recent study to reconcile the PEDOT film formation debate, employed optical microscopy to observe the “state” of the oxidant film under vacuum VPP conditions.25 The study concluded that the oxidant layer was liquid-like and film polymer growth was a bottom up process. In this study, oxidant solutions were prepared using no additive, a pyridine additive, or a glycol-based surfactant additive, with a series of alkanol solvents, for use in the vacuum VPP procedure. Additional supporting evidence to the original visual observations25 is provided by quartz crystal microbalance (QCM) experiments which were conducted under vacuum chamber conditions. Evidence for an enhanced morphological form of PEDOT is provided by means of X-ray diffraction (XRD) and atomic force microscopy (AFM), with resulting sheet resistances as low as 45 Ω/□ (ca. 3400 S·cm−1) and a transparency in the visible spectrum of >80% at 60 nm thickness. Tuning of the film thickness has resulted in sheet resistances less than 25 Ω/□ with concomitant optical transmission values of >70%. Engineering this enhanced morphological form of PEDOT allows it to act as both the transparent electrode, and/or double as the electrochromically active material, foregoing the need for ITO or any of the currently touted replacement materials.

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The electrochromic device was assembled using an electroactive electrode made of PEDOT prepared from the 5800 Da copolymer containing oxidant solution polymerized onto a cellulose acetate sheet (flexible overhead transparency film). A counter electrode was prepared by employing a magnetron cosputtered transition metal alloy thin film to act as a corrosion resistant electrode. For the transmission switching of the PEDOT film, the central region of the transition metal alloy electrode was removed to allow light to transmit through the device. In the electrochromic cell, the PEDOT electroactive electrode was the working electrode, the transition metal alloy electrode was the auxiliary electrode, and a Ag/AgCl reference electrode was used as the reference. The transmission switching of the cell was measured using a HunterLab UltraScan Pro spectrophotometer. The photopic transmission (%Txph) of the coatings was measured as the tristimulus value Y in the A/2 standard illuminant/observer scale. During the switching of the cell, cyclovoltammetry (CV) measurements were made by measuring the current flow between the working (PEDOT) and auxiliary electrodes (transition metal alloy) as a function of the applied voltage difference.

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RESULTS AND DISCUSSION Across the three oxidant solutions examined the oxidant film with no additive was a solid, the pyridine additive film showed gel-like relaxation without flow, and the glycol-based surfactant additive produced a viscous liquid oxidant film. Evidence for the earlier visual observations25 was provided by QCM experiments which showed that the largest change in frequency (while reducing the pressure in the chamber) occurred for the film with no additive (see Supporting Information Figure S1). This relatively large change in frequency is hypothesized to represent a greater loss of solvent from the oxidant film. Of the three oxidant films tested under vacuum VPP conditions used in this study, only the glycol-based surfactant additive resulted in the formation of a PEDOT film. This observation is in agreement with the work by Mueller et al., where the additive free oxidant was not expected to yield a PEDOT film in the absence of “free water”.26 Comparable studies performed at atmospheric pressure showed that both the pyridine and glycolbased surfactant additives resulted in liquid(-like) oxidant films which subsequently yielded PEDOT films. From this, it was concluded that the oxidant film must possess a liquid(-like) behavior at the operating pressure used in vacuum VPP experiments in order to produce confluent PEDOT films. Given that the desirable outcome is for an oxidant film that remains as a viscous liquid even under vacuum VPP conditions, an understanding of how the components of the oxidant solution behave is important. First, amphiphilic molecules such as PEG-PPG-PEG block copolymers are known to spontaneously self-assemble to form thermodynamically stable configurations.27 The PEG moiety has an affinity for “water” (hydrophilic domain), and the PPG moiety an affinity for “oil” (hydrophobic domain); thus the ratio of PEG to PPG in the block copolymer serves as a means to qualitatively define the way in which different PEG-PPG-PEG copolymers partition hydrophilic and hydrophobic domains.28 This partitioning of domains controls the redistribution of constituents within the solution depending on their relative polar or nonpolar nature. The polar Fe3+ ions as well as the polar ends of the tosylate ligands will reside in the PEG rich domain, while the nonpolar aryl portion of the tosylate ligand and the alkanol solvent(s) will reside in the PPG rich domain. It is hypothesized that during polymerization the monomer and intermediate PEDOT oligomers are also portioned within the structured domains. To date the 2900 Da PEG-PPG-PEG block copolymer (PEG/PPG ratio of 0.87:1) has proved advantageous in the

MATERIALS AND METHODS

Fe(III) tosylate was received from HC Stark as a 40 wt % CB40 solution in n-butanol. 3,4-Ethylenedioxythiophene (EDOT) monomer, the block copolymers poly(ethylene glycol−propylene glycol− ethylene glycol) (PEG-PPG-PEG) in Mw = 1900, 2900, 4400, and 5800, and alkanols C1 to C8 were obtained from Aldrich. The ionic liquid contained within the electrochromic device was 1-butyl-1methyl pyrrolidinium bis(trifluor-methylsulfonyl) imid and used as supplied from Merck, Germany. PEDOT films were synthesized on cleaned microscope slides using a method described elsewhere.20 A series of oxidant solutions were prepared using the pristine oxidant solution and diluting to 16 wt % Fe(III) tosylate using the C1 to C8 alkanols. Block copolymers (Mw = 1900, 2900, 4400, and 5800) were added, producing a 1:1 molar ratio with respect to the Fe(III) tosylate. Quartz crystal microbalance (QCM) (Maxtek, RQCM) experiments were performed by spin coating the respective oxidant films directly onto the quartz crystal at 1500 rpm for 25 s, heated to 70 °C for 30 s, and then mounted in the crystal holder within the vacuum oven. The resonance frequency was measured while the pressure was reduced from atmospheric to 45 mbar. A glass microscope slide was weighed before and after spin coating and heating to determine the relative differences in oxidant thickness for the three oxidants investigated. X-ray diffraction (XRD) (PANanalytical X’Pert Pro) was performed on a rotating sample puck using Co Kα X-rays having wavelength λ = 0.17902 nm. X-ray photoelectron spectroscopy (XPS), 4-point probe conductivity, and atomic force microscopy (AFM) measurement details are described elsewhere.20 The transmission across the visible spectrum was measured using a HunterLab UltraScan Pro spectrophotometer. 3999

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maintained even for this higher molecular weight block copolymer. In Figure 1d, XPS spectra indicated the presence of both Fe3+ and Fe2+ species on the surface of the unwashed PEDOT film immediately after removal from the vacuum VPP chamber. The presence of the liquid-like oxidant, even under vacuum conditions, facilitates the interaction between the monomer and the reactants enabling the polymerization. XRD of the PEDOT provides insights into the influence that the oxidant film has on the molecular length scale. PEDOT formed without the addition of the block copolymer PEG-PPGPEG (i.e., a binary system comprising iron tosylate and nbutanol) exhibited a small diffraction peak located at 2θ = 7.38°, with an intensity of 2300 (au) (Figure 2). This peak has

synthesis of PEDOT. The use of this polymerization media reduces the number of defects which form along the conjugated backbone of the polymer by suppressing the crystallization of the Fe(Tos)3 oxidant as well as moderating the polymerization rate.20 However, given the amphiphilic nature of the PEG-PPGPEG block copolymers,29 combined with the presence of a liquid oxidant film under vacuum VPP conditions, a new approach arises by which to manipulate the growth of the conducting polymer. This approach now employs PEG-PPGPEG block copolymers which have the ability to partition hydrophilic and hydrophobic domains30 in such a way as to direct the growth of the resultant PEDOT (analogous to the work of Hulvat et al. for the liquid crystal templated electropolymerization of PEDOT31). The term structure directing is defined as the property of an additive in the precursor oxidant film that when present yields a change in the final structure of the PEDOT film, be it molecular structure or film morphology, when compared to the polymer grown without such an additive. Of primary interest in this study is the 5800 Da PEG-PPG-PEG block copolymer (PEG/PPG ratio of 0.58:1) which was shown by Zhao et al. to direct the growth structure of meso-porous metal oxides such as silica and titania.30 This PEG-PPG-PEG block copolymer has a similar PEG molecular weight to that in the 2900 Da PEG-PPG-PEG block copolymer, but the PPG component is larger, leading to a smaller PEG/PPG ratio (0.58:1 compared to 0.87:1 for the previously studied 2900 Da copolymer). In Figure 1a−c we confirmed that with this new oxidant composition (5800 Da PEG-PPG-PEG block copolymer

Figure 2. Molecular order observations in vacuum VPP PEDOT. XRD diffraction peaks located at 2θ = 7.38°, corresponding to a d100 spacing of 13.91 Å; (red) no block copolymer addition; (blue) block copolymer Mw = 2900 Da; and (green) block copolymer Mw = 5800 Da Solvent carrier was ethanol. Note the distinct change in the morphology of the PEDOT film from nodular to lamellar when using block copolymer Mw = 2900 Da and Mw = 5800 Da, respectively.

been assigned as the d100 spacing between successive PEDOT backbone layers normal to the substrate surface (Figure 2, inset) and corresponds to a spacing of 13.91 Å, which is in line with other reports.32,33 With the addition of the 2900 Da block copolymer the intensity of the d100 peak increased to 2950 (au) and the full width half-maximum decreased, but the spacing did not change. Importantly, previous studies8,20 have used XPS to show the presence of the block copolymer in the washed PEDOT matrix but despite this the d100 spacing remained unchanged. Even with the addition of the larger 5800 Da block copolymer the d100 spacing remained constant. Furthermore, XRD experiments to elucidate the manner by which the d010 spacing may have been affected returned a near constant value of 3.45 Å across these samples, again in agreement with reported experimental and theoretical results.32,33 Interestingly the appearance of d200 and d300 diffraction peaks for the PEDOT prepared with the 5800 Da block copolymer additive suggests the PEDOT has lamellar-like structure,30 thus implying the block copolymer has induced a change in the PEDOT film morphology rather than crystallinity (i.e., at a longer length scale than that commonly probed by XRD). Inspection of the PEDOT film morphology over a 500 nm by 500 nm area, using an AFM (Figure 2 insets), confirms that the morphology has indeed changed and now resembles a sheetlike structure with the use of the 5800 Da PEG-PPG-PEG block

Figure 1. Verification of viscous liquid-like oxidant layer (Mw = 5800 Da). Time evolution images of oxidant layer, a, t = 0 min, b, t = 2 min, c, t = 4 min. d, Wide scan XPS spectra for unwashed PEDOT film polymerized for 25 min. Inset shows Fe 3p peak and the presence of Fe3+ (blue trace) and Fe2+ (red trace) species which reside on the surface of the film.

additive) liquid-like behavior was maintained within the oxidant film. A scratch was scribed through the oxidant layer and optical microscope snapshots taken two minutes apart. The optical images showed the displaced oxidant film relaxing (highlighted with arrows), indicating that liquid-like behavior was 4000

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copolymer. This is the first reported observation of PEDOT fabricated with sheet-like film morphology, regardless of the synthesis protocol used. Thus the 5800 Da copolymer has directed the structure/morphology of the PEDOT to be sheetlike, while the molecular spacing of the PEDOT is consistent with the theoretically predicted and experimental measured spacing for a range of different additives and oxidant anions.33−35 UV−vis spectra can be used to qualitatively compare the physical/electronic state of differently prepared PEDOT samples,36 in this specific case PEDOT made with either the 5800 Da or 2900 Da copolymers (Supporting Information Figure S2). The absorption shift shown in Supporting Information Figure S2 to longer wavelengths for the 5800 Da produced PEDOT infers an increase in one, or possibly all, of the following: conjugation length; doping levels; and intermolecular electronic interactions. The question that occurred to us was, whether this sheet-like morphology would result in a concomitant increase in conductivity? With the observation of the structure-directing effect from the 5800 Da block copolymer as well as the liquid oxidant film in mind, we set about optimizing the polymer growth by varying the alkanol solvents to produce an oxidant layer having differing hydrophilic/hydrophobic domain volumes and different liquid viscosity and vapor pressure (Supporting Information Figure S3). Figure 3 shows that for the 5800 Da copolymer a

morphology change to sheet-like (for morphology of the methanol carrier solvent see Supporting Information Figure S4) and a step increase in conductivity up to a high of ca. 3400 S·cm−1. The maximum conductivity of ca. 3400 S·cm−1 was obtained with the addition of (60 wt %) ethanol to the stock nbutanol oxidant mixture (note that this blend also resulted in the maximum conductivity for the 2900 Da copolymer). The variation in conductivity with alkanol solvent did not correlate with morphology changes, and we hypothesize that the alkanol solvent modifies the liquid properties of the oxidant film which aids in the vacuum VPP PEDOT film growth. For ICPs, the calculation of conductivity is an essential criterion in assessing the performance of that polymer. The calculation is typically a two-step process of measuring the sheet resistance and obtaining an accurate film thickness. ICPs are typically analyzed as thin films of thickness from 10 to 100 nm22 and by their very nature are considered to be mechanically soft when compared to inorganic and metallic thin films. The ease with which conductivity values can be miscalculated and over inflated was recently highlighted by Fabretto et al.38 who questioned the use of mechanical profilometry for measuring the film thickness of soft ICPs. Measurement tip forces as low as 0.2 mN produced indentations on the polymer surface, which resulted in underestimated film thickness and therefore overestimated conductivity. With consideration to this, great care was taken in measuring the sheet resistance and film thickness of the PEDOT films used herein. In Figure 4, the PEDOT film was

Figure 3. PEDOT conductivity as a function of added alkanol solvent chain length (C1−C8) and block copolymer molecular weight and PEG/PPG ratio. The standard deviation of each data point is from multiple measurements (>9) across large areas of the PEDOT film and several samples. Inset: Literature values reported for PEDOT conductivity using various treatment/process methods; post treatment;37 base inhibiting;15 rate moderating;20 structure-directing; ITO films.10

Figure 4. Film thickness measurements for V-VPP PEDOT. a, AFM image showing a plan view of the scan area and sheet-like structure, b, line scan of PEDOT film thickness showing distinct steps, and c, 3-D image of the edge of the PEDOT film. PEDOT film synthesized using Mw = 5800 Da block copolymer.

significant increase in conductivity ranging from 120% to 200% of that obtained for the 2900 Da grown PEDOT was obtained across all the alkanol solvents used. Similar experiments employing other PEG-PPG-PEG block copolymers with similar PEG molecular weights revealed that both the total molecular weight and PEG/PPG ratio play a role in directing the structural growth and, thus, the final conductivity that is achieved. The PEG-PPG-PEG block copolymers of 1900 Da (PEG/PPG ratio =1.38:1) and 4400 Da (PEG/PPG ratio =0.57:1) yielded similar film morphology and conductivity to the 2900 Da grown PEDOT, while the 8400 Da (PEG/PPG = 5.3:1) could not be processed via this technique because the resulting oxidant was a gel that could not be spin-coated. Increasing the molecular weight from 4400 to 5800 Da for a near constant PEG/PPG ratio yielded a

polymerized using the 5800 Da block copolymer and the sheetlike structure is clearly observed by the steps in the film height. Note the step heights shown in the film do not correlate with the d100 spacing from the XRD experiments but simply demonstrate the film’s layered structure. A section of the film was carefully removed and a step height “film/no film” measurement made using an AFM in semicontact mode. To ensure film thickness accuracy each scan was preceded by a calibration scan using a 102 nm step-height calibration grid, with each film thickness value averaged from multiple measurements (>9) made across a large area of the PEDOT film and across multiple samples (>3). 4001

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cyclovoltametry curves observed for this material within the electrochromic cell (Supporting Information Figure S5). A limitation in the performance of this particular device is the asymmetric switching time of the PEDOT due to the change in conductivity of the PEDOT when in either the reduced or oxidized state. As shown in Figure 5, in the fully reduced (darkened) state the photopic (human eye’s perception of optical) transmission

The directed structural change has resulted in a new morphological form of PEDOT which is among the most conductive reported to date (25 Ω/□ herein compared to 100 Ω/□ for equivalent film thickness recently reported in ref 39) and now rivals conductivity commonly associated with transparent ITO (resistivity of 300 μΩ·cm10). A comparison of the PEDOT reported herein with ITO, and materials proposed as replacements for ITO, is provided in Table 1. Table 1. Comparison of PEDOT with ITO and the Popular Electromaterials Designed To Replace ITO material

conductivity

visible T%

spray coated graphene films11,a

∼1.5 S·cm−1 ∼72 S·cm−1 ∼286 S·cm−1 ∼2150 S·cm−1 ∼3300 S·cm−1 ∼3400 S·cm−1

>96% 85% ∼ 94% >85% >80%

carbon nanotube sheets12 sputtered Al doped zinc oxide40 sputtered ITO10 vacuum VPP PEDOTb a

Figure 5. ITO-free electrochromic device with a flexible electrode. a, The change in transmission for the electrochromic device made with the high conductivity PEDOT as a function of wavelength. b, The electrochromic device in the “bleached” state. c, The electrochromic device in the “darkened” state. The PEDOT film thickness in this device is 120 nm.

The conductivity for the high transparency graphene is calculated from the reported sheet resistance of 2 × 107 Ω/□ and an assumed film thickness of 0.34 nm. bThe reported values of conductivity and T % are for the same 65 nm thick vacuum VPP PEDOT film.

Upon the basis of the conductivity, sheet resistance (PEDOT = 45 Ω/□ and ITO = 20−25 Ω/□), and the transmission over the visible spectrum, PEDOT compares favorably with ITO, especially when the touted replacement materials are examined in the comparison. The electrical properties of the other organic materials (i.e., carbon nanotube sheets and graphene) are 1 to 2 orders of magnitude lower in conductivity. As shown in Table 1, the conductivity for graphene can be increased (by an increase in film thickness); however, owing to graphene being an optically dense material, the transparency will be concomitantly reduced. In addition, the electrical properties of PEDOT also out perform other similar ICPs such as polyaniline13 ca. 1900 S·cm−1 and polypyrrole14 ca. 1600 S·cm−1. Noteworthy here is that for applications that require an optically active material or a p-type conductor (hole conductor), such as in organic electronic devices, the metallike conductivity of PEDOT negates altogether the need for an additional transparent electrode in the device structure. Thus the removal of the transparent electrode alleviates issues related to interfacing PEDOT with the electrode. These include work function mismatch41 and PEDOT wetting on the electrode itself.42 In addition to the high electrical conductivity of this PEDOT, the material has well-known electrochromic properties.8 For example, when placed into an electrochromic device, the optical properties of PEDOT can be switched via appropriate electrical stimulation. The demonstration example shown (Supporting Information Movie S1) of an electrochromic device, where the benefits of employing a conducting polymer as both the electrode material as well as the optically active element, has resulted in decreased device complexity, and importantly, the elimination of cost sensitive ITO layers altogether. The electrochromic device had PEDOT coated directly onto a plastic substrate to form a flexible electrode, which then performed the role of both electrode and optically switchable layer, with its optical properties switched by electrically changing its doping level (driving ions from an ionic liquid in and out of the PEDOT via an electric field). The applied voltages to oxidize/reduce the PEDOT were determined from

for the device was %Txph = 13%. In the fully oxidized (bleached) state the photopic transmission for the device was % Txph = 64%, which yielded a total transmission change for the device of Δ%Txph = 51% (this is not the transmission change for the PEDOT electrode only).

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CONCLUSION Having demonstrated the ability to tailor the structure-directing properties of the oxidant solution by means of altering the additives used, the methodology establishes a new paradigm for the synthesis of conducting polymers using vacuum VPP. The significant increase in conductivity reported here (ca. 3400 S·cm−1) opens the door for PEDOT to compete with traditional transparent electrode materials such as ITO as well as other competing organic alternatives. This development represents a significant step forward in the realization of flexible conducting polymers as a viable alternative to rigid transparent electrode materials, a market which up to now has been dominated by ITO. Indeed for organic electronic devices, as has been demonstrated herein, PEDOT is able to perform the function of both the electrode and the active optical material. This reduces device complexity by eliminating the need for ITO within the device. Importantly, implementing the fundamental insight of utilizing a structure-directing template, in conjunction with the emerging vacuum VPP technique, has the potential to change the manner by which future high conductivity ICPs are synthesized for use in the rapidly growing organic electronics arena.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S5 and Movie S1. This material is available free of charge via the Internet at http://pubs.acs.org. 4002

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Ph: +61 8 83025719. Fax: +61 8 8302 3683. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Michelmore for assistance with X-ray photoelectron spectroscopy. This work was supported in part by ITEK Pty Ltd, the commercialization company of the University of South Australia. PEDOT, poly(3,4-ethylenedioxythiophene); ITO, indium tin oxide; VPP, vapor phase polymerization; PEG, poly(ethylene oxide); PPG, poly(propylene oxide); AFM, atomic force microscopy; QCM, quartz crystal microbalance; ICP, inherently conducting polymer

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REFERENCES

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dx.doi.org/10.1021/cm302899v | Chem. Mater. 2012, 24, 3998−4003