Highly Conductive and Conformal Poly(3,4-ethylenedioxythiophene

May 20, 2014 - Electrical conductivity of these textured films is routinely above 1000 S cm–1, with the most conductive films exceeding 3000 S cm–...
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Highly Conductive and Conformal Poly(3,4-ethylenedioxythiophene) (PEDOT) Thin Films via Oxidative Molecular Layer Deposition Sarah E. Atanasov,† Mark D. Losego,† Bo Gong,† Edward Sachet,‡ Jon-Paul Maria,‡ Philip S. Williams,† and Gregory N. Parsons*,† †

Department of Chemical and Biomolecular Engineering and ‡Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27607, United States S Supporting Information *

ABSTRACT: This work introduces oxidative molecular layer deposition (oMLD) as a chemical route to synthesize highly conductive and conformal poly(3,4-ethylenedioxythiophene) (PEDOT) thin films via sequential vapor exposures of molybdenum(V) chloride (MoCl5, oxidant) and ethylene dioxythiophene (EDOT, monomer) precursors. The growth temperature strongly affects PEDOT’s crystalline structure and electronic conductivity. Films deposited at ∼150 °C exhibit a highly textured crystalline structure, with {010} planes aligned parallel with the substrate. Electrical conductivity of these textured films is routinely above 1000 S cm−1, with the most conductive films exceeding 3000 S cm−1. At lower temperatures (∼100 °C) the films exhibit a random polycrystalline structure and display smaller conductivities. Compared with typical electrochemical, solution-based, and chemical vapor deposition techniques, oMLD PEDOT films achieve high conductivity without the need for additives or postdeposition treatments. Moreover, the sequential-reaction synthesis method produces highly conformal coatings over high aspect ratio structures, making it attractive for many device applications.

P

ization (VPP) and oxidative chemical vapor deposition (oCVD). In VPP, a substrate with a spun-coated oxidant is exposed to a continuous flux of EDOT monomer vapor. In the oCVD process, the oxidant is introduced in the vapor simultaneously with the EDOT monomer. Compared to solution methods, vapor processing promotes monomer transport on the growth surface, resulting in more crystallization of the PEDOT polymer. Also, therefore, vaporprocessed materials are generally more conductive than those made in solution. To achieve the highest conductivity, PEDOT is often mixed with chemical additives and/or treated after polymerization. For example, pyridine is added as a base inhibitor during VPP or oCVD to moderate strong oxidants and neutralize acidic polymerization byproducts, such as HCl. The highly acidic HCl is undesirable because it saturates the carbon and promotes unconjugated, nonconductive PEDOT.8 Other materials, such as glycol-based surfactants, are also often added during VPP to prevent oxidant crystallization.9 Postpolymerization acid rinses are also known to improve film conductivity and stability. Table 1 summarizes oxidants, additives, and resulting electrical conductivities for PEDOT films grown by previous vapor phase methods and for films described here (vide infra).

oly(3,4-ethylenedioxythiophene), or PEDOT, is one of the most economically significant conductive polymers used today, with several tons per year being commercially produced for capacitors and other products.1 PEDOT’s chemical stability, visible transparency, and high electrical conductivity1 make it useful for electronic applications including organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), and electrolytic capacitor electrodes.1,2 The high electrical conductivity of PEDOT manifests from a conjugated bond structure that permits π-orbital overlap along the polymer backbone. Through an oxidative polymerization and doping process, the conductive polycation form of the polymer is stabilized by anions that promote charge carrier generation at room temperature.3,4 The π-stacking between PEDOT polymer chains provides an avenue for hole mobility, enhancing overall conductivity.5 Analogous to pyrrole, PEDOT is often synthesized in solution or by vapor phase methods through a series of oxidation and deprotonation steps.4,6,7 Polymerization starts by the recombination of oxidized monomers. After oligomers have formed, the oxidized monomer is capable of reacting directly with the neutral oligomer to propagate the polymerization.6 Therefore, in vapor phase methods, the oxidant serves to both promote monomer polymerization and to subsequently oxidize the neutral polymer to form the conductive polycation (Scheme 1). Anions then coordinate to the polycation, stabilizing the charge. PEDOT materials have previously been formed by vapor methods1 including vapor phase polymer© XXXX American Chemical Society

Received: March 7, 2014 Revised: May 20, 2014

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Scheme 1. Oxidative Polymerization of EDOT Followed by Oxidative Doping Leads to the Conductive Form of PEDOT

Table 1. Comparison of Different VPP Methods for Depositing PEDOT

a

method

oxidant

additives

comments

highest reported σ [S cm·1]a

ref

oCVD VPP oCVD VPP oCVD oCVD VPP VPP oMLD

Fe(III)Cl3 Fe(Tos)3 bromine Fe(Tos)3 Fe(III)Cl3 Fe(III)Cl3 Fe(Tos)3 Fe(Tos)3 MoCl5

pyridine H2O − pyridine − − PEG−PPG−PEG PEG−PPG−PEG −

− − − − − post-polymerization acid rinse (ion exchange) − − this work

100 400 400 1000 1000 1500 1500 2500 1200−5300b

7 10 11 8, 12 13 14 15 16

Conductivities were rounded to the nearest hundred. bRange of highest conductivity found in this study.

phene and polyfuran films by VPP.22 This selection allows the oMLD reaction to proceed readily at low temperatures (≤150 °C), making the process compatible with a large range of substrates including polymers. Similar to FeCl3, MoCl5 will generate chlorine counterion dopants in the PEDOT films.13

This work introduces oxidative molecular layer deposition (oMLD) as a chemical route to form highly conductive, conformal PEDOT thin films with controlled crystal structure and orientation. High electrical conductivity is achieved at moderate deposition temperatures (∼150 °C) without the use of additives or postpolymerization processing. As an additional benefit, the saturating vapor-phase process leads to highly conformal coatings, as demonstrated on silica fiber substrates. The effect of alcohol, water, and dilute acid rinses on film composition and conductivity are also characterized. The oMLD approach introduces monomer and oxidant vapors sequentially in a reactor chamber under viscous flow at reduced pressures (∼0.8 Torr), where reactant exposures are separated by an inert gas (N2) purge step. The process is a chemical extension of atomic and molecular layer deposition, where self-limiting half reactions proceed in a binary cycle to build up conformal thin films with submonolayer precision.17−20 The purge removes excess monomer, oxidant, and any volatile polymerization byproducts.21 Iron trichloride (FeCl3) and iron tosylate are common oxidants for PEDOT synthesis, but they have low vapor pressures and are not readily introduced into the vapor phase. Therefore, a more volatile oxidant, molybdenum pentachloride (MoCl5), was selected, which was previously used to produce conductive polythio-

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RESULTS AND DISCUSSION Figure 1 summarizes the structure of as-deposited PEDOT thin films synthesized by oMLD. Representative FTIR spectra from oMLD PEDOT formed at 100 and 150 °C are given in Figure 1a. The spectra show peaks at 1519, 1410, and 1360 cm−1 (corresponding to CC and CC stretch of the thiophene ring), 1197, 1083, 1052 cm−1 (COC moiety), and 830 cm−1 (CS stretch), among others, consistent with reported p-doped PEDOT films formed electrochemically23,24 or by oCVD.7 Figure 1b shows a photograph of an ∼100 nm thick PEDOT film on a planar fused quartz substrate exhibiting its characteristic blue color transmission. Figure 1c shows the C 1s and S 2p X-ray photoelectron spectroscopy (XPS) spectra for oMLD PEDOT films deposited at 100 and 150 °C. The broad, asymmetric C 1s peak indicates the presence of carbon atoms from the thiophene and ether moiety of the monomer, and the single S 2p doublet at 164 eV is consistent with moderately oxidized material.25 The Mo 3d 5/2 peak (Supporting B

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Figure 1. (a) FTIR spectra of two PEDOT films deposited at 100 and 150 °C. (b) Photograph of PEDOT deposited at 100 °C on a fused quartz disc. (c) C 1s and S 2p XPS spectra for two PEDOT films, one deposited at 150 °C (green) and the other at 100 °C (blue). (d) SEM image of PEDOT coated silica wool fiber via oMLD. Coating was deposited at 100 °C.

Information Figure S1) at ∼232 eV indicates that a mixture of oxidation states is present, dominated by Mo(VI) for films deposited at 100 and 150 °C. MoCl4 could oxidize upon reaction with degraded monomer. However, since the XPS analysis is performed ex situ after deposition, it is more likely that the highly oxidized Mo results from exposure to ambient, which coincides with the excess of oxygen in the PEDOT film, as seen in the XPS elemental analysis in Supporting Information Table S1. The excess carbon content is also attributed to exposure to ambient. Highly oxidized MoO3 is known to be a poor conductor,26 so it will not add conductive pathways in the film. The ability for oMLD to coat high aspect features is shown in the SEM image in Figure 1d. The image shows a uniform PEDOT film on a single 10 μm diameter silica fiber extracted from a silica fiber wool sample after coating on the entire fiber substrate. The sequential exposure promotes uniform surface coverage on all examined fibers, similar to uniform inorganic coatings achieved on fibrous substrates using atomic layer deposition.27,28 Coating uniformity around the entire fiber surface demonstrates oMLD’s ability to allow both the monomer and oxidant precursors to reach non-line-of-sight positions in the fabric structure. Results from in situ quartz crystal microbalance (QCM) are presented in Figure 2. These experiments track the areal mass gain during each precursor delivery and gas purge step. Microbalance measurements were made at 100, 125, and 150 °C (Figure 2a). Growth proceeds linearly with the number of cycles after an initial incubation period. Figure 2b shows that an

Figure 2. (a) Quartz crystal microbalance (QCM) measurements for PEDOT films deposited at 100, 125, and 150 °C. (b) Magnified view of steady state QCM data. Left (solid) line indicates MoCl5 dose, and the right (dashed) line indicates EDOT dose.

C

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Figure 4. If there was a significant buildup of metal at the interface, a high contrast layer would be visible between the

increase in deposition temperature tends to decrease the growth rate and increase the growth incubation time. This is ascribed to an increase in the precursor desorption at higher temperature.29 The MoCl5 mass gain decreases from 170 ng cm−2 at 100 °C to 30 and 16 ng cm−2 at 125 and 150 °C, respectively. We also note that after each EDOT dose, the mass appears to decrease during the purge cycle step. This is consistent with the loss of HCl molecules as expected during polymerization and doping (Scheme 1). We measured the electrical conductivity of oMLD PEDOT films deposited on planar oxidized silicon substrates at room temperature using a four-point probe, and the results are plotted in Supporting Information Figure S2. Samples were stored in laboratory air, and the conductivity was measured at regular intervals over 2 weeks. The conductivity shows good stability over the ∼14 day measurement period. Figure 3 plots

Figure 4. Cross-sectional TEM images of PEDOT films deposited on SiO2/Si substrates at (a) 100 °C and (b) 150 °C. PEDOT films deposited at 150 °C appear more homogeneous. Magnified views show film structure at the film/substrate interface. Neither film shows significant compositional heterogeneity at the interface.

deposited polymer and the substrate. However, the images show a single contrast change at the interface, indicating relatively uniform film composition. To further analyze film uniformity, we used time-of-flight SIMS analysis (Supporting Information Figure S3) to examine composition versus film depth for films deposited at 100 and 150 °C. These results show uniform C, Cl, and Mo profiles with film depth, demonstrating no spatial buildup of Mo. To determine if the crystal structure changed substantially as the film growth proceeded, we used X-ray diffraction to probe the crystal structure versus thickness for samples produced at 100 and 150 °C. The positions of the diffraction peaks are not affected as film thickness increased from ∼100 nm to ∼1 μm, indicating uniform crystal structure during growth. On the basis of these results, we conclude that the thickness dependence of the conductivity in Figure 3 cannot be ascribed to crystal or composition variations but may result from defect structure that promotes surface charge accumulation and apparent higher conductivity in the thinner films. Gleason et al. reported that the conductivity of PEDOT formed by oCVD increased by 2 orders of magnitude with increasing deposition temperature from 34 to 85 °C.7 The improved conductivity was ascribed to HCl removal at high temperature, leading to a more basic reaction environment. Likewise, high temperatures will promote HCl removal in the oMLD process, but the active purge steps during oMLD also help prevent HCl buildup, suggesting that other factors may contribute to the improved conductivity at high temperature. Furthermore, the FTIR and XPS results are similar for films deposited at 100 and 150 °C, indicating that composition and extent of polymerization are the same at both deposition temperatures. In the IR spectra, the intensity of the CC peak at ∼1520 cm−1 is similar at both temperatures, consistent with the same extent of conjugation and polymerization.13 Likewise, the C and S signal intensities in the XPS spectra in Figure 1 are similar at 100 and 150 °C. We used X-ray diffraction to probe the crystallographic ordering of as-formed PEDOT films deposited at 100 and 150 °C, and results are shown in Figure 5. The PEDOT crystal

Figure 3. Conductivity plotted versus sample thickness for films deposited at 150, 135, 125, and 100 °C. Error bars in the figure correspond to the standard deviation of four measurements.

electrical conductivity as a function of film thickness for several deposition temperatures. Corresponding tabulated data is given in Supporting Information Table S2. The thinnest films all show relatively poor conductivity, with a rapid increase in conductivity as thickness increases. This increase is likely due to film coalescence and/or reduced surface or interface scattering as the layer builds up. For films deposited at 150 °C and ∼100 nm thickness, we measure conductivity (within 1 day of deposition) exceeding 5000 S cm−1, which is five times higher than typical PEDOT and nearly two times better than previous reports of exceptional quality material.16 For thicker films, the conductivity became more independent of film thickness. Thicker films deposited at 150 °C showed values of ∼1000 ± 200 S cm−1. Some films, including those with conductivity greater than 1000 S cm−1, were measured after being stored in the lab environment for more than 10 months after film preparation. The films generally retained approximately 80− 90% of their conductivity over this extended period. We hypothesized that the observed peak in electrical conductivity as a function of thickness could result from (i) a buildup of Mo at the film/substrate interface; (ii) improved crystallinity during the initial stages of growth; and/or (iii) a charge accumulation surface layer that dominates conductivity for the thinner films. To test these possibilities, we performed control experiments to examine composition and crystal uniformity in the surface normal film-growth direction. First, representative PEDOT films were imaged using cross-sectional transmission electron microscopy (TEM). Images collected from both low and high temperature coatings are shown in D

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reasonable compared to 14 Å reported for other PEDOT materials 30 and is attributed to the incorporation of molybdenum ions in the polymer. Using the results shown here, we illustrate the most likely oMLD PEDOT crystalline arrangement in Figure 5d. At low temperatures, the PEDOT films exhibit a random polycrystalline structure, whereas at higher temperatures the PEDOT grows preferentially along the [010] direction. This marked difference in crystal structure with deposition temperature, combined with the similar composition and bonding structure observed from spectroscopic analysis, leads us to conclude that the high conductivity obtained for films deposited at 150 °C results from the well-aligned crystalline structure obtained at that deposition temperature. To confirm that the deposition temperature was important in achieving high conductivity, a series of control experiments were performed, where films deposited at lower temperature (100 °C and less) were systematically annealed in air or inert gas to 150 °C for times consistent with the film deposition time (typically 30−60 min). None of the films deposited at low temperatures showed a significant increase in film conductivity after annealing, indicating that the film deposition reaction process is important to achieve the high conductivity structure. The higher growth temperature combined with the sequential reaction kinetics in the cyclic oMLD process may promote the more favorably aligned crystal structure with larger overall conductivity. After polymerization, PEDOT is often rinsed to remove unreacted oxidants or metal salts or to facilitate dopant ion exchange.7,14 We therefore analyzed the conductivity, elemental composition, and thickness of oMLD films after rinsing with DI water, methanol, and dilute acid. After all rinses, profilometry shows no change in film thickness. Tables 2 and 3 show the

Figure 5. (a) X-ray diffraction spectra of low (100 °C) and high (150 °C) temperature PEDOT films deposited on thermal oxide coated silicon wafers. (b) Pole figures of the 020 reflection for a 100 °C PEDOT film (top) indicating random ordering and a 150 °C PEDOT film (bottom) showing crystalline texturing. Schematic illustration of the perceived crystalline structure of oMLD PEDOT thin films deposited at 100 °C (c) and 150 °C (d). Arrows indicate the PEDOT planes as described by Aasmundtveit et al.30

structure changes markedly as deposition temperature increases from 100 to 150 °C. Both diffraction patterns in Figure 5 exhibit crystalline structure consistent with the orthorhombic phase identified in prior experiments.30 For the oMLD films deposited at 100 °C, three main diffraction peaks appear at scattering angles of 2θ ∼ 4.9°, 9.8°, and 26°. These peaks can be assigned to the 100, 200, and 020 reflections of PEDOT, respectively. These diffraction peak positions and intensities match that of a randomly oriented polycrystalline PEDOT material. Because the 200 reflection is better resolved than the 100 reflection, it was used to calculate the “a” lattice parameter. For the oMLD films deposited at 150 °C, only the 020 reflection is present, suggesting crystalline fiber-texturing with the [010] crystal direction perpendicular to the substrate plane. PEDOT films exhibiting a similar 020 reflection have been previously reported with oCVD.14 A smaller 010 peak may be present at ∼13°, but it is too faint to be readily visible in the spectra. As discussed above, the diffraction pattern did not change with film thickness. Furthermore, the XRD spectra show no evidence for MoOx or MoClx crystals in the films. To further verify fiber-texturing, 020 pole-figures were collected for low and high temperature films (Figure 5b). For the higher temperature film, the 020 reflection aligns with the surface normal and has a full width at half-maximum (fwhm) of 2.608°. For films deposited at 100 °C, the 020 reflection shows no preferred orientation. Using the above scattering angles, the corresponding lattice parameters are calculated to be a = 18.01 Å and b = 6.84 Å. Supporting Information Table S3 provides the lattice parameter values. The b lattice parameter corresponds to the distance between stacked polymer chains and is consistent with literature.30 Along the a-axis, the stacked polymer chains are separated by doping ions. The a lattice parameter is known to change with the dopant ion,31 so the value of ∼18 Å is

Table 2. Conductivity Data for Individual PEDOT Films Pre- and Post-Rinsea conductivity (S cm−1) deposition temperature

rinse

before rinse

after rinse

100 °C

methanol DI water 0.5 M H2SO4 methanol DI water 0.5 M H2SO4 1 M HBr

84 ± 12 75 ± 10 71 ± 4 995 ± 8 1000 ± 8 1440 ± 9 1790 ± 10

79 ± 9 8±7 68 ± 7 711 ± 6 190 ± 5 1810 ± 14 2250 ± 14

150 °C

a

Uncertainty represents standard deviation for each sample.

conductivity and composition results. The DI water rinse removed Mo and Cl, and the conductivity decreased by >80% for both 100 and 150 °C substrates, as expected upon removal of the Cl dopant. The methanol rinse removed some Cl, which may be due to water present in the methanol. This led to some decrease in conductivity, especially in the film deposited at 150 °C. The H2SO4 rinse also removed Mo and Cl but led to sulfur incorporation in sulfate groups (XPS spectra, Figure 6). Moreover, conductivity increased by >20% for the 150 °C substrate after the H2SO4 rinse, consistent with sulfate replacing the Cl as a dopant.14 For the 100 °C film, the composition analysis also shows dopant exchange upon H2SO4 rinse, where the conductivity before and after rinse were similar, likely due to the reduced crystallinity in the low temperature films. A 1 M HBr acid rinse of a 150 °C substrate also shows dopant E

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sequential vapor exposure likely boosts precursor surface mobility and byproduct desorption, thereby promoting thermodynamically preferred ordering of the crystal structure. Postdeposition dilute acid rinses enhance conductivity, facilitate dopant ion exchange, and aid in the removal of spent oxidant. In addition to high conductivity, the oMLD approach provides well-controlled and adjustable film thickness and highly conformal film coverage on nonplanar substrates. This ability to form highly conductive films and deposit onto high aspectratio surfaces may promote advances in the design and performance of novel organic electronics and electrochemical energy storage systems.

Table 3. Elemental Analysis for Films Post-Rinsing from XPS and EDS Analysisa element (atom %) deposition temperature 100 °C

150 °C

rinse

C

O

S

Cl

Mo

Br

no rinseb methanolb DI waterc 0.5 M H2SO4b no rinseb methanolb DI waterb 0.5 M H2SO4b 1 M HBrb

52 51 72 60 49 56 64 58 65

33 41 22 28 35 31 27 30 27

5 4 5 12 7 8 8 11 7

4