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Water-Assisted Vapor Deposition of PEDOT Thin Film Hilal Goktas, Xiaoxue Wang, Asli Ugur, Karen K. Gleason*

The synthesis and characterization of poly(3,4-ethylenedioxythiophene) (PEDOT) using waterassisted vapor phase polymerization (VPP) and oxidative chemical vapor deposition (oCVD) are reported. For the VPP PEDOT, the oxidant, FeCl3, is sublimated onto the substrate from a heated crucible in the reactor chamber and subsequently exposed to 3,4-ethylenedioxythiophene (EDOT) monomer and water vapor in the same reactor. The oCVD PEDOT was produced by introducing the oxidant, EDOT monomer, and water vapor simultaneously to the reactor. The enhancement of doping and crystallinity is observed in the water-assisted oCVD thin films. The high doping level observed at UV–vis–NIR spectra for the oCVD PEDOT, suggests that water acts as a solubilizing agent for oxidant and its byproducts. Although the VPP produced PEDOT thin films are fully amorphous, their conductivities are comparable with that of the oCVD produced ones.

1. Introduction Organic electronic materials have received a great deal of interest since the discovery of electrical conductivity in π-conjugated polymers (CPs). Due to their ease of application on a variety of substrates, cost effectiveness, and unique properties, including mechanical flexibility, CPs offer a means to replace the traditional inorganic conductors.[1] The physical and chemical properties of poly(3,4ethylenedioxythiophene) (PEDOT), such as its electronic, optical, and stability make it the most industrially promising and widely studied CP.[2] Moreover, tuning of the conjugation length and doping levels make the properties of PEDOT compatible with a wide range of applications including organic thin film transistor, [3] light emitting diodes (OLED),[4] solar cells,[5] electrochromic mirrors,[6] bioProf. H. Goktas, X. Wang, Dr. A. Ugur, Prof. K. K. Gleason Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, MA 02139, USA E-mail: [email protected] Macromol. Rapid Commun. 2015, 36, 1283−1289 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

sensors,[7] and capacitors.[8] Since the application space for PEDOT is so diverse, the need for producing its thin films with the desired combination of electronic and optical properties is still an active research area. Vapor synthesis, including vapor phase polymerization (VPP) and oxidative chemical vapor deposition (oCVD), are oxidative step growth processes and have become common methods to produce PEDOT thin films. Vapor synthesized PEDOT has displayed high conductivity,[9] high transparency,[10] the ability to be grafted,[11] and the ability to deposit as an ultrathin conformal layer,[12] to form well-defined patterns,[13] and long-term stability.[14] The VPP and the oCVD techniques have recently been reviewed by Bhattacharyya et al.[15] The VPP of CPs is a two-step process and based on the chemical reaction between an oxidant and the monomer obtained from vapor phase.[16] Typically, the oxidant (such as Fe(III) Tosylate, FeCl3) is first deposited on the substrate by spin coating from a solution followed by a drying step to evaporate the solvent. The solid thin film of oxidant on the substrate is then exposed to monomer vapor.[17] In some reports, VPP proceeds without drying the oxidant

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DOI: 10.1002/marc.201500069

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resulting in growth of the PEDOT thin films on a liquid droplet oxidant.[9] The first demonstration for using VPP to produce PEDOT from 3,4-ethylenedioxythiophene (EDOT) and FeCl3 as oxidant in a reaction chamber under ambient conditions reported a conductivity of around 70 S cm−1.[18] By optimizing the process of VPP and using several additives, the conductivity of the VPP produced PEDOT improved substantially. For example, in order to eliminate the nonPEDOT impurities formed in the structure due to high acid nature of the oxidant, WintherJensen et al. added pyridine as a base inhibitor;[16] Zuber et al. added glycol-based surfactants to suppress the crystal growth of the oxidant and improved the proton scavenging by optimizing the humidity levels inside the reaction chamber during polymerization.[19] The highest conductivity attained for the PEDOT thin film produced under atmospheric condition by adding glycol-based surfactants and varying the humidity is around 800 S cm–1.[19] Fabretto et al. introduced water vapor into the reaction chamber during PEDOT polymerization via VPP at an operating pressure ranging from 3.7 to 26 Torr and added glycol-based surfactant to the oxidant to investigate the role of water.[20] Under these conditions, polymerization did not occur when water vapor was not present within the chamber,[20] and concluded that the water vapor acts as a proton scavenger on the EDOT dimers, allowing the polymerization cycle to repeat itself. The oCVD technique is a facile route to deposit thin films of CPs by flowing monomer vapors simultaneously while sublimating an oxidizing agent which reacts at the substrate via oxidative step growth polymerization. The oxidant vapor also dopes the conjugated backbone to achieve conductive films.[21] Since the deposition can be achieved at a moderate vacuum (≈0.1–0.01 Torr) and at a low substrate temperature (≈20–150 °C), oCVD provides a means to deposit conformal thin films on any kind of substrate such as planar or nonplanar surfaces, paper, textile, etc. with uniformity and thickness control.[13] However, the oCVD PEDOT produced at moderate pressure and temperature without water inclusion is conductive but amorphous.[22] Here, we present the synthesis of PEDOT thin films via VPP and oCVD by introducing water vapor to the reactor simultaneously with the monomer to enhance the morphology and doping level of the produced films. As it is proposed, if the water vapor acts as a proton scavenger by removing the hydrogen from the polymerization environment as hydronium, the crystallinity of the produced PEDOT polymer should be improved through the enhancement of conjugation and doping level. Moreover, since the oxidant, FeCl3 is highly soluble in water, it is thought that the water vapor will dissolve the oxidant to produce Fe+, Cl− ions, which will prevent the crystallization of the oxidant and provide a higher doping concentration.

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An enhancement of the crystallinity and doping level is anticipated to lead to higher conductivity. Hence, to investigate the doping structure of the PEDOT, the oxidant, FeCl3, was employed in two different ways; a) by sublimation of oxidant onto the substrate prior to feeding the monomer and water vapor to the reactor, namely vapor phase deposition (VPP) and b) simultaneously exposing the substrate to all three components (oxidant, monomer, and water vapor), which is referred to as oCVD.

2. Experimental Section The VPP and oCVD process to deposit PEDOT thin films were performed in a custom-built vacuum chamber that has two inlet ports (for monomer and water vapor) and an exhaust to a pump. The details of the experimental setup and deposition procedure of the oCVD system which is also used for VPP were previously reported.[21,23] Briefly, the oxidizing agent, FeCl3, which was used as purchased from Sigma-Aldrich, is placed in a resistively heated crucible at the bottom of the reactor and was sublimed at 350 °C for all the experiments. Above the oxidant crucible, glass slides and silicon wafers substrates were attached to a downward-facing stage which was held at a constant temperature of 70, 90, or 110 °C for the series of experiments. EDOT monomer and deionized water vapor were introduced into the chamber from side inlet ports on the reactor and the chamber pressure was held constant at 25 mTorr. The monomer jar was kept at 140 °C and the deionized water bubbler at room temperature with argon gas passed through the bubbler as the carrier gas. The reactor body temperature was held at least 30 °C hotter than that of the stage for all experiments. All the process parameters were kept constant for the performed two main series of experiments; VPP (the oxidant sublimed first on the substrate then the vapors introduced to the reactor) and oCVD (the oxidant sublimation and the vapors feed to the chamber simultaneously to react at or near the substrate). Since the vapor pressure of dimers is typically ≈1000× lower than for the corresponding monomer[24] and the EDOT monomer itself has low volatility, it is difficult to imagine the polymerization of PEDOT in the vapor phase occurring to any significant extent. In both cases, the deposition time was 25 min. The samples were subsequently removed from the vacuum chamber, and immediately rinsed with methanol (≥99.9%, Sigma-Aldrich) for 5 min to remove unreacted monomer and/or oxidant. Polymer characterization: UV−vis−NIR absorption spectra of the PEDOT thin films on glass substrates were measured with a Cary 5000 (Varian) UV−vis−NIR dual-beam spectrophotometer, which was calibrated using a blank substrate. The thicknesses of the thin films were measured using a Veeco Dektak 150 surface profilometer with a stylus having a radius of 2.5 μm and a force of 1.5 mg using delta average step height analytical function. The sheet resistances of the films deposited on glass substrates were measured with a Jandel four-point probe where the samples were subjected to five average reading at each forward and reverse current measurements. The conductivity of the produced samples was calculated by using the measured sheet resistance and the thickness measured with the profilometer.

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The Fourier transform infrared (FTIR) spectra of the thin films on Si wafers were obtained by using Nexus 870, Thermo Electron Corp. spectrophotometer. Atomic force microscopy (AFM) was performed in a Veeco Multimode noncontact height mode. The multipurpose Rigaku SmartLab diffractometer operated at 45 kV and 200 mA was used to generate CuKα1 (0.154 nm) radiation for X-ray diffraction (XRD) data collection.

3. Results and Discussion 3.1. XRD Figure 1 compares the XRD spectra for the polymer films deposited on Si(001) (the bare spectra of Si wafer given in Figure S1, Supporting Information) substrates by the two different processes. In Figure 1a, the oxidant is first sublimated on the substrate followed by EDOT and H2O vapors (VPP), and in Figure 1b the oxidant and the vapors are introduced simultaneously (oCVD). For the former, the films are completely amorphous with no sign of crystallinity. In the VPP process, water reacts only with the oxidant at the surface since there is no continuous flow of FeCl3. Therefore, water dissolves a small amount of FeCl3, which results in small quantities of free Fe+, Cl− ions. In this scenario, crystallization is not facilitated and the polymer film structure remains totally amorphous as for the PEDOT produced without water vapor (Figure S2b, Supporting Information). In this case, the adsorption and/or condensation of the monomer onto the oxidant surface not only limits the rate of deposition but also may prevent the formation of any favorable orientation. However, the situation changes dramatically for the latter case, wherein the crystallinity of the polymer films was enhanced compared with the films produced under the same conditions without using water vapor (Figure S2a, Supporting Information). XRD reveals the distribution of crystallite orientations through two types of reflections on these samples, (h00) and (0k0), where the dominant reflection is the (h00).[25] (100) peak corresponds to edge-on configuration for PEDOT corresponding to a d-spacing of 1.38 nm. (020) peak corresponds to face-on

configuration for PEDOT with d-spacing of 0.34 nm. The peak at 26° becomes smaller with temperature but the first peak at 6.4° is increasing which shows an improvement in crystallinity in edge-on configuration. The enhancement of (h00) peak suggests that the water vapor may act as proton scavenger leading to an increase in the conjugation length, which may in turn provide a favorable arrangement of the PEDOT subunits. For the case of waterassisted oCVD, water vapor continuously reacts with FeCl3 since they are both introduced to the chamber at the same time in vapor phase. Therefore, the water vapor solvates most of the FeCl3 to Fe+, Cl− ions in agreement with UV– vis–NIR results given below. It is possible that the electrostatic effect arising from Fe+, Cl− ions with the polar nature of water molecule induce a new crystallite orientation, (0k0) peak at the water-assisted oCVD deposition. Since this peak was not observed for the PEDOT films produced without water vapor (Figure S2a, Supporting Information), it can be concluded that the crystallinity is a direct consequence of the water vapor addition. Moreover, the intensity of the peak decreases with substrate temperature, which is a direct consequence of water vapor volatility. 3.2. AFM We used AFM to study the surface morphology of the polymer films. Figure 2a–c) are AFM scans for water-assisted oCVD produced PEDOT films and d–f) are for films synthesized using the water-assisted VPP method. All the samples were deposited with water vapor introduced through a bubbler. The surface roughness is listed in Table 1. The surface roughness of oCVD and VPP produced PEDOT films with and without water vapor addition are heavily influenced by substrate temperature (Figure S3 and Table S1, Supporting Information) and exhibit similar trends. The roughness of oCVD PEDOT samples increase with temperature. The XRD results (Figure 1) revealed that the water vapor and high substrate temperature enhance the crystallinity of oCVD PEDOT. Thus, the higher

Figure 1. XRD spectra of PEDOT thin-films vapor deposited at three successive temperatures a) by VPP; and b) oCVD.

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Figure 2. AFM scans of PEDOT films synthesized using VPP and oCVD techniques. a–c) are the samples synthesized by VPP at a stage temperature of 70, 90, and 110 °C, respectively; d–f) are the samples synthesized via oCVD and under 70, 90, and 110 °C, respectively.

crystallinity leads to a higher surface roughness for oCVD samples. The roughness of oCVD water-free samples is close to the oCVD water-assisted one. This phenomenon may indicate that the water vapor has limited influence on the surface roughness of oCVD samples. On the contrary, for the VPP samples, the roughness of the surface decreases with substrate temperature, and the roughness of VPP samples is in general, smaller than their oCVD counterparts. Previous reports indicated that the surface morphology of VPP PEDOT is associated with the redox Table 1. Data collected from image analysis of AFM given at Figure 2.

T [°C] VPP

oCVD

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Roughness Particle diameter Height [nm] [μm] [nm]

activity of the oxidants.[26] High FeCl3 redox activities lead to rapid polymerization, which provides defect sites, and therefore increase the film roughness. Since the VPP polymerization rate is much slower than that of the oCVD process due to slow dissociation rate of FeCl3, and combined effect with the amorphous structure of VPP samples, the roughness of VPP PEDOT is smaller than that of the oCVD PEDOT. Moreover, the roughness of the water-assisted VPP samples (90 and 110 °C) is smaller than the roughness of water free VPP samples (Table S1, Supporting Information) at the same temperatures. In this case, the water vapor may remove any residual, or defect established during polymerization. Hence, in VPP process, the surface roughness is influenced both by temperature and water vapor.

70

4.31

0.087

8.147

90

2.73

0.103

4.223

3.3. UV–Visible–NIR Spectroscopy

110

0.704

0.05

1.89

70

6.89

0.092

7.293

90

5.87

0.078

4.267

110

17.4

0.171

14.928

Thickness normalized UV–vis–NIR absorption spectra of the as-grown PEDOT (Figure 3) depends both on the FeCl3 sublimation process (VPP or oCVD) and deposition temperature. For VPP, Figure 3a shows a red shift and an intensity increase at π–π* transitions (224 and 315 nm)

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a low level doping and/or high oligomer content present in the structure.[20,27] On the contrary, the UV–vis–NIR spectra (see Figure 3b) of the PEDOT produced by oCVD shows a characteristic spectra like the electrochemically oxidized PEDOT at around 1.0 V electrode potential, where a high doping induced transition is observed at NIR region.[28] As seen in Figure 3a,b, the relative intensity of π–π* transitions in oCVD PEDOT decreases when compared with VPP PEDOT with a more intense absorption peak at around 800 nm, which strongly suggests that a high doping level is achieved in oCVD PEDOT[21] along with the introduction of polaronic energy states.[29] The UV– vis–NIR spectra revealed not only the doping level of the PEDOT produced via water-assisted oCVD is higher than that of the VPP PEDOT but also it is higher than that of the oCVD film produced without water vapor (see Figure 3c). As a result, the continuous flow of oxidant and water vapor provide a high doping level via the disintegration of the oxidant and leading to a more extended conjugation of p-doped PEDOT. Even, the feed of water vapor at the VPP case give relatively higher doping level with respect to the VPP case (Figure 3c) produced without water vapor, as well. The sheet resistance (Rsh), thickness, and conductivity (σ) of water-assisted VPP and oCVD-produced PEDOT thin films are given in Table 2 where the deposition time was kept constant for both techniques. The decrease in Rsh and increase in σ with the increase of substrate temperature are consistent with the red shift of π–π* transitions of UV– vis–NIR result.[21] At all three temperatures, the oCVD provides substantially lower sheet resistance as desired for Figure 3. Thickness normalized UV–vis–NIR spectra of PEDOT thin films synthesized via electrode applications. Indeed, the oCVD deposited at the lowest temperature a) VPP; and b) oCVD at 70, 90, and 110 °C stage temperature; and c) with and without water vapor at 110 °C. has a lower sheet resistance than the VPP film deposited at the highest temperature. In the case of VPP, the low thickness provides with the increase of substrate temperature and a shoulder high transparency (≥90%)@550 nm and the oCVD PEDOT peak which red-shifts toward 370 nm (see the inset figure). with higher thickness resulted in transparency lower While the red shift indicates an increase in the conjugation than 10%. Although the oCVD film thickness is 20 times length providing higher conductivity, the observation of thicker than that of the PEDOT produced via VPP, the higher absorption in the UV–vis region (π–π* transitions) conductivities are at the same order of magnitude at with a lower absorption plateau in the NIR region suggests

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Table 2. Sheet resistance (Rsh), and conductivity of VPP and oCVP produced PEDOT.

VPP

oCVD

70

90

110

70

90

110

°C−1]

8.5 k

1.4 k

700

320

170

80

Thickness [nm−1]

20 ± 3.0

23 ± 3.0

8 ± 4.0

460

485

245

−1

58 ± 9

310 ± 40

1785 ± 892

68

121

1042

Substrate Temp [°C] Rsh [Ω

σ [S cm ]

each substrate temperature. This ability to achieve high conductivity for thicker films is important for achieving low Rsh values. And, even though the UV–vis–NIR result revealed that the oCVD PEDOT is more doped than the VPP PEDOT, their conductivities are almost similar, which suggests that the doping level is not the major factor responsible for the change in conductivity. The similar conductivity could arise from a combination of the following; hopping mechanism, physical/electronic states, conjugation length, doping levels, and intermolecular electronic interactions. Lastly, both VPP and oCVD processes produced PEDOT without introducing water vapor to the reactor, their Rsh value are approximately an order of magnitude higher and the conductivity is again an order lower than that of the one produced via water-assistance. Hence, the water vapor enhances the conductivity in both deposition techniques.

4. Conclusion The influence of water vapor on PEDOT thin films produced by VPP and oCVD has been studied. The FTIR, XRD, and UV– vis–NIR spectra showed that the molecular structure of the thin film produced via VPP and oCVD techniques have similar molecular structure, with different doping level and degree of crystallinity. The efficient use of Fe+ and Cl− ions is obtained by dissolving FeCl3 with water vapor on or near the surface resulting in better doping levels compared with the case where no water vapor is used. In the case of VPP deposition, the introduction of water vapor decreased the sheet resistance and increased the conductivity an

3.4. FTIR Spectroscopy The FTIR spectra of the thin films produced in both methods have the same absorption bands with a slight shift and various absorption intensities (see Figure 4a). The observed FTIR bands fit well with the spectra previously reported for oCVD[21] and electrochemically oxidized PEDOT.[30] The proposed band assignments for the thickness normalized spectra are shown in Figure 4b are: 1520, 1470 cm−1 C C stretching vibration, 1365 cm−1 C H stretching vibration at oxy-methylene, 1310 cm−1 interring stretching vibration of thiophene, 1130 cm−1 C C stretch at oxy, 1000–1060 cm−1 C O stretching at oxy and/or thiophene cycle deformation, 920–960 cm−1 stretching symmetric C S C deformation; 880, 820 cm−1 oxyethylene ring deformation, 700–720 cm−1 CH2 rocking oxy-methylene. The p-doped PEDOT always has a band at approximately 1520 cm−1, which is observed at both spectra suggesting that even the sublimed oxidant on the substrate can give rise to some level of doping.[30] This doping effect can enhance the conductivity of the thin films which is in-agreement with the 4-point probe measurement. The high relative intensity ratio of the bands near 720 cm−1, the methylene rocking/C H out-of-plane bending vibrations bands, suggest that while the oCVD PEDOT has a crystalline fraction, the VPP produced films do not.[31] This observation was confirmed via XRD spectra as well (Figure 1).

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Figure 4. The FTIR spectra of water-assisted oCVD and VPP PEDOT a) full range (4000–650 cm−1); and b) the normalized spectra for 1600–650 cm−1 wavenumbers.

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order of magnitude compared with the films produced without water vapor. The addition of H2O during the oCVD process further promotes the packing of the PEDOT chains perpendicular to the plane of the substrate. Four-point probe and UV–vis–NIR results revealed that since the conductivities of the thin films produced with both deposition techniques are very close to each other, the doping level is not a major contributor to the observed results. Morphological studies by XRD, AFM show that increased substrate deposition temperature with introduction of water vapor facilitates better alignment of molecules for oCVD PEDOT and a smoother surface for the VPP PEDOT.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: Hilal Goktas, on-leave from Physics Department of Canakkale Onsekiz Mart University, was supported by TUBITAK Turkey. This work was supported in part by the U.S. Army Research Laboratory and the U.S. Army Research Office through the Institute for Soldier Nanotechnologies, under contract number DAAD-19–02D-0002. Received: February 4, 2015; Revised: March 12, 2015; Published online: April 17, 2015; DOI: 10.1002/marc.201500069 Keywords: conductivity; morphology; poly(3; ethylenedioxythiophene); vapor deposition; water vapor

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Water-Assisted Vapor Deposition of PEDOT Thin Film.

The synthesis and characterization of poly(3,4-ethylenedioxythiophene) (PEDOT) using water-assisted vapor phase polymerization (VPP) and oxidative che...
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