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Semiconductor-Metal Transition in Poly (3,4-Ethylenedioxythiophene): Poly(Styrenesulfonate) and its Electrical Conductivity While Being Stretched.

INTRODUCTION

Various studies have been carried out on poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), some of which are cited here [1-17]. Aleshin et al. [1] showed that PH of the PEDOT:PSS colloidal solution from which free-standing polymer films were prepared affected their electrical conductivity at room temperature. The highest conductivity that they were able to achieve was 20.6 S [cm.sup.-1] for a film prepared from a solution with PH 1.23 [1]. Moreover, the conductivity of their PEDOT: PSS films changed in an exponential fashion with temperature [mathematical expression not reproducible] over the temperature range 300-6 K [1].

Kim et al. [2] fabricated various PEDOT:PSS free-standing films from PEDOT:PSS aqueous dispersion containing different solvents (tetrahydrofuran [THF], N,N-dimethyl formamide [DMF], dimethylsulfoxide [DMSO], or [H.sub.2]O) and investigated the effect of those solvents on the electrical conductivity of PEDOT:PSS films. Among various solvents used, DMSO improved the conductivity the most ([sigma] ~ 80 S [cm.sup.-1]) because of its higher dielectric constant leading to a stronger screening effect between charge carriers and counterions [2]. Through optical absorbance measurements and X-ray diffraction (XRD) characterization, they discovered that the structure of PEDOT:PSS films was independent of the solvent added to the solution [2]. Further, while DC conductivity of the PEDOT:PSS film prepared from pristine PEDOT:PSS aqueous dispersion changed exponentially with respect to temperature [mathematical expression not reproducible] over the temperature range 300-10 K, the electrical conductivity of PEDOT:PSS films prepared from solutions with organic solvents had a power-law dependence on temperature [2].

Na et al. [8] fabricated a variety of PEDOT:PSS films from PEDOT:PSS aqueous dispersion to which DMSO was added in various concentrations and characterized them in order to study the effect of DMSO on the properties of PEDOT:PSS films. Their research revealed that DMSO rendered the distribution of PEDOT molecules more uniform throughout the PEDOT:PSS film and reduced the insulating PSS-rich areas between conductive PEDOT-rich regions and hence facilitated charge transport and increased the electrical conductivity of the film [8].

Liu et al. [9] studied the effect of additives such as ethylene glycol (EG) and DMSO on the electrical conductivity of PEDOT: PSS films. To this end, they prepared several PEDOT:PSS freestanding films from PEDOT:PSS aqueous dispersion with different volume ratios of DMSO or EG [9]. From DC conductivity and Hall effect measurements carried out on all samples, they deduced that the addition of EG or DMSO to the polymer aqueous dispersion increased the electrical conductivity of PEDOT: PSS films, primarily by enhancing the mobility of charge carriers without notably changing their concentration [9]. In addition, the temperature dependence of the conductivity of all samples was characteristic of a semiconductor [9].

Zhou et al. [11] researched the temperature dependence of the electrical conductivity of PEDOT:PSS films, prepared from pristine PEDOT:PSS aqueous dispersion, over the temperature range -150 to 250 [degrees]C (123-523 K). Their findings indicated that the polymer film was in the semiconducting state over the entire temperature range [11]. Khasim et al. [12] improved the electrical conductivity of PEDOT:PSS thin films by adding sorbitol to the polymer aqueous dispersion before film preparation. The results of DC conductivity measurements over the temperature range 20-200 [degrees]C (293-4-73 K) proved that all the polymer films prepared from solutions containing different amounts of sorbitol were semiconductors [12].

Chanda et al. [13] were able to increase the electrical conductivity of PEDOTPSS films by adding DMSO to PEDOT:PSS aqueous dispersion before film deposition. From the results of X-ray photoelectron spectroscopy (XPS) measurements made on PEDOT:PSS films and theoretical considerations, they inferred that the conductivity enhancement was a result of thinner energy barriers between conductive regions and PSS phase segregation on the surface of the films following the addition of DMSO to the polymer dispersion [13].

To our knowledge, there were few studies in which researchers observed metallic behavior in PEDOT:PSS films over limited temperature ranges [14-17]. For instance, in one study the metallic state was detected in some PEDOT:PSS films, prepared from DMSO-treated PEDOT:PSS aqueous dispersion, at very low temperatures (1.8-10 K) [15]. In another study, the PEDOT:PSS films treated with a [gamma]-butyrolactone (GBL) solution of methylammonium iodide (MAI) or a DMF solution of MAI showed metallic behavior over the temperature range 320-350 K [16]. Also, PEDOT:PSS films (fabricated from PEDOT:PSS dispersion treated with DMSO) behaved like a metal only above ~350 K when deposited on poly(ethyleneterephthalate) (PET) non-woven microfibers [14] and over short temperature ranges far below 380 K when deposited on cotton fabrics [17].

Flexible and stretchable electronics can open the door to many applications that would not be achieved with their rigid counterparts [18, 19]. Various techniques were developed in order to produce stretchable electrical conductors based on conductive materials such as gold [20-30], silicon [31-33], single-walled carbon nanotubes (SWCNTs) [34-40], copper [41, 42], silver nanowires [43-46], and PEDOT:PSS [47-52]. Polydimethylsiloxane (PDMS) has been widely used in the form of a relaxed and un-patterned [20, 21, 23, 27-29, 48], prestretched and un-patterned [22, 23, 25, 31-33, 35, 42], or relaxed and surface-patterned substrate [24, 26, 29] on which conductors or circuits were deposited in order to make them stretchable. There have also been other ways of fabricating stretchable conductors such as integrating gold nanoparticles and polyurethane (PU) into a composite [30], enclosing conductors with PDMS [40, 41, 44, 46], combining DMSO-treated PEDOT:PSS and PU into stretchable composite fibers [51], or spray coating poly(ethylene terephthalate) (PET) substrates with a solution made of PEDOT:PSS aqueous dispersion, water, EG, and isopropyl alcohol (IPA) [49]. Another example is preparing elastic composite films from SWCNTs, an ionic liquid and a fluorinated copolymer [34, 36] using two different methods one of which produces a more elastic film that is printable on a PDMS substrate [36], whereas the other one produces a film that needs to be perforated and coated with PDMS to gain higher stretchability [34]. Preparation of stretchable or flexible conductors paves the way for the fabrication of stretchable and/or flexible electronics including, but not limited to, metal oxide semiconductor field-effect transistors (MOSFETs) [31-33], acoustic devices [53], light-emitting diode (LED) displays [36], energy storage devices [37, 54, 55], pressure sensors [29, 48], wearable sensors [46, 52], electrocardiography (ECG) monitoring systems [56], organic solar cells [48, 49], and polymer light-emitting diodes (PLEDs) [50].

In this paper, we present the results of the electronic characterization of PEDOT:PSS films deposited on glass and stretchable knitted fabric. The experimental results indicate that PEDOT:PSS exhibits metallic properties at room temperature and conducts electricity while under stretch. The research results reported in this paper are part of the first author's doctoral dissertation [57].

EXPERIMENT

Materials

An aqueous dispersion of PEDOT:PSS (Clevios[TM] PH1000) was purchased from Heraeus (USA). DMSO was purchased from Sigma Aldrich. Microscope glass slides were purchased from Fisher. A free sample of fine copper wire was generously given to us by California Fine Wire Company. Spandex fabric was donated by Lubrizol.

Sample Preparation

A solution consisting of 5 wt% DMSO and 95 wt% PEDOT: PSS aqueous dispersion was prepared and sonicated for 10 min. A thick PEDOT:PSS film (9.55 [micro]m thick) was prepared from the DMSO-treated PEDOT:PSS aqueous dispersion using a method similar to the one that Massonnet et al. [58] developed for the preparation of thick PEDOT:PSS films (10-12 [micro]m thick) from the pristine PEDOT:PSS dispersion. First, a 2.5 cm x 2.5 cm glass slide was sonicated in deionized (DI) water, ethanol, and acetone, respectively. The first two cleaning steps were followed by a rinse with DI water. Next, 0.8 mL of the aqueous dispersion of PEDOT: PSS treated with 5 wt% DMSO was drop cast on the glass substrate and was left under a hood at room temperature for ~48 h. Then, the sample was placed in an oven in an ambient atmosphere at 80 [degrees]C for 24 h.

The method used to fabricate conductive fabric was developed by Sotzing [59]. First a 2 cm x 1 cm rectangular piece was cut from Spandex knitted fabric and treated with Oxygen/Argon plasma for 20 s. Next, it was put in a glass Petri dish and the DMSO treated PEDOT:PSS aqueous dispersion was drop cast onto it. After letting the fabric soak in the solution for 30 min, it was hung for an hour at room temperature. Then, the fabric was placed in an oven in an ambient atmosphere at 110 [degrees]C for 1 h. When the fabric was taken out of the oven, the whole process was repeated one more time to obtain the desired conductive fabric.

Sample Characterization

The electrical sheet resistance, DC conductivity, Hall mobility and charge carrier concentration of the PEDOT:PSS film were measured using van der Pauw method [60]. These measurements were carried out utilizing an HP 4145B Semiconductor Parameter Analyzer. For Hall effect measurements, a magnet with magnetic field strength of 0.7 Tesla was also used. A Quantum Design I Physical Property Measurement System EverCool-II[R] was utilized in order to measure the electrical resistance of the PEDOT: PSS film on glass while the temperature was being decreased from 380 K to 10 K. For this characterization, a small piece of the prepared sample was cut, on which a row of four contacts were made by applying silver paste to attach fine copper wires to the surface of the PEDOT:PSS film. The silver paste was allowed to dry by placing the sample in an oven at 60 [degrees]C for an hour. Then, the copper wires were soldered to the sample holder before resistance measurements at different temperatures were taken. The measurements were finally performed using four-point probe technique. A scanning electron microscope (JEOL JSM-6335F) was utilized to check the uniformity of the thickness of the polymer film on glass.

RESULTS AND DISCUSSION

PEDOT.PSS Film on Glass

Figure 1a shows the electrical resistance of the PEDOT:PSS film on the glass substrate at different temperatures [57: p.28]. The electrical resistance of the PEDOT:PSS film changed variously with respect to temperature over three ranges of temperature. This indicates that there are different charge transport mechanisms in the PEDOT: PSS film over different ranges of temperature. In order to proceed with further analysis of Fig. 1a, each range of temperature, over which the temperature dependence of the resistance followed a different trend, was studied separately. Figure lb displays that the electrical resistance of the PEDOT:PSS film decreased while the temperature was being decreased from 380 K to 292 K, which is indicative of a metallic behavior [57: p.31 ]. Over this temperature range, the resistance of the PEDOT:PSS film has a fifth order polynomial dependence on the temperature, R = -4E-11 [T.sup.5] 6E-08 [T.sup.4] - 4E -05 [T.sup.3] + 0.0137 [T.sup.2]-2.3089 T + 155.58. However, by further decreasing the temperature below 292 K, the electrical resistance started increasing with decreasing temperature. From 290 K to 258 K, the electrical resistance of PEDOT:PSS is a power function of temperature (R = 6.9518 [T.sup.-0505]). Such a relationship between the resistance and temperature indicates that PEDOT:PSS is in the critical regime at the boundary of metal-semiconductor transition over this temperature range [61, 62]. This regime is depicted in Fig. 1c [57: p.33].

For temperatures lower than 258 K, the resistance changed in an exponential fashion with temperature [mathematical expression not reproducible]), which shows that the charge carrier transport mechanism in the PEDOT: PSS film over the temperature range 10-258 K can be explained by the three dimensional variable range hopping (3D VRH) model [63, 64]. The [T.sub.0] value obtained from the equation Ln(R) = 3.4511 [T.sup.-1/4] - 1.736 that was fitted to the experimental data in Fig. Id is approximately 141.85 K [57: p.34].

Conductivity and Hall effect measurements were also performed on the PEDOT:PSS film on glass at room temperature. The results are presented in Table 1. The electrical conductivity of the PEDOT:PSS film is high because of the presence of DMSO in the solution from which it was prepared. As a result of the addition of DMSO to PEDOT:PSS aqueous dispersion, PSSH-DMSO hydrogen bonds are formed which are stronger than water solvation energy, PSS-PEDOT, PSS-PSS, and PEDOT-DMSO interactions, hence the phase separation of PSS from PEDOT [65].

Additionally, the charge carrier concentration of the PEDOT: PSS film in this study is higher than the one previously reported for PEDOT:PSS films [9]. Also, because the charge carrier concentration of metallic polymers is usually in the 2-5 x [10.sup.21] [cm.sup.-3] range [66], the value of the charge carrier concentration of our sample is another indicator that the polymer film is a metal at room temperature.

PEDOT:PSS Film on Stretchable Knitted Fabric

Owing to the anisotropic structure of the knitted fabric, the electrical conductivity and sheet resistance of the conductive fabric coated with PEDOT:PSS layers are not scalar. Even though van der Pauw technique was initially developed for isotropic conductors, Price [67] proved that it could also be applied to aniso-tropic ones with tensor conductivity in order to measure the geometric mean of the conductivity tensor's principal components in the plane of the conductors, provided that the x and y axes are along the conductivity tensor's principal axes. Banaszczyk et al. [68] also showed that van der Pauw technique could be employed for measuring the sheet resistance of conductive fabrics.

In this study, the fabric was stretched in its course direction which was considered as x axis. Figure 2 shows that the geometric mean of the principal components of the conductive fabric's electrical conductivity decreased in an exponential fashion, [([[sigma].sub.x][[sigma].sub.y]).sup.1/2] = 121.08[e.sup.(-0.057][epsilon])], as the sample was being stretched to higher amounts of strain up to [epsilon] = 48.7% [57: pp.47,48]. It also displays the geometric mean of the principal components of its sheet resistance increasing exponentially with increasing strain, [([R.sub.s,x] [R.sub.s,y]).sup.1/2] = 16.515[e.sup.(00577[epsilon])]. The sheet resistance of the conductive spandex fabric in the relaxed state was ~14.6 [ohm] [sq.sup.-1]. This value of sheet resistance is much lower than that of the para-aramide fabric coated with polypyrrole [68]. Because the fabric was entirely covered with PEDOT:PSS, the thickness of the conductive fabric was used in the calculation of its conductivity. Stretching the conductive fabric lengthens some of the chemical bonds in PEDOT:PSS and breaks them at sufficiently large strains, which constantly decreases the electrical conductivity of the sample as the applied strain is increased. It is worth noting that PEDOT:PSS only constituted 4.42% of the total weight of the conductive Spandex fabric. Our stretchable fabric coated with DMSO-treated PEDOT:PSS exhibits a great potential to be applied in wearable technology upon optimization.

CONCLUSIONS

In summary, PEDOT:PSS aqueous dispersion was first treated with 5 wt% DMSO from which a uniform PEDOT:PSS film was prepared on glass. The temperature dependence of the electrical resistance of the PEDOT:PSS film was studied over the temperature range 380-10 K. The resistance versus temperature graph indicates that PEDOT:PSS is in the metallic state over the temperature range 380-292 K below which the polymer crosses over to the semiconducting state. High values were obtained for the charge carrier concentration and DC conductivity of the PEDOT: PSS film from Hall effect and conductivity measurements, respectively, which further confirm that PEDOT:PSS is a metal at room temperature. Moreover, a conductive stretchable textile was fabricated by simply coating stretchable knitted fabric with DMSO treated PEDOT:PSS. This conductive textile was then electrically characterized while being in relaxed and stretched states. The obtained results showed that PEDOT:PSS on stretchable fabric was electrically conductive under the applied strains.

ACKNOWLEDGMENTS

The authors would like to thank Dr. Menka Jain, Dr. Ali Gokirmak, and Dr. Helena Silva for allowing us to use their research instruments in order to characterize our samples. We also thank S. Yin for his experimental assistance with R versus T measurements.

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Neda Paziresh (iD), (1) Gregory Allen Sotzing (1,2,3)

(1) Department of Physics, University of Connecticut, Storrs, Connecticut, 06269

(2) Department of Chemistry, University of Connecticut, Storrs, Connecticut, 06269

(3) Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, 06269

Correspondence to: N. Paziresh; e-mail: neda.paziresh@uconn.edu

DOI 10.1002/pen.25074

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. (a) Temperature dependence of the electrical resistance of the PEDOT:PSS film prepared from DMSO treated PEDOT:PSS dispersion on a glass substrate [57: p.28]. (b) The metallic regime [57: p.31]. (c) The critical regime [57: p.33]. (d) The semiconducting regime [57: p.34].

Caption: FIG. 2. The principal components' geometric mean of the electrical conductivity and sheet resistance tensors of the spandex fabric coated with PEDOT: PSS while in the relaxed and stretched states [57: pp.47,481. The squares represent conductivity data points and the triangles represent the sheet resistance ones.
TABLE 1. Electrical sheet resistance, conductivity,
Hall mobility of charge carriers and charge carrier
concentration of the PEDOT:PSS film deposited on
the glass substrate.

Sheet                     Conductivity         Hall mobility
resistance                  ([sigma])        ([[mu].sub.Hall])
([R.sub.s])                ([cm.sup.2]        (n) ([cm.sup.2]
([ohm] [sq.sup.-1])    (S [cm.sup.-1])   [V.sup.-1] [s.sup.-1])

0.80                          1300                  2.7

Sheet                    Charge carrier
resistance                concentration
([R.sub.s])                    (n)
([OMEGA] [sq.sup.-1])     ([cm.sup.-1])

0.80                     3 x [10.sup.21]
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Author:Paziresh, Neda; Sotzing, Gregory Allen
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:May 1, 2019
Words:4468
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