Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

1 of 9

M A T E R I A L S S C I E N C E

Fully soluble self-doped poly(3,4-ethylenedioxythiophene) with an electrical conductivity greater than 1000 S cm−1

Hirokazu Yano1,2, Kazuki Kudo1, Kazumasa Marumo1, Hidenori Okuzaki1*

Wet-processable and highly conductive polymers are promising candidates for key materials in organic electronics. Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is commercially available as a water dis-persion of colloidal particles but has some technical issues with PSS. Here, we developed a novel fully soluble self-doped PEDOT (S-PEDOT) with an electrical conductivity as high as 1089 S cm−1 without additives (solvent ef-fect). Our results indicate that the molecular weight of S-PEDOT is the critical parameter for increasing the number of nanocrystals, corresponding to the S-PEDOT crystallites evaluated by x-ray diffraction and conductive atomic force microscopic analyses as having high electrical conductivity, which reduced both the average distance be-tween adjacent nanocrystals and the activation energy for the hopping of charge carriers, leading to the highest bulk conductivity.

INTRODUCTIONAlthough investigations of electrically conductive conjugated poly-mers date back to the early 1960s, particularly on polypyrrole and polyaniline (1), the development of conductive polymers in the 1970s by A. G. MacDiarmid, A. J. Heeger, and H. Shirakawa (2000 Nobel Prize in Chemistry), where electrical conductivity of polyacetylene films increased remarkably upon iodine doping (2, 3), accelerated explosively the use of organic electronics in low-cost, lightweight, and flexible electronic devices such as organic light-emitting diodes (OLEDs), organic field effect transistors, and organic photovoltaics (OPVs) (4, 5) since the early 1980s. Furthermore, in the era of the “Internet of things”, the field of organic electronics is developing via increased interest in printed electronics, stretchable electronics, and, recently, wearable electronics for use in flexible displays and touch panels, soft sensors, and actuators (6, 7). In such devices, a flexible, wet-processable, and highly conductive polymer would be a prom-ising candidate as a key material.

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS), developed by Bayer AG in 1990 (8, 9) and commercially available as a water dispersion of colloidal particles, is one of the most successful conductive polymers of the last two decades. Be-cause of its high electrical conductivity (up to 1000 S cm−1), trans-parency, and thermal stability, PEDOT:PSS has been used in a wide field of applications, including antistatic coatings, hole injection layers in OLEDs and OPVs, cathode materials in solid aluminum electrolytic capacitors (10), and transparent electrodes for various electronic devices, as an alternative to indium tin oxide (11, 12). However, PEDOT:PSS has some technical issues that impede its further practical application; these issues stem from the use of PSS as external ions to compensate the positive charges on PEDOT. Since the hydrophobic PEDOT core is surrounded by a shell of excess hydrophilic PSS (13) for water dispersion as a colloidal particle with a diameter of several tens of nanometers (11), (i) forming ultrathin layers thinner than the colloidal particle size is difficult (14), (ii) the PEDOT:PSS colloids aggregate and precipitate when stored for an

extended period, and (iii) the pristine PEDOT:PSS exhibits poor electrical conductivity (<1 S cm−1) (15); additives, such as ethylene glycol (EG), dimethyl sulfoxide (DMSO), and sorbitol, are therefore indispensable for improving electrical conductivity, namely, by the “solvent effect” (15−20), which can be interpreted in terms of the crystallization of the PEDOT core and the removal of the insulating PSS shell of the colloidal particles to enhance the transport of charge carriers between the PEDOT nanocrystals by a hopping mechanism (21−24). That is, the insoluble and intractable nature of PEDOT is responsible for these drawbacks of PEDOT:PSS having a 2.5-fold excess of PSS over PEDOT by weight (24). Therefore, prescient and intensive studies on fully soluble self-doped PEDOT (S-PEDOT) have been reported in the literature (25−29). Zotti and co-workers (26) synthesized an EDOT derivative bearing a sodium alkylsulfonate side chain, sodium 4-[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy]butane-1-sulfonate (fig. S1A), and polymerized it to obtain S-PEDOT with a mass average molecular weight (Mw) of 5000 g mol−1 and a maximum electrical conductivity of 1 to 5 S cm−1. On the other hand, Konradsson, Berggren, and co-workers reported chemical polymerizations of the same S-PEDOT and found that the electrical conductivities were 12 S cm−1 (27) and 30 S cm−1 (28), respectively. Since these conductivity values were two or three orders of magni-tude lower than that of the highly conductive PEDOT:PSS (up to 1000 S cm−1), improving the electrical conductivity of S-PEDOT has thus far been considered impossible.

In this study, we first synthesized a novel EDOT derivative (S-EDOT) bearing a sodium alkylsulfonate side chain, sodium 4-[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy]butane- 2-sulfonate (fig. S1B). From the viewpoints of safety, low environmental load, and mass production for commercialization, the S-EDOT mono-mer was designed because of the lower toxicity of 2,4-buthanesultone (table S1) (30) used in the synthetic process. Furthermore, the fully soluble S-PEDOT was synthesized by the oxidative polymerization of different concentrations (CS-EDOT) of the S-EDOT monomer in a range of 1 to 10 weight % (wt %) (Fig. 1) and characterized by means of gel permeation chromatography (GPC), UV-vis-NIR (ultraviolet– visible–near infrared) spectroscopy, x-ray diffraction analysis (XRD), conductive atomic force microscopy (cAFM), and the four-point tech-nique. It was found that the electrical conductivity achieved as high

F1

1Graduate Faculty of Interdisciplinary Research, University of Yamanashi, 4-4-37 Takeda, Kofu 400-8510, Japan. 2Organic Materials Research Laboratory, Tosoh Cor-poration, 4560 Kaisei-cho, Shunan 746-8501, Japan.*Corresponding author. Email: okuzaki@yamanashi.ac.jp

Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

2 of 9

as 1089 S cm−1, which is two orders of magnitude higher than those of previously reported S-PEDOTs (25−29), and exceeded the con-ductivity of PEDOT:PSS (up to 1000 S cm−1). The results led us to conclude that the molecular weight of S-PEDOT is the critical pa-rameter for increasing the number of nanocrystals, corresponding to the S-PEDOT crystallites evaluated by XRD and cAFM, which re-duced both the average distance between adjacent nanocrystals and the activation energy for the hopping of charge carriers, leading to the highest bulk conductivity.

RESULTSMolecular weight and solution propertiesS-PEDOT was synthesized by the oxidative polymerization of dif-ferent concentrations (CS-EDOT) of the S-EDOT monomer in a range of 1 to 10 wt % using (NH4)2S2O8 and FeSO4 as oxidant and catalyst, respectively (Fig. 1). The GPC elution curves of various S-PEDOTs are shown in Fig. 2A. Since the GPC measurement is not applicable to the PEDOT:PSS colloidal dispersions, these results provide clear evidence that the S-PEDOT is fully soluble, similar to other self-doped conductive polymers (25−29, 31−36). A flow-through test was carried out using a membrane filter with a pore diameter of 0.1 m [Optimizer V-47, 0.1 m hydrophobic ultrahigh molecular weight polyethylene (UPE), Entegris]. An S-PEDOT water solution (0.5 wt %) permeated without resistance, whereas a PEDOT:PSS water dispersion (0.5 wt %; Clevios PH1000, Heraeus) hardly passed through the membrane filter; only transparent water was permeated under pressurization (fig. S2). The GPC curves were unimodal with polydispersity indices (PDIs) of 3.15 to 4.15, indicative of a broad molecular weight distribution. This is probably because the termina-tion reaction is due to recombination rather than disproportiona tion (9). The weight average molecular weight (Mw) and the number av-erage molecular weight (Mn) of the S-PEDOT reached maximum values of 22,300 and 5380 g mol−1, respectively, at CS-EDOT = 5 wt %. Here, the number average degree of polymerization (DP) was ca. 18, which was more than twice higher than the value of the previously reported S-PEDOT (8 units average) measured by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (26). The decrease of both Mw and Mn at CS-EDOT > 5 wt % was likely due to a steep increase in the viscosity of the reaction solution; for example,

at CS-EDOT = 10 wt %, the stirring of the reaction solution became difficult in 30 min after adding the (NH4)2S2O8 solution, which may decrease the diffusion of reactants such as monomer, oxidant, and catalyst, leading to the inhomogeneous polymerization and/or low monomer conversion. Figure 2B shows that the viscosity of the S-PEDOT solutions (1 wt %) reached a maximum (24.3 mPa·s) at CS-EDOT = 5 wt %, reflecting the molecular weight, whereas the pH values were nearly constant (1.86 to 2.04). These values are compa-rable to those of the PEDOT:PSS water dispersion (1 wt %) with a viscosity and pH of 16 mPa·s and 1.89, respectively. As shown in Fig. 2C, the UV-vis-NIR spectra of the S-PEDOT water solutions (0.01 wt %) were less dependent on CS-EDOT, demonstrating the same electronic structure: A broad shoulder at approximately 800 nm is consistent with the low energy absorbance of neutral PEDOT, and a free carrier of bipolarons in the NIR region corresponds to PEDOT in the oxidized state (37−39). To compare the optical properties in more detail, absorption and transmittance spectra of S-PEDOT and PEDOT:PSS (Clevios PH1000, Heraeus) thin films were measured (fig. S3). The absorption of the S-PEDOT thin film (125 nm thick) was slightly higher than that of the PEDOT:PSS thin film (128 nm thick) in the whole experimental range of wavelength, which is as-sociated with the higher optical density of PEDOT chromophore in S-PEDOT than in PEDOT:PSS. On the other hand, the transmittance of the S-PEDOT thin film was lower than that of the PEDOT:PSS, where the value at 550 nm, near the center of visible light, was 83%. Furthermore, while PEDOT:PSS can be dispersed only in water and in several mixed solvents of water and alcohol, S-PEDOT is soluble not only in water but also in various organic solvents (table S2), which is a great advantage for use in organic electronic devices easily de-graded by water and for the fabrication of composites with water- insoluble polymers.

Crystalline structureXRD analysis was performed to clarify the structure of S-PEDOT; the results are shown in Fig. 3A. Although S-PEDOT is a new polymer, the diffraction patterns are simpler than the PEDOT:PSS (13,40) and similar to those of PEDOTs doped with perchlorate (PEDOT:ClO4), p-toluenesulfonate (PEDOT:TsO), and hexafluorophosphate (PEDOT:PF6), which indicates that the S-PEDOT unit cell belongs to an orthorhombic system (40−43). Therefore, the diffraction peaks

FeSO4 (0.6 eq.)(NH4)2S2O8 (2.0 eq.)

H2SO4 (1 M)20 , 20 hours in N2

TODEP-STODE-S

CS-EDOT = 1–10 wt %

n

–

Fig. 1. Synthesis of S-PEDOT by oxidative polymerization of the S-EDOT monomer.

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

3 of 9

at 2 = 5.3° (d = 16.6 Å), 10.5° (d = 8.4 Å), and 24.7° (d = 3.6 Å) can be assigned to the diffractions from the (100), (200), and (020) planes of the orthorhombic unit cell of the S-PEDOT crystal with lattice pa-rameters of a = 16.6 Å, b = 7.2 Å, and c = 7.8 Å. Since the main chain

of the S-PEDOT matches the c axis, the c value corresponds to the repetition period of two monomer units (7.8 Å) similar to that of PEDOT (40−43). However, the -stacking distance between two S-PEDOT chains, representing one-half of the value of b (3.6 Å), is

21 23 25 27 29 31 33

Inte

nsity

Retention time (min)

0

0.5

1

1.5

2

200 400 600 800 1000 1200

Abs

orba

nce

Wavelength (nm)

0

10

20

30

Visc

osity

(mP

as)

1

1.5

2

2.5

3

0 2 4 6 8 10

pHCS-EDOT (wt %)

CS-EDOT(wt %)

Mw(g mol–1)

Mn(g mol–1) PDI

1 14,400 3,480 4.143 17,700 4,660 3.795 22,300 5,380 4.157 19,000 4,790 3.968 15,900 4,300 3.70

10 10,600 3,380 3.15

BA

C

Fig. 2. Solution properties of S-PEDOT. (A) GPC elution curves, weight average molecular weight (Mw), number average molecular weight (Mn), and PDI of S-PEDOT synthesized with different CS-EDOT. (B) Viscosity and pH dependencies of the S-PEDOT water solutions (1 wt %) on CS-EDOT. (C) UV-vis-NIR spectra of the S-PEDOT water solutions (0.01 wt %) synthesized with different CS-EDOT.

0 10 20 30 40

X-ra

y in

tens

ity (a

.u.)

2θ (°)

60

65

70

75

Xc (

%)

0

2

4

6

8

10

0 2 4 6 8 10

Cry

stal

lite

size

(nm

)

CS-EDOT (wt %)

(020)(200)

1

3

5

7

8

10

CS-EDOT(wt %)

(100)

D100

D020

a

c

b

BA

Fig. 3. Crystalline structure of S-PEDOT. (A) X-ray diffraction (XRD) patterns of S-PEDOT films with different CS-EDOT and a possible crystalline structure of S-PEDOT (inset). (B) Crystallinity (Xc) and crystallite size (D100 and D020) dependencies of S-PEDOT films on CS-EDOT. a.u., arbitrary units.

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

4 of 9

slightly longer than that of PEDOT (3.4 Å) (40−43) due to the steric hindrance and/or electrostatic repulsion between the S-PEDOT side chains. Unlike polyalkylthiophenes, in which the dopant ions are present in the vicinity of the polymer chains (44), PEDOT exhibits dopant ions between the -stacked columns along the a axis direction (40−43). Thus, the a value (16.6 Å) of the S-PEDOT crystal is de-termined by the length of the alkylsulfonic acid side chains. Since the unit lattice contains four monomer units with the side chains alternatingly arranged in opposite directions with respect to the polymer chain, the theoretical density of the S-PEDOT crystal is calculated to be 1.59 g cm−3, which is consistent with the actual density measured by the method of Archimedes (1.50 to 1.53 g cm−3). Assuming that a broad halo in the 2 range of 15° to 35° corresponds to the amorphous S-PEDOT, the degree of crystallinity (Xc) can be expressed as the ratio of total area of crystalline peaks to total area of all diffraction peaks. As shown in Fig. 3B, the Xc value of the S-PEDOT film reached 72% at CS-EDOT = 5 wt %, the tendency of which agrees with the molecular weight and viscosity results (Fig. 2B). However, the result is inconsistent with the trend observed in common polymers (such as polyethylene); because of the entanglement of the molecular chains, a higher molecular weight results in a lower Xc (45). This trend suggests that the S-PEDOT exhibits less entan-glement in water since the polymer chains have an extended coil structure due to the rigid -conjugated backbone and electrostatic repulsion between the alkylsulfonic acid side chains, favoring crys-tallization by -stacking in the solid state. Notably, the Xc values are substantially higher than those of PEDOT:PSS (24) and are as-sociated with the higher weight fractions of PEDOT main chains in S-PEDOT (45 wt %), the absence of amorphous PSS chains (the weight fraction of PEDOT in PEDOT:PSS is only 29%), and/or the side chains of S-PEDOT being highly ordered and incorporated in the crystal-line phase. Assuming that the doping level of S-PEDOT is the same

as that of PEDOT (0.22 to 0.35) (46), corresponding to one charge per three to four monomer units, 22 to 35% of the sulfonic acid groups of the S-PEDOT side chains dissociate and compensate for the posi-tive charges on S-PEDOT main chains as counter ions. However, 65 to 78% of the undissociated sulfonic acid groups in the S-PEDOT side chains can form intermolecular hydrogen bonds and stabilize the crystal-line structure, as shown in the inset of Fig. 3A. Furthermore, the crys-tallite sizes normal to the (100) and (020) planes calculated using the Scherrer’s equation were D100 ≈ 6 nm and D020 ≈ 3 nm, respectively, regardless of the CS-EDOT (Fig. 3B). Therefore, the high Xc value can be explained in terms of the large number of crystallites, in which approximately four columns of eight S-PEDOT molecules -stacked in the b axis direction are accumulated in the a axis direction (fig. S4).

Surface morphology and current imagesAs shown in Fig. 4A, the surface morphologies of S-PEDOT thin films were similar irrespective of CS-EDOT, where numerous particles with an average particle size (Dp) of ca. 11 nm were densely packed to form a coherent solid film with a particle number (Np), defined as the number of particles in 1 m2, of 3000 to 3600 (Fig. 4B). In addi-tion, S-PEDOT thin films were smooth, with a surface roughness (Ra) of ca. 0.4 nm, compared to PEDOT:PSS films (1 ≤ Ra ≤ 2 nm) (21), because S-PEDOT is not dispersed as a colloid but is fully dis-solved in water. Figure 5A shows current images representing the distribution of local conductivity measured by cAFM; these images reveal that the highly conductive S-PEDOT nanocrystals (shown in the bright region) are sparsely distributed in less conducive amorphous matrices (shown in the blue region), similar to PEDOT:PSS (24). The average size of S-PEDOT nanocrystals (Dnc) was found to be an almost constant value of ca. 5 nm (Fig. 5B), coincident with the crystallite sizes (D100 and D020) in Fig. 3B, suggesting that the S-PEDOT nanocrystal corresponds to a single crystal. However,

0

5

10

15

0

1000

2000

3000

4000

5000

6000

Dp

(nm

)

Np

(µm

–2)

0

0.2

0.4

0.6

0.8

0 2 4 6 8 10

Ra

(nm

)

CS-EDOT (wt %)

10 wt %

CS-EDOT = 1 wt % 3 wt %

5 wt % 7 wt %

8 wt %

Dp

Np

BA

Fig. 4. AFM height images of S-PEDOT. (A) Surface morphologies of S-PEDOT thin films with different CS-EDOT. (B) Average particle size (Dp), number of particles (Np), and surface roughness (Ra) dependencies of S-PEDOT thin films on CS-EDOT.

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

5 of 9

the number of nanocrystals in 1 m2 (Nnc) shows a maximum of 5800 m−2 at CS-EDOT = 5 wt %. The tendency of Nnc is similar to those of the molecular weight (Fig. 2A) and the Xc (Fig. 3B), demonstrating that one particle in Fig. 4A consists of ca. two nanocrystals. Therefore,

the average distance between adjacent nanocrystals (Lnc) reached a minimum (3.9 nm) at CS-EDOT = 5 wt %. This interparticle distance is suitable for the hopping of charge carriers between adjacent nano-crystals, which is critical to achieving high bulk conductivity (24).

0

5

10

15

0

1000

2000

3000

4000

5000

6000

Dnc

(nm

)

Nnc

(µm

–2)

3

4

5

6

0 2 4 6 8 10

Lnc

(nm

)

CS-EDOT (wt %)

10 wt %

CS-EDOT = 1 wt % 3 wt %

5 wt % 7 wt %

8 wt %

Dnc

Nnc

LncNC NC

BA

Fig. 5. cAFM images of S-PEDOT. (A) Current mapping images of S-PEDOT thin films with different CS-EDOT. (B) Average nanocrystal size (Dnc), number of nanocrystals (Nnc), and average length between adjacent nanocrystals (Lnc) dependencies of S-PEDOT thin films on CS-EDOT.

0

200

400

600

800

1000

1200

0 2 4 6 8 10

Con

duct

ivity

(S c

m–1

)

CS-EDOT (wt %)

0

5

10

15

3.5 4 4.5 5 5.5 6

Ea

(meV

)

Lnc (nm)

CS-EDOT(wt %)

Lnc(nm)

σ77K(S cm–1)

σ293K(S cm–1)

Ea(meV)

1 5.33 91 300 10.7

3 4.27 445 877 6.1

5 3.95 677 1098 4.4

7 4.76 322 683 6.8

8 5.13 270 584 7.0

10 5.55 66 219 10.8

LncNC NC

C

BA

Fig. 6. Electrical conductivity and carrier transport properties of S-PEDOT. (A) Electrical conductivity dependency of S-PEDOT films measured by the four-point method on CS-EDOT. (B) Electrical conductivity and activation energy (Ea) dependencies of S-PEDOT films on Lnc. (C) Lnc, electrical conductivities at 77 K (77K) and 293 K (293K), and Ea of S-PEDOT films with different CS-EDOT. The vertical and horizontal axes of (B) (top) correspond to the vertical axis of (A) and the horizontal axis of (B) (bottom), respectively.

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

6 of 9

Electrical conductivity and transport of charge carriersTo clarify the correlation between the current images at the nanoscale and the transport of charge carriers in the bulk state, electrical con-ductivity was measured by the four-point technique. We should emphasize that an electrical conductivity as high as 1089 S cm−1 at CS-EDOT = 5 wt % (Fig. 6A) was attained without additives (solvent effect). This value is more than two orders of magnitude higher than that of the S-PEDOT (25−29) and exceeds the conductivity of PEDOT:PSS with additives such as EG and DMSO (up to 1000 S cm−1). In addition, the S-PEDOT was stable in nitrogen and air, where the electrical conductivity of the film held in air at room temperature for 1 year was almost constant to be 1058 S cm−1. On the other hand, the electrical conductivity was reduced to 836 S cm−1 after the film was soaked in water for 3 days. The value is still higher than those of the oxidative chemical vapor–deposited zwitterionic PEDOT thin films before (668 S cm−1) and after water soaking (172 S cm−1) (47), demonstrating that the S-PEDOT is stable even in water.

A strong correlation is observed between Lnc and electrical con-ductivity (Fig. 6B), demonstrating that the distribution of nanocrystals plays an important role in electrical conduction. According to the

nearest-neighbor hopping model (48), the activation energy (Ea) required for hopping of charge carriers can be calculated by measur-ing the electrical conductivity () at room temperature (T = 298 K) and that at liquid-nitrogen temperature (T = 77 K) and using the equation  = 0 exp.(−Ea/kBT), where 0 and kB are the conductivity factor and the Boltzmann constant (8.617 × 10−5 eV K−1), respec-tively (21). A linear relationship between Lnc and Ea demonstrates that the longer the distance between adjacent nanocrystals, the lower the probability of charge carrier hopping (Fig. 6B). Since the values of Ea (Fig. 6C) are lower than the thermal energy at room temperature, ca. 26 mV, the charge carriers in S-PEDOT have sufficient energy for hopping, which is responsible for the extremely high electrical conductivities at room temperature.

DISCUSSIONFigure 7A shows the hierarchical structure of S-PEDOT. The S-EDOT monomer (primary structure), sodium 4-[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy]butane-2-sulfonate, was newly synthesized and chemically polymerized into S-PEDOT, where Mw (secondary

0

200

400

600

800

1000

1200

3.5 4 4.5 5 5.5 6

Con

duct

ivity

(S c

m–1

)

Lnc (nm)

0

1

2

3

4

5

6

0

5

10

15

20

3000 4000 5000 6000

Lnc

(nm

)

Ea

(meV

)

Nnc (µm–2)

60

65

70

75

80

85

0

1000

2000

3000

4000

5000

6000

1.0 × 104 1.5 × 104 2.0 × 104 2.5 × 104

Xc

(%)

Nnc

(µm

–2)

Mw (g mol–1)

0 × 100

1 × 104

2 × 104

3 × 104

0 2 4 6 8 10CS-EDOT (wt %)

Tertiary structure(nanocrystal)

Quaternary structure

Primary structure(S-EDOT)

Secondary structure(S-PEDOT)

Xc

Nnc

Lnc

Ea

S

OO

O

SO3H

S

OO

O

SO3H

SO3–

O

O O

Sn

+

5.0 × 103

(bulk film)

Mw

(g m

ol–1

)

Fig. 7. Correlation between the hierarchical structure and properties of S-PEDOT. (A) Chemical structures of the S-EDOT monomer (primary structure) and S-PEDOT (secondary structure), the S-PEDOT nanocrystal (tertiary structure), and the distribution of S-PEDOT nanocrystals (quaternary structure). (B) Relation between CS-EDOT and Mw. (C) Dependencies of Xc and Nnc on Mw. (D) Dependencies of Lnc and Ea on Nnc. (E) Relation between Lnc and electrical conductivity.

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

7 of 9

structure) reached a maximum (22,300 g mol−1) at CS-EDOT = 5 wt % (Fig. 7B). Furthermore, the values of Xc and Nnc (tertiary structure) in-creased in proportion to Mw, reaching 72% and 5800 m−2, respec-tively (Fig. 7C). Here, the larger the Nnc, the lower the Lnc (3.9 nm) (quaternary structure) and Ea (4.4 meV) (Fig. 7D), which resulted in a remarkable increase in bulk conductivity up to as high as 1089 S cm−1 (Fig. 7E). Thus, the results led us to conclude that molecular weight was the key parameter responsible for improving electrical conductivity, which arose from the fact that the Mw of the S-PEDOT with low conductivity (1 to 5 S cm−1) was only 5000 g mol−1 (26). In addition, synthesis of S-PEDOT with a narrow molecular weight distribution by optimization of polymerization conditions will be important for improving the electrical conductivity. Notably, the relation between Lnc and electrical conductivity (Fig. 7E) has not yet saturated, sug-gesting that a further improvement in conductivity can be achieved by decreasing the Lnc value (facilitating hopping of charge carriers); further investigations are currently underway.

MATERIALS AND METHODSSynthesis of S-EDOTHydroxymethyl EDOT (HMEDOT) was synthesized by the reac-tion of 3,4-dihydroxythiophene-2,5-dicarboxylic acid diethyl ester (Sigma- Aldrich) and 2,3-dibromo-1-propanol (Tosoh Finechem), with subsequent hydrolysis and decarboxylation. The S-EDOT monomer, sodium 4-[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy]butane-2- sulfonate (fig. S1B), was synthesized by the sul-fonation of HMEDOT with 2,4-buthanesultone (Fujifilm Wako Pure Chemical), where the overall yield was 42%, and the purity evaluated by liquid chromatographic analysis was 99.2% (fig. S5) (49). The molecular structure of the as-synthesized S-EDOT monomer was fully characterized by 1H nuclear magnetic resonance (NMR), 13C NMR, and Fourier transform infrared spectroscopies and by elemental anal-ysis (fig. S6).

Polymerization of S-PEDOTS-PEDOT was synthesized by the oxidative polymerization of the S-EDOT monomer at different concentrations (CS-EDOT) in the range of 1 to 10 wt % (Fig. 1). A typical polymerization procedure at CS-EDOT = 5 wt % was as follows. The S-EDOT monomer (50 g, 151  mmol) and FeSO4·7H2O (25.2  g, 90.6  mmol; Fujifilm Wako Pure Chemical) were dissolved in a 1 M sulfuric acid water solution. Meanwhile, (NH4)2S2O8 (68.9 g, 302 mmol; Fujifilm Wako Pure Chemical) was dissolved in pure water (41.2 wt %) and added drop-wise to the S-EDOT solution. The total weight of the reaction solu-tion was typically 1 kg, and the oxidative polymerization was carried out under vigorous stirring at 20°C for 20 hours in a nitrogen atmo-sphere. After impurity ions were removed using cation and anion exchange resins (Lewatit Monoplus S108H and MP62WS, Lanxess), the solution was homogenized at 70 MPa using a high-pressure ho-mogenizer (R5 10.38, SMT). At different CS-EDOT, the polymeriza-tion procedure was exactly the same except that the FeSO4 and (NH4)2S2O8 were 0.6- and 2-fold, respectively, to the S-EDOT monomer. 1H NMR measurement of the S-PEDOT was carried out, but clear spectrum could not be obtained similarly to the previous work (26). This can be explained in terms of the Knight shift, where NMR line shift and line broadening are brought about probably by the in-crease of Pauli paramagnetic spin susceptibility (50) in the doped highly conductive S-PEDOT.

GPC and solution characteristicsThe Mw, Mn, and PDI of S-PEDOT were measured by GPC on an in-strument (Prominence, Shimadzu) equipped with a TSKgel -M col-umn and guard column (Tosoh); polystyrene sulfonate standards (PSS, American polymer standard) were used. The S-PEDOT aqueous solution (1 wt %) was neutralized with N,N- diisopropylamine (Junsei Chemical), dried on a hot plate at 75°C, and then dried in a vacuum at 65°C for 2 hours. The GPC measurements were performed on DMSO solutions of the neutralized S-PEDOT (5 × 10−3 wt %) using a DMSO solution with 10 mM tetrabutylammonium bromide (Tokyo Chemical Industry) and 10 mM N,N-diisopropylamine as an eluent at flow rate, column temperature, and injection volume of 0.3 ml min−1, 50°C, and 20 l, respectively. The UV-vis-NIR spectra of the S-PEDOT water solutions and spin-coated thin films were measured with a spectrophotometer (V-670, Jasco). The viscosity and pH of the S-PEDOT water solutions (1 wt %) were measured at 25°C with a torsional oscillation-type vis-cometer (Viscomate VM-10A-L, Sekonic) and a pH meter (Laqua F-74S, Horiba), respectively.

Electrical conductivityThe solid films were fabricated by casting the S-PEDOT water solu-tion (1 wt %) onto a glass substrate (35 mm long, 25 mm wide, and 1 mm thick) at 120°C with a moisture analyzer (MOC-120H, Shimadzu) and subsequent heating at 200°C for 30 min in a vacuum. The electrical conductivity of the S-PEDOT films (10 mm2, 7.2 to 10.2 m thick) was evaluated by the normal four-point method with a Loresta-GP (MCP-T610, Mitsubishi Chemical Analytech), where the thickness of the films was measured with a stylus profilometer (D-100, KLA-Tencor) in dry air.

XRD analysisThe XRD patterns were measured by an x-ray diffractometer (MiniFlex600, Rigaku) equipped with a one-dimensional high-speed detector (D/tex Ultra, Rigaku) at 40 kV and 15 mA. The values of Xc and Dhkl were esti-mated by the profile fitting method and Scherrer’s equation, respec-tively, using the integrated x-ray powder diffraction software (PDXL 2, Rigaku).

Scanning probe microscopyThe S-PEDOT water solutions (1 wt %) were spin coated onto n-doped Si wafers (0.5 mm thick, resistivity was 0.02 ohm cm) at 2000 rpm with a spin coater (MS-A150, Mikasa); the coated wafers were then heat treated at 200°C for 10 min in air to stabilize the structure of the S-PEDOT. The thermal stability of the S-PEDOT film was measured in nitrogen at a constant heating rate of 10°C min−1 with thermogravimetry and a differential thermal analyzer (TG-DTA2000S, NETZSCH Japan), where the decomposition temperature (Td) of the S-PEDOT film was ca. 240°C (fig. S7). The cAFM measurements were carried out with a scanning probe microscope (SPM-9600, Shimadzu) equipped with a con-ductive probe (CONTPt, NanoWorld), where height and current im-ages were measured in contact mode under a bias voltage of −0.1 V.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/4/eaav9492/DC1Table S1. Oral median lethal dose (LD50) of various sultones in rats.Table S2. Dispersibility of PEDOT:PSS and solubility of S-PEDOT in various solvents.Fig. S1. Chemical structure of S-EDOT isomers.Fig. S2. Flow-through test.Fig. S3. Optical properties of thin films.

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

8 of 9

Fig. S4. Possible structure of the S-PEDOT nanocrystal.Fig. S5. Synthetic route of the S-EDOT monomer.Fig. S6. Characterization of the S-EDOT monomer.Fig. S7. TG-DTA curves of the S-PEDOT film.

REFERENCES AND NOTES 1. S. C. Rasmussen, Electrically conducting plastics: revising the history of conjugated

organic polymers, in 100+ Years of Plastics. Leo Baekeland and Beyond, E. T. Strom, S. C. Rasmussen, Eds. (ACS Symposium Series 1080, American Chemical Society, 2011), chap. 10, pp. 147–163.

2. H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang, A. J. Heeger, Synthesis of electrically conducting organic polymers: Halogen derivatives of polyacetylene, (CH)x. J. Chem. Soc. Chem. Commun. 1977, 578–580 (1977).

3. C. K. Chiang, C. R. Fincher Jr., Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, A. G. MacDiarmid, Electrical conductivity in doped polyacetylene. Phys. Rev. Lett. 39, 1098–1101 (1977).

4. H. Klauk, Organic Electronics: Materials, Manufacturing and Applications (Wiley, 2006). 5. H. Klauk, Organic Electronics Ii: More Materials and Applications (Wiley, 2010). 6. H. Okuzaki, PEDOT: Material Properties and Device Applications (Science & Technology,

2012). 7. K. Asaka, H. Okuzaki, Soft Actuators: Materials, Modeling, Applications, and Future

Perspectives (Springer, 2012). 8. F. Jonas, W. Krafft, B. Muys, Poly(3,4-ethylenedioxythiophene): Conductive coatings,

technical applications and properties. Macromol. Symp. 100, 169–173 (1995). 9. A. Elschner, S. Kirchmeyer, W. Lövenich, U. Merker, K. Reuter, PEDOT: Principles and

Applications of an Intrinsically Conductive Polymer (CRC Press, Boca Raton, 2010). 10. T. Wakabayashi, M. Katsunuma, K. Kudo, H. Okuzaki, pH-tunable high-performance

PEDOT:PSS aluminium solid electrolytic capacitors. ACS Appl. Energy Mater. 1, 2157–2163 (2018).

11. S. Kirchmeyer, K. Reuter, Scientific importance, properties and growing applications of poly(3,4-ethylenedioxythiophene). J. Mater. Chem. 15, 2077–2088 (2005).

12. D. Hohnholz, H. Okuzaki, A. G. MacDiarmid, Plastic electronic devices through line patterning of conducting polymers. Adv. Funct. Mater. 15, 51–56 (2005).

13. T. Takano, H. Masunaga, A. Fujiwara, H. Okuzaki, T. Sasaki, PEDOT nanocrystal in highly conductive PEDOT: PSS polymer films. Macromolecules 45, 3859–3865 (2012).

14. H. Yan, S. Arima, Y. Mori, T. Kagata, H. Sato, H. Okuzaki, Poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate): correlation between colloidal particles and thin films. Thin Solid Films 517, 3299–3303 (2009).

15. H. Shi, C. Liu, Q. Jiang, J. Xu, Effective approaches to improve the electrical conductivity of PEDOT:PSS: A review. Adv. Electron. Mater. 1, 1500017 (2015).

16. S. Ashizawa, R. Horikawa, H. Okuzaki, Effect of solvent on carrier transport in poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate). Synth. Met. 153, 5–8 (2005).

17. J. Ouyang, Q. Xu, C. -W. Chu, Y. Yang, G. Li, J. Shinar, On the mechanism of conductivity enhancement in poly(3,4-ethylenedioxythiophene): Poly(styrene sulfonate) film through solvent treatment. Polymer 45, 8443–8450 (2004).

18. J. Y. Kim, J. H. Jung, D. E. Lee, J. Joo, Enhancement of electrical conductivity of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) by a change of solvents. Synth. Met. 126, 311–316 (2002).

19. A. M. Nardes, R. A. J. Janssen, M. Kemerink, A morphological model for the solvent-enhanced conductivity of PEDOT:PSS thin films. Adv. Funct. Mater. 18, 865–871 (2008).

20. S. K. M. Jönsson, J. Birgerson, X. Crispin, G. Greczynski, W. Osikowicz, A. W. Denier van der Gon, W. R. Salaneck, M. Fahlman, The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)polystyrenesulfonic acid (PEDOT-PSS) films. Synth. Met. 139, 1–10 (2003).

21. T. Horii, Y. Li, Y. Mori, H. Okuzaki, Correlation between the hierarchical structure and electrical conductivity of PEDOT/PSS. Polym. J. 47, 695–699 (2015).

22. H. Okuzaki, Y. Harashina, H. Yan, Highly conductive PEDOT/PSS microfibers fabricated by wet-spinning and dip-treatment in ethylene glycol. Eur. Polym. J. 45, 256–261 (2009).

23. M. Yamashita, C. Otani, M. Shimizu, H. Okuzaki, Effect of solvent on carrier transport in poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) studied by terahertz and infrared-ultraviolet spectroscopy. Appl. Phys. Lett. 99, 143307 (2011).

24. T. Horii, H. Hikawa, M. Katsunuma, H. Okuzaki, Synthesis of highly conductive PEDOT:PSS and correlation with hierarchical structure. Polymer 140, 33–38 (2018).

25. O. Stéphan, P. Schottland, P. -Y. L. Gall, C. Chevrot, C. Mariet, M. Carrier, Electrochemical behavior of 3,4-ethylenedioxythiophene functionalized by a sulfonate group. Application to the preparation of poly(3,4-ethylenedioxythiophene) having permanent cation-exchange properties. J. Electroanal. Chem. 443, 217–226 (1998).

26. G. Zotti, S. Zecchin, G. Schiavon, L. B. Groenendaal, Electrochemical and chemical synthesis and characterization of sulfonated poly(3,4-ethylenedioxythiophene): A novel water-soluble and highly conductive conjugated oligomer. Macromol. Chem. Phys. 203, 1958–1964 (2002).

27. R. H. Karlsson, A. Herland, M. Hamedi, J. A. Wigenius, A. Åslund, X. Liu, M. Fahlman, O. Inganäs, P. Konradsson, Iron-catalyzed polymerization of alkoxysulfonate-functionalized 3,4-ethylenedioxythiophene gives water-soluble poly(3,4-ethylenedioxythiophene) of high conductivity. Chem. Mater. 21, 1815–1821 (2009).

28. K. M. Persson, R. Karlsson, K. Svennersten, S. Löffler, E. W. H. Jager, A. Richter-Dahlfors, P. Konradsson, M. Berggren, Electronic control of cell detachment using a self-doped conducting polymer. Adv. Mater. 23, 4403–4408 (2011).

29. K. M. Persson, R. Gabrielsson, A. Sawatdee, D. Nilsson, P. Konradsson, M. Berggren, Electronic control over detachment of a self-doped water-soluble conjugated polyelectrolyte. Langmuir 30, 6257–6266 (2014).

30. H. Yano, M. Nishiyama, S. Hayashi, Water-soluble self-doped conducting polymers. TOSOH Res. Technol. Rev. 61, 67–76 (2017).

31. A. O. Patil, Y. Ikenoue, F. Wudl, A. J. Heeger, Water soluble conducting polymers. J. Am. Chem. Soc. 109, 1858–1859 (1987).

32. Y. Ikenoue, Y. Saifa, M.-A. Kira, H. Tomozawa, H. Yashima, M. Kobayashi, A facile preparation of a self-doped conducting polymer. J. Chem. Soc. Chem. Commun. 1990, 1694–1695 (1990).

33. X. -L. Wei, Y. Z. Wang, S. M. Long, C. Bobeczko, A. J. Epstein, Synthesis and physical properties of highly sulfonated polyaniline. J. Am. Chem. Soc. 118, 2545–2555 (1996).

34. K. Faïd, M. Leclerc, Functionalized regioregular polythiophenes: Towards the development of biochromic sensors. Chem. Commun. 1996, 2761–2762 (1996).

35. M.-R. Choi, T.-H. Han, K.-G. Lim, S.-H. Woo, D. H. Huh, T.-W. Lee, Soluble self-doped conducting polymer composites with tunable work function as hole injection/extraction layers in organic optoelectronics. Angew. Chem. Int. Ed. 50, 6274–6277 (2011).

36. K.-G. Lim, S. Ahn, H. Kim, M.-R. Choi, D. H. Huh, T.-W. Lee, Self-doped conducting polymer as a hole-extraction layer in organic-inorganic hybrid perovskite solar cells. Adv. Mater. Interfaces 3, 1500678 (2016).

37. A. Kumar, D. M. Welsh, M. C. Morvant, F. Piroux, K. A. Abboud, J. R. Reynolds, Conducting poly(3,4-alkylenedioxythiophene) derivatives as fast electrochromics with high-contrast ratios. Chem. Mater. 10, 896–902 (1998).

38. J. C. Gustafsson, B. Liedberg, O. Inganäs, In situ spectroscopic investigations of electrochromism and ion transport in a poly(3,4-ethylenedioxythiophene) electrode in a solid state electrochromical cell. Solid State Ionics 69, 145–152 (1994).

39. H. W. Heuer, R. Wehrmann, S. Kirchmeyer, Electrochromic window based on conducting poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate). Adv. Funct. Mater. 12, 89–94 (2002).

40. H. Okuzaki, M. Ishihara, Spinning and characterization of conducting microfibers. Macromol. Rapid Commun. 24, 261–264 (2003).

41. M. Granström, O. Inganäs, Electrically conductive polymer fibres with mesoscopic diameters: 1. Studies of structure and electrical properties. Polymer 36, 2867–2872 (1995).

42. K. E. Aasmundtveit, E. J. Samuelsen, L. A. A. Pettersson, O. Inganäs, T. Johansson, R. Feidenhans’l, Structure of thin films of poly(3,4-ethylenedioxythiophene). Synth. Met. 101, 561–564 (1999).

43. L. Niu, C. Kvarnström, K. Fröberg, A. Ivaska, Electrochemically controlled surface morphology and crystallinity in poly(3,4-ethylenedioxythiophene) films. Synth. Met. 122, 425–429 (2001).

44. T. J. Prosa, M. J. Winokur, J. Moulton, P. Smith, A. J. Heeger, X-ray-diffraction studies of the three-dimensional structure within iodine-intercalated poly(3-octylthiophene). Phys. Rev. B 51, 159–168 (1995).

45. L. J. Tung, S. Buckser, The effect of molecular weight on the crystallinity of polyethylene. J. Phys. Chem. 62, 1530–1534 (1958).

46. G. Zotti, S. Zecchin, G. Schiavon, F. Louwet, L. Groenendaal, X. Crispin, W. Osikowicz, W. Salaneck, M. Fahlman, Electrochemical and XPS studies towards the role of monomeric and polymeric sulfonate counterions in the synthesis. Composition, and properties of poly(3,4-ethylenedioxythiophene). Macromolecules 36, 3337–3344 (2003).

47. M. Wang, P. Kovacik, K. K. Gleason, Chemical vapor deposition of thin, conductive, and fouling-resistant polymeric films. Langmuir 33, 10623–10631 (2017).

48. N. F. Mott, E. A. Davis, Electronic Processes in Non-Crystalline Materials (Clarendon, Oxford, UK, 1979).

49. H. Yano, M. Nishiyama, S. Hayashi, K. Kudo, H. Okuzaki, Synthesis and characterization of a novel self-doped water-soluble conducting polymer. Kobunhsi Ronbunshu 75, 607–612 (2018).

50. M. Peo, H. Förster, K. Menke, J. Hockter, J. A. Gradner, S. Roth, K. Drandfeld, Absence of Knight-shift in the metallic state of polyacetylene. Mol. Cryst. Liq. Cryst. 77, 103–110 (1981).

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

Yano et al., Sci. Adv. 2019; 5 : eaav9492 12 April 2019

S C I E N C E A D V A N C E S | R E S E A R C H A R T I C L E

9 of 9

Acknowledgments: We greatly appreciate H. Shirakawa at the University of Tsukuba for the advice and useful comments on the synthesis of S-PEDOT. Funding: This work was financially supported, in part, by Grant-in-Aid for Scientific Research (B), JSPS KAKENHI grant number 16H04203. Author contributions: H.O. proposed and supervised this research. H.Y. designed and synthesized the S-EDOT monomer and wrote the manuscript. The polymerization and characterization of S-PEDOT were carried out by K.K. K.M. mainly contributed to the analysis of molecular weight by GPC measurements. Competing interests: H.O. and H.Y. are inventors on a patent application related to this work filed by the University of Yamanashi and Tosoh Corporation (no. P2018-123213A, filed on 31 January 2017). The other authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are

present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Submitted 4 November 2018Accepted 25 February 2019Published 12 April 201910.1126/sciadv.aav9492

Citation: H. Yano, K. Kudo, K. Marumo, H. Okuzaki, Fully soluble self-doped poly(3,4-ethylenedioxythiophene) with an electrical conductivity greater than 1000 S cm−1. Sci. Adv. 5, eaav9492 (2019).

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from

1−greater than 1000 S cm Fully soluble self-doped poly(3,4-ethylenedioxythiophene) with an electrical conductivity

Hirokazu Yano, Kazuki Kudo, Kazumasa Marumo and Hidenori Okuzaki

DOI: 10.1126/sciadv.aav9492 (4), eaav9492.5Sci Adv 

ARTICLE TOOLS http://advances.sciencemag.org/content/5/4/eaav9492

MATERIALSSUPPLEMENTARY http://advances.sciencemag.org/content/suppl/2019/04/08/5.4.eaav9492.DC1

REFERENCES

http://advances.sciencemag.org/content/5/4/eaav9492#BIBLThis article cites 43 articles, 0 of which you can access for free

PERMISSIONS http://www.sciencemag.org/help/reprints-and-permissions

Terms of ServiceUse of this article is subject to the

is a registered trademark of AAAS.Science AdvancesYork Avenue NW, Washington, DC 20005. The title (ISSN 2375-2548) is published by the American Association for the Advancement of Science, 1200 NewScience Advances

License 4.0 (CC BY-NC).Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of

on May 1, 2020

http://advances.sciencemag.org/

Dow

nloaded from