−総合報文−

SELFTRON®: A Novel Fully Soluble Self-Doped Highly
Conductive Polymer
Hidenori OKUZAKI ＊ 1
Hirokazu YANO ＊ 2

We developed a novel fully soluble self-doped poly(3,4-ethylenedioxythiophene) (SELFTRON®) with an electricalconductivity greater than 1000 S/cm. It was found that the molecular weight is the critical parameter for increasing
the number of nanocrystals, corresponding to the crystallites evaluated by X-ray diffraction and conductive atomic
force microscopic analyses having a high electrical conductivity, which reduced both the average distance between
adjacent nanocrystals and the activation energy for the hopping of charge carriers, leading to the highest bulk
conductivity.

S/cm), transparency and thermal stability, PEDOT:PSS

1． Introduction

has been used in a wide field of applications, including

The development of conductive polymers in 1970s by

antistatic coatings, hole injection layers in OLEDs

A. G. MacDiarmid, A. J. Heeger and H. Shirakawa (2000

and OPVs, cathode materials in solid aluminium

Nobel Prize in Chemistry), where electrical conductivity

electrolytic capacitors and transparent electrodesof polyacetylene films increased remarkably uponfor various electronic devices, as an alternative toiodine doping, 1 accelerated explosively the use ofindium tin oxide (ITO). 6 However, PEDOT:PSS hasorganic electronics in low-cost, lightweight and flexible

some technical issues that impede its further practicalelectronic devices such as organic light-emitting diodes

application; these issues stem from the use of PSS as(OLEDs), organic field-effect transistors (OFETs) andthe external ions to compensate the positive charges2

on PEDOT. Since the hydrophobic PEDOT core isorganic photovoltaics (OPVs) since the early 1980s.

Furthermore, in the era of the‘Internet of Things’

surrounded by a shell of excess hydrophilic PSS7 for(IoT), the field of organic electronics is developing via

water dispersion as a colloidal particle with a diameterincreased interest in printed electronics, stretchableof several tens of nanometres, (I) forming ultra-

electronics, and, recently, wearable electronics for use

thin layers thinner than the colloidal particle size isin flexible displays and touch panels, soft sensors anddifficult,8 (II) the PEDOT:PSS colloids aggregate andactuators.3,4 In such devices, a flexible, wet-processable,

precipitate when stored for an extended period, andand highly conductive polymer would be a promising(III) the pristine PEDOT:PSS exhibits poor electricalcandidate for a key material.

conductivity (< 1 S/cm),3,5 additives, such as ethylene

Poly(3,4-ethylenedioxythiophene):poly(4-glycol (EG), dimethyl sulfoxide (DMSO), and sorbitol,

styrenesulfonate) (PEDOT:PSS), developed by Bayerare therefore indispensable for improving electrical5

AG in 1990 and commercially available as a water

conductivity, namely, by the“solvent effect”,3,9 which

dispersion of colloidal particles, is one of the mostcan be interpreted in terms of the crystallization of thesuccessful conductive polymers of the last two decades.

PEDOT core and the removal of the insulating PSS

Because of its high electrical conductivity (up to 1000shell of the colloidal particles to enhance the transport＊ 1 山梨大学大学院総合研究部 教授＊ 2 有機材料研究所 ファインケミカルグループ グループリーダー

of charge carriers between the PEDOT nanocrystals
by a hopping mechanism. 10-12 That is, the insoluble
3

TOSOH Research & Technology Review Vol.64（2020）

and intractable nature of PEDOT is responsible for1) and characterized by means of gel permeationthese drawbacks of PEDOT:PSS having a 2.5-fold

chromatography (GPC), UV-vis-NIR spectroscopy,

excess of PSS over PEDOT by weight. Therefore,

X-ray diffraction analysis (XRD), conductive atomic

prescient and intensive studies on fully soluble self-

force microscopy (cAFM), and the four-point technique.

13-17

doped PEDOT have been reported in the literature.

It was found that the electrical conductivity achieved

Zotti and coworkers synthesized an EDOT derivative

as high as 1089 S/cm, 20 which is two orders ofbearing a sodium alkylsulfonate side chain, sodium

magnitude higher than those of previously reported

4-[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy]

self-doped PEDOTs13-17 and exceeds the conductivity

butane-1-sulfonate and polymerized it to obtain self-of PEDOT:PSS (up to 1000 S/cm). The results led us

doped PEDOT with a mass-average molecular

to conclude that the molecular weight is the criticalweight (Mw) of 5000 g/mol and a maximum electricalparameter for increasing the number of nanocrystals,

14

On the other hand,corresponding to the crystallites evaluated by XRD andKonradsson and Berggren et al. reported chemicalcAFM, reduced both the average distance between

polymerizations of the same self-doped PEDOT andadjacent nanocrystals and the activation energy for the15

hopping of charge carriers, leading to the highest bulk

conductivity of 1 − 5 S/cm.

found that the electrical conductivities were 12 S/cm

and 30 S/cm,16 respectively. Since these conductivity

conductivity.

values were two or three orders of magnitude lower
than that of the highly conductive PEDOT:PSS (up2．Experimental

to 1000 S/cm), improving the electrical conductivityof self-doped PEDOT has thus far been considered

Synthesis of SELFTRON

impossible.

SELFTRON was synthesized by the oxidative

In this study, we first synthesized a novel monomer

polymerization in different monomer concentrations

bearing a sodium alkylsulfonate side chain, sodium(Cm) between 1 − 10 wt% (Fig. 1). The weight-average

4-[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy]

molecular weight (M w), number-average molecularbutane-2-sulfonate (Fig. 1). From the viewpoints ofweight (M n), and polydispersity index (PDI) were

safety, low environmental load, and mass production

measured by GPC on an instrument (Prominence,for commercialization, the monomer was designed due

Shimadzu) equipped with a TSK gel α-M column andto the lower toxicity of 2,4-buthanesultone

18,19

used inguard column α(Tosoh); polystyrene sulfonate

the synthetic process. Furthermore, the fully soluble

standards (PSS, American polymer standard) wereself-doped PEDOT (SELFTRON) was synthesized byused. The UV-vis-NIR spectra were measured with athe oxidative polymerization of different monomer

spectrophotometer (V-670, Jasco). The viscosity and pH

concentrations (Cm) in a range of 1 − 10 wt% (Fig.

of SELFTRON water solutions (1 wt%) were measured

SO3
O

SO3Na
O
O

O

H2SO4(1M)
20℃，20h in N2

S
Cm＝1－10wt％

Monomer

O

S

O

S

S
O

n
O

O

O
SELFTRON

Fig.１

4

O

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

O
SO3H

Synthesis of SELFTRON by oxidative polymerization.

SO3H

東ソー研究・技術報告

at 25°C with torsional oscillation-type viscometer

第 64 巻（2020）

Scanning probe microscopy
The SELFTRON water solutions (1 wt%) were spin

(Viscomate VM-10A-L, Sekonic) and pH meter (Laqua
F-74S, Horiba), respectively.

coated onto doped Si wafers (0.5 mm thick, resistivity

Electrical conductivity

was 0.02 Ω cm) at 2000 rpm with a spin coater (MS-The solid films were fabricated by casting theA150, Mikasa); the coated wafers were then heat treated

SELFTRON water solution (1 wt%) onto a glassat 200°C for 10 min in air to stabilize the structure of

substrate (35 mm long, 25 mm wide, and 1 mm thick) at

SELFTRON. The cAFM measurements were carried

120°C with a moisture analyser (MOC-120H, Shimadzu)

out with a scanning probe microscope (SPM-9600,

and subsequently heating at 200°C for 30 min in a

Shimadzu) equipped with a conductive probe (CONTPt,

vacuum. The electrical conductivity of the films (10NanoWorld), where height and current images were

mm square, 7.2 − 10.2 µm thick) was evaluated by themeasured in contact mode under a bias voltage of − 0.1

normal four-point method with a Loresta-GP (MCP-T610,

V.

Mitsubishi Chemical Analytech), where the thicknessof films was measured with a stylus profilometer (D-100,

3．Results and discussion

KLA-Tencor) in dry air.

The GPC elution curves of various SELFTRON

X-ray diffraction analysis
The XRD patterns were measured by an X-ray

solutions are shown in Fig. 2A. Since the GPC

diffractometer (MiniFlex600, Rigaku) equipped with ameasurement is not applicable to the PEDOT:PSS

1D high-speed detector (D/tex Ultra, Rigaku) at 40 kV

colloidal dispersions, these results provide clear

and 15 mA. The values of Xc and Dhkl were estimatedevidence that the SELFTRON is fully soluble, similarby the profile fitting method and Scherrer’s equation,

to other self-doped conductive polymers. 13-17 The

respectively, using the integrated X-ray powder

GPC curves were unimodal with PDI of 3.15 − 4.15,

diffraction software (PDXL 2, Rigaku).

indicative of a broad molecular weight distribution.
This is probably because the termination reaction is

30

21

23

25

27

29

31

Viscosity (mPa・s)

Intensity

Cm
Mw
Mn
PDI
(wt%) (g/mol) (g/mol)
114400 3480 4.14
317700 4660 3.79522300 5380 4.15
719000 4790 3.96
815900 4300 3.70
1010600 3380 3.15

2.5

pH

Absorbance

1.5

1

2

1.5

0.5

0
200

400

600

800

Wavelength (nm)

Fig.２

10

0
3

33

Retention time (min)

2

20

1000

1200

1

0

2

4

6

8

10

Cm (wt%)Solution properties of SELFTRON. (A) GPC elution curves, weight-average molecular weight(Mw), number-average molecular weight (Mn), and polydispersity index (PDI) of SELFTRON
synthesized with different Cm. (B) The viscosity and pH dependencies of SELFTRON water
solutions (1 wt%) on Cm. (C) UV-vis-NIR spectra of SELFTRON water solutions (0.01 wt%)
synthesized with different Cm.

5

TOSOH Research & Technology Review Vol.64（2020）

due to recombination rather than disproportionation.5

of water and alcohol, SELFTRON is soluble not only

The Mw and Mn of the SELFTRON reached maximum

in water but also in various organic solvents, which isvalues of 22300 g/mol and 5380 g/mol, respectively,a great advantage for use in organic electronic devices

at C m = 5 wt%. Here, the number average degree of

easily degraded by water and for the fabrication ofpolymerization (DP) was ca. 18, which was more thancomposites with water-insoluble polymers.

twice higher than the value of the previously reported

XRD analysis was performed and the results are

self-doped PEDOT (8 units average) measured byshown in Fig. 3A. Although the SELFTRON is a new

matrix-assisted laser desorption ionization time-of-flightpolymer, the diffraction patterns are simpler than

14

(MALDI-TOF) mass spectrometry. The decrease of

PEDOT:PSS7,10 and similar to those of PEDOTs doped

both Mw and Mn at Cm > 5 wt% was likely due to a steepwith perchlorate (PEDOT:ClO 4), p -toluenesulfonate

increase in the viscosity of the reaction solution. Fig.

(PEDOT:TsO) and hexafluorophosphate (PEDOT:PF6),

2B shows that the viscosity of SELFTRON solutions

which indicates that the unit cell belongs to an(1 wt%) reached a maximum (24.3 mPa·s) at Cm = 5

orthorhombic system. 22-24 Therefore, the diffraction

wt%, reflecting the molecular weight, whereas the pH

peaks at 2θ = 5.3 °(d = 16.6 Å), 10.5 °(d = 8.4 Å), andvalues were nearly constant (1.86 − 2.04). These values

24.7°(d = 3.6 Å) can be assigned to the diffractions fromare comparable to those of the PEDOT:PSS water

the (100), (200), and (020) planes of the orthorhombic

dispersion (1 wt%) with a viscosity and pH of 16 mPa·s

unit cell of the crystal with lattice parameters of a =and 1.89, respectively. As shown in Fig. 2C, the UV-vis-

16.6 Å, b = 7.2 Å, and c = 7.8 Å. Since the main chain

NIR spectra of SELFTRON water solutions (0.01 wt%)

matches the c axis, the c value corresponds to thewere less dependent on Cm, demonstrating the same

repetition period of two monomer units (7.8 Å) similarelectronic structure: a broad shoulder at approximately

to that of PEDOT. However, the π-stacking distance

800 nm is consistent with the low energy absorbance

between two chains, representing one-half of theof neutral PEDOT, and a free-carrier of bipolarons invalue of b (3.6 Å), which is slightly longer than that of

the NIR region corresponds to PEDOT in the oxidized

PEDOT (3.4 Å)22-24 due to the steric hindrance and/or

state.

21

Furthermore, while PEDOT:PSS can beelectrostatic repulsion between the side chains. Unlike

dispersed only in water and in several mixed solventspolyalkylthiophenes in which the dopant ions are

75c

X c (%)

70

a

(100)

(200)

60
10

Cm
(wt%)

(020)

1

Crystallite size (nm)X-ray intensity (a.u.)

65

b

3
5
7

8

8

D100

6
4

D020

2

10
0

10

20

2θ (°)

Fig.３

6

30

40

0

0

2

4

6

8

10

Cm (wt%)Crystalline structure of SELFTRON films. (A) X-ray diffraction (XRD) patterns of different Cm
and a possible crystalline structure of SELFTRON (inset). (B) The crystallinity (Xc) andcrystallite size (D100 and D020) dependencies on Cm.

東ソー研究・技術報告第 64 巻（2020）

present in the vicinity of the polymer chains,25 PEDOTwt%), the absence of amorphous PSS chains (the weight

exhibits dopant ions between the π-stacked columns

fraction of PEDOT in PEDOT:PSS is only 29%), and/along the a-axis direction. Thus, the value (16.6 Å) isor the side chains of SELFTRON being highly ordered

determined by the length of the alkylsulfonic acid sideand incorporated in the crystalline phase. Assuming

chains. Since the unit lattice contains four monomer

that the doping level of SELFTRON is the same as that

units with the side chains alternatingly arranged inof PEDOT (0.22 − 0.35),26 corresponding to one charge

opposite directions with respect to the polymer chain,

per 3 − 4 monomer units, 22 − 35% of sulfonic acidthe theoretical density of the SELFTRON crystal isgroups of the side chains dissociate and compensate

3

calculated to be 1.59 g/cm , which is consistent with thefor the positive charges on main chains as counter

actual density measured by the method of Archimedesions. However, 65 − 78% of undissociated sulfonic3(1.50 − 1.53 g/cm ). Assuming that a broad halo in theacid groups in the side chains can form intermolecular2θ range of 15 − 35 °corresponds to the amorphoushydrogen bonds and stabilize the crystalline structure,

chains, the degree of crystallinity (Xc) can be expressedas shown in the inset of Fig. 3A. Furthermore, theas the ratio of total area of crystalline peaks to total

crystallite sizes normal to the (100) and (020) planes

area of all diffraction peaks. As shown in Fig. 3B, the Xc

calculated using the Scherrer’s equation were D 100

value reached 72% at Cm = 5 wt%, the tendency of which≈ 6 nm and D 020 ≈ 3 nm, respectively, regardless ofagrees with the molecular weight and viscosity results

the Cm (Fig. 3B). Therefore, the high Xc value can be(Fig. 2B). This trend suggests that the SELFTRON

explained in terms of the large number of crystallites, inexhibits less entanglement in water since the polymer

which approximately four columns of eight SELFTRON

chains have an extended coil structure due to the rigid

molecules π-stacked in the b-axis direction are

π-conjugated backbone and electrostatic repulsion

accumulated in the a-axis direction.

between the alkylsulfonic acid side chains, favouring

As shown in Fig. 4A, the surface morphologies of thincrystallization by π-stacking in the solid state. Notably,films were similar irrespective of Cm, where numerous

the X c values are substantially higher than those ofparticles with an average particle size (Dp) of ca. 11

12

PEDOT:PSS and are associated with the higher weight

nm were densely packed to form a coherent solid film

fractions of PEDOT main chains in SELFTRON (45with a particle number (Np), defined as the number of

Cm = 1 wt%

15

3 wt%

6000

Dp

5000

Np (/µm2)

4000

D p (nm)

10

3000

Np

2000

5

5 wt%

7 wt%

1000
0
0.8

0

10 wt%Ra (nm)

0.6

8 wt%

0.4

0.2

0

0

2

4

6

8

10

Cm (wt%)Fig.４

AFM height images of SELFTRON thin films. (A) Surface morphologies of different Cm.
(B) The average particle size (Dp), number of particles (Np) and surface roughness (Ra)dependencies on Cm.

7

TOSOH Research & Technology Review Vol.64（2020）

particles in 1 µm2, of 3000 − 3600 (Fig. 4B). In addition,

To clarify the correlation between the current images

thin films were smooth, with a surface roughness (Ra)at the nanoscale and the transport of charge carriers inof ca. 0.4 nm, compared to PEDOT:PSS films (1 ≤ Ra

the bulk state, electrical conductivity was measured by≤ 2 nm)10

the four-point technique. We should emphasize that an

because SELFTRON is not dispersed as acolloid but is fully dissolved in water. Fig. 5A showselectrical conductivity as high as 1089 S/cm at Cm = 5current images representing the distribution of local

wt% (Fig. 6A) was attained without additives (solvent

conductivity measured by cAFM; these images reveal

effect). This value is more than two orders of magnitude

that the highly conductive nanocrystals (shown in thehigher than those of the self-doped PEDOT 13-17 andbright region) are sparsely distributed in less conducive

exceeds the conductivity of PEDOT:PSS with additives

amorphous matrices (shown in the blue region), similarsuch as EG and DMSO (up to 1000 S/cm).

to PEDOT:PSS.

12

The average size of nanocrystals

A strong correlation is observed between L nc and(Dnc) was found to be an almost constant value of ca. 5

electrical conductivity (Fig. 6B), demonstrating that

nm (Fig. 5B), coincident with the crystallite sizes (D100the distribution of nanocrystals plays an importantand D020) in Fig. 3B, suggesting that the nanocrystal

role in electrical conduction. According to the nearest-

corresponds to a single crystal. However, the number ofneighbour hopping model, 10 the activation energy

nanocrystals in 1 µm2 (Nnc) shows a maximum of 5800

(E a) required for hopping of charge carriers can be2/µm at Cm = 5 wt%. The tendency of Nnc is similar to

calculated by measuring the electrical conductivity

those of the molecular weight (Fig. 2A) and the Xc (Fig.

(σ) at room temperature (T = 298 K) and that at liquid-

3B), demonstrating that one particle in Fig. 4A consists

nitrogen temperature (T = 77 K) and using the equationof ca. two nanocrystals. Therefore, the average distanceσ = σ0 exp( − E a / k BT), where σ0 and k B are thebetween adjacent nanocrystals (Lnc) reached a minimum

conductivity factor and Boltzmann constant (8.617 ×(3.9 nm) at Cm = 5 wt%. This interparticle distance is10-5 eV/K), respectively.10 A linear relationship between

suitable for the hopping of charge carriers between

Lnc and Ea demonstrates that the longer the distanceadjacent nanocrystals, which is critical to achieving highbetween adjacent nanocrystals, the lower the probability

12bulk conductivity.

of charge carrier hopping (Fig. 6B). Since the values

Cm = 1 wt%

6000

15

3 wt%

5000

5 wt%

4000

10

Nnc (/µm 2)

Dnc (nm)

Nnc

3000

Dnc

5

2000

7 wt%

1000
0

0
6

Lnc

NC

NC

8 wt%

10 wt%

Lnc (nm)

5

4

3

0

2

4

6

8

10

Cm (wt%)Fig.５

8

Conductive AFM (cAFM) images of SELFTRON thin films. (A) Current mapping images ofdifferent Cm. (B) The average nanocrystal size (Dnc), number of nanocrystals (Nnc) and average
length between adjacent nanocrystals (Lnc) dependencies on Cm.

東ソー研究・技術報告第 64 巻（2020）

1000
800
600
400
200
0

Fig.６

0

2

4

6

8

Cm
(wt%)

Lnc
(nm)

σ77K
(S/cm)

σ293K(S/cm)

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

Cm (wt%)

10 15

10

Ea (meV)

Conductivity (S/cm)

1200

Lnc

NC

NC

5

0
3.5

4

4.5

5

5.5

6

Lnc (nm)Electrical conductivity and carrier transport properties of SELFTRON films. (A) The electricalconductivity dependency measured by the four-point method on Cm. (B) The electricalconductivity and activation energy (Ea) dependencies on Lnc. (C) Lnc, electrical conductivities
at 77 K (σ77K) and 293 K (σ293K) and the Ea of different Cm.

of Ea (Fig. 6C) are lower than the thermal energy at

which is responsible for the extremely high electricalroom temperature, ca. 26 mV, the charge carriers

conductivities at room temperature.

in SELFTRON have sufficient energy for hopping,

85

6000
5000

Nnc

3000

70
65

3x104

4000

Nnc (/µm2 )Xc (%)

80
75

Quaternary structure(Bulk film)

2000

Xc

1000

Tertiary structure(nanocrystal)

60
0
5.0x103 1.0x104 1.5x104 2.0x104 2.5x104

2x104

0

2

4

6

8

Conductivity (S/cm)

1x104

0x100

1200

Secondary structure(SELFTRON)

10

Cm (wt%)

5

20

Lnc

15
4

Lnc (nm)Primary structure(monomer)

6

3

1000
800
600
400
200
0
3.5

4

4.5

5

5.5

6

Lnc (nm)E a (meV)

Mw (g/mol)Mw (g/mol)

Ea

10

2
5
1
0
3000

4000

5000

0
6000

Nnc (/µm2)

Fig.７

Correlation between the hierarchical structure and properties of SELFTRON. (A) Chemical
structures of the monomer (primary structure) and SELFTRON (secondary structure), thenanocrystal (tertiary structure) and the distribution of nanocrystals (quaternary structure).
(B) Relation between Cm and Mw. (C) Dependencies of Xc and Nnc on Mw. (D) Dependenciesof Lnc and Ea on Nnc. (E) Relation between Lnc and electrical conductivity.

9

TOSOH Research & Technology Review Vol.64（2020）６）S. Kirchmeyer, K. Reuter, J. Mater. Chem ., 15, 2077

Conclusion(2005).

Fig. 7A shows the hierarchical structure of
SELFTRON. The monomer (primary structure),

７）
T. Takano, H. Masunaga, A. Fujiwara, H. Okuzaki, T.
Sasaki, Macromolecules , 45, 3859 (2012).

sodium 4-[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)

８）H. Yan, S. Arima, Y. Mori, T. Kagata, H. Sato, H.

methoxy]butane-2-sulfonate, was newly synthesized andOkuzaki, Thin Solid Films , 517, 3299 (2009).

chemically polymerized into SELFTRON, where M w９）S. Ashizawa, R. Horikawa, H. Okuzaki, Synth. Met.,(secondary structure) reached a maximum (22300 g/mol) at Cm = 5 wt% (Fig. 7B). Furthermore, the valuesof Xc and Nnc (tertiary structure) increased in proportion2

to Mw, reaching 72% and 5800 /µm , respectively (Fig.
7C). Here, the larger the N nc, the lower the L nc (3.9nm) (quaternary structure) and E a (4.4 meV) (Fig.
7D), which resulted in a remarkable increase in bulk

153, 5 (2005).
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1414）G. Zotti, S. Zecchin, G. Schiavon, L. B. Groenendaal,

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Löffler, E. W. H. Jager, A. Richter-Dahlfors, P.

suggesting that a further improvement in conductivity

Konradsson, M. Berrgren, Adv. Mater ., 23, 4403

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currently underway.

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