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Synthesis of highly conductive PEDOT:PSS and correlation with hierarchical structure
Polymer 140 (2018) 33e38
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Synthesis of highly conductive PEDOT:PSS and correlation with hierarchical structure Tatsuhiro Horii, Hanae Hikawa, Masato Katsunuma, Hidenori Okuzaki* University of Yamanashi, 4-4-37 Takeda, Kofu 400-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 December 2017 Received in revised form 4 February 2018 Accepted 14 February 2018 Available online 16 February 2018
The water dispersions of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonic acid) (PEDOT:PSS) as colloidal particles were synthesized by oxidative polymerization with different composition ratios of repeating units for PSS and PEDOT (a ¼ 1.4e8.3). The role and effect of the PSS on hierarchical structure and electrical conductivity of the PEDOT:PSS were investigated systematically by means of X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), and conductive atomic force microscopy (c-AFM). It was found that the PEDOT:PSS colloidal particles stably dispersed in water at a 2.3, while small primary particles aggregated to form large secondary particles in water at a ¼ 1.4 because the zeta potential dropped owing to the less PSS. The PEDOT:PSS showed paracrystalline structure where highly conductive PEDOT nanocrystals uniformly distributed in the less conductive PSS matrices. The electrical conductivity was strongly dependent on the composition ratio and attained as high as 700 S/cm at a ¼ 2.3 where a positive correlation was seen between the electrical conductivity and number of conductive particles favorable for hopping of charge carriers in between the nanocrystals. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Conducting polymer Hierarchical structure PEDOT:PSS
1. Introduction The organic electronics originated from low cost, lightweight, and flexible electronics is developing through printed electronics, stretchable electronics, and recently into wearable electronics for Internet of Things (IoT) applications such as flexible displays, smart sensors, and soft actuators. Here, wet-processable, flexible, and highly conductive polymers are of great advantages to the organic electronics. Poly(3,4-ethylenedioxythiophene) doped with poly(4styrenesulfonate acid) (PEDOT:PSS), commercially available in the form of water dispersion as colloidal particles, can be applied to antistatic coatings, solid electrolytic capacitors, and organic LEDs [1e5]. Moreover, because of the high electrical conductivity, transparency, and thermal stability, the PEDOT:PSS has attracted considerable attention for transparent electrodes of flexible displays, touch panels, and solar cells as an alternative of indium tin oxide (ITO) [6]. Importantly, such excellent electrical characteristics of the PEDOT:PSS essential for the organic electronics are strongly dependent on its hierarchical structure [7e16]. Furthermore, the
* Corresponding author. E-mail address: [email protected] (H. Okuzaki). https://doi.org/10.1016/j.polymer.2018.02.034 0032-3861/© 2018 Elsevier Ltd. All rights reserved.
electrical conductivity of the PEDOT:PSS is improved significantly by two orders of magnitude upon adding high boiling point solvents such as ethylene glycol (EG) and dimethyl sulfoxide, so-called ‘secondary dopant’ [17e22]. The mechanism was explained in terms of changes in the hierarchical structure: Crystallization of the PEDOT molecules, removal of the insulating PSS surrounding the PEDOT core, and aggregation of the PEDOT:PSS colloidal particles, which affect both intra- and inter-particle transport of charge carriers [9,23,24]. For further improvement of the electrical conductivity, a correlation between the hierarchical structure and electrical conductivity of the PEDOT:PSS should be clarified in more detail. In particular, the secondary structure (poly-ion complex) is a key structure affecting both tertiary (colloidal gel particle) and quaternary structures (aggregation) in order to optimize the transport of charge carriers in the solid state [15]. However, a few studies on synthesis and characterization of the PEDOT:PSS reported extremely low electrical conductivities [26,27], and therefore the effect of composition ratio between the PEDOT and PSS on the hierarchical structure and high electrical conductivity of the PEDOT:PSS is still a ‘black box’. In this study, we newly synthesized highly conductive PEDOT:PSS water dispersions by oxidative polymerization with
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various feed composition ratios of PSS repeating unit to EDOT monomer (afeed ¼ 1e10) and changes in the hierarchical structure and electrical conductivity of the PEDOT:PSS were investigated by means of X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), and conductive atomic force microscopy (c-AFM). The results allowed us to conclude that the composition ratio was crucially important for the higher-ordered structure and electrical conductivity of the PEDOT:PSS, in which a number of conductive PEDOT nanocrystals uniformly distributed in the insulating PSS matrices were favorable for hopping of charge carriers and led to the highest bulk conductivity as high as 700 S/cm. 2. Experimental 2.1. Synthesis of PEDOT:PSS The water dispersions of PEDOT:PSS colloids with various feed composition ratios (afeed), defined as the molar ratio of PSS repeating unit to 3,4-ethylenedioxythiophene (EDOT) monomer, from 1 to 10 were synthesized by oxidative polymerization of EDOT (Aldrich) in the presence of various concentrations of the PSS (Mw ¼ 75,000 g/mol, Aldrich). The EDOT (0.5 wt%) and PSS (0.2e6.5 wt%) were mixed in pure water containing 0.98 wt% of Na2S2O8 (Junsei Chemical) as an oxidant and 0.2 wt% of FeSO4 7H2O (Junsei Chemical) as a catalytic agent. The total volume of reaction solution was typically 1.2 L and the oxidative polymerization was carried out under vigorous stirring in a nitrogen stream at 20 C for 24 h. After polymerization, the resulting sodium, iron, and sulfate ions were removed by cation exchange (Lewatit Monoplus S108H, Lanxess) and anion exchange resins (Lewatit MP62WS, Lanxess). The solid films were fabricated by casting the PEDOT:PSS water dispersion (1 wt%) containing 5 wt% of ethylene glycol (EG) as a secondary dopant [9,23,24] on a glass substrate at 200 C with a moisture analyzer (MOC-120H, Shimadzu).
and PSS anions where the PSS has two functions: One is the dopant to compensate positive charges of the PEDOT in the state of polaron or bipolaron. The other is the dispersant for stably dispersing hydrophobic PEDOT in water as the poly-ion complex [28]. Therefore, the composition ratio of the poly-ion complex will be one of the key parameters affecting the dispersion state of the PEDOT:PSS colloidal particles. Fig. 1 shows XPS spectra of the PEDOT:PSS in the energy range of the S 2p signal, where two peaks at binding energies of 163 and 167 eV assigned to the sulfur atoms of PEDOT and PSS, respectively [29]. It was found that relative intensity of the PSS to PEDOT increased with increasing the afeed, where actual composition ratios (a) evaluated from the peak areas of the XPS spectra were in good agreement with the values of afeed as shown in the inset of Fig. 1. 3.2. Median diameter and zeta potential of colloidal particles (tertiary structure) The effect of composition ratio on median diameter (D50) and zeta potential of the PEDOT:PSS colloidal particles was investigated by means of the DLS as shown in Fig. 2. At a ¼ 2.3e8.3, a sharp monodisperse peak was observed with the D50 of 17e29 nm, similarly to the commercial grades of the PEDOT:PSS (PH500 and PH1000, Heraeus) [12], where zeta potential was found to be 86e89 mV. This indicates that a negatively charged PSS-rich layer covers on the surface of the PEDOT:PSS colloidal particles, which is responsible for the stable dispersion in water. On the other hand, at a ¼ 1.4, the D50 significantly increased to 218 nm while the zeta potential dropped, demonstrating aggregation of the PEDOT:PSS colloidal particles caused by poor dispersibility in water due to the less PSS. 3.3. Crystalline structure of PEDOT (tertiary structure) In order to elucidate the dependence of the secondary structure
2.2. Measurements The median diameter (D50) and zeta potential of the PEDOT:PSS colloidal particles were evaluated by a dynamic light scattering (DLS) method at 25 C with particle size analyzer (Nanotrac UPAUT151, MicrotracBEL) and zeta potential analyzer (DelsaNano C, Beckman Coulter), respectively. The actual composition ratio (a) of repeating units for PSS and PEDOT in the PEDOT:PSS thin films spin-coated on n-Si wafer was evaluated by X-ray photoelectron spectroscopy (XPS) with a XPS photometer (JPS-9200, JEOL) using monocromatized Al (Ka) X-ray with an incident angle of 10 . The Xray diffraction (XRD) patterns were measured using an imaging plate (R-AXIS DS3C, Rigaku) at 40 kV and 30 mA with an exposure time of 1 h. The conductive atomic force microscopic (c-AFM) measurements were carried out with a scanning probe microscope (SPM-9600, Shimadzu) equipped with a conductive probe, where height and current images were measured by a contact mode under a bias voltage of 0.1 V. The electrical conductivity of the PEDOT:PSS film was measured by a normal four-point method with a LorestaGP (MCP-T610, Mitsubishi Chemical Analytech), where thickness of the PEDOT:PSS film was evaluated with a stylus profilometer (D100, KLA-Tencor). 3. Results and discussion 3.1. Composition ratio of poly-ion complex (secondary structure) The secondary structure of PEDOT:PSS poly-ion complex is formed through electrostatic interactions between PEDOT cations
Fig. 1. XPS spectra of PEDOT:PSS with various feed composition ratios (afeed) of PSS repeating unit to EDOT monomer in the energy range of the S 2p signal. Inset: Relation between afeed and actual composition ratio (a) of repeating units for PSS and PEDOT estimated from the XPS spectra.
T. Horii et al. / Polymer 140 (2018) 33e38
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a screening effect [33]. This may induce a spontaneous stacking of the linear and planar PEDOT molecules, which favors the formation of a crystalline phase [9,14]. It is also seen from Fig. 3 that the crystallinity (Xc) decreases with increasing the a, which is attributed to the decrease of the PEDOT fraction in the PEDOT:PSS (broken line). The smaller Xc values than the broken line suggest that the PEDOT is not completely crystallized and/or the X-ray scattering factors of the PEDOT and PSS are different. Therefore, the Xc is a relative value depending on the change in the a values. 3.4. Aggregation of PEDOT:PSS colloidal particles (quaternary structure)
Fig. 2. Dependence of median diameter (D50) and zeta potential of the PEDOT:PSS colloidal gel particles on composition ratio (a) of repeating units for PSS and PEDOT measured by DLS.
of the composition ratio on tertiary structure in the solid state, we performed XRD measurements on the PEDOT:PSS films and the results are shown in Fig. 3. The Laue images show paracrystalline structure in which the Debye-Scherrer ring at 2q ¼ 26 (d ¼ 3.4 Å) can be assigned to the (020) planes of the orthorhombic unit cell of PEDOT crystals [30e32]. Here, the EG having high boiling point and high dielectric constant as the secondary dopant may decrease the electrostatic interactions between PEDOT cations and PSS anions by
Fig. 4 shows surface morphology of the PEDOT:PSS thin films with different composition ratios. At a ¼ 1.4, the surface roughness (Ra) was 0.9 nm where numerous PEDOT:PSS particles with an average particle size (Dp) of 32 nm were densely and randomly packed with a particle number (Np), defined as the number of particles in 1 mm2, ca. 800. This suggests that the large colloidal particles in water (D50 ¼ 218 nm in Fig. 2) correspond to aggregates consisting of small primary particles (Dp ¼ 32 nm). On the other hand, an increase of the a increases both values of Dp and Ra but decreases Np, demonstrating that less number of larger primary particles loosely aggregate with higher surface roughness. The AFM image is affected by a tip locus effect where typical resolution in the horizontal and vertical directions, depending on the tip curvature radius of the cantilever, are ca. 10 nm and 0.1 nm, respectively. Taking into account of the resolution of the AFM image, the results seem to be reasonable but are still inconsistent with the fact that the values of D50 were nearly constant (17e29 nm) at a ¼ 2.3e8.3 (Fig. 2). This can be explained in terms of the core-shell structure of the PEDOT:PSS colloidal particle [9,10]: The D50 measured by DLS may related to the hydrophobic PEDOT core size since the hydrophilic PSS shell swells in water. Indeed, at a ¼ 6.9 and 8.3 the primary particles are not clearly visible but obvious the fibril structure which may correspond to the PSS shell surrounding the PEDOT core. Furthermore, local conductivity was evaluated by c-AFM in which the current flowing though the PEDOT:PSS thin film between substrate and cantilever under a bias voltage of 0.1 V. As shown in Fig. 5, the current images reveal that the conductive PEDOT nanocrystals (red area) distributed spatially in the insulating PSS matrices (blue area), which supports the phase segregation of PEDOT:PSS [34]. Regardless of the composition ratio, the conductive particle size (Dcp) is ca. 10 nm or less by considering the resolution. The Dcp values are smaller than corresponding D50 (Fig. 2) and Dp (Fig. 4), while the Ncp are three times more than Np. This demonstrates that one PEDOT core in the colloidal particle is consisting of a few nanocrystals. It should be noted the Ncp shows maximum at a ¼ 2.3, indicating a number of smaller nanocrystals are densely and uniformly distributed in the PSS matrices. 3.5. Electrical conductivity
Fig. 3. Dependence of crystallinity (Xc, symbols) measured by XRD and weight fraction of the PEDOT in the PEDOT:PSS (broken line) on composition ratio (a) of repeating units for PSS and PEDOT. Inset: Laue images of the PEDOT:PSS films with different a values.
A clear indication of the importance of composition ratio on electrical conductivity of the PEDOT:PSS is demonstrated in Fig. 6. It is found that the electrical conductivity increases linearly with decreasing the a and attained as high as 700 S/cm at a ¼ 2.3, which can be attributed to the increase of the PEDOT fraction. A further decrease of a, however, results in a drop of the conductivity at a ¼ 1.4, indicative of an optimum composition between PEDOT and PSS. Assuming the doping ratio of the PEDOT to be 0.35 for the PEDOT:PSS [35], the a value of 1.4 is high enough to produce charge carriers in the PEDOT as polarons and bipolarons. Therefore, the drop in electrical conductivity can be explained by the decrease of
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Fig. 4. Changes in surface morphology, particle size (Dp), and particle number (Np) of the PEDOT:PSS on composition ratio (a) of repeating units for PSS and PEDOT.
Fig. 5. Changes in current image, conductive particle size (Dcp), and conductive particle number (Ncp) of the PEDOT:PSS on composition ratio (a) of repeating units for PSS and PEDOT measured under a bias voltage of 0.1 V.
carrier mobility but carrier density. Since the electrical conductivity of conductive polymers depends on transport of charge carriers between conductive particles or fibrils [25], a strong correlation between the conductivity and Ncp as shown in the inset of Fig. 6
clearly demonstrates that a number of small conductive PEDOT nanocrystals uniformly distributed in the insulating PSS matrices is favorable for hopping of charge carriers between the PEDOT nanocrystals responsible for the highest bulk conductivity.
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Fig. 6. Dependence of electrical conductivity measured by four-point method on composition ratio (a) repeating units for PSS and PEDOT and on Ncp (inset).
4. Conclusion The PEDOT:PSS water dispersions with different composition ratios were synthesized and correlation between hierarchical structure and electrical conductivity was systematically investigated by means of XPS, DLS, XRD, and c-AFM analyses. It was found that the PEDOT:PSS colloidal particles stably dispersed in water at a 2.3, where D50 and zeta potential were 17e29 nm and 86e89 mV, respectively. The PEDOT:PSS formed paracrystalline structure and the PEDOT nanocrystals with Dcp of ca. 10 nm or less distributed spatially in the PSS matrices. Furthermore, the electrical conductivity attained as high as 700 S/cm at the optimum composition ratio at a ¼ 2.3. The results allowed us to conclude that the secondary structure of composition ratio between the PEDOT and PSS significantly affected the tertiary and quaternary structures with the strong correlation between electrical conductivity and Ncp. Acknowledgements This work was supported in part by Grant-in-Aid for Scientific Research (B) with JSPS KAKENHI Grant Number 16H04203 from The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. References [1] F. Jonas, J.T. Morrison, 3,4-Polyethylenedioxythiophene (PEDT): conductive coatings technical applications and properties, Synth. Met. 85 (1997) 1397e1398. €s, Electrochromic and highly stable [2] Q. Pei, G. Zuccarello, M. Ahlskog, O. Ingana poly(3,4-ethylenedioxythiophene) switches between opaque blue-black and transparent sky blue, Polymer 35 (1994) 1347e1351. [3] H.W. Heuer, R. Wehrmann, S. Kirchmeyer, Electrochromic window based on conducting poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate), Adv. Funct. Mater. 12 (2002) 89e94. € m, M. Berggren, O. Ingan€ [4] M. Granstro as, Micrometer- and nanometer-sized polymeric light-emitting diodes, Science 267 (1995) 1479e1481. [5] Y. Cao, G. Yu, C. Zhang, R. Menon, A.J. Heeger, Polymer light-emitting diodes with polyethylene dioxythiophene-polystyrene sulfonate as the transparent anode, Synth. Met. 87 (1997) 171e174.
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Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Synthesis of highly conductive PEDOT:PSS and correlation with hierarchical structure Tatsuhiro Horii, Hanae Hikawa, Masato Katsunuma, Hidenori Okuzaki* University of Yamanashi, 4-4-37 Takeda, Kofu 400-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 December 2017 Received in revised form 4 February 2018 Accepted 14 February 2018 Available online 16 February 2018
The water dispersions of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonic acid) (PEDOT:PSS) as colloidal particles were synthesized by oxidative polymerization with different composition ratios of repeating units for PSS and PEDOT (a ¼ 1.4e8.3). The role and effect of the PSS on hierarchical structure and electrical conductivity of the PEDOT:PSS were investigated systematically by means of X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), and conductive atomic force microscopy (c-AFM). It was found that the PEDOT:PSS colloidal particles stably dispersed in water at a 2.3, while small primary particles aggregated to form large secondary particles in water at a ¼ 1.4 because the zeta potential dropped owing to the less PSS. The PEDOT:PSS showed paracrystalline structure where highly conductive PEDOT nanocrystals uniformly distributed in the less conductive PSS matrices. The electrical conductivity was strongly dependent on the composition ratio and attained as high as 700 S/cm at a ¼ 2.3 where a positive correlation was seen between the electrical conductivity and number of conductive particles favorable for hopping of charge carriers in between the nanocrystals. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Conducting polymer Hierarchical structure PEDOT:PSS
1. Introduction The organic electronics originated from low cost, lightweight, and flexible electronics is developing through printed electronics, stretchable electronics, and recently into wearable electronics for Internet of Things (IoT) applications such as flexible displays, smart sensors, and soft actuators. Here, wet-processable, flexible, and highly conductive polymers are of great advantages to the organic electronics. Poly(3,4-ethylenedioxythiophene) doped with poly(4styrenesulfonate acid) (PEDOT:PSS), commercially available in the form of water dispersion as colloidal particles, can be applied to antistatic coatings, solid electrolytic capacitors, and organic LEDs [1e5]. Moreover, because of the high electrical conductivity, transparency, and thermal stability, the PEDOT:PSS has attracted considerable attention for transparent electrodes of flexible displays, touch panels, and solar cells as an alternative of indium tin oxide (ITO) [6]. Importantly, such excellent electrical characteristics of the PEDOT:PSS essential for the organic electronics are strongly dependent on its hierarchical structure [7e16]. Furthermore, the
* Corresponding author. E-mail address: [email protected] (H. Okuzaki). https://doi.org/10.1016/j.polymer.2018.02.034 0032-3861/© 2018 Elsevier Ltd. All rights reserved.
electrical conductivity of the PEDOT:PSS is improved significantly by two orders of magnitude upon adding high boiling point solvents such as ethylene glycol (EG) and dimethyl sulfoxide, so-called ‘secondary dopant’ [17e22]. The mechanism was explained in terms of changes in the hierarchical structure: Crystallization of the PEDOT molecules, removal of the insulating PSS surrounding the PEDOT core, and aggregation of the PEDOT:PSS colloidal particles, which affect both intra- and inter-particle transport of charge carriers [9,23,24]. For further improvement of the electrical conductivity, a correlation between the hierarchical structure and electrical conductivity of the PEDOT:PSS should be clarified in more detail. In particular, the secondary structure (poly-ion complex) is a key structure affecting both tertiary (colloidal gel particle) and quaternary structures (aggregation) in order to optimize the transport of charge carriers in the solid state [15]. However, a few studies on synthesis and characterization of the PEDOT:PSS reported extremely low electrical conductivities [26,27], and therefore the effect of composition ratio between the PEDOT and PSS on the hierarchical structure and high electrical conductivity of the PEDOT:PSS is still a ‘black box’. In this study, we newly synthesized highly conductive PEDOT:PSS water dispersions by oxidative polymerization with
34
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various feed composition ratios of PSS repeating unit to EDOT monomer (afeed ¼ 1e10) and changes in the hierarchical structure and electrical conductivity of the PEDOT:PSS were investigated by means of X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), and conductive atomic force microscopy (c-AFM). The results allowed us to conclude that the composition ratio was crucially important for the higher-ordered structure and electrical conductivity of the PEDOT:PSS, in which a number of conductive PEDOT nanocrystals uniformly distributed in the insulating PSS matrices were favorable for hopping of charge carriers and led to the highest bulk conductivity as high as 700 S/cm. 2. Experimental 2.1. Synthesis of PEDOT:PSS The water dispersions of PEDOT:PSS colloids with various feed composition ratios (afeed), defined as the molar ratio of PSS repeating unit to 3,4-ethylenedioxythiophene (EDOT) monomer, from 1 to 10 were synthesized by oxidative polymerization of EDOT (Aldrich) in the presence of various concentrations of the PSS (Mw ¼ 75,000 g/mol, Aldrich). The EDOT (0.5 wt%) and PSS (0.2e6.5 wt%) were mixed in pure water containing 0.98 wt% of Na2S2O8 (Junsei Chemical) as an oxidant and 0.2 wt% of FeSO4 7H2O (Junsei Chemical) as a catalytic agent. The total volume of reaction solution was typically 1.2 L and the oxidative polymerization was carried out under vigorous stirring in a nitrogen stream at 20 C for 24 h. After polymerization, the resulting sodium, iron, and sulfate ions were removed by cation exchange (Lewatit Monoplus S108H, Lanxess) and anion exchange resins (Lewatit MP62WS, Lanxess). The solid films were fabricated by casting the PEDOT:PSS water dispersion (1 wt%) containing 5 wt% of ethylene glycol (EG) as a secondary dopant [9,23,24] on a glass substrate at 200 C with a moisture analyzer (MOC-120H, Shimadzu).
and PSS anions where the PSS has two functions: One is the dopant to compensate positive charges of the PEDOT in the state of polaron or bipolaron. The other is the dispersant for stably dispersing hydrophobic PEDOT in water as the poly-ion complex [28]. Therefore, the composition ratio of the poly-ion complex will be one of the key parameters affecting the dispersion state of the PEDOT:PSS colloidal particles. Fig. 1 shows XPS spectra of the PEDOT:PSS in the energy range of the S 2p signal, where two peaks at binding energies of 163 and 167 eV assigned to the sulfur atoms of PEDOT and PSS, respectively [29]. It was found that relative intensity of the PSS to PEDOT increased with increasing the afeed, where actual composition ratios (a) evaluated from the peak areas of the XPS spectra were in good agreement with the values of afeed as shown in the inset of Fig. 1. 3.2. Median diameter and zeta potential of colloidal particles (tertiary structure) The effect of composition ratio on median diameter (D50) and zeta potential of the PEDOT:PSS colloidal particles was investigated by means of the DLS as shown in Fig. 2. At a ¼ 2.3e8.3, a sharp monodisperse peak was observed with the D50 of 17e29 nm, similarly to the commercial grades of the PEDOT:PSS (PH500 and PH1000, Heraeus) [12], where zeta potential was found to be 86e89 mV. This indicates that a negatively charged PSS-rich layer covers on the surface of the PEDOT:PSS colloidal particles, which is responsible for the stable dispersion in water. On the other hand, at a ¼ 1.4, the D50 significantly increased to 218 nm while the zeta potential dropped, demonstrating aggregation of the PEDOT:PSS colloidal particles caused by poor dispersibility in water due to the less PSS. 3.3. Crystalline structure of PEDOT (tertiary structure) In order to elucidate the dependence of the secondary structure
2.2. Measurements The median diameter (D50) and zeta potential of the PEDOT:PSS colloidal particles were evaluated by a dynamic light scattering (DLS) method at 25 C with particle size analyzer (Nanotrac UPAUT151, MicrotracBEL) and zeta potential analyzer (DelsaNano C, Beckman Coulter), respectively. The actual composition ratio (a) of repeating units for PSS and PEDOT in the PEDOT:PSS thin films spin-coated on n-Si wafer was evaluated by X-ray photoelectron spectroscopy (XPS) with a XPS photometer (JPS-9200, JEOL) using monocromatized Al (Ka) X-ray with an incident angle of 10 . The Xray diffraction (XRD) patterns were measured using an imaging plate (R-AXIS DS3C, Rigaku) at 40 kV and 30 mA with an exposure time of 1 h. The conductive atomic force microscopic (c-AFM) measurements were carried out with a scanning probe microscope (SPM-9600, Shimadzu) equipped with a conductive probe, where height and current images were measured by a contact mode under a bias voltage of 0.1 V. The electrical conductivity of the PEDOT:PSS film was measured by a normal four-point method with a LorestaGP (MCP-T610, Mitsubishi Chemical Analytech), where thickness of the PEDOT:PSS film was evaluated with a stylus profilometer (D100, KLA-Tencor). 3. Results and discussion 3.1. Composition ratio of poly-ion complex (secondary structure) The secondary structure of PEDOT:PSS poly-ion complex is formed through electrostatic interactions between PEDOT cations
Fig. 1. XPS spectra of PEDOT:PSS with various feed composition ratios (afeed) of PSS repeating unit to EDOT monomer in the energy range of the S 2p signal. Inset: Relation between afeed and actual composition ratio (a) of repeating units for PSS and PEDOT estimated from the XPS spectra.
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a screening effect [33]. This may induce a spontaneous stacking of the linear and planar PEDOT molecules, which favors the formation of a crystalline phase [9,14]. It is also seen from Fig. 3 that the crystallinity (Xc) decreases with increasing the a, which is attributed to the decrease of the PEDOT fraction in the PEDOT:PSS (broken line). The smaller Xc values than the broken line suggest that the PEDOT is not completely crystallized and/or the X-ray scattering factors of the PEDOT and PSS are different. Therefore, the Xc is a relative value depending on the change in the a values. 3.4. Aggregation of PEDOT:PSS colloidal particles (quaternary structure)
Fig. 2. Dependence of median diameter (D50) and zeta potential of the PEDOT:PSS colloidal gel particles on composition ratio (a) of repeating units for PSS and PEDOT measured by DLS.
of the composition ratio on tertiary structure in the solid state, we performed XRD measurements on the PEDOT:PSS films and the results are shown in Fig. 3. The Laue images show paracrystalline structure in which the Debye-Scherrer ring at 2q ¼ 26 (d ¼ 3.4 Å) can be assigned to the (020) planes of the orthorhombic unit cell of PEDOT crystals [30e32]. Here, the EG having high boiling point and high dielectric constant as the secondary dopant may decrease the electrostatic interactions between PEDOT cations and PSS anions by
Fig. 4 shows surface morphology of the PEDOT:PSS thin films with different composition ratios. At a ¼ 1.4, the surface roughness (Ra) was 0.9 nm where numerous PEDOT:PSS particles with an average particle size (Dp) of 32 nm were densely and randomly packed with a particle number (Np), defined as the number of particles in 1 mm2, ca. 800. This suggests that the large colloidal particles in water (D50 ¼ 218 nm in Fig. 2) correspond to aggregates consisting of small primary particles (Dp ¼ 32 nm). On the other hand, an increase of the a increases both values of Dp and Ra but decreases Np, demonstrating that less number of larger primary particles loosely aggregate with higher surface roughness. The AFM image is affected by a tip locus effect where typical resolution in the horizontal and vertical directions, depending on the tip curvature radius of the cantilever, are ca. 10 nm and 0.1 nm, respectively. Taking into account of the resolution of the AFM image, the results seem to be reasonable but are still inconsistent with the fact that the values of D50 were nearly constant (17e29 nm) at a ¼ 2.3e8.3 (Fig. 2). This can be explained in terms of the core-shell structure of the PEDOT:PSS colloidal particle [9,10]: The D50 measured by DLS may related to the hydrophobic PEDOT core size since the hydrophilic PSS shell swells in water. Indeed, at a ¼ 6.9 and 8.3 the primary particles are not clearly visible but obvious the fibril structure which may correspond to the PSS shell surrounding the PEDOT core. Furthermore, local conductivity was evaluated by c-AFM in which the current flowing though the PEDOT:PSS thin film between substrate and cantilever under a bias voltage of 0.1 V. As shown in Fig. 5, the current images reveal that the conductive PEDOT nanocrystals (red area) distributed spatially in the insulating PSS matrices (blue area), which supports the phase segregation of PEDOT:PSS [34]. Regardless of the composition ratio, the conductive particle size (Dcp) is ca. 10 nm or less by considering the resolution. The Dcp values are smaller than corresponding D50 (Fig. 2) and Dp (Fig. 4), while the Ncp are three times more than Np. This demonstrates that one PEDOT core in the colloidal particle is consisting of a few nanocrystals. It should be noted the Ncp shows maximum at a ¼ 2.3, indicating a number of smaller nanocrystals are densely and uniformly distributed in the PSS matrices. 3.5. Electrical conductivity
Fig. 3. Dependence of crystallinity (Xc, symbols) measured by XRD and weight fraction of the PEDOT in the PEDOT:PSS (broken line) on composition ratio (a) of repeating units for PSS and PEDOT. Inset: Laue images of the PEDOT:PSS films with different a values.
A clear indication of the importance of composition ratio on electrical conductivity of the PEDOT:PSS is demonstrated in Fig. 6. It is found that the electrical conductivity increases linearly with decreasing the a and attained as high as 700 S/cm at a ¼ 2.3, which can be attributed to the increase of the PEDOT fraction. A further decrease of a, however, results in a drop of the conductivity at a ¼ 1.4, indicative of an optimum composition between PEDOT and PSS. Assuming the doping ratio of the PEDOT to be 0.35 for the PEDOT:PSS [35], the a value of 1.4 is high enough to produce charge carriers in the PEDOT as polarons and bipolarons. Therefore, the drop in electrical conductivity can be explained by the decrease of
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Fig. 4. Changes in surface morphology, particle size (Dp), and particle number (Np) of the PEDOT:PSS on composition ratio (a) of repeating units for PSS and PEDOT.
Fig. 5. Changes in current image, conductive particle size (Dcp), and conductive particle number (Ncp) of the PEDOT:PSS on composition ratio (a) of repeating units for PSS and PEDOT measured under a bias voltage of 0.1 V.
carrier mobility but carrier density. Since the electrical conductivity of conductive polymers depends on transport of charge carriers between conductive particles or fibrils [25], a strong correlation between the conductivity and Ncp as shown in the inset of Fig. 6
clearly demonstrates that a number of small conductive PEDOT nanocrystals uniformly distributed in the insulating PSS matrices is favorable for hopping of charge carriers between the PEDOT nanocrystals responsible for the highest bulk conductivity.
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Fig. 6. Dependence of electrical conductivity measured by four-point method on composition ratio (a) repeating units for PSS and PEDOT and on Ncp (inset).
4. Conclusion The PEDOT:PSS water dispersions with different composition ratios were synthesized and correlation between hierarchical structure and electrical conductivity was systematically investigated by means of XPS, DLS, XRD, and c-AFM analyses. It was found that the PEDOT:PSS colloidal particles stably dispersed in water at a 2.3, where D50 and zeta potential were 17e29 nm and 86e89 mV, respectively. The PEDOT:PSS formed paracrystalline structure and the PEDOT nanocrystals with Dcp of ca. 10 nm or less distributed spatially in the PSS matrices. Furthermore, the electrical conductivity attained as high as 700 S/cm at the optimum composition ratio at a ¼ 2.3. The results allowed us to conclude that the secondary structure of composition ratio between the PEDOT and PSS significantly affected the tertiary and quaternary structures with the strong correlation between electrical conductivity and Ncp. Acknowledgements This work was supported in part by Grant-in-Aid for Scientific Research (B) with JSPS KAKENHI Grant Number 16H04203 from The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. References [1] F. Jonas, J.T. Morrison, 3,4-Polyethylenedioxythiophene (PEDT): conductive coatings technical applications and properties, Synth. Met. 85 (1997) 1397e1398. €s, Electrochromic and highly stable [2] Q. Pei, G. Zuccarello, M. Ahlskog, O. Ingana poly(3,4-ethylenedioxythiophene) switches between opaque blue-black and transparent sky blue, Polymer 35 (1994) 1347e1351. [3] H.W. Heuer, R. Wehrmann, S. Kirchmeyer, Electrochromic window based on conducting poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate), Adv. Funct. Mater. 12 (2002) 89e94. € m, M. Berggren, O. Ingan€ [4] M. Granstro as, Micrometer- and nanometer-sized polymeric light-emitting diodes, Science 267 (1995) 1479e1481. [5] Y. Cao, G. Yu, C. Zhang, R. Menon, A.J. Heeger, Polymer light-emitting diodes with polyethylene dioxythiophene-polystyrene sulfonate as the transparent anode, Synth. Met. 87 (1997) 171e174.
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