PEDOT/PSS nanoparticles: synthesis and properties

Abstract

Conductive polymer consisting of poly(3,4-ethylenedioxythiophene)/cross-linked poly(p-styrenesulfonate) (PEDOT/CL-PSS) particles was synthesized, and its dispersion and film properties were evaluated. CL-PSS was synthesized by sulfonation of poly(styrene-co-divinylbenzene) prepared by emulsion polymerization, and PEDOT/CL-PSS was synthesized by oxidative polymerization of EDOT in the presence of CL-PSS particles. The dispersion of PEDOT/CL-PSS exhibited lower viscosity than that of the usual PEDOT/PSS, enabling the highly concentrated dispersion of the conductive polymer in water. The electrostabilities of the PEDOT/CL-PSS film against heat and humidity were significantly improved by the cross-linkage, although the conductivity decreased with the increase of crosslink density. PEDOT/CL-PSS was miscible with PEDOT/PSS and could be used as a surface roughness-controlling agent for PEDOT/PSS film.

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Introductions

Transparent conductive materials (TCMs) play an important role in their application as the transparent electrodes of a wide range of electronic devices. Liquid crystal display, light-emitting diodes, touch screen panels,¹ e-paper^2–4 and photo voltaic cells^5,6 are obvious examples of the use of TCMs as transparent electrodes. There are many research studies concerning TCMs, including both inorganic materials and organic materials. The most widely used inorganic and organic TCMs are tin-doped indium oxide (ITO)^7–11 and poly(3,4-ethylenedioxythiophene)/poly(p-styrenesulfonate) (PEDOT/PSS),^12–16 respectively. ITO has advantageous properties such as higher conductivity and higher transparency¹⁷ than PEDOT/PSS. However, the formation of ITO film always suffers from its relatively low production efficiency and high production cost because the film has to be formed by sputtering method, which must be carried out under vacuum condition. Although it is not difficult to deposit ITO on flexible substrates, the conductivity of the film easily decreases with mechanical deformation, whereas flexible transparent conductive films are highly required components in industry nowadays.^18,19 Therefore, it is expected that solution-processible organic TCM would be a promising material that is flexible and can be processed under ambient condition, leading to higher production efficiency and lower production cost. Among those solution-processible organic TCMs, PEDOT/PSS is widely used because it exhibits sufficient transparency at the visible region, tough mechanical flexibility, decent thermal stability^7–11 and high hole-transporting property.^20,21 PEDOT/PSS is generally synthesized by oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) monomer in water with linear PSS as a stabilizer and a dopant for PEDOT polymer.⁹ Oxidative polymerization of EDOT generates the cationic charged PEDOT, which strongly interacts with the sulfonate ion of PSS to form a stable PEDOT/PSS complex. Although PSS is an indispensable component of PEDOT/PSS, the linear PSS is so hygroscopic that the water-resistant property and environmental reliability of PEDOT/PSS film is poor, and its conductivity easily changes depending on the temperature and humidity condition.^22–24 Nevertheless, there is an application of PEDOT/PSS utilizing its high sensitivity to humidity,²⁵ although it is usually recognized as an unfavorable feature of PEDOT/PSS. In order to improve the environmental reliability of the material, we investigated using a cross-linked PSS (CL-PSS) particle instead of linear PSS as a stabilizer and dopant for PEDOT. It is expected that the water resistivity of PEDOT/CL-PSS will be superior to that of PEDOT/PSS because CL-PSS is less hygroscopic owing to its cross-linkage.

The shape of the complex of PEDOT and CL-PSS will be rigid particles, unlike that of linear PEDOT/PSS, because CL-PSS particles will serve as seed particles (Fig. 1). The formation of particles rather than linear polymer is expected to have some advantages. The dispersion of PEDOT/CL-PSS particles will exhibit lower viscosity compared to a PEDOT/PSS dispersion, enabling the dispersion of higher concentrations.

Fig. 1 Schematic image of a liner PEDOT/PSS polymer and a PEDOT/CL-PSS particle.

Examples of PEDOT adsorbed on particles have already been reported by a few groups.^28,29 In the reports, PEDOT was synthesized on silica core particles and attempted in various applications including as a photonics catalyst, biocatalyst and drug delivery system. However, there is no report that utilizes PEDOT particles as TCMs. This is because PEDOT directly deposited on a silica particle would not exhibit high conductivity without dopants. The combination of PEDOT and CL-PSS will not only provide PEDOT particle with, but also improve, conductivity because CL-PSS contains a lot of dopant units, i.e. sulfonate groups. We also expect that the PEDOT particle is applicable as a roughness-controlling agent for the hole-transporting layer of organic photovoltaic (OPV) cells. A hole-transporting layer is placed between the electrode and the organic active layer of OPV cells, and its surface roughness influences the strength of adhesion between the hole-transporting layer and active layer, light absorption ratio, and charge collection ratio.^26,27 The addition of the highly conductive PEDOT particle to the PEDOT/PSS layer will improve the surface roughness without reducing electronic properties.

In this article, synthetic procedures for CL-PSS and PEDOT/CL-PSS are shown, and properties of the PEDOT/CL-PSS dispersion in water and its films were evaluated. Also, the effect of the addition of PEDOT/CL-PSS particles on the surface roughness of PEDOT/PSS layer was examined.

Experimental

Synthesis of CL-PS

Cross-linked polystyrene particles (CL-PS) were prepared by emulsion co-polymerization of styrene and divinylbenzene (DVB). As a typical procedure, styrene (5.0 g, 48 mmol), DVB (0.11 g, 0.48 mmol) and sodium dodecylsulfate (SDS, 0.35 g, 1.2 mmol) were placed into a four-necked round-bottom flask, and 100 g of water was added. The mixture was purged with nitrogen gas for 15 min and then heated to 80 °C. An aqueous solution (2.0 ml) of potassium persulfate (0.1 g, 0.37 mmol) was added to the flask, and the mixture was kept at 80 °C for 2 h with vigorous stirring.

The diameter of CL-PS was evaluated by dynamic light scattering (DLS, Otsuka Electronics, FPAR-1000).

Synthesis of CL-PSS

The dispersion of CL-PS was dried by a rotary vacuum evaporator to obtain its powder. CL-PS powder (1.0 g, 9.57 mmol) was dissolved in anhydrous 1,2-dichloroethane (20 ml). Acetic anhydride (3.0 g, 28.7 mmol) and concentrated sulfuric acid (3.2 g, 31.7 mmol) was added. The mixture was stirred for 5 h at 50 °C. After the reaction, a brown-colored deposit was obtained. The deposit was rinsed three times with 1,2-dichloroethane and dried in a vacuum oven at 120 °C for 1 h; a gum-like solid of CL-PSS polymer was obtained. The solid (3.61 g) was redispersed in 100 ml water, and the dispersion was directly used in the next reaction. The diameter of CL-PSS in the dispersion was also evaluated by DLS.

Ratio of carbon, sulfur and hydrogen in CL-PSS was evaluated by chemical elemental analysis (CE Instruments, EA1110), and sulfonation ratios for each CL-PSS were calculated.

Synthesis of PEDOT/CL-PSS

EDOT (0.24 g, 1.69 mmol) and 3.61 wt% CL-PSS dispersion in water (21.5 g) was placed into a flask. The mixture was purged with nitrogen gas and kept at 18 °C. An aqueous solution of 1 wt% aq. iron(III) sulfate (1.38 g) and 11 wt% sodium persulfate solution (3.45 g) were added to the mixture. The mixture was stirred for 23 h at 18 °C. After stirring, cationic ion exchange resin (Mitsubishi, SK-1BH, 20 g) and anionic ion exchange resin (Mitsubishi, SA-10AOH, 25 g) were added to absorb free ions. After filtration of the resin, dispersion of PEDOT/CL-PSS was obtained. Solid content of PEDOT/CL-PSS dispersion was 0.98 wt% by gravimetric measurement. PEDOT/PSS was also synthesized for comparison. Viscosities of dispersions were evaluated by an E-type viscometer (Thermo Scientific).

Ink formulation and coating procedure

Coating inks of PEDOT/CL-PSS and PEDOT/PSS were prepared by adding 50 wt% of 2-propanol and 5 wt% of dimethylsulfoxide (DMSO). Solid content of the ink was 0.44 wt%. 2-Propanol was added to decrease the surface tension of the dispersion to improve wettability, and DMSO was added to enhance conductivity as a secondary dopant. The film formation was carried out by bar coating in this article. We have also confirmed the availability of other methods such as spray coating, gravure coating, slot die coating and so on. Coating ink was coated on polyethylene terephthalate (PET) film substrates by bar (#9) to an estimated 20 μm wet-coating thickness. Coated film was obtained after the evaporation of water and solvent at 120 °C in a drying oven. Thickness of the films was measured with surface stylus profiling (Tencor P-6) and estimated to be approximately 100 nm for each sample. For the evaluation of PEDOT/CL-PSS as a surface roughness-controlling agent, PEDOT/CL-PSS ink and PEDOT/PSS ink were combined and stirred with a magnetic stirrer for 5 min prior to coating.

Evaluation of PEDOT/CL-PSS films

Morphology of the films was observed by field emission scanning electron microscope (FE-SEM, Hitachi, FE-4700) measurement. Surface roughness of the films was evaluated by atomic force microscopy (AFM, Shimadzu, SPM-9600). Conductivity was calculated from film thickness and sheet resistance (4-point probe method, Mitsubishi, MCP-T610). Environmental reliabilities, i.e. heat resistivity and humidity resistivity, of the films were measured by R/R₀, with storage under 90 °C for 250 h or 60 °C and 95% relative humidity for 250 h, respectively, where R stands for the resistivity after the test and R₀ stands for the initial resistivity.

Results and discussion

Synthesis of PEDOT/CL-PSS and properties of the dispersion

As a component of PEDOT/CL-PSS, the ratio of PEDOT to CL-PSS, the size, crosslinking ratio, and the concentration of sulfonic acid group of CL-PSS are expected to be important factors for the overall property. Since there would be a lot of choices for the synthetic method of CL-PSS, we synthesized CL-PS particles by emulsion polymerization and conducted the sulfonation reaction to obtain CL-PSS. Emulsion polymerization provides relatively small (<0.1 μm) particles, which would be suitable for use as a TCM that requires low haze and sufficient smoothness of the film.

Scheme 1 shows the synthetic procedure for CL-PS, CL-PSS and PEDOT/CL-PSS. CL-PS was synthesized by emulsion co-polymerization of styrene and DVB. Sulfonation of CL-PS was carried out by the Friedel–Crafts type reaction with sulfuric acid and acetic anhydride to obtain CL-PSS. And finally, EDOT was polymerized in the presence of CL-PSS with sodium persulfate and iron(III) sulfate as oxidant to give PEDOT/CL-PSS.

Scheme 1 Synthesis of CL-PS, CL-PSS, and PEDOT/CL-PSS.

As shown in Table 1, the diameters of CL-PS were around 40 nm irrespective of the crosslinking ratio. After the sulfonation, CL-PSS with low DVB content showed a significantly large diameter compared to that of CL-PS, suggesting that the particle easily absorbed water and swelled. The swelling ratio of CL-PSS was highly dependent on the DVB content, indicating that the degree of crosslinking increases with the increased content of DVB and that the cross-linkage on the PS main chain did not suffer from the sulfonation reaction. Since the swelling of CL-PSS containing 10% DVB was prevented by the cross-linkage, the change of diameter was almost ignorable. After the polymerization of EDOT, the diameters of PEDOT/CL-PSS were far larger than that of CL-PSS. This result suggested the formation of aggregated particles of PEDOT/CL-PSS, which is probably due to the hydrophobic interaction of PEDOT-rich domains on the surface of PEDOT/CL-PSS particles.

Table 1 Diameters of CL-PS, CL-PSS and PEDOT/CL-PSS particles^a

Content of DVB (%)	CL-PS (nm)	CL-PSS (nm)	PEDOT/CL-PSS (nm)
a Determined by DLS measurement.
0.5	45	304	1100
1	44	230	984
3	40	100	886
5	40	95	762
10	39	45	406

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Table 2 shows the results of chemical elemental analyses of CL-PSS with different DVB contents and calculated sulfonated ratio versus monomer unit. The sulfonation ratio obtained from the sample without DVB was 73%. In the case of PSS containing 3% DVB, the sulfonation ratio slightly decreased from 73% to 69%. PSS containing 10% DVB showed further decrease to 58%. These results suggested that co-polymerization of DVB would increase steric hindrance and decrease the sulfonation ratio. The relatively low sulfonation ratio of highly cross-linked CL-PSS would also be responsible for the smaller swelling ratio in water. Nevertheless, CL-PSS with 10% DVB was moderately sulfonated even with the high crosslink density, and it was expected to possess sufficient sulfonic acid groups to work as a dopant of PEDOT.

Table 2 Carbon, sulfur content and calculated sulfonation ratio of CL-PSS

Content of DVB (%)	Carbon content (%)	Sulfur content (%)	Sulfonation ratio (%)
0	25.1	5.06	73
3	31.1	5.76	69
10	35.4	5.16	58

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The ratios of PEDOT to CL-PSS were not precisely constant for each sample because the sulfonation ratio varied. According to the reaction condition, the feed ratio of EDOT to sulfonic acid group was calculated to be between 1 : 1.8 and 2.1. We consider that a ratio variation of this range will not make a serious difference in the electronic properties of each sample.

PEDOT/CL-PSS with varied contents of DVB were casted on glass plates, and their morphologies were observed by FE-SEM. As shown in Fig. 2, PEDOT/CL-PSS containing more than 3% DVB was clearly observed to be a coagulum of particles, whereas PEDOT/PSS without crosslinkage showed no specific morphology and PEDOT/CL-PSS with 1% DVB was composed of fused particles. As the crosslinking ratio of CL-PSS increases, the uniformity of the film decreases, and voids and particle boundary remain inside the films. The average diameter of the primary particles was observed to be around 40 nm from each SEM image and was almost identical to that of CL-PS as shown in Table 1. This result indicated that the sulfonation reaction and subsequent complexation with PEDOT did not affect the crosslinking and did not give rise to change of primary particle size. Also, the result agreed with the assumption that PEDOT/CL-PSS particles formed aggregated secondary particles.

Fig. 2 SEM images of (a) PEDOT/PSS film, (b) PEDOT/CL-PSS (1% DVB) film, (c) PEDOT/CL-PSS (3% DVB) film, and (d) PEDOT/CL-PSS (10% DVB) film.

It is well known that conventional PEDOT/PSS suffers high viscosity when it is slightly concentrated. Too-high viscosity of the dispersion is a serious defect in its use as a coating solution. As shown in Fig. 3, the viscosity of the dispersions of PEDOT/CL-PSS decreased when the content of DVB increased. This result would be responsible for the decrease of intermolecular polymer chain entanglement because all PSS chains were crosslinked to form particles. In the case of PEDOT/CL-PSS with 10% DVB, the increase in viscosity was so small that it was easy to obtain a highly concentrated dispersion of up to 3.5% under 10 mPa s⁻¹. The availability of high-concentration PEDOT dispersion is one of the most advantageous features of the present research.

Fig. 3 Relationship between solid concentration of PEDOT/CL-PSS dispersions and their viscosities.

Film formation of PEDOT/CL-PSS and conductivity of the films

Coating ink was prepared by adding additives (2-propanol and DMSO) to PEDOT/CL-PSS dispersions, which were then coated on a PET substrate by wire bar. As shown in Fig. 4, the conductivity of PEDOT/CL-PSS films decreased with the crosslink density increased. The lower density of sulfonic acid group in the highly cross-linked CL-PSS particle would affect the conductivity of PEDOT/CL-PSS films. Also, the films of highly crosslinked PEDOT/CL-PSS contains a lot of voids and micro-cracks as shown in the SEM pictures (Fig. 2c and d), which would be another reason for the low conductivity. Nevertheless, the result clearly showed the difficulty of forming uniform films with high conductivity by cross-linked PEDOT/CL-PSS.

Fig. 4 Relationship between content of DVB and the conductivity of PEDOT/CL-PSS films.

Environmental reliability of the films was tested by storing them at high temperature (90 °C) and high humidity (60 °C, 95% RH) conditions for 250 h, and the effects on resistivity were measured. The relationship between the content of DVB and relative resistivity is shown in Fig. 5. PEDOT/PSS without DVB was easily affected by both humidity and heat. Under the heat storage test, it was found that the value of R/R₀ decreased as the crosslink density increased. In the humidity storage test, it is well known that dedoping of PEDOT/PSS occurs due to the hygroscopicity of PSS.³⁰ However, R/R₀ became stable by adding only 0.5% DVB. These results indicated that the dedoping of PEDOT/PSS was prevented by the steric hindrance and less hygroscopic nature of the crosslinked PSS. Thus, it can be concluded that introduction of cross-linking significantly improved the environmental reliability of PEDOT/CL-PSS films.

Fig. 5 Relationship between the content of DVB and relative resistivity (R/R₀) of the films.

The results showed that PEDOT/CL-PSS is a more stable and reliable material than PEDOT/PSS. Whereas the cross-linkage also caused the decrease of conductivity, and the sample with 1% DVB exhibited sufficient conductivity, significantly decreased viscosity, and improved environmental reliability at the same time. High environmental reliability is expected to enrich various device applications in extreme environmental conditions such as a touchscreen panel application in automotive and OPVs for outdoor use.

Surface roughness and conductivity of PEDOT/PSS + PEDOT/CL-PSS composite films

One of the well-known applications of PEDOT/PSS is as a hole-transporting layer for OPVs. Although PEDOT/PSS usually has very smooth surface as shown in Fig. 6a, increasing the surface roughness of PEDOT/PSS as a hole-transporting layer is expected to improve OPV efficiency due to increasing surface area, higher light-scattering property, and improved adhesion toward the active layer.^26,27 The film of conventional PEDOT/PSS has smooth surface with R_a of 0.88 nm (Table 3). On the other hand, the mixture of 1% PEDOT/CL-PSS (DVB 10%) + 99% PEDOT/PSS gave rise to increased surface roughness with R_a of 1.15 nm. The more PEDOT/CL-PSS particles were added, the rougher the surface that was obtained. The composite film with 90% PEDOT/PSS + 10% PEDOT/CL-PSS exhibited higher R_a up to 6.29 nm. From Fig. 6b, it can be seen that PEDOT/CL-PSS particles were well dispersed in PEDOT/PSS film without aggregation, probably because they have same functional groups and therefore they are miscible with each other. Moreover, the conductivities of 100% PEDOT/PSS film and composite film of 90% PEDOT/PSS + 10% PEDOT/CL-PSS were 200 S cm⁻¹ and 198 S cm⁻¹, respectively, indicating that the addition of PEDOT/CL-PSS particles did not reduce the conductivity of PEDOT/PSS films at all. The conductivity of the composite films was retained because PEDOT/CL-PSS uniformly dispersed in PEDOT/PSS film and electricity was mainly conducted through the PEDOT/PSS region.

Fig. 6 AFM images of (a) PEDOT/PSS surface and (b) of PEDOT/PSS:PEDOT/CL-PSS surface (DVB 10%) = 90 : 10.

Table 3 Surface roughness and conductivity of PEDOT/PSS + PEDOT/CL-PSS composite film

PEDOT/PSS:PEDOT/CL-PSS (DVB 10%)	Ra of film surface^a (nm)	Conductivity (S cm^−1)
a Determined by AFM measurement.
100 : 0	0.88	200
99 : 1	1.15	200
95 : 5	3.23	198
90 : 10	6.29	198

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The results demonstrated that PEDOT/CL-PSS particles can be used as a surface roughness-controlling additive of PEDOT/PSS film without affecting conductivity of the hole-transporting layer itself. Higher roughness of the film is expected to enhance the adhesion between the PEDOT/PSS film and the next layer fabricated onto the film, i.e. active layer. Thus, this novel method will be readily applicable to improving the reliability and durability of organic electronic devices that contain the PEDOT/PSS layer, such as OPV cells.

Conclusions

CL-PSS was synthesized by the emulsion co-polymerization of styrene and DVB and the subsequent sulfonation of thus obtained CL-PS. Then, PEDOT/CL-PSS was synthesized by oxidative polymerization of EDOT with the existence of CL-PSS in water. The viscosity of the dispersions decreased with increased crosslink density. It was revealed that the shape of the PEDOT/CL-PSS complex was an aggregated dispersion of stable primary particles unlike that of linear PEDOT/PSS. The low viscosity of the PEDOT/CL-PSS dispersion enabled the preparation of a high-concentration dispersion, up to 3.5%, which could not be achieved by conventional PEDOT/PSS. Although the film of highly cross-linked PEDOT/CL-PSS exhibited relatively low conductivity, the conductivity was stable against high temperature and humidity, indicating the improvement of environmental reliability by the introduction of cross-linkage on the PSS chain. The PEDOT/CL-PSS particles were easily mixed with PEDOT/PSS, and the composite film with rough surface was obtained without decrease of conductivity.

Those interesting properties of this material will lead to various applications. For example, the PEDOT/PSS + PEDOT/CL-PSS composite with increased surface roughness will readily be applicable as a hole-transporting layer of OPV, with improved reliability and durability as well as increased surface area, higher light-scattering property, and improved adhesion toward the active layer. Also, it is expected to be used in applications such as conductive additives for conductive dispersant, conductive filler and coloring filler.

Acknowledgements

This research was partly supported by Japan Science and Technology Agency (JST), A-Step program AS2524023M.

Notes and references

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