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Roles of Nonionic Surfactant Additives in PEDOT:PSS Thin Films Sung-Soo Yoon, and Dahl-Young Khang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12043 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 12, 2016

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The Journal of Physical Chemistry

Roles of Nonionic Surfactant Additives in PEDOT:PSS Thin Films Sung-Soo Yoon and Dahl-Young Khang* Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea

ACS Paragon Plus Environment

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ABSTRACT

The effects of nonionic surfactant additive, Triton X-100, on the properties of PEDOT:PSS thin films has been investigated. The detailed mechanism for the well-known conductivity enhancement upon the addition of high boiling point nonionic surfactant has been elucidated based on various characterization results. The surfactant additive has been found to enhance the π-π stacking of PEDOT segment, leading to enhanced electrical conductivity. In addition, the added surfactant has facilitated the removal of insulating PSS during the methanol spin washing, which contributes to further enhancement in electrical conductivity. Interestingly, the remaining surfactant in the dried PEDOT:PSS thin films plays the role of plasticizer. Upon aging of the films under room ambient (RH~30%), the conductivity of the film has been gradually increased until the humidity effect comes in. As a plasticizer, the surfactant has been found to enhance lamella stacking during the aging. Also, the mechanical modulus of the films has been characterized by buckling method, which proves the softening effect of residual surfactant for PEDOT:PSS films. On the other hand, the reinforcing filler effect of PEDOT crystallites has been found to be negligible compared to plasticizer effect.


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INTRODUCTION Poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is one of the most important conducting polymers,1 which is widely available under various commercial name. Here, the electrically conductive part is PEDOT, while the PSS is an electrical insulator. Due to hydrophobic nature of low molecular weight PEDOT segments (MW: 1000~2500), the hydrophilic high molecular weight (~400,000) PSS chains are used as a carrier for water soluble dispersion. The interaction between PEDOT and PSS is known to be ionic, and thus the PEDOT:PSS is also called as polyelectrolyte complex (PEC) of polycationic PEDOT segments and polyanionic PSS chain. The use of insulating PSS enables the easy dispersion of the complex in water, it hinders the electrical conduction of the final, dried film, however. To enhance the electrical conductivity of PEDOT:PSS films, various approaches has been suggested and called as ‘secondary doping’.1-4 These methods are mostly based on the addition of solvents such as sorbitol,5 ethylene glycol (EG),6 dimethylsulfoxide (DMSO),7 polyethylene glycol (PEG),8 dimethylsulfate,9 etc. These solvents are believed to reduce the ionic interaction between PEDOT and PSS, which facilitates the phase separation and the conformational change of PEDOT from benzoid to quinoid and thus crystalline packing. Also, the prepared films are usually treated with solvent to further enhance the conductivity by washing off the insulating shell of PSS.10-12 Another line of additives is amphiphiles or surfactants.13,14 Surfactant additives have also been found to enhance the electrical conductivity. The amphiphilic nature of surfactant molecules is effective to weaken the ionic stitching between PEDOT and PSS, which seems to play quite similar roles to those of solvent additives mentioned above. Furthermore, recent demonstration of viscoelasticity of surfactant-added PEDOT:PSS may extend the applicability beyond typical ones, such as mallable or deformable electronics.15,16


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In this work, we have investigated the role of nonionic surfactant additive, Triton X-100 (shortened as TX hereafter), in PEDOT:PSS films by various analytical techniques. Although there have been a few reports on the conductivity enhancement effect upon the addition of TX into PEDOT:PSS, its roles have been elucidated based on electrical, structural, mechanical and compositional analyses. The surfactant molecules have been found to weaken the ionic interaction between PEDOT and PSS and expedite phase separation into separate phases, leading to enhanced crystallization of PEDOT segments and thus electrical conductivity. At the same time, the surfactant facilitates the removal of PSS during the following methanol washing step, which enhances the electrical conductivity further. The high boiling point surfactant residing in the dried PEDOT:PSS film acts as a plasticizer, leading to unexpected gradual increase in electrical conductivity of the film upon aging under room ambient, until the humidity effect dominates the electrical conductivity. The gradual conductivity enhancement is due to the increased lamella stacking of PEDOT and PSS during aging. Therefore, the surfactant additive contributes to increased molecular π-π coupling among PEDOT segments in solution and during film formation, while it increases intergrain coupling during aging by endowing chain mobility in dried films, in addition to facilitate easy removal of insulating PSS during methanol washing. All these contribute to the enhanced conductivity of PEDOT:PSS film. Finally, the change of mechanical modulus upon the addition of surfactant and methanol washing has been characterized by buckling method. Regardless of methanol washing, the modulus decreases with the increase of surfactant content due to its softening effect. And the reinforcing filler effect by PEDOT crystallites has been found to be negligible, compared to the plasticizer effect. The indepth understanding on the roles of surfactant additive can be a valuable help in diverse applications of PEDOT:PSS thin films.


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EXPERIMENTAL METHODS Nonionic surfactant (Triton X-100, Aldrich) was added into PEDOT:PSS (Clevios PH-1000, Heraeus) dispersion at specific concentrations (0, 0.1wt% or 2wt%), and stirred with magnetic bar overnight. These solutions were used to form thin films on cleaned Si or glass substrates. The film thickness was controlled by varying the spin speed, 300rpm or 3000rpm, which resulted in ~130nm and ~50nm-thick films in the case of pure PEDOT:PSS dispersion. Surfactant-added PEDOT:PSS solution has led to thicker film(~165nm and ~185nm for 0.1wt% and 2wt% solutions, respectively) due to the high viscosity of surfactant. Then, the coated samples were annealed at various temperatures on a hot plate, at room temperature, 60oC or 120oC, for 10min. Some of these samples were then washed with methanol drops while spinning at 8000rpm for 30sec. After the methanol washing, the samples were annealed again at the same temperature of the first annealing for 5min. Although the main focus of this work is on the roles of TX additive, methanol washing has been adopted in some cases because most of practical applications require highly conductive PEDOT:PSS.10,12,14 The thickness of PEDOT:PSS layer was measured by surface profiler (Dektak XT, Bruker). The sheet resistance of PEDOT:PSS layer was measured by four points probe (Laresta-GP, Mitsubishi Chemical Analytech); for these measurements, the PEDOT:PSS layer was prepared on transparent and insulating glass substrate. X-ray photoelectron spectroscopy (XPS) spectra of S(2p) has been characterized using Thermo K-alpha with Al-Kα X-ray with detector at 45o from the surface normal. The Raman spectra of films were measured using LabRam HR (Horiba Jobin-Yvon) system with a 632.8nm HeNe laser as the excitation source. The laser power was 0.1mW and the resolution was 2cm-1. Atomic force microscopy (AFM; MFP-3D, Asylum) have been used for sample surface imaging. The X-ray diffraction (XRD) patterns were obtained using


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a Rigaku SmartLab diffractometer in conventional θ-2θ geometry, with Cu-Kα radiation (λ~1.54Å) at 40kV and 200mA. For the buckling experiments,17-19, flat slab of elastomeric polydimethylsiloxane (PDMS) was prepared using Sylgard 184 (Dow). A 10:1 mixture (in weight) of base resin to curing agent was de-gassed and cured at 70oC for >4hrs in a plastic petri dish. To make the PDMS surface hydrophilic for the coating of pristine (i.e, without surfactant addition) PEDOT:PSS solution, the elastomer surface was floated on HCl solution for >15hrs.18 The coated films were dried at 60oC for 10min, except for otherwise stated samples. Some of these samples were washed with methanol while spinning at 8000rpm for 30sec and annealed again at the same temperature for 5min. For the buckling, home-made compression stage was used under an optical microscope (OM; Oylmpus, BX51). Small (~1.5%) compressive strain was applied to generate buckling wave on the film surface, and the OM images were taken on the surface (Excessive, >5%, compression may lead to nonlinear buckling, where the buckling wavelength depends on the applied compressive strain19). Buckling wavelength was measured from the total distance between multiples of waves divided by the number of waves contained, which yields more accurate and less scattered value than that obtained from the single wavelength measurement. The film thicknesses were measured by the surface profiler. From the measured values of buckling wavelength and film thickness, together with known material parameters of elastomer PDMS (Es~1.8MPa, νs=0.5), the following equation was used to extract the mechanical modulus:17,18

Ef

3 Es = 1 −ν s2

(1 −ν ) ( 2 f

)

 λ   2π h

  

3

(1)


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Here, λ is the buckling wavelength measured from OM images, h the film thickness, Es the Young’s modulus, and the subscripts f and s denote the film and substrate, respectively. RESULTS AND DISCUSSION Considering the high boiling point (270oC) of added surfactant, the residual additive in PEDOT:PSS film can have some effects on the properties of the film. The change of the sheet resistance of PEDOT:PSS films upon exposure to room ambient (relative humidity (RH) ~ 30%) are shown in Figure 1 (Similar plots for the change of film conductivity, instead of relative resistance change, can be found in Supporting Information, Figure S1). Notably, the sheet resistance decreases with exposure time, which is quite contrary to common sense that the resistance increases upon exposure to humidity due to the hygroscopic nature of PEDOT:PSS, except for the thin (~50nm) PEDOT:PSS films. Figure 1a shows the resistance change with time for PEDOT:PSS films annealed at room temperature, i.e., simply dried at RT for 10min. after the spin coating. All the films show similar trend: resistance decreases with exposure time, up to 100~120hrs, regardless of surfactant addition. The film prepared from pristine (without surfactant) solution shows ~10% decrease in sheet resistance, while other films prepared from surfactant-added (0.1 or 2 wt%) solutions do ~20% decrease in resistance. The decrease in resistance of pristine film is likely due to the hydration effect of adsorbed water molecules. The condensed water molecules on film surface, upon exposure to room ambient, seem to enable the film surface to relax or rearrange more energetically favorable state. The rearrangement would be more intimate contact between conducting PEDOT-rich grains at the surface, which leads to ~10% reduction in resistance. Other mechanisms have been proposed in literature,20-23 but the indepth discussion on the exact mechanism of the humidity effect is beyond the scope of the present work. Upon prolonged exposure, however, the water vapor can have enough time to


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diffuse deep into the film and induce the swelling of PSS-rich shells embedding the PEDOT-rich core. The swelling makes the electrical contact among PEDOT-rich cores worse, leading to increase in sheet resistance. Note here that the difference in the degree of resistance change between pristine (~10%) and surfactant-added (~20%) cases cannot be completely explained by the hydration and rearrangement effect by water. There should be some other mechanisms for such additional drop in resistance during aging for surfactant-added cases.

Figure 1. Change of relative electrical resistance in PEDOT:PSS films. (a) dried films at room temperature, (b) annealed films at 60oC, (c) annealed at 120oC, and (d) thin (~50nm) films annealed at 60oC. Similar trend has been found for films annealed at 60oC, as shown in Figure 1b. Here again, film prepared from pristine solution showed ~10% decrease in resistance upon exposure to air up


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to 100~120hrs, then started to increase again for the longer exposure time. Included in the Figure 1b is the change of resistance for the film prepared from pristine solution without methanol washing, which shows abrupt increase in resistance as soon as the air exposure starts. This is due to the hygroscopic PSS in the film, which is abundant on the film surface due to the absence of washing with methanol. In other words, the methanol washing indeed removes PSS preferentially from the top surface of film. Shown in Figure 1c is the resistance change for the films annealed at 120oC. Similar trend was observed, but with less decrease in the resistance compared to Figure 1a,b. In this case, the pristine and surfactant-added films show ~7% and ~15% decrease in resistance, respectively. Figure 1d shows the change in sheet resistance of thin (~50nm) PEDOT:PSS films. In these very thin films, the change was observed at the very initial stage of air exposure. Then, the resistance starts to increase in 1-2hrs. For these thin films, condensed water vapor can diffuse deep into the film in very short time and thus the hydration effect is observed at much shorter time scale. The rapid penetration of water into these thin films then induces swelling of PSS, leading to the typical resistance increase.


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Figure 2. (a) XPS spectra for S(2p) on different PEDOT:PSS film surfaces. (b) Raman spectra of PEDOT:PSS films. (c), (d) AFM height images of PEDOT:PSS film surfaces prepared from pristine (c) and 2wt% TX-added (d) solutions. (scan size: 1µm*1µm, z-scale: ±7nm for both images) To further elucidate the role of nonionic surfactant additive in PEDOT:PSS, XPS spectra of S(2p) has been checked and shown in Figure 2a. The bimodal peaks at ~164.5eV and 165.6eV are from S in the thiophene ring of PEDOT, while peak at ~168eV is from S in PSS.24-26 The change in peak height at those peak positions can shed light on the change of composition at the film surface. As shown, the height of PEDOT peaks increases upon methanol washing treatment. This confirms that the washing with methanol enriches PEDOT at the film surface by the preferential removal of PSS.10 Further increase in PEDOT peak height has been observed for the surfactant-added PEDOT:PSS film. Unlike the pristine film, where the PEDOT and PSS are tied


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together by strong ionic interaction, the interaction between PEDOT and PSS weakens by the added surfactant, forming TX-PEDOT and TX-PSS complexes. That is, the surfactant expedites the phase separation of PEODT:PSS into separate phases, which is quite similar to other solvent additives.27-29 The TX-PSS complex seems to be easily removed by methanol washing, compared to PSS removal by methanol washing in pristine PEDOT:PSS film. This enhanced removal of PSS by methanol washing contributes to the additional, though small, increase in conductivity of the PEDOT:PSS films. Further, the addition of surfactant has led to the red-shift of Raman spectrum compared to pristine solutions, as shown in Figure 2b, which is in good agreement with literature.14,30 Also, washing of the coated film with methanol does not have any effect on Raman spectra of PEDOT:PSS films; the spectrum from methanol-washed film (red line) almost completely overlap with non-washed (black line) one, as shown in Figure 2b. Therefore, the change of PEDOT conformation from benzoid to quinoid is not by methanol washing but by the addition of nonionic surfactant. The nonionic surfactant used in this work, Triton X-100, has aromatic hydrophobic group and hydrophilic polyethylene oxide chain. Upon addition into PEDOT:PSS/water dispersion, the amphiphilic surfactant can interact with both PEDOT and PSS. This interaction of the surfactant molecules with both types of polyelectrolytes means that the breakage of ionic bond between PEDOT and PSS, again which means the conformational change of those released PEDOT segments from benzoid into quinoid form. The freed, rigid rodlike PEDOT segments can now assemble/pack into crystalline state. On the other hand, the washing with methanol has no effect on the conformation of PEDOT; rather, it simply washes away or removes insulating PSS or TX-PSS complex, leading to steep increase in electrical conductivity due to increased direct contact among conducting PEDOT crystallites.10


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The surface morphology of PEDOT:PSS films prepared from different solutions has been investigated by AFM, as shown in Figure 2c, d. As a reference, Figure 2c shows the height image of PEDOT:PSS film surface prepared from pristine solution (i.e., no surfactant addition), which shows small (less than 20nm) granular surface. On the other hand, PEDOT:PSS film surface prepared from 2wt% surfactant-added solution shows much larger (>40nm), ellipsoidal grains, as shown in Figure 2d. They look like somewhat elongated, possibly nanofibril-like shapes.14 In accordance with previous literature, the formation of larger, elongated grains upon the addition of surfactant leads to higher conductivity via the formation of larger crystallites.

Figure 3. XRD patterns of PEDOT:PSS films, (a) right after the film preparation and (b) aged at room ambient for 3 days. We have performed XRD analysis to further check the effect of surfactant additive. Shown in Figure 3 are the XRD patterns of PEDOT:PSS films prepared from different solutions, right after the film preparation (Figure 3a) and aged under room ambient for 3 days (Figure 3b), respectively. We compare diffraction peaks at 2θ=~3.8o and ~26o.11,24,30 The former peak represents the alternate lamella stacking of PEDOT and PSS, which has lattice spacing of ~ 23Å. The latter peak at the higher diffraction angle denotes the interchain π-π stacking of PEDOT segments having lattice spacing of ~ 3.4Å. For the as-prepared films shown in Figure 3a, the


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surfactant-added film shows clear increase in peak intensity at 2θ=~26o, meaning that the enhanced crystalline packing of PEDOT segment, which leads to the enhanced electrical conductivity of TX-added PEDOT:PSS film. Upon aging under room ambient for 3 days as shown in Figure 3b, the pristine film does not show any meaningful change in diffraction pattern. On the other hand, the surfactant-added film shows increased peak intensity at 2θ=~3.8o, meaning that the enhanced lamella packing and thus grain growth with increased intergrain coupling. Therefore, the electrical resistance of the film decreases with time, as shown in Figure 1. That is, the additional ~10% reduction of resistance in surfactant-added films shown in Figure 1 is due to the enhanced lamella stacking of the films during the room-temperature aging. The high boiling point and low vapor pressure of the surfactant leads not to complete removal upon annealing, even annealing at 120oC. The residual surfactant in those films plays the role of plasticizer, facilitating the gradual increase in lamella packing by endowing the film with plasticity.


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Figure. 4. Optical microscopy images of buckled PEDOT:PSS films on an elastomer. (a) and (b) are images of films without methanol washing, and (c) and (d) are with methanol washing. (e) tabulated values of film thicknesses, buckling wavelengths, and mechanical modulus values for samples with or without surfactant additive. Scale bars denote 30µm for all images. Figure 4 shows the OM images of buckled PEDOT:PSS surface on PDMS by applying small(~1.5%) compressive strain. From these images, the measured film thicknesses and the corresponding buckling wavelengths are tabulated in Figure 4e. Shown also are the calculated modulus values in the last column of Figure 4e. For the modulus calculation using eq.(1), the Poisson ratio of PEDOT:PSS film was set to be 0.33,16 and the modulus of PDMS substrate was


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used to be 2MPa. Although the Poisson ratio of PEDOT:PSS film, especially for the films with surfactant additive, is not known a priori, the calculated modulus values are not sensitive to the Poisson ratio of the film. First of all, methanol washing has led to decrease in film thicknesses for all cases investigated here. That is, the washing removes materials from the film surface, mostly TX-PSS complexes. Also, the higher the surfactant concentration, the larger the degree of decrease in film thickness, ~4% decrease for pristine film, ~8% decrease for 0.1wt% film, and ~9% decrease for 2wt% film, respectively. These results agree well with XPS data shown in Figure 2, which shows the added surfactant makes it easier to remove PSS from the film surface. Second, the modulus decreases with an increase in surfactant concentration, for both non-washed and washed films with methanol. For example, the pristine film without methanol washing shows ~0.55GPa, while the film with 2wt% TX does ~0.08GPa. This is due to the plasticizer or softening effect of the added surfactant molecules. Although there are some scattering on the value of the mechanical modulus of PEDOT:PSS in literature,15,16,32,33 the present value for pristine films (0.55GPa for non-washed and 0.51GPa for washed cases, respectively) shows reasonable agreement with literature15,33 within the experimental error range considering the differences in PEDOT:PSS grades used and in experimental techniques and detailed conditions. It would be interesting to check what the effect of surfactant additive on the mechanical modulus values of films. On the one hand, the added surfactant helps preferential removal of PSS from the film surface, which may reduce the mechanical modulus. This is due to large difference in molecular weight between PSS and PEDOT: the high molecular weight of PSS tends to determine the modulus of PEDOT:PSS complex.18 Further, the surfactant additive plays the role of softening agent, leading to the reduction of mechanical modulus. On the other hand, enhanced crystalline packing of PEDOT segments has been confirmed upon the addition of


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surfactant. Then, PEDOT crystallites can play the role of reinforcing fillers in the film as in composites.34,35 Therefore, it would be interesting to determine which effect is dominant factor for the mechanical modulus of various films. As shown in Figure 4e, the modulus values of PEDOT:PSS films decrease with the surfactant addition, from 0.51GPa for pristine down to 80MPa for 2wt% TX added films. That is, the plasticizer effect of surfactant overwhelms the reinforcing filler effect of PEDOT crystallites. The reinforcing filler effect is very small due to nanoscale size of crystallized PEDOT segments.

Figure 5. Schematic drawings for the events that occur in surfactant-added PEDOT:PSS. i) In pristine PEDOT:PSS dispersion, gels particles of PSS-rich shell embedding PEDOT-rich core are dispersed in water. ii) Upon addition of nonionic surfactant, Triton X-100, electrostatic interaction between PEDOT and PSS is weakened, forming PEDOT-TX and PSS-TX complexes in water/TX solvent. This phase separation accompanies the conformational change of PEDOT from benzoid to quinoid, facilitating ordered packing of PEDOT molecules. iii) When coated on a substrate, the film rapidly dries and the complexes shrink a lot. iv) The PSS-TX complex is preferentially removed by following methanol washing, leading to enhanced conductivity of the film. v) Due to residual TX molecules in the film, enhanced lamella packing and grain growth


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occurs, i.e., the residual TX plays the role of plasticizer, leading to conductivity enhancement upon aging under room ambient until the deleterious effect of humidity becomes dominant. Based on all the experimental results, we propose model for the role of nonionic surfactant additive in PEDOT:PSS, as shown in Figure 5. In the pristine dispersion (i), low molecular weight PEDOT segments are attached to high molecular weight PSS chains via electrostatic interaction. The polyelectrolyte complex forms gel particles, PEDOT-rich core with PSS-rich shell due to drastic difference in hydrophilicity. Note here that the distance between positive/negative charges on PEDOT and PSS backbones are not the same. To mitigate this disparity in the distances, most of the PEDOT segment takes the benzoid conformation.36 In benzoid form, the PEDOT is flexible and thus conforms onto PSS backbone even with the difference in the distance between charges on each chain. Upon addition of nonionic surfactant (ii), the PEDOT:PSS complex is disintegrated into PEDOT-TX and PSS-TX phases due to amphiphilic nature of the surfactant.27-29 The phase separation accompanies the conformational change of PEDOT segments from benzoid to quinoid, facilitating an ordered crystalline packing of PEDOT via π-π interaction (probably incorporating surfactant molecules in it; lamella packing occurs too, though not explicitly shown in Fig. 5(ii)).36-39 When coated onto a substrate, the low boiling point solvent (water in the present case) rapidly evaporates and dried further by following thermal annealing. Now the PEDOT:PSS gel particles and TX-PSS complexes shrink into more compacted form, and stay as solvated in the residual high boiling point surfactant as a solvent. The conformational change into quinoid and thus the formation of crystalline packing leads to the enhanced electrical conductivity (XPS, Raman, and AFM data in Fig. 2). The following washing with methanol preferentially removes TX-PSS complexes from the film, as shown in (iv), leading to additional increase in the conductivity. Regardless of washing with methanol, the


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added surfactant plays the role of plasticizer or softening agent, leading to decrease in mechanical modulus with the increase in surfactant concentration. The plasticizer effect overwhelms the reinforcing filler effect of the PEDOT crystallites (Figure 4). While the stages from (i) to (iv) describe the effect of surfactant additive until the formation of dried film on a substrate, the final step (v) in Figure 5 shows its effect during aging as a plasticizer.13,16 Although the residual surfactant in the dried film has very high viscosity (~270 cP at 25oC), there are continual changes inside the film. Upon aging, the lamella stacking and thus enhanced intergrain coupling progress (XRD data shown in Figure 3), leading to grain growth.13,14,30 The grain growth is responsible for the gradual reduction of electrical resistance upon aging, up to 100~120hrs (Figure 1). Prolonged exposure to humid air, however, induces the swelling of PSS by water uptake, which increases the resistance again. Therefore, optimization of post-annealing, temperature and time, is one of key steps in obtaining high conductivity PEDOT:PSS film containing nonionic surfactant. Based on our experimental results shown in Figure 1, it takes long (~100hrs) time to reach the highest conductivity for low (room) temperature drying. On the other hand, high (>100oC) temperature annealing may lead to early evaporation of surfactant. Considering these findings, one can find the optimum temperature and time for post-annealing, which can shorten the process time and minimize the adverse effect of early evaporation of surfactant from the film. CONCLUSION The effect of nonionic surfactant additive on the properties of the resulting PEDOT:PSS films has been discussed. First of all, the added nonionic surfactant weakens the ionic interaction between PEDOT and PSS, leading to the phase separation into TX-PEDOT and TX-PSS


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complexes. The released PEDOT segments changes from benzoid to quinoid forms, facilitating crystalline packing of PEDOT. Then, the formed TX-PSS complex is easily removed by the following methanol washing, compared to PSS removal by methanol washing from ionically stitched PEDOT:PSS film. These enhance the electrical conductivity of the resulting PEDOT:PSS film. Interestingly, the remaining TX in the film acts as a plasticizing agent, increasing the lamella packing of alternate PEDOT and PSS upon aging under room ambient. This increases the conductivity of the films further, until the deleterious effect of humidity comes in. Finally, the mechanical modulus of films prepared from various different conditions has been measured by the buckling based method. The in-depth understanding of the effect of nonionic surfactant additive into PEDOT:PSS solution can be a great help in designing and implementing novel applications of PEDOT:PSS. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Change of electrical conductivity (PDF) AUTHOR INFORMATION Corresponding Author *Telephone: +82-2-2123-5835; E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Research Foundation Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-0029061).

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Significant

Vertical

Phase

ethylenedioxythiophene):Poly(styrene

Separation sulfonate)

in

Solvent-Vapor-Annealed

Composite

Films

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Poly(3,4to

Better

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Silicon/Poly(3,4-ethylenedioxythiophene):Poly(styrene

sulfonate)

Heterojunction Solar Cells. J. Phys. Chem. C 2013, 117, 9049-9055.


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Table of Contents Graphic


26

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Roles of nonionic surfactant additives in PEDOT:PSS thin films

Department of Materials Science and Engineering Yonsei University

Sung-Soo Yoon, Dahl-Young Khang



(a) 1.00

(b)2.4

pristine TX 0.1 wt% TX 2 wt%

pristine TX 0.1 wt% TX 2 wt% pristine (w/o MeOH washing)

2.2

0.90

R/R0

R/R0

0.95

1.2

0.85

1.0

0.80 0.8

0

40

80

120

160

0

200

40

80

120

160

200

Aging time [hr]

Aging time [hr] 1.8

(c) 1.00

(d)



1.6

0.95 1.4

0.90

R/R0

R/R0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 28 of 32

1.2

1.1

0.85

1.0

1.0 0.9

0.80

0

3

6

9

12

0.8

0

40

80

120

160

200

0

30

60

Aging time [hr]

Aging time [hr]

Fig. 1.


90

120

Page 29 of 32

10000

(a)

(b) Raman intensity [a.u.]

pristine, no washing pristine, washing TX 2 wt%, washing

8000

Counts [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


6000

4000

2000

0 175

170

165

160

155

1.00

pristine, no washing pristine, washing TX 2wt.%, washing

0.75

0.50

0.25

0.00 1300

1350

1400

1450

Wavenumber [cm-1]

Binding energy [eV]

(c)

(d)

pristine

TX 2wt%

Fig. 2.


1500


20

20

(a)

(b)

pristine TX 2 wt%

Intensity [103 counts]

12

8

4

0

pristine TX 2 wt% 16

16

Intensity [103 counts]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 30 of 32

5

10

15

20

25

30

12

8

4

0

5

10

15

2θ [°]

2θ [°]

Fig. 3.


20

25

30

Page 31 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


(a)

TX 0.1wt% No washing

(b)

TX 2wt% No washing

(c)

TX 0.1wt% Washing

(d)

TX 2wt% Washing

(e) pristine

0.1wt% TX

2wt% TX

Film thickness 105 nm

Buckling wavelength 2.82 µm

100 nm

2.63 µm

510 MPa

w/o wash

119 nm

2.42 µm

240 MPa

washed

110 nm

2.17 µm

217 MPa

w/o wash

140 nm

2.07 µm

92 MPa

washed

127 nm

1.81 µm

80 MPa

w/o wash washed

Fig. 4.


Modulus 550 MPa


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

i

ii

iv

v

iii

Fig. 5.


Page 32 of 32

Roles of Nonionic Surfactant Additives in PEDOT:PSS Thin Films - The

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