Conducting and Stretchable PEDOT:PSS Electrodes: Role of Additives

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Conducting and Stretchable PEDOT:PSS Electrodes: Role of Additives on Self-Assembly, Morphology and Transport emilie dauzon, Ahmed E Mansour, Muhammad R. Niazi, Rahim Munir, Detlef-M. Smilgies, Xavier Sallenave, Cédric Plesse, Fabrice Goubard, and Aram Amassian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00934 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Materials & Interfaces

Conducting and Stretchable PEDOT:PSS Electrodes: Role of Additives on Self-Assembly, Morphology and Transport Emilie Dauzon,†‡ Ahmed E. Mansour,† Muhammad R. K. Niazi,† Rahim Munir,† Detlef-M. Smilgies,‖ Xavier Sallenave,‡ Cedric Plesse, ‡ Fabrice Goubard, ‡ Aram Amassian§* AUTHOR ADDRESS. †Organic

Electronics & Photovoltaics Group, Physical Science and Engineering Division, King

Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‖Cornell

High Energy Synchrotron Source Cornell University Ithaca, NY 14853, USA

‡Laboratoire

de Physicochimie des Polymères et des Interfaces, Université de Cergy-Pontoise,

95000 Cergy, France §Department

of Materials Science and Engineering, North Carolina State University, Raleigh,

N.C., 27695, USA KEYWORDS. Conducting Polymers, Charge Transport, Self-Assembly, Stretchable Electronics, PEDOT:PSS, In Situ Characterization, Grazing Incidence Wide-Angle X-ray Scattering

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ABSTRACT. The addition of DMSO and Zonyl into PEDOT:PSS can be combined to achieve excellent electrical, optical and mechanical properties. We demonstrate that it is possible to produce highly transparent conducting electrodes (FoM > 35) with low Young’s modulus and high carrier density. We investigated the relationship between the transport properties of PEDOT:PSS and the morphology and microstructure of these films by performing Hall effect measurement, atomic force microscopy (AFM) and grazing incidence wide-angle X-ray scattering (GIWAXS). Our analysis reveals distinctive impact of the two additives on the PEDOT and PSS components in the solid-state PEDOT:PSS films. Both additives induce fibrillar formation in the film and the combination of the two additives only enhances the fibrillary nature and the aggregations of both PEDOT and PSS components of the film. In situ GIWAXS allows to time-resolve the morphology evolution. It highlight that investigation performed during the spin-coating and annealing steps show that the presence of the additives influences the aggregation behaviors of the PEDOT and PSS components directly during the transition from wet to dry film, i.e., during solvent removal, and do not evolve further during subsequent annealing. These results indicate that the additives directly influence the self-assembly behaviors of PEDOT and PSS during the ink-to-solid phase transformation.

Introduction Transparent conducting electrodes (TCEs) are a critical component in many optoelectronic devices, ranging from touch screens and displays,1 to photodetectors2 and solar cells.3,4 Increasingly, such devices are also more and more designed and integrated onto flexible and stretchable platforms, for instance, in the context of wearable electronic applications. The need for mechanical properties favoring flexible and stretchable devices challenges rigid TCE materials, such as indium tin oxide (ITO), currently the most commonly used TCE. In recent years,


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alternative TCE materials exhibiting mechanical properties more compatible with the needs of stretchable electronics have emerged. For instance, silver nanowires,5 carbon nanotubes,6 conjugated polymers7 have been developed for wearable, biological, medical and skin-like technologies.8,9 Among these, conducting and stretchable polymers, such as poly(3,4ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), are particularly interesting because their mechanical, electrical and optical properties can be tuned with the help of additives in the ink formulation, making them compatible with low-cost solution coating and printing. Moreover, they exhibit unique emergent properties, such as mixed transport of electrons and ions, making them also suitable for bioelectronic applications.10,11 A variety of methods have been reported to successfully increase the electrical conductivity of PEDOT:PSS,12 including by addition to the ink of polar solvents (dimethylsulfoxide (DMSO),13–15 dimethylformamide (DMF),16 ethylene glycol (EG),17 xylitol,18 waterborne polyurethane (WPU)19 and surfactants, such as Zonyl-FS 300.13,14,20 Post treatment of PEDOT:PSS films with salts,21 acids,22 or solvents3 has also been shown to improve conductivity. Moreover, the processing conditions have also been suggested to influence the conductivity of PEDOT:PSS.23 Yet, despite the popularity of PEDOT:PSS and the development of myriad recipes and formulations for tuning its properties, the conductivity of this material remains a bit below that of ITO, with the highest reproducible conductivity on the order of 4380 S/cm24 compared to ITO’s 800-5000 S/cm.25 This deficiency is not easy to address given that very little is actually understood about the mechanism of transport in PEDOT:PSS. The role of additives on its structuration which ultimately leads to high conductivity and, in some instances, simultaneous mechanical stretchability, remains vague. The basic question of whether the additives enhance the conductivity by inducing morphological changes, by doping or due in some way to the intrinsic properties of PEDOT:PSS has not been addressed to this day.


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Numerous studies have attempted to explain the enhancement of conductivity, often presenting differing and sometimes competing conclusions.3,24,26–34 Hence, a complete and coherent picture remains elusive. The proposed mechanisms include PEDOT chemical conformation changes,27 removal of PSS,3,30 phase separation of PEDOT and PSS,31,32 screening effects,16 as well as morphology and microstructure changes, such as crystallinity and crystal orientation.26,35 In spite of all these efforts, the field lacks consensus regarding the origin of conductivity improvements. Some claim the improvement of conductivity is due to a change of the PEDOT conformation from benzoid to quinoid during EG treatment.27 Others seem to agree that the addition of a polar solvent increases the conductivity because of the removal of PSS3,28–30 and a better phase separation between PEDOT and PSS.31,32 A screening effect has been as well evoked by Kim et al.16 Also a change of morphology,24,33 the improvement of the crystallinity34 and orientation of the crystalline domains26,35 have been put forth as explanations for the enhancement of conductivity. A number of reports in the literature have also recently speculated that a core-shell structure forms in highly conducting PEDOT:PSS, wherein PEDOT constitutes the core and PSS enriches the shell.28,36,37 Clearly, understanding the mechanisms behind the formation of highly conducting PEDOT:PSS would lead to improvements of the electrical properties by optimization of the structuration of PEDOT:PSS through the development of suitable formulations and fabrication processes. Here, we investigate the addition of two very different additives on the self-assembly, structuration, transport and mechanical properties of PEDOT:PSS. The polar solvent DMSO is proven to improve the conductivity, while Zonyl FS 300 is a surfactant which remains in the film, improving both the conductivity, the wettability on elastomer-type material and the stretchability of PEDOT:PSS.13,14 Our combined in situ and ex situ analyses shed light on the distinctive impact of the two additives on the aggregation of PEDOT and PSS components in solid-state PEDOT:PSS


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film. The DMSO preferentially solubilizes PSS and thus enhances the aggregation of PEDOT and its phase separation from PSS. Meanwhile, Zonyl preferentially interacts with PSS, providing a pathway for phase separation and introduces order into PSS domains. Both additives induce fibril formation in the film, but their combination dramatically enhances the fibrillar characteristics, such as the length, width and mesoscale arrangement, and promotes the formation of core-shell fibrils which form a percolated conducting network. These results show that both additives impact – in their own way – the microstructure, phase separation and ultimately the electrical and mechanical properties of PEDOT:PSS film. DMSO promotes phase separation by initiating PEDOT fibrils growth, while Zonyl promotes PSS phase segregation and the self-assembly of core-shell PEDOT:PSS fibrils within an plasticizer matrix, increasing the stretchability of the material. The addition of both additives causes a synergistic effect which further improves the electrical transport and the stretchability through a self-assembly mechanism of highly percolated core-shell fibrils.

Results 1. Electrical, optical and mechanical properties of PEDOT:PSS

Figure 1a depicts the conductivity () behavior of solid-state PEDOT:PSS films in the presence of DMSO polar solvent, Zonyl surfactant and their mixtures in the PEDOT:PSS ink. The weight percent (wt%) is defined as the ratio between the weight of additives to the weight of the total solution (wt% = 100 ∗ Wadditive/(Wadditive + WPEDOT:PSS)). The conductivity was measured by Hall Effect and all samples were measured in the Van der Pauw geometry with four square contacts (inset Figure 1c). Special care was taken to minimize artefacts38–41 and resistances from Hall Effect and 4-pt probe techniques were found to be in good quantitative agreement (Figure S1), indicating the reliability of the Hall Effect data. Nevertheless, an anomalous negative Hall coefficient was


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obtained for some of the samples known to exhibit very low conductivity (i.e. pristine and 0.1% Zonyl), despite PEDOT:PSS being a p-type material. This highlights the limitations of the Hall Effect measurement for the low conductivity samples. However, as the conductivity of samples prepared with additives increased by orders of magnitude, the data was reproducible and its accuracy is expected to be within 5%. We find that addition of DMSO co-solvent increases the conductivity by 4 orders of magnitude from 0.06 to 700 S.cm-1, with most of the variation occurring for 0-5 wt% DMSO, in agreement with previous reports.16,31 Zonyl also enhances conductivity, but its effect is more modest in comparison to DMSO, as the conductivity peaks at 67 S.cm-1 for 5 wt% Zonyl addition.13,20 The combination of the two additives in the high conductivity regime can be seen by varying the amount of Zonyl while the concentration of DMSO is kept constant at 5 wt%. The conductivity is very slightly decreased with the addition of Zonyl to DMSO, but it remains far better than films prepared with Zonyl alone. It is important here to note that while DMSO is a co-solvent which evaporates from the film upon annealing, as will be shown later, Zonyl remains embedded in the film and represents a significant volume of the film, in some cases equal to or surpassing the amount of PEDOT:PSS. Indeed, adding 5 wt% Zonyl in a 1 wt% PEDOT:PSS solution represents ca. 80 wt% of Zonyl in the solute and, eventually, the dried film. Given the PEDOT:PSS ratio is 1:2.5, the final film is expected to be made up of a whopping 94 wt% insulators (Zonyl and PSS), assuming that solute composition in the solution does not change in the solid-state film. It is quite remarkable then that the conductivity of the films is maintained essentially unchanged with the addition of so much Zonyl and suggests that the PEDOT:PSS content must be very well percolated and possess excellent intrinsic conductivity for the conductivity of the macroscopic film to remain so high.


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The conductivity (𝜎) is directly related to the free carrier density (n) and the mobility () of the latter through the relationship 𝜎 = 𝑛𝑒𝜇, where e is the elementary charge. To distinguish if the enhancement of the conductivity is due to an increased availability of charge carriers or changes in their mobility, both carrier density and mobility have been extracted from Hall Effect measurements. The carrier density and carrier mobility are summarized in Figure 1b and 1c, respectively. We observe that free carrier density increased similarly by 6 orders of magnitude, from 1016 to 1022 cm-3 with the addition of either DMSO or Zonyl. However, the increase of carrier density is steeper with addition of DMSO co-solvent than with Zonyl and while it remains high for increasing concentrations of DMSO, it peaks at 5 wt% of Zonyl and decreases. The Hall mobility suffers in all cases by addition of either additive into the PEDOT:PSS ink, decreasing by 2 to 3 orders of magnitude. With the exception of data points at the lowest conductivity, the measured mobilities and carrier densities are within the norm of what has been reported in the literature for PEDOT:PSS, including mobility in the range of 0.04 to 4 cm2.V-1.s-1 for different treatments,24,42,43 and carrier density between 1018 and 1022 cm-3.24,42 The decrease of mobility is attributed in part to the increase of the number of charge carriers which produce more scattering centers.44 This is probably not the only factor influencing the Hall mobility, which has a steeper decreasing trend with Zonyl addition and is typically an order of magnitude lower than with DMSO addition alone, despite yielding a lower carrier density. We can attribute this to the presence of a large fraction of the insulating Zonyl in the PEDOT:PSS film, which is likely to disrupt some of the conducting pathways in the film and could additionally explain this decreasing trend. On the whole, the drastic increase in carrier density easily overcomes the decreases in mobility, explaining the net increase of the conductivity whether DMSO, Zonyl or a mixture of the two is added. Nevertheless, it is clear that the one order of magnitude reduction


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of Hall mobility for Zonyl vs. DMSO, and slower increase of carrier density with Zonyl addition are the culprits for the lower conductivity achieved in Zonyl-based films. Although such a large volume fraction of insulating material is used, the ability of PEDOT:PSS to maintain its conductivity is impressive. Mixing Zonyl with DMSO leads to much higher carrier density, closer to that of DMSO only, and the mobility values remain higher than Zonyl alone for an amount superior at 3 wt%, which is also impressive. This suggests that the large amount of Zonyl present in the film does not disrupt the charge transport pathways as effectively when the DMSO cosolvent is present or that the presence of DMSO also helps enhance the percolating conduction pathway in the overall film. We turn our attention to the study of mechanical properties for the different ink formulations described above (Figure 2a). We have measured the Young’s modulus (tensile modulus) from the slope at origin of strain-stress curves on thick films (Figure S2). The Young’s modulus is found to be ca. 1.15 GPa for pristine PEDOT:PSS (0 wt% DMSO or Zonyl). The addition of Zonyl plasticizes the film and drastically reduces the tensile modulus, with 1 wt% Zonyl decreasing it by a factor of 4 to ca. 338 MPa. By contrast, addition of DMSO, even in large amounts, has a marginal effect on the mechanical properties of the conducting film. The mixture of Zonyl and DMSO improves the plasticity of the film, further reducing the Young’s modulus of the film to 35) with broadly tailored mechanical properties and plasticity through a mixture of DMSO and Zonyl. Indeed, the implied ability to decouple optoelectronic and mechanical properties, the latter being highly tunable by an order of magnitude for 0 < [Zonyl] < 2 wt%, while the former is maintained essentially constant, is remarkable and likely beneficial technologically. It opens up tremendous opportunities to produce complex designs achieving uniform optical and electrical properties, which are crucial for device operation and aesthetics, but with different mechanical properties, which makes it possible to achieve complex mechanical functionalities by adjusting, for instance, to different mechanical properties and stretchabilities in different parts of a wearable and/or stretchable device. The plasticizing effect of Zonyl is probably due to a hidden microstructural features which increase the integrity of the films under stress, as will be revealed later in this study.


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Even as stretchability is a sought after property, the same electrodes must achieve excellent TCE properties, including low sheet resistance and high transparency in the visible, which have thus far made ITO a particularly interesting material. We compare the sheet resistance-transmittance of our best stretchable TCE to commercial ITO in Figure 2c. We have prepared dozens of PEDOT:PSS films with different thicknesses using the optimal formulation (5 wt%/1 wt% DMSO/Zonyl) for high conductivity and low Young’s modulus. The scatter plot of sheet resistance vs. visible transmittance (550 nm) in Figure 2c combine our data with those of commercial ITO51 and other reports on PEDOT:PSS.3,13 The plot shows that increasing the layer thickness lowers the transmission but also decreases the sheet resistance. The data reveal that sheet resistance 5 wt%) compromises the percolation, but for smaller amounts the fibrils become more


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prominent in numbers, size (length and diameter) and closely connected, indicating that the Zonyl presence participates directly in fibril formation and interconnectivity. Fibril formation and their interconnection has been seen in polymer-based solar cells, where liquid-liquid phase separation helps the formation of a polymer scaffold.67 Additives also induce the selective aggregation of the polymer and thus enhance the phase separation by selectively solubilizing the acceptor molecule.68 For instance, the high boiling point 1,8-octanedithiol for P3HT:PCBM was first chosen to preferably solubilize the fullerene acceptor and causes the P3HT polymer to aggregate when the main solvent (chlorobenzene) evaporates to form a network in which the fullerene is solvated. When the additive eventually evaporates, the acceptor aggregates and solidifies in the surrounding of pre-formed P3HT domains.64 Our in situ experimental results in this study have clearly shown the selective solubility of PSS31 and the earlier aggregation of PEDOT in the presence of DMSO. They also show that in the absence of DMSO, water surrounds PSS and PEDOT in the same way and provides insufficient driving force for PEDOT to separate from PSS, thus hindering the formation of extended and hierarchically connected fibrillary networks. The system is thus quenched. In this way, the solubility of the components in the solvent additive plays an important role in aggregation, and whether fibrils can form and interconnect. When DMSO is added to the system, in situ experimental data suggest that while DMSO is in the presence of a lot of water it is a co-solvent. However, by the time the film is on the hot plate and the solvent composition is in favor of DMSO due to its higher boiling point, the solution and its self-assembly become entirely dominated by DMSO. Hence, PEDOT and PSS are self-assembled in an environment where there is preferential solubility toward PSS by DMSO and less toward PEDOT. The polar co-solvent causes chain unfolding and promotes unhindered growth of PEDOT-rich grains by promoting its phase separation from PSS and providing a driving force for


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aggregation via selective reduced solubility.26,31 This is a classical way of getting PEDOT to aggregate and form fibrils, while PSS will subsequently deposit around them, perhaps even forming a shell due to electrostatic interactions. In contrast to DMSO, Zonyl is a surfactant which behaves like a self-assembly agent. Zonyl is an amphiphilic oligomer that can interact with PEDOT and PSS by bonding with the electronegative fluorine and through hydrogen bonding. The OH groups on Zonyl and the PSS anion may also exhibit a preferential interaction. Zonyl does not preferentially solubilize in either DMSO or water, but it is solubilized by DMSO, as is PSS, while PEDOT is not a priori. This means that as water evaporates faster than DMSO and the DMSO concentration increases, DMSO solubilizes PSS and the Zonyl with which PSS interacts while PEDOT is free to aggregate. We propose a liquid-liquid phase separation between PEDOT-rich and PSS/Zonyl-rich regions, allowing PEDOT to nucleate and grow fibrils without disruption, surrounded by the solvated anionic PSS. In situ/ex situ GIWAXS confirmed a significant PSS aggregation in the presence of Zonyl only, along with a small increase in PEDOT aggregation. The aggregation of PEDOT occurs earlier and is further enhanced when DMSO is also present, allowing PSS and Zonyl to remain soluble while PEDOT aggregates. Given the amphiphilic nature of Zonyl, it is indeed likely that hydrophobic PEDOT and hydrophilic PSS self-assemble around it, thus forming core-shell fibrils with Zonyl present at the interface. Other amphiphilic surfactants such as Triton X have previously shown to promote nanofibrils with a PEDOT core and at the interface with a PSS shell.69 The mechanism proposed behind the fibrous structure formation was attributed to the miscibility of the surfactant preferentially with the PSS at low concentration and PEDOT at higher concentration. Thus, adding a surfactant promoted phase separation and larger grains in most of the case.13,21 Despite the fact Zonyl has a low surface tension (~23 mN.m-1)70 and is used to help PEDOT:PSS


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wet hydrophobic surfaces, we note that the contact angles of films decreased with small amount of Zonyl (Figure S12), reinforcing the notion that much of the Zonyl is incorporated into the PEDOT:PSS fibrils rather than decorating their outer surface at low content. A large fraction of Zonyl (>1 wt%) extends slowly the distribution of the surfactant outside the interfibers indicating by an increase of hydrophobicity and a smoothing effect detected on AFM roughness measurement. The importance of the interfacial incorporation of Zonyl as opposed to surface coating of fibrils cannot be overstated in the context of conducting materials and the need for a percolating pathway. Zonyl and DMSO both promote phase separation and fibril formation, resulting in cores-shell fibrils which can be electronically connected, for instance through exposed PEDOT and PEDOT:PSS sections of the fibrils, to form a conducting network. The Zonyl surfactant can thus mechanically soften the fibrils, but can also mechanically bridge adjacent fibrils without encapsulating them with an insulator. This may explain partly why with DMSO only, fibrils appear quite a bit narrower than with Zonyl or Zonyl/DMSO mixture. With Zonyl and DMSO both present, the crystalline correlation length (CCL) of PEDOT and PSS increases, possibly indicating thicker cores and shells in addition to higher crystallinity. This would then contribute to fibril thickness in addition to the presence of the amphiphilic Zonyl at the interface and partly at the surface too. The fact that carrier concentration is nearly the same for Zonyl-only and DMSO-only formulations indicates that the doping is similar at the local molecular level. However, the reduced crystalline order in PEDOT regions when using Zonyl only and the loss of conduction pathway can explain the one order of magnitude lower hall mobility when Zonyl is used as additive. Based on the above discussion, we propose a mechanism of formation of fibrils in the presence of polar solvent and surfactant additives, as shown in Figure 6. In presence of DMSO, core-shell


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fibrils are nucleated due to selective solubility of PSS, phase segregation and aggregation of PEDOT, leading to a highly aggregated PEDOT core and a dispersed PSS. In the presence of Zonyl, core-shell fibrils are also formed due to Zonyl’s affinity with PSS and its amphiphilic nature and hydrophobic/hydrophilic interactions with PEDOT and PSS, respectively. However, PEDOT and PSS are still co-solubilized in water and they solidify simultaneously, with a limited driving force for segregation. The core-shell fibrils are elongated but do not form a highly aggregated PEDOT core. When both additives are present, DMSO helps Zonyl to separate and arrange PEDOT and PSS into a core-shell fibril with Zonyl present both at the interface and between fibrils. The fibrils are well-connected electrically through conducting bridges.

Conclusion The mechanisms of structuration of PEDOT:PSS film with different additives have been studied. A precise correlation was highlighted between the morphology and Hall Effect measurement giving new perspectives for futures applications in stretchable transparent conducting electrode. In situ experiments show a valuable approach towards understanding mechanisms of film formation. PEDOT:PSS phase separation is improved by the synergetic effect of the two DMSO and Zonyl. DMSO improves the structuration and the crystallinity of PEDOT, creating a pathway for the carrier transport. Zonyl additive involves twisted fibrils with thicker PEDOT core. The addition of both additives generates a synergetic effect producing large fibrils well oriented. This enhanced understanding of the crystallization dynamics should help to pull up the challenges in achieving highly stretchable TCE.


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Experimental Section Materials. We purchased the PEDOT:PSS aqueous solution (Clevios PH1000) from Heraeus containing 1-1.3% of solid and had a PEDOT to PSS weight ratio of 1:2.5. Zonyl FS-300 (Zonyl) was purchased from abcr. Dimethylsulfoxide (DMSO) was purchased from Merck. Preparation of PEDOT:PSS film. Different quantities of Zonyl (0 to 15 wt%) and DMSO (0 wt% or 15 wt%) were incorporated into a commercial solution of PH1000 before casting. Thus, all the concentrations given are expressing hereafter in mass fraction or mass percent (noted wt%) compared to total weight of solution. The mixture was stirred for 2-3 h before coating on precleaned glass for optoelectronic measurements. Afterwards, the solution of PEDOT:PSS containing the additives was spin-coated at 1000 rpm for 50 s on different substrates. Finally, on a hot plate at 110°C the coated films were annealed for 30 min. Resistance and conductivity measurement. 4 points probes were used to determine the sheet resistance. The thickness was measured by Dektak 150 profilometer. Hall-effect measurements were performed using a Lakeshore 7700 system on film using the Van der Pauw configuration. Measurements were conducted by applying a reversible sweep of 5 kG magnetic fields. Transmittance. To examine transparency, UV-Visible spectra were measured using a UV-visNIR spectrophotometer (JASCO V-570). The values of transmittance were measured at the wavelength of 550 nm. Glass substrate was used as reference. Young’s modulus. Young’s modulus was measured by traction on Q800 Dynamic mechanical analysis (DMA) from TA Instruments. Self-supporting PEDOT:PSS was clamped and a preload force of 0.05 N was applied until 0.05% strain. The mode strain rate at 20%/min to failure was used. The tensile modulus was deduced from the stress versus strain curve as the slope at origin, generally between 0 to 0.5%.


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AFM Imaging. Atomic force microscopy (AFM) was used to image the surface topography of a sample by analyzing point by point with a local probe scan. An AFM Bruker® device is used. The images were taken in ScanAsyst mode (constant force, frequency of 2 kHz). Ex situ and In Situ GIWAXS Characterization. In situ and ex situ two-dimensional (2D) GIWAXS experiments were performed at D1 beamline at the Cornell High Energy Synchrotron Source. The beamline energy was 1.17 Å and the sample detector distance was 167.5 mm. An exposure time of 5 s was used for ex situ experiments. Ex situ GIWAXS experiments were performed at variable angles ranging from angles below the critical angle to above the critical angle. Practically, in situ experiments during spin coating were performed on a miniature spin coater with a glass substrate was placed on the x-ray beam path. A laser was used to verify good sample alignment and low wobble during rotation. The spin-coater and sample were also aligned with the incident x-ray beam and an incidence angle of 0.17o was used for in situ experiments. In situ measurements and the spin coating process were initiated remotely and lasted 300 s, using a time resolution of 0.5 to 1 s for in situ measurements. 2D GIWAXS images were transformed with a geometric correction and two-dimensional intensity reshaping using GIXGUI software. In situ thickness. PEDOT:PSS films was monitored using spectroscopic ellipsometry (M2000XI, J. A. Woollam Co., Inc) measurements. This technique only works when the solution thickness approaches 10 microns. FIGURES.


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Figure 1. Effect of the DMSO (red), Zonyl (blue) and their mixture (yellow; DMSO amount is kept constant at 5 wt.% as Zonyl concentration is varied) on the electrical properties of PEDOT:PSS as a function of additives concentration in solution: (a) conductivity, (b) carrier concentration (inset: schematic illustration of Hall effect measurement configuration), (c) mobility measured by Hall effect. Inset in (c) presents Hall mobility on a logarithmic scale.


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Figure 2. (a) Young’s modulus and (b) TCE figure of merit (FoM) with respect to the additive composition for Zonyl (blue), DMSO (red) and the mixture of Zonyl with 5 wt% DMSO (yellow). (c) Sheet resistance-transmittance (550 nm) plot for highly conducting and stretchable PEDOT:PSS films (1 wt% Zonyl and 5 wt% DMSO) and its comparison with commercial ITO and previous reports using the same additives.3,13


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Table 1: Transparent, stretchable, conductive PH1000 based electrode in the literature Young’s modulus (GPa)

Conditions

Ref

2.2(72)

Buckling

71,72

0.9-2.8(45,73)

Tensile test

45,73

0.01

1.2

Tensile test

this work

97(13)

33(13)

N/A

Buckling

13,71

75

91

53

0.95

Tensile test

this work

380 x103

N/A

N/A

0.66

Buckling

20

150x103

90

0.03

0.3

Tensile test

this work

79260(13,14,20)

95-97(13,14)

2743(13,14)

3.14(20)

Buckling

13,14,20

94

91.5

43.8

0.11

Tensile test

this work

PH1000 + 1%EMIM TCB+ 1 wt% FS-31

80

91.5

51.8

0.038

Tensile test

53

PH1000 + 45.5 wt% STEC1

59

96

142

0.055

Tensile test

54

Compositions

R (Ω.sq-1)

T (%)

FoM

(71)

N/A

N/A

300 x103

87

190370(13,71)

630 x103 PH1000

PH1000 + 5 wt% DMSO

PH1000 + 1 wt% Zonyl

PH1000 + 1 wt% Zonyl + 5 wt% DMSO

Figure 3. Topographic AFM images of PEDOT:PSS films prepared (a) with no additives, (b) with DMSO (5 wt%), (c) with Zonyl (1 wt%) and (d) with a mixture of 5 wt% DMSO and 1 wt% Zonyl.


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Dimensions 2μm x 2μm. (e) The root mean squared (RMS) roughness values as a function of the additives content (Zonyl in blue, DMSO in red and the mixture in yellow).

Figure 4. Time-resolved GIWAXS performed in situ during (a) spin-coating at 1000 rpm and (b) annealing at 110C of a drop-cast film. (c) In situ thickness measurement of spin-cast films during annealing from 24 to 50C. From top to bottom: PEDOT:PSS formulation with no additives, 5 wt% DMSO, 1 wt% Zonyl, and 1 wt% Zonyl/5 wt% DMSO.


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Figure 5. 2D GIWAXS images for PEDOT:PSS films with (a) no additives, (b) 5 wt% DMSO, (c) 1 wt% Zonyl, and (d) 1 wt% Zonyl/5 wt% DMSO. Full integration linecuts for PEDOT:PSS thin films with (e) DMSO and (f) Zonyl in comparison with the mixture. Crystalline characteristics, including (g) displacement of center maximum of PEDOT pi-stacking scattering feature and (h) relative crystallinity of PSS domains.

Figure 6. Schema of additives effects and fibrils structuration of PEDOT:PSS (PH1000) films. ASSOCIATED CONTENT. Supporting Information. The following files are available free of charge. Description of Hall Effect measurement; comparison of conductivities measurement methods;


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Stress-Strain curves; UV-Visible Transmittance; Supplementary AFM images; complementary in situ GIWAXS during annealing; In situ thickness at high temperature; GIWAXS fitting analysis; wetting test (PDF)

AUTHOR INFORMATION Corresponding Author * Email: [email protected] ACKNOWLEDGMENT This work was supported by the King Abdullah University of Science and Technology (KAUST) and the Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI). We acknowledge the additional support of CHESS by the NSF & NIH/NIGMS via NSF award DMR-1332208. ABBREVIATIONS FoM, Figure of Merit; atomic force microscopy, AFM; grazing incidence wide-angle X-ray scattering, GIWAXS; Transparent conducting electrode, TCE; indium tin oxide, ITO; poly(3,4ethylenedioxythiophene), PEDOT; polystyrene sulfonate, PSS; dimethylsulfoxide, DMSO; dimethylformamide, DMF; ethylene glycol, EG; waterborne polyurethane, WPU; weight percent, wt%; liquid crystal display, LCD; organic photovoltaic devices, OPV; root mean squared, RMS; field effect transistor, FET; bulk heterojunction, BHJ; crystalline correlation length, CCL. REFERENCES


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Conducting and Stretchable PEDOT:PSS Electrodes: Role of Additives

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