Effect of the Rheological Properties of Carbon ... - ACS Publications

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Effect of the Rheological Properties of Carbon Nanotube Dispersions on the Processing and Properties of Transparent Conductive Electrodes Laurent Maillaud, Philippe Poulin, Matteo Pasquali, and Cécile Zakri* CNRS, Centre de Recherche Paul Pascal, Université de Bordeaux, 115 Avenue Schweitzer, 33600 Pessac, France S Supporting Information *

ABSTRACT: Transparent conductive films are made from aqueous surfactant stabilized dispersions of carbon nanotubes using an up-scalable rod coating method. The processability of the films is governed by the amount of surfactant which is shown to alter strongly the wetting and viscosity of the ink. The increase of viscosity results from surfactant mediated attractive interactions between the carbon nanotubes. Links between the formulation, ink rheological properties, and electro-optical properties of the films are determined. The provided guidelines are generalized and used to fabricate optimized electrodes using conductive polymers and carbon nanotubes. In these electrodes, the carbon nanotubes act as highly efficient viscosifiers that allow the optimized ink to be homogeneously spread using the rod coating method. From a general point of view and in contrast to previous studies, the CNTs are optimally used in the present approach as conductive additives for viscosity enhancements of electronic inks.

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has to be sufficiently high to avoid dewetting before the film dries. Addition of surfactants to CNT inks plays a positive role as surfactants can promote wetting and viscosity increase of the ink. Nevertheless, the underlying mechanisms of viscosifying remained to be elucidated. Dan et al.16 proposed two main plausible mechanisms: a synergetic association between the surfactants present in solution,19,20 leading to morphological changes of surfactant micelles, or aggregation of CNTs in response to interactions mediated by the surfactant molecules.16,21,22 These two mechanisms could explain increases of viscosity but have to be clearly identified so that inks can be optimized in future applications. Indeed, even if different mechanisms can induce increases of viscosity, they can differently alter the structure and properties of the resulting transparent electrodes. Therefore, a systematic investigation of the role played by the surfactant remains critical. The effect of the ink formulation on its viscosity and on the homogeneity of the coatings is investigated in the present work. The obtained results support the second scenario proposed by Dan et al. Indeed, the increase of viscosity is shown to result from the formation of loose CNT aggregates. These aggregates are formed in response to weak depletion attractive interactions and can be easily broken when the ink is sheared during film deposition. The present findings suggest a route toward the optimization of aqueous conductive inks using conductive polymer particles as depleting agents and CNTs as highly

INTRODUCTION Transparent conductive films made via the deposition of carbon nanotube (CNT) liquid dispersions are extensively studied for various applications, such as flexible displays, light-emitting diodes, or organic photovoltaic devices.1,2 The use of CNT inks is considered as one of the most promising routes to replace inorganic materials.3−6 The conductivity of CNT films is known to depend on a number of factors including CNT electronic properties, type of nanotubes, chemical doping, and functionalization.3,7 Achievement of homogeneous films of low thickness, typically between 5 and 50 nm, combining a low surface resistivity and a high optical transparency, remains challenging. The final properties of the films depend not only on the CNT features but also on their assembly and deposition methods.8 There are numerous deposition techniques, based upon dry or wet processing; due to their high versatility and scale-up capacity, the solution methods, in particular waterbased ones, are generally preferred. Indeed, aqueous dispersions of CNTs can be manipulated via known methods, such as membrane filtration,9 spin or dip coating,10−12 spray,13 or inkjet printing.14 These methods allow the preparation of homogeneous thin films provided that the CNT dispersion and the deposition process are appropriately designed and controlled. The rod coating method15 is particularly promising as it can be easily scaled up and is already widely used in paint industry. It consists in moving a metal rod wound with a metal wire that spreads the liquid onto a substrate. Rod coating has already been successfully used with CNT inks.16−18 The wetting and rheological properties of the ink are critical for the preparation of homogeneous films.16 In particular, the viscosity of the ink © XXXX American Chemical Society

Received: March 10, 2015 Revised: May 6, 2015

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DOI: 10.1021/acs.langmuir.5b00887 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Viscosity versus shear rate measurements of CNT/SDS/TX100 dispersions containing 1 wt % of SDS and 0.2 wt % of CNT (a) or 0.4 wt % of CNT (b). Photographs of rod coated films made from three of the above dispersions containing 0.2 wt % of CNT and 0 wt % (c), 6 wt % (d), and 8 wt % of TX100 (e) (lateral size of each image 8 cm). a poly(ethylene terephthalate) (PET) substrate. The contact angle is measured using a goniometer Teclis Tracker version DC1. Coating Method and Film Characterization. The rod coating apparatus is an Auto-Draw II Automatic Drawdown Machine (Gardco, model DP-8301). It is made of a glass drawdown pad and stainless steel coating rods with wires of diameters 810 μm around them. These tightly wound wires form regularly spaced grooves at the surface of the rod. Thin films are made by depositing ∼500 μL of the CNT inks on a cleaned PET substrate. Then, the stainless rod rolls over the substrate and spreads the coating liquid. The final thickness of the film is controlled by the size of the grooves.23 The CNT film on PET is dried at room temperature. All the air-dried films are rinsed in a water:ethanol bath (1:1 by volume) overnight and then dried for 30 min at 60 °C. Transmittance of CNT films at a wavelength of 550 nm is measured using a Unicam UV4-100 spectrophotometer. The sheet resistance (Rs) of CNT films is measured with a four-probe configuration using gold sputtered contacts.8

conductive viscosity enhancers. The formation of CNT aggregates allows the viscosity of the ink to be sufficiently increased so that it can be deposited by the rod coating method. This approach is shown to be efficient at achieving high performances transparent electrodes. From a general point of view and in contrast to previous studies, here the CNTs are not only used for their electronic properties but as conductive additives for viscosity enhancements of electronic inks.

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EXPERIMENTAL PART

CNT Inks. Stock aqueous dispersions of CNTs are first prepared. 0.4 wt % of CNTs (Elicarb PR0920 single-walled nanotubes purchased from Thomas SWAN, batch 79816/203) is dispersed and stabilized in water either with 1 wt % sodium dodecyl sulfate (SDS purchased from Sigma-Aldrich, critical micellar concentration, cmc = 0.2 wt %) or with 0.4 and 0.8 wt % poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS aqueous solution purchased from AGFA, Orgacon HBS5 grade, batch 5585376/04, with a concentration of 1.1 wt %). The stock dispersions are homogenized by tip sonication for 1 h with a Branson Sonifier S-205A supplying 20 W of acoustic power. Different amounts, from 1 to 8 wt %, of Triton X-100 (TX100 purchased from Acros Organics) are added to the aqueous dispersions stabilized with 1 wt % SDS and 0.4 wt % PEDOT:PSS. TX100 is a nonionic surfactant with a hydrophilic poly(ethylene oxide) chain head and an aliphatic hydrophobic tail containing 14 carbons (cmc = 0.013 wt %). Addition of this surfactant to CNT dispersions was shown to enhance their viscosity. TX100 is homogenized with mechanical stirring for 5 min and in a bath sonicator for 3 min. Rheological characterizations are conducted on a stress-controlled rheometer (TA Instruments, AR2000) in a cone-plane geometry with a cone diameter of 60 mm and a large contact surface area. Contact angle measurements are performed with the dispersions deposited on

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RESULTS AND DISCUSSION

CNT/SDS/TX100 Formulation. Test films prepared from the stock CNT/SDS suspensions are first realized. Such suspensions exhibit a low viscosity close to that of water. No homogeneous films could be obtained from these dispersions, even when the CNT concentration was increased. This is due to the fast dewetting of the suspensions from the substrate. TX100 is then added to the above dispersions in order to increase the viscosity and slow down dewetting. With a spreading speed of about 10 cm/s, the rod coating setup is working at shear rates around 10 s−1. The optimal viscosity of the solution should be in the range 0.01−1 Pa·s.23 In order to B

DOI: 10.1021/acs.langmuir.5b00887 Langmuir XXXX, XXX, XXX−XXX

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Langmuir determine such optimum, steady state flow measurements of CNT/SDS/TX100 dispersions are performed. Figure 1 shows the viscosity η as a function of the shear rate γ̇ for various TX100 concentrations and a fixed CNT concentration of 0.2 wt % (Figure 1a) or 0.4 wt % (Figure 1b). The SDS concentration is kept constant at 1 wt %. The dispersions exhibit a shear-thinning behavior, at all CNT concentrations.24 The viscosity is also markedly dependent on the TX100 concentration. In the 10−100 s−1 shear rate range and for low TX100 concentrations, the viscosity of the different dispersions is barely higher than the viscosity of water (ηwater = 1 mPa·s). However, strong thickening is observed above 3 wt % of TX100. At a higher CNT concentration (Figure 1b), the trend is the same but with higher values of viscosity. Figure 1 shows that dispersions at 0.2 or 0.4 wt % of CNTs and 8 wt % of TX100 have an appropriate viscosity to be rod coated. The two rod coated films on the left (Figures 1c,d) processed from low-viscosity suspensions show evidence of dewetting during drying. The resultant films are not homogeneous. In contrast, the coating (Figure 1e), made from a dispersion containing 8 wt % of TX100, is uniform, showing that no dewetting occurred during drying. In order to establish the role played by the surfactants, the viscosity of a solution of 1 wt % of SDS and 8 wt % of TX100 in the absence of CNTs is measured. The increase of viscosity of a surfactant solution due to the addition of another surfactant has previously been studied and is often ascribed to morphological changes of micelles, for example from spherical to wormlike structures.19,20 The rheological characterizations of the present surfactant mixtures are shown in Figure 2. They clearly show a

and TX100 molecules. It is therefore deduced that the CNT play a critical role in the presently observed viscosity enhancements. All the suspensions studied in the present work are well above the critical micellar concentration (cmc) of the two surfactants. It has been shown in previous studies that at high concentrations surfactant micelles induce depletion attraction between nanotubes.22,25 The strength of the attraction is in first approximation proportional to the osmotic pressure of the micelles, which is itself proportional to the micelle concentration. These attractive interactions induce the formation of weak network-like aggregates, which in turn induce an increase of viscosity.26,27 The presence of clusters can be revealed by optical micrographs of dispersions with the same CNT fraction and increasing amounts of surfactant. Figures 3a−c show such micrographs for a suspension containing 1 wt % of SDS, 0.4 wt % of CNTs, and different TX100 fractions. When the TX100 concentration is increased, the nanotube aggregates become larger and the associated gel-like texture is more pronounced.28,29 From the above micrographs and in agreement with previous studies,30 we see that 1 wt % of SDS alone is not enough to induce aggregation of CNTs. Aggregates are formed only when a sufficient amount of TX100 is added. Moreover, the optical appearance of the texture matches the significant increase of viscosity in Figure 2 at 3 wt % of TX100. Figure 4 shows the relative viscosity ηr,10, taken at a shear rate of 10 s−1 as a function of the TX100 weight fraction for

Figure 4. Relative viscosity (at 10 s−1 shear rate) plotted semilogarithmically as a function of TX100 mass fraction for two suspensions at 1 wt % SDS, containing 0.2 wt % of CNTs (circles) or 0.4 wt % of CNTs (squares). ηr,10 is obtained by dividing the value of viscosity at a given fraction xTX100 by the value of viscosity at xTX100 = 0.

Figure 2. Viscosity versus shear rate measurements of SDS/TX100 dispersions containing 1 wt % of SDS and 8 wt % of TX100.

Newtonian behavior. The viscosity of this solution is low and independent of shear rate, suggesting the absence of large anisotropic surfactant structures (like wormlike micelles). The rise of viscosity of the CNT inks cannot be ascribed to morphological changes of micelles or to the association of SDS

suspensions containing CNTs. ηr,10 is obtained by normalizing the viscosity at a given fraction of TX100 by the value of viscosity in the absence of TX100. The data are extracted from

Figure 3. Optical micrographs of suspensions containing a fixed fraction of CNTs (0.4 wt %) and SDS (1 wt %) and three different fractions of TX100: (a) 0, (b) 3, and (c) 8 wt %. C

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Figure 5. Viscosity versus shear rate of CNT/PEDOT:PSS dispersions containing 0.4 wt % of CNT and 0.4 wt % of PEDOT:PSS (a) or 0.8 wt % of PEDOT:PSS (b).

surfactant materials used in the previous section. Like surfactant micelles, PEDOT:PSS particles are therefore expected to enhance viscosity via the depletion-induced aggregation of carbon nanotubes. Figure 5 shows steady measurements on different CNT/PEDOT:PSS dispersions (0.4 wt % CNTs) in order to characterize their viscosity η as a function of the shear rate γ̇. Figure 5a shows the viscosity of a dispersion at 0.4 wt % of CNT and 0.4 wt % of PEDOT:PSS with increasing amounts of TX100. As expected, η raises markedly with the TX100 concentration. 2 wt % of TX100 is sufficient to reach a viscosity suitable to make uniform films by rod coating (to be compared to the 8 wt % used in the first part). This result confirms that the polymer plays also a role on the aggregation of the carbon nanotubes by enhancing depletion interactions between the nanotubes. Data provided in Supporting Information S1 show that the viscosity raise is not due to the polymer and the TX100 alone. The carbon nanotubes govern the viscosifying mechanism. This is confirmed by the texture observed in the optical micrograph in Supporting Information S2. Figure 5b also shows what happens when no surfactant is used. Instead, only 0.8 wt % of polymer is added to CNT in the solution, and a proper viscosity range is reached to rod-coat the solution. However, it is not possible to directly make a homogeneous film with such a solution because of its poor wetting properties. Indeed, while surfactants are undesirable because they are electrically insulating, they remain valuable as wetting agents. They indeed decrease the surface tension of the ink and the contact angle with the substrate. Further improvement of wetting is achieved by a UV−O3 plasma to clean the PET surface and degrade adsorbed organic residues. In addition, UV−O3 plasma treatments increase the PET surface energy by creating polarized functional groups. Figure 6 shows a 0.8 wt % PEDOT:PSS solution deposited on raw PET (left) and 15 min UV−O3 pretreated PET (right). On raw PET, the measured contact angle is 73°, and after 15 min of U-VO3 pretreatment it drops to 44°. The dispersion presented in Figure 5b is suitable to make homogeneous coatings. Such films have been prepared by the rod coating method on PET substrates and their electrical properties measured.

Figures 1a,b. They form, both the suspensions containing 0.2 or 0.4 wt % of CNTs, an apparent straight line when plotted in a semilog scale. This apparent exponential dependence is in agreement with a thermally activated flow with η ∝ exp(−Ea/ kT), where Ea is an activation energy and kT the thermal energy.22 The present behavior is believed to result from the disruption of CNT aggregates as the system is flowing. The bonds involved in the present aggregates are generated by depletion interactions. The activation energy is expected in such conditions to scale linearly with the concentration of depleting agents. The observed experimental behavior is well consistent with this expectation. It is also interesting to note that the slopes of the curves in Figure 4 for two different CNT concentrations are very close. This is again in agreement with the fact that the viscosity increase is directly related to the osmotic pressure of the surfactant micelles. Viscosity increases of colloidal suspensions in response to depletion attractions have been already reported for spheres,31,32 using nonadsorbing polymers as depleting agents.33,34 But viscosity enhancements are by far much more pronounced in the case of CNTs compared to spherical colloids. It is for example necessary to reach particle concentrations of at least 40 vol % to achieve viscosity levels that compare with viscosities of CNT suspensions at volume fractions which do not exceed 0.7 vol %. The CNTs can therefore be viewed as highly efficient viscosifiers. In addition, they are electrically conductive. They are thus ideal constituents that play the role of additives for viscous electronics inks to be spread by techniques that require adequate viscosity. CNT/PEDOT−PSS Formulation. The above model system containing CNTs and surfactants allowed us to understand the mechanisms of viscosity increase. However, when the depletion is generated by the addition of electrically insulating surfactants (such as Triton X), the resulting formulation is ill-suited for the fabrication of high-performance conductive films. Following the same concepts, the formulation is thus consequently changed in order to replace the insulating surfactants by a conductive polymer, namely PEDOT:PSS. This polymer is water-soluble and can efficiently stabilize CNTs in water via repulsive electrostatics interactions at short range.35 It can also create at longer range attractive interactions, in close analogy with D

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more quantitatively compare the different films independently of their thickness, we have fitted the data in Figure 7 by the equation36−38 −2 ⎛ Z σop ⎞ T = ⎜1 + 0 ⎟ 2R s σdc ⎠ ⎝

(1)

where Z0 is the impedance of free space (≃377 Ω) and σop and σdc are the optical and the dc conductivity of the material, respectively. The different films can be compared by the ratio σdc/σop which is defined as a figure of merit (FOM). The higher the FOM, the better the performance of the electrode is. The FOM of F1 before washing is 0.18 ± 0.02 and rises up to 0.58 ± 0.13 after washing. As expected, the use of a washing post-treatment helps in removing the surfactant from the film. But as shown in Figure 8, surfactants cannot be completely

Figure 6. Pictures showing 1 mL of 0.8 wt % PEDOT:PSS solution deposited on raw PET (left) and 15 min UV-O3 pretreated PET (right).

C. Electrooptical Properties of CNT/SDS/TX100 and CNT/PEDOT−PSS Films. Based upon the previous characterizations, three different CNT dispersions are used to prepare homogeneous films (named F1, F2, and F3, described in Table 1). Table 1. Composition of Dispersions Used To Make Transparent Conductive Films

SWNT (wt %) SDS (wt %) PEDOT:PSS (wt %) TX100 (wt %)

F1 SWNT/SDS/ TX100

F2 SWNT/PEDOT/ TX100

F3 SWNT/ PEDOT

0.4 1

0.4

0.4

0.4

0.8

8

3

Figure 8. SEM picture of F1 (SWNT/SDS/TX100 film) after washing. Inset: higher magnification. Black arrows show CNT bundles still coated by surfactants and white arrows show residual surfactant aggregates.

The three types of coatings are characterized by optical transmittance and electrical conductivity measurements. Several films of each type with different thicknesses are made using the rod coating setup. The obtained results of transmittance vs surface resistivity are shown in Figure 7. As classically observed, the transmittance increases with the surface resistivity as the films become thinner. The transmittance and surface resistivity are measured on raw films or after a washing procedure that consists in dipping the films in water:ethanol (1:1 by volume) bath overnight. This washing was found to be an efficient method to remove the surfactants without any peeling of the conductive film from the substrate. As evidenced in Figure 7a, CNT/SDS/TX100 films properties are clearly improved after washing. In order to

removed using this washing step. This is probably due to strong interactions between SDS, TX100, and the CNTs. Actually even high-resolution SEM does not allow a clear visualization of the CNT bundles as they are coated by surfactant molecules. The use of PEDOT:PSS increases the FOM of the film, as shown in Figure 7b. The FOM of the raw films is 0.32 ± 0.09 and that of washed films 0.98 ± 0.16. Again, the washing step probably allows a partial elimination of the insulating surfactant (here the TX100) and a decrease of the surface resistivity. Note

Figure 7. Optical transmittance at the wavelength of 550 nm versus sheet resistance of F1 (a), F2 (b), and F3 (c). The solid line on graph c is an example of fit using the FOM eq 1. Because of the semilogarithmic scale, errors are not represented on the graphs, but they are estimated as ±1% for the transmittance and ±5% for the sheet resistance. E

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that this result was not a priori obvious as washing can also reduce the doping of the conducting polymer by reducing the overall acidity of the film. This hypothesis cannot be ruled out, but apparently it does not mortgage the positive effect of the surfactant removal. Nevertheless, it is clear that optimization of the washing process, using different solvents for example, could certainly be a route toward improvements of properties. Very interestingly, the performances of the films are markedly improved when all the surfactants are replaced by PEDOT:PSS (Figure 7c). In this last case, the FOM reaches 1.84 ± 0.58 before washing and is 1.27 ± 0.20 after washing. In this case dedoping of the polymer can explain the negative effect of washing. Nevertheless, for a transmittance of T = 80%, the surface resistivity is 1 order of magnitude lower than that of a film made from a CNT/PEDOT/TX100 ink. Lastly, we note that the present FOM could certainly be improved by using longer and defect-free nanotubes such as those achieved in solutions of reduced carbon nanotubes39 or solutions in superacids.12

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CONCLUSION

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ASSOCIATED CONTENT

M.P.: The Smalley Institute for Nanoscale Science & Technology, Rice University, 6100 Main Street, Houston, TX 77005. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge fundings from the ANR in the frame of the IMPEC project and from the AMADEus Labex ANR-10LABX-0042-AMADEus.

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(1) Chopra, K. L.; Major, S.; Pandya, D. K. Transparent ConductorsA Status Review. Thin Solid Films 1983, 102 (1), 1−46. (2) Granqvist, C. G.; Hultåker, A. Transparent and Conducting ITO Films: New Developments and Applications. Thin Solid Films 2002, 411 (1), 1−5. (3) Hecht, D. S.; Hu, L.; Irvin, G. Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures. Adv. Mater. 2011, 23 (13), 1482−1513. (4) Du, J.; Pei, S.; Ma, L.; Cheng, H.-M. 25th Anniversary Article: Carbon Nanotube- and Graphene-Based Transparent Conductive Films for Optoelectronic Devices. Adv. Mater. 2014, 26 (13), 1958− 1991. (5) Park, S.; Vosguerichian, M.; Bao, Z. A Review of Fabrication and Applications of Carbon Nanotube Film-Based Flexible Electronics. Nanoscale 2013, 5 (5), 1727−1752. (6) Cao, Q.; Rogers, J. A. Ultrathin Films of Single-Walled Carbon Nanotubes for Electronics and Sensors: A Review of Fundamental and Applied Aspects. Adv. Mater. 2009, 21 (1), 29−53. (7) Yang, S. B.; Kong, B.-S.; Jung, D.-H.; Baek, Y.-K.; Han, C.-S.; Oh, S.-K.; Jung, H.-T. Recent Advances in Hybrids of Carbon Nanotube Network Films and Nanomaterials for Their Potential Applications as Transparent Conducting Films. Nanoscale 2011, 3 (4), 1361−1373. (8) Maillaud, L.; Zakri, C.; Ly, I.; Pénicaud, A.; Poulin, P. Conductivity of Transparent Electrodes Made from Interacting Nanotubes. Appl. Phys. Lett. 2013, 103 (26), 263106. (9) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; et al. Transparent, Conductive Carbon Nanotube Films. Science 2004, 305 (5688), 1273−1276. (10) Meitl, M. A.; Zhou, Y.; Gaur, A.; Jeon, S.; Usrey, M. L.; Strano, M. S.; Rogers, J. A. Solution Casting and Transfer Printing SingleWalled Carbon Nanotube Films. Nano Lett. 2004, 4 (9), 1643−1647. (11) Jang, E. Y.; Kang, T. J.; Im, H. W.; Kim, D. W.; Kim, Y. H. Single-Walled Carbon-Nanotube Networks on Large-Area Glass Substrate by the Dip-Coating Method. Small 2008, 4 (12), 2255− 2261. (12) Mirri, F.; Ma, A. W. K.; Hsu, T. T.; Behabtu, N.; Eichmann, S. L.; Young, C. C.; Tsentalovich, D. E.; Pasquali, M. High-Performance Carbon Nanotube Transparent Conductive Films by Scalable Dip Coating. ACS Nano 2012, 6 (11), 9737−9744. (13) Kaempgen, M.; Duesberg, G. S.; Roth, S. Transparent Carbon Nanotube Coatings. Appl. Surf. Sci. 2005, 252 (2), 425−429. (14) Song, J.-W.; Kim, J.; Yoon, Y.-H.; Choi, B.-S.; Kim, J.-H.; Han, C.-S. Inkjet Printing of Single-Walled Carbon Nanotubes and Electrical Characterization of the Line Pattern. Nanotechnology 2008, 19 (9), 095702. (15) Mayer, C. W. Coating-Machine. US1043021 A, Oct 29, 1912. (16) Dan, B.; Irvin, G. C.; Pasquali, M. Continuous and Scalable Fabrication of Transparent Conducting Carbon Nanotube Films. ACS Nano 2009, 3 (4), 835−843. (17) Li, X.; Gittleson, F.; Carmo, M.; Sekol, R. C.; Taylor, A. D. Scalable Fabrication of Multifunctional Freestanding Carbon Nanotube/polymer Composite Thin Films for Energy Conversion. ACS Nano 2012, 6 (2), 1347−1356.

Some scalable and efficient coating methods require viscous and wetting materials to achieve homogeneous coatings. This is the case of the popular rod bar coating method. Transparent electrodes have been successfully made by these techniques by using water-based thick electronic inks. These inks are composed of surfactant and conducting polymer stabilized dispersions of carbon nanotubes. The present study has shown that the processability and overall homogeneity of the films are governed by the amount of surfactant or conducting polymer which strongly alter the wetting and viscosity of the ink. It has been evidenced that the raise of viscosity results from attractive interactions between the carbon nanotubes mediated by the surfactant. It is possible to finely tune these interactions to reach the suitable range of viscosity allowing the fabrication of homogeneous films. However, because the use of insulating surfactants is not suitable to get electrically conductive electrodes, we have generalized the above concepts to the case of conductive depleting agents. We have used PEDOT:PSS to fabricate optimized electrodes using conductive polymers and carbon nanotubes. The provided results could help in the fabrication of transparent conductive films on a large scale or other printed devices by using industrial deposition methods (slot die, slide, roll coating, etc.) that require thick inks to be used.

S Supporting Information *

S1: viscosity versus shear rate measurements of aqueous solutions containing 0.4 wt % of PEDOT:PSS and various amounts of TX100; S2: optical micrograph of CNT/ PEDOT:PSS/TX100 dispersions containing 0.4 wt % of CNT, 0.4 wt % of PEDOT:PSS, and 2 wt % of TX100. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00887.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(C.Z.) E-mail [email protected]. F

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DOI: 10.1021/acs.langmuir.5b00887 Langmuir XXXX, XXX, XXX−XXX