Synthesis of Continuous Conductive PEDOT:PSS Nanofibers by

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Synthesis of continuous conductive PEDOT:PSS nanofibers by electrospinning: a conformal coating for optoelectronics Bastien Bessaire, Mathieu Maillard, Vincent Salles, Taguhi Yeghoyan, Caroline Celle, Jean-Pierre Simonato, and Arnaud Brioude ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13453 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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ACS Applied Materials & Interfaces

Synthesis of continuous conductive PEDOT:PSS nanofibers by electrospinning: a conformal coating for optoelectronics Bastien BESSAIRE 1,2, Mathieu MAILLARD1*, Vincent SALLES1, Taguhi YEGHOYAN1, Caroline CELLE2, Jean-Pierre SIMONATO2, Arnaud BRIOUDE1 1

Université de Lyon, Université Claude Bernard LYON1, Laboratoire des Multimatériaux et Interfaces, UMR CNRS 5615, F-69622 Villeurbanne, France 2

Univ. Grenoble Alpes, CEA, LITEN/DTNM/SEN/LSIN, F-38054 Grenoble

Keywords: electrospinning, PEDOT/PSS, transparent electrode, conformant coating, conductivity, nanofibers

Abstract A process to synthesize continuous conducting nanofibers were developed using PEDOT:PSS as a conducting polymer and an electrospinning method. Experimental parameters were carefully explored to achieve reproducible conductive nanofibers synthesis in large quantities. In particular, relative humidity during the electrospinning process was proven to be of critical importance, as well as doping post-treatment involving glycols and alcohols. The synthesized fibers were assembled as a mat on glass substrates, forming a conductive and transparent electrode and their optoelectronic have been fully characterized. This method produces a conformable conductive and transparent coating that is well adapted to non-planar surfaces, having very large aspect ratio features. A demonstration of this property were made using surfaces having deep trenches and high steps, were conventional transparent conductive materials fail because of a lack of conformability.

1. Introduction Conductive polymers have been recently widely developed because of their outstanding properties as transparent and conductive materials1–7. Among this family, poly(3,4-

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ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a good candidate for flexible and portable electronics8,9. Lots of materials have been studied in this field, like transparent conductive oxides10–13, metallic nanowires14–16 or carbon based nanomaterials17,18. However, their poor mechanical properties19 combined with the difficulty to deposit continuous films of materials on rough or non-planar surfaces are major issues for their use in portable electronics. It has been demonstrated that thin films of PEDOT:PSS can reach similar performances to Indium-Tin Oxide (ITO) electrodes, leading to ITO-free highly conductive organic electrodes8,9. Nevertheless, PEDOT:PSS having a fairly high absorption coefficient (figure S1), transparency can be further improved using a similar strategy than with metallic nanowires20,21, i.e. by replacing an absorbing film by a network of percolating nanowires, having a diameter larger than the film thickness. The obvious advantage of such strategy is to keep the amount of conducting material constant while creating highly transparent voids on the substrate. In our case, it consists in patterning the substrate with percolating fibers of PEDOT:PSS, having conductivity of raw PEDOT:PSS. High surface coverage with conductive nanofibers (NFs) has already shown good sensing22,23, thermochromic24 or conductive25 properties and can lead to the market of portable electronics or intelligent textiles. Electrospinning is a low cost, reproducible and simple method to produce homogeneous fibers at the nanometer scale for various applications26–28. It is the balance of two opposite forces: electrostatic force tends to form a thin fiber of polymer to separate charges and transport them to a counter electrode, and surface tension minimizing the polymer solution surface. Many solution parameters affect morphology of the obtained NFs, including viscosity and visco-elasticity but environmental parameters are also important to obtain reproducible NFs29,30. Indeed, temperature and humidity play a key role in the evaporation of solvent between needle and collector, leading to various morphologies (from wet fibers to beads-on-string structures)31,32. The poor rheological properties of the conductive polymer, mostly a low viscosity, require using carrier polymers to process it. Poly(ethylene oxide) (PEO)33, Polyvinylpyrrolidone (PVP)22, chitosan34 and Polyvinyl alcohol (PVA)23 are the most used binders in the literature. Alternatively, a recent study demonstrated that PEDOT:PSS could be complexed with Mg2+ ions but polymer conductivity is still to be proven35.


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In our work, electrospinning offers the possibility to produce soft nanomaterials, which have high value for non-planar transparent electrodes. While electrospinning gives flexible 1D nanofibers, liquid-state deposition technique produces thin films that can hardly adapt to the morphology of the substrate, especially when having high aspect ratio features like steps, grooves or channels. Indeed, electrospun PEDOT:PSS NFs can be easily shaped according to the form of the surface, being deposited on very localized areas, which is not the case for spin-(or spray-) coated nanomaterials36,37. Herein, the influence of humidity of the spinning atmosphere on the morphology of electrospun PEO:PEDOT:PSS NFs, their optoelectronic performances and the deposition of this nanofibers mat on patterned substrates have been investigated. A non-destructive chemical treatment on the 1D nanostructure has also been developed to obtain a conductive network for ITO free electrodes. To our knowledge, this work is the first to show no loss of conductivity through obstacles on non-planar surfaces using conductive polymers.

2. Experimental 2.1.

Materials

Poly(Ethylene Oxide) Mw 900,000 (PEO), N,N-Dimethylformamide (DMF), ethylene glycol (EG) were purchased from Sigma-Aldrich, ethanol was purchased from Carlo-Erba Reagants. Clevios PH1000 PEDOT:PSS was provided by Heraeus, constisting in an aqueous dispersion of colloidal polymer in water (1.3wt%). All these chemical reagents were used as received. 1 inch square glass substrates were provided by CarlRoth, Germany. 2.2.

Electrospinning of PEDOT:PSS/PEO NFs

The solution was prepared as follows: 0.067g of PEO was dissolved in 2.560g of PEDOT:PSS solution under stirring at room temperature. Then, 0.345g of DMF was added to the prepared solution and kept under agitation for 24h at room temperature to obtain a homogeneous and spinnable solution. This specific composition is the result of an extensive optimization work, varying parameters such as polymer nature, molecular weight, concentrations, polymer to PEDOT/PSS ratio and additives (S2). In order to get comparable results, PEDOT:PSS thin films were spin-coated on glass patterned substrates at 5000 rpm for 60s. Deposited thin films were



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stabilized at 70°C for 10 minutes. Then a chemical treatment, consisting in an immersion in various solvents, was applied to both NFs and spin-coated substrates. The as-prepared solution was loaded in a plastic syringe for electrospinning connected with a 21G metallic needle (Braun, 0.5mm internal diameter). Electrospinning was performed in a homemade system to control humidity, using a high voltage power supply (Spellman SL30, Spellman, USA), a syringe pump (KDS-100, kD Scientific, USA) and a closed electrospinning box of 64L (see figure 1). The applied voltage was 8.5kV and the distance between the tip of the needle and the grounded collector (aluminum foil for SEM characterization and 1-inch square glass substrates stuck on aluminum foil for optoelectronic measurements) was 10cm. The flow rate of the solution was kept constant at 0.2mL/h. Current temperature and humidity of the chamber atmosphere were monitored using a Dickson FH525 logger (± 0.1RH, ±0.1°C). Blue silica-gel and a water spray were used to control relative humidity (RH) in the chamber. RH was measured prior to each deposition process and remained stable during the whole process; variations due to water evaporation during the process were never up to 2-3%, which is consistent with the feeding rate of the solution.

Figure 1 : Schematic illustration of the electrospinning box used for low humidity electrospinning

2.3.

Characterization 4 ACS Paragon Plus Environment

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The composite PEO/PEDOT:PSS NFs were characterized using scanning electron microscopy (SEM, FEI Quanta FEG 250) with an accelerating voltage of 10kV. Elemental analyses (EnergyDispersive X-ray Spectroscopy, EDS) were carried on the same microscope. Transmittance measurements were performed on Varian Cary 5000 spectrophotometer using the substrate as reference whereas haze measurements where carried using an integrating sphere DRA 2500 integral. Haze is the ratio between diffuse transmittance and total transmittance. In that case, it is possible for us to distinguish specular and diffuse transmittance from electrospun mats. The values given are average values of 3 samples after an equivalent treatment. Sheet resistance was measured using a four pin probe with a Loresta EP resistivity meter. At least 10 distinct measurements were performed on each sample to confirm homogeneity of the nanofiber mat. For non-planar substrates, the measurement was performed such as probing pins were in contact with planar areas, whereas at least one feature was located between pins and creating an obstacle through the entire sample length. The average diameter of nanostructures was obtained using ImageJ software by taking at least 50 different measurements per sample. 2.4.

Estimated transmittance and sheet resistance

Theoretical transmittance and sheet resistance have been calculated based on a simple model, assuming that specular transmittance is mostly due to fiber coverage surface fraction and transmittance through fiber and sheet resistance is due to an effective PEDOT thickness. Light scattering from the fiber network was estimated by discrete dipole approximation calculations. For surface of area S covered by a fiber of total length L and Diameter D, surface coverage is given by Φ =

௅஽ ௌ

. If fibers have an absorption coefficient α, specular transmittance %Tspec is

equal to the surface fraction not covered by fibers %ܶ௩௢௜ௗ = ሺ1 − Φሻ and transmittance through the fibers %ܶ௙௜௕௘௥௦ = Φ10ି஑ୈ. If we consider that fiber is made of N=L/l section of fibers of length l with a scattering cross section ߪ௦௖௔௧ , then transmission due to scattered light is given by %ܶ௦௖௔௧ =

௅ൗ ௟ ߪ ௌ ௦௖௔௧

஍

= ஽௟ ߪ௦௖௔௧ . Specular transmission will thus be lowered due to light scattering

and be given by %ܶ௦௣௘௖ = 1 − Φ ቀ1 − 10ି஑ୈ + coverage and transmittance Φ =

ଵି%୘౩౦౛ౙ

. ഑ ଵିଵ଴షಉీ ା ೞ೎ೌ೟

ఙೞ೎ೌ೟ ஽௟

ቁ leading to a relation between surface

Scattering efficiency has been calculated using

ವ೗



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discrete dipole approximation and corresponds to a haze of about 50% (see S3 for detailed results). Now scattering is a critical parameter to avoid only if imaging through the electrode is mandatory but can be beneficial for other applications like transparent electrode for photovoltaic panel38. For this reason we considered both specular and overall transmission, as the present material is not limited to a single application field. For a homogeneous thin film, sheet resistance Rsq is equal to Rsq=ρ/e with ρ the material resistivity and e the film thickness. Now if we consider e as an effective thickness, meaning the thickness corresponding to the volume v of PEDOT in a fiber of length L on a surface S, then గ

గ

௩

గ

‫ܦ = ݒ‬ଶ ‫ܵܦ = ܮ‬Φ and thus ݁ = = ‫ܦ‬Φ Based on this result we can deduce sheet resistance ସ

ସ

ௌ

versus transmittance ܴ௦௤ =

ସ

഑ ସఘቀଵିଵ଴షಉీ ା ೞ೎ೌ೟ ቁ

గ஽ሺଵି%்ሻ

ವ೗

(1). In the limit condition of an opaque and non

scattering fiber, which is true for small metallic nanowires, we retrieve the relation ܴ௦௤ = ସఘ

. This theoretical calculation assumes a negligible electrical contact resistance between

గ஽ሺଵି%்ሻ

fibers, which underestimates the effective sheet resistance of such mats. Nevertheless, conductive mat of PEDOT/PSS being made of a single fiber and fiber being a polymer, we can reasonably neglect contact resistance in first approximation.

3. Results and discussion 3.1.

Morphology characterization of NFs

Figure 2 (A-E) shows the obtained morphology of NFs electrospun under air at various relative humidities. Figure 3 represents the evolution of composite NFs diameter and beads diameter as a function of relative humidity. The average diameter of NFs decreased from around 157 nm to 105 nm as humidity raised from 7% RH to 42% RH , while beads diameter tend to increase in the same time.


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Figure 2 : SEM images of PEDOT :PSS/PEO NFs electrospun at : (A) 7%RH, (B) 20%RH, (C) 25%RH, (D) 33%RH, (E) 42%RH from aqueous solution

Figure 3 : Average diameter of electrospun PEDOT:PSS/PEO NFs at different humidity

It is important to note that when electrospinning is performed at 7%RH, perfectly linear and beads-free NFs are obtained, which can be explained by a fast evaporation of water due to a low water pressure in the spinning atmosphere. On the opposite, at higher relative humidity, evaporation rate slows down and allows the jet to elongate on a longer distance; producing thinner fibers32. A thin liquid fiber is, unfortunately, thermodynamically less stable due to surface 7 ACS Paragon Plus Environment


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tension and if evaporation rate is too low, beads are formed between segments of fibers, due to Rayleigh-Plateau type instabilities39. Because the present solvent is actually water these fibers have not been synthesized in a totally moisture free environment like a glove box but it could eventually further benefit from this kind of environment, as it has been proved with non aqueous solvents40 that it improves surface roughness from the fibers and thus reduce light scattering. 3.2.

Optoelectronic performances

Optoelectronic performances of as-spun NFs are quite poor as shown in table 1. In our case, the resistance is between 18 and 50kΩ for as-spun fibers with transmittance under 10% in each case. However, it is known that raw PEDOT:PSS is almost insulating without doping agents. Alcohols, glycols and other chemicals have recently proved their efficiency in increasing spin coated PEDOT:PSS conductivity41,42. Table 1 : Optoelectronic performances of as-spun PEDOT:PSS NFs (samples with different electrospinning deposition times, from 30min to 1h)

Sample

R□ (kOhm)

%T

a

18.0

3.5

b

21.5

7.7

c

31.9

8.8

Different treatments have been applied to the as-spun NFs in order to decrease sheet resistance from the mat and at the same time increase the overall sample transparency. It is critical that such a liquid post-treatment does not destruct the nanostructure, as the objective is to finally obtain conductive NFs on non-planar surfaces. The SEM images in figure 4 shows the stability of PEDOT:PSS NFs after immersion in ethanol and EG solutions after an optimized elapsed duration of 10 min. The soaking time has been chosen after different trials, indicating that longer soaking time did not lead to better results. Diameter from fibers before and after treatment remains around 150 nm, which shows that nanostructuration of the fibers has not been destroyed. Moreover, for each of these treatments by immersion, NFs remain dispersed, without any bundling due to capillary forces during solvent drainage. 8 ACS Paragon Plus Environment

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Figure 4: SEM images of: (A) as-spun PEDOT:PSS NFs, (B) NFs after ethanol soaking, (C) NFs after EG soaking

Optoelectronic performances measured after chemical treatments are shown in figure 5. It appears that, as for usual deposition methods with PEDOT:PSS thin films, doping PEDOT:PSS NFs leads to more transparent and more conducting mats. Indeed, we assume that the soaking induces two major effects: PEO removal and PEDOT:PSS doping. Doping is likely due to polymeric conformations changes, transforming the PEDOT chains from benzoid to quinoid structures as explained by Frey et al 43. In that case, the PEDOT chains finally get an expandedcoil (e.g. almost linear) structure which is conductively favorable. We observed from experimental results that he effect of EG is slightly higher than that of EtOH, because the driving force of conformation change is the dipole interaction between PEDOT and the dopant. In our case, EG has a higher dipole moment (µ=2,20D) than EtOH (µ=1.66D) which leads to higher conformation changes of PEDOT. Moreover, the experimental values of doped PEDOT:PSS NFs are close to the theory in which the conductive polymer is doped by DMSO which is known as the best dopant for PEDOT because of its higher dipole moment (µ=3,96D). The partial fusion of the nanofibers junctions during the immersion in the solvent solutions can be another hypothesis for the conductivity enhancement mechanism, leading to better fiber-to-fiber contacts.



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Figure 5 : Optoelectronic performances of electrospun PEDOT:PSS NFs : as-spun (circles) and after doping considering specular transmittance only (stars) and total transmittance (triangles). Lines are the theoretical values using our prediction model for : PEDOT:PSS before doping (solid) and PEDOT:PSS after doping (dashed) for fibers of diameter D=150nm and length l=1.5µm, corresponding to a fiber coverage from 0 to about 60% according to equation 1 .

Now because we measured specular transmittance %Tspecular, our transmittance results do not exactly reflect the total transmittance due to both specular transmission and light scattering %Ttotal = %Tscattering + %Tspecular. For this reason we performed Haze measurements on electrospun samples. Haze is defined as the ratio between diffuse transmittance %Tscattering and total transmittance %Ttotal. It has been performed using a UV-Visible spectrometer with an integration sphere having a removable mirror, allowing to measure sequentially both the total absorption %Ttotal when mirror is on and diffuse transmittance when mirror is removed. (Figure 6a shows the evolution of haze after immersion EG for different times.) In the present samples, light scattering inducing Haze is due to both intrinsic scattering from the materials used itself and light scattering form the 1D objects. As solvent treatment induces negligible change in the fibers morphology, it is reasonable to conclude that a decrease in the Haze value is directly linked to a partial dissolution of PEO which is a major contributor to light scattering. Indeed, PEDOT/PSS is a colored polymer but highly transparent, whereas PEO is known to be non-absorbing but being semi-crystalline44, which make it highly non transparent due to massive light scattering. In that sense, intrinsic scattering from PEO is reduced when extracting PEO from the PEDOT/PSS/PEO nanofibers. Of course scattering from the fibers themselves remains similar because refractive index is only slightly modified by PEO extraction. The model used to obtain results from figure 5 10 ACS Paragon Plus Environment

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includes scattering cross sections obtained by DDA calculations from nanofibers of composition corresponding to actual fibers, as-spun and after PEO extraction (n=1.47 and 1.52 respectively). We assume that extraction of PEO occurs from the surface of the NFs, as haze decreases while immersing the samples for different times. A maximum decrease of 35% is observed for long time EG soaked samples. The same results are observed with EtOH treatment, leading to more transparent networks as illustrated in figure 6b.

Figure 6 : (a) Evolution of haze after different soaking times in EG, (b) Images of the samples after different immersion times in EtOH from 0 to 5 minutes

Figure 7a displays the evolution of C/S ratio measured by Energy-Dispersive X-ray Spectroscopy before and after both chemical treatments. It shows that solvent treatment leads to a significant decrease of the C/S ratio that can be explained by a partial surface removal of insulating PEO, highly soluble in both EG and EtOH (and not in DMSO). A decrease of 16 % from the C/S ratio is measured for an expected decrease of 41% (in case of complete PEO removal). Based on this result, we can estimate that 40 % of the PEO has been removed by the solvent. As a consequence, the improvement of the performances of the electrospun mat can be related to that partial PEO removal, which enables to have more percolating PEDOT:PSS chains on the surface. Surprisingly fibers remain similar after solvent extraction but it is easily explained by a very limited reduction of the fiber volume (S4).

Now this removal improves both

conductance and transparency, as PEO is both an insulating polymer and a source of light scattering. The extinction of the characteristic absorption band of PEDOT:PSS/PEO at 2880cm-1 is another proof of the partial PEO removal after immersion in EG or EtOH.



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Figure 7 : (a) Evolution of C/S EDX ratio of NFs after EtOH and EG treatment, (b) FTIR spectra of : PEDOT:PSS/PEO thin film (dashed lines), PEDOT:PSS/PEO as spun nanofibers (red) and after immersion in EG for 7min (blue), 15 min (orange), 25 min (green) and 45 min (pink)

3.3.

Morphology and conductivity on non-planar surfaces

Electrospinning deposition has also been conducted on patterned glass substrates having high aspect ratio features to prove the efficiency of nanofibres compared to thin films. Experiments have been conducted on two types of channeled glass substrates: indented (60µm deep trenches obtained using a diamond saw) and raised (10µm high and 500µm large polyimide tape) (see figure 8).

Figure 8 : Patterned glass substrates for electrospinning deposition (first : indented, second : raised)

In order to compare, these substrates have also been spin-coated with PEDOT:PSS followed by the same post-treatment but exhibited no measurable conductivity across the obstacles for both surfaces. These results are not surprising since liquid cannot coat homogeneously the vertical sides of the obstacle and form an electrical connection between horizontal areas for these high aspect ratio features.


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Figure 9 : SEM images of NFs on the patterned substrates: a) indented, b) raised. Highlighted areas with dashed lines correspond to vertical sides (normal incidence)

Figure 9 shows SEM images of nanofibers on the patterned substrates. It clearly illustrates the nanofibers coating on the Kapton walls and the polymeric channels between the different parts of the substrates.

Figure 10 : a) Evolution of the sheet resistance of PEDOT:PSS thin films or nanofibers on high aspect ratio patterned substrates : for thin films (black squares), for nanofibers (red circles), in insert : low magnification SEM pictures of as-spun nanofibers on channels and steps of the substrates. b) 4point-probe measurements through the channels, c) 4point-probe measurements through the steps.

Such samples exhibits surface resistivity of around 13kΩ for 60%T and 39kΩ for 69%T for indented substrates, after treatments as shown in figure 10. For raised substrates, the transparency has not been measured, but the value of surface resistivity of each side is similar to the one 13 ACS Paragon Plus Environment


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measured through the strip and is around 22kΩ. One can assume that transparency is comprised between 55 and 65% according to previous results. All these data confirm that electrospinning gave us the opportunity to obtain conductive and transparent nanostructures on substrates of variable geometry and high aspect ratio obstacles, where conventional techniques fail. This deposition method has also been successfully applied to fabric to make them conductive as well as to produce self-standing conductive film. In the latter case the conducting mat is deposited onto a non-sticking coating and simply peeled off after drying.

4. Conclusion In summary, this paper demonstrates a controlled electrospinning process combined with a chemical treatment to prepare conductive nanowires assembled into nanostructures on any surfaces including non-planar ones. Compared to conventional thin film deposition techniques, the obtained nanofibers mats provided remarkable conductivities and transparencies on positively and negatively patterned substrates. The networks prepared in this work exhibited surface resistance of 12kΩ at 60% whereas no conductivity can be measured on equivalent spin-coated substrates. In addition, the relative humidity control of the spinning atmosphere has been confirmed as a key parameter to obtain a robust and reproducible process. These nanostructures give a much wider flexibility to manufacturing processes and thus pave the way of promising integration for transparent electronics or solar cells on atypical surfaces. Supporting Information :

PEDOT/PSS absorption spectra (S1), exploratory experimental parameters (S2), theoretical calculations using DDA (S3), and extraction impact on fibers diameter (S4) * Corresponding author: Mathieu Maillard [email protected] Acknowledgments Authors would like to acknowledge the CTµ (Centre Technologique des Microstructures, microscopies.univ-lyon1.fr) for the access to the SEM microscopes used in this work. Catherine Marichy and Danièle Blanc-Pelissier are gratefully acknowledged for their help in four-point probe measurements, Guillaume Sudre and René Fulchiron from Université de Lyon for the fruitful discussion on light scattering from polymers. 14 ACS Paragon Plus Environment

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