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Structure and Dopant Engineering in PEDOT Thin Films: Practical Tools for a Dramatic Conductivity Enhancement Magatte N. Gueye, Alexandre Carella, Nicolas Massonnet, Etienne Yvenou, Sophie Brenet, Jerome Faure-Vincent, Stéphanie Pouget, Francois Rieutord, Hanako Okuno, Anass Benayad, Renaud Demadrille, and Jean-Pierre Simonato Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01035 • Publication Date (Web): 29 Apr 2016 Downloaded from http://pubs.acs.org on May 3, 2016
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Chemistry of Materials
Structure and Dopant Engineering in PEDOT Thin Films: Practical Tools for a Dramatic Conductivity Enhancement. Magatte N. Gueye1,2, Alexandre Carella1,*, Nicolas Massonnet1, Etienne Yvenou1, Sophie Brenet1, Jérôme Faure-Vincent2, Stéphanie Pouget3, François Rieutord3, Hanako Okuno3, Anass Benayad1, Renaud Demadrille2 and Jean-Pierre Simonato1,*. 1
Magatte N. Gueye, Dr. Alexandre Carella, Dr. Nicolas Massonnet, Etienne Yvenou, Sophie Brenet and Dr. JeanPierre Simonato. Univ. Grenoble Alpes, CEA/LITEN/DTNM, MINATEC Campus, F-38054 Grenoble, France. 2
Magatte N. Gueye, Dr. Jérôme Faure-Vincent and Dr. Renaud Demadrille.
a) Univ. Grenoble Alpes, CEA, INAC, F-38000 Grenoble, France b) CNRS, Alpes, INAC-SPRAM, F-38000 Grenoble, France. c) CEA, INAC-SPRAM, F-38000 Grenoble, France. 3
Dr. Stéphanie Pouget, Dr. François Rieutord and Dr. Hanako Okuno.
a) Univ. Grenoble Alpes, INAC-MEM, F-38000 Grenoble, France. b) CEA, INAC-MEM, F-38000 Grenoble, France. ABSTRACT: Poly(3,4-ethylenedioxythiophene) (PEDOT) is certainly the most known and most used conductive polymer since it is commercially available and shows great potential for organic electronic, photovoltaic and thermoelectric applications. Studies dedicated to PEDOT films have led to high conductivity enhancements. However, an exhaustive understanding of the mechanisms governing such enhancement is still lacking, hindered by the semi-crystalline nature of the material itself. In this article, we report the development of highly conductive PEDOT films by controlling the crystallization of the PEDOT chains and by a subsequent dopant engineering approach using iron(III) trifluoromethanesulfonate as oxidant, N-methyl pyrrolidone as polymerization rate controller and sulfuric acid as dopant. XRD, HRTEM, Synchrotron GIWAXS analyses and conductivity measurements down to 3 K allowed us to unravel the organization, doping and transport mechanism of these highly conductive PEDOT materials. N-methyl pyrrolidone promotes bigger crystallites and structure enhancement during polymerization while sulfuric acid treatment allows the replacement of triflate anions by hydrogenosulfate and increases the charge carrier concentration. We finally propose a charge transport model that fully corroborates our experimental observations. These polymers exhibit conductivities up to 5400 S cm-1 and thus show great promise for room temperature thermoelectric applications or ITO alternative for transparent electrodes.
INTRODUCTION Conductive polymers have received tremendous attention in optoelectronics,1–3 spintronics,4,5 and thermoelectricity6–9 due to their abundance, low cost, ease of processability and high performance. Among them, poly(3,4ethylenedioxythiophene) (PEDOT) has focused interest owing to its stability and tunable conductivity which ranges from 0.1 S cm-1 for undoped PEDOT:PSS (poly(styrenesulfonate)) films up to 8797 S cm-1 for PEDOT monocrystals.2,10–18 Reported conductivities spread over a wide range, however an exhaustive understanding of the origin of such differences is still lacking, and subsequently designing optimized polymer films remains chal-
lenging. The origin of the high conductivities could be investigated by unravelling conduction pathways governing the electrical properties of these materials. However, accurate assessment of the charge-transport mechanisms in PEDOT films is arduous due to the semi-crystalline nature of the material.19,20 In this work, thin films of highly conductive PEDOT stabilized with trifluoromethanesulfonate CF3SO3- (OTf) counter-anions (PEDOT:OTf) are studied,21 their structural organization is solved and their heterogeneous transport properties are disclosed. A first chemical treatment involving a high boiling point co-solvent increases the conductivity from 1200 to 3600 S cm-1 and further acid
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treatment leads to the highest conductivity ever reported for PEDOT films, that is 5400 S cm-1. Such high conductivities are achieved with thin films where monocrystalline domains are separated by amorphous regions, as shown by synchrotron GIWAXS (Grazing-Incidence Wide-Angle X-Ray Scattering) measurements and HRTEM (High Resolution Transmission Electron Microscopy) images. XPS (X-Ray Photoelectron Spectroscopy) measurements outline a strengthening of the primary doping induced by the acid treatment. Temperature dependent conductivity experiments from 3 to 315 K unravel the transport mechanism which appears to be a heterogeneous conduction model.
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co-solvent addition and the influence of the acid treatment.
RESULTS AND DISCUSSION Electrical properties. PEDOT:OTf films were obtained by spin-coating an ethanolic solution containing 3,4ethylenedioxythiophene (EDOT) , the oxidative reagent Fe(III)(OTf)3, and the polymerization rate controller PEGPPG-PEG14 on glass substrates. The formulation with high boiling point solvent was obtained by adding NMP (NMethyl-2-pyrrolidone) as co-solvent in the aforementioned solution, hence obtaining PEDOT:OTf-NMP films. Detailed explanation using other co-solvents can be found in the Supporting Information, Figure S1. Organic solvents have proven to induce phase separation in PEDOT:PSS between PEDOT-rich and PSS-rich domains, and to reduce the excess of insulating PSS,22–24 thus enhancing the conductivity. However, their influence on PEDOT obtained with smaller counter-anions has not been investigated yet, though they are expected to refine the structure of the polymer chains in addition to the PEG-PPG-PEG action.25 Figure 1a shows conductivity values obtained on PEDOT:OTf-NMP with different amounts of NMP. Maximum conductivities of 3600 +/- 200 S cm-1 are reached for a loading of 7 – 8 wt. %. These values are among the highest reported for conducting polymer films. PEDOT films were also treated with dilute sulfuric acid in order to further enhance the conductivity. Such treatment has been proven efficient on both PEDOT:PSS and PEDOT:OTf films.15,21,26,27 PEDOT:OTf and PEDOT:OTfNMP films were immersed into 1 M sulfuric acid aqueous solution for 30 min before being heated at 120 °C for 20 min without rinsing. Resulting samples were named PEDOT:Sulf and PEDOT:Sulf-NMP respectively. The conductivity of PEDOT:Sulf-NMP was measured at 5400 +/- 400 S cm-1 (Figure S4 in the Supporting Information), which is remarkably high when compared to reported values of PEDOT films so far. Explanation of the chemical mechanisms responsible for this extensive enhancement is presented hereafter. Oxidation state. XPS provides insights into the chemical composition at the surface of the films and their oxidation states.28 We investigated PEDOT:OTf-NMP and PEDOT:Sulf-NMP in order to determine the effect of the
Figure 1. Electrical properties of the PEDOT materials. a) Conductivity and b) thickness of PEDOT:OTf-NMP thin films as a function of the quantity of NMP. Error bars include the average value over three samples.
High resolution core level spectra confirmed the presence of fluorine, nitrogen, carbon, oxygen and sulfur in both PEDOT:OTf-NMP and PEDOT:Sulf-NMP, while fluorine was scarcely present in the latter as observed in the survey spectra (Figure 2a & Figure S7 in the Supporting Information). The oxidation level of PEDOT films can be estimated thanks to the ratio of thiophene units to other sulfur based components.29 The S2p spectra of PEDOT:OTfNMP and PEDOT:Sulf-NMP are depicted in Figure 2b,c. The S2p3/2-1/2 doublet from the thiophene units of PEDOT is distinct from that of the counterions. Both thiophenes and sulfonates are detected in the S2p spectrum of PEDOT:OTf-NMP. An oxidation level of 25.6 % was measured, which is in good agreement with the oxidation level of PEDOT:OTf which is about 27.8 %.21 The co-
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Chemistry of Materials
solvent addition does not alter the oxidation of PEDOT, and the conductivity enhancement is more likely due to structural and morphological changes rather than a doping process. Regarding
NMP (upper curve) and PEDOT:Sulf-NMP (lower curve). b) S2p XPS spectrum of PEDOT:OTf-NMP. c) S2p XPS spectrum of PEDOT:Sulf-NMP.
the acid treated films, not only were thiophenes and sulfonates detected, but also sulfates. This, along with the reduced amount of fluorine found in the films, can be explained by the replacement of triflate by hydrogenosulfates as counter-anions. The degree of oxidation of PEDOT:Sulf-NMP is about 39.1 % which means that not only some CF3SO3- are replaced by HSO4-, but also the film undergoes further oxidation. The use of a less steric counter-anion introduced after acid treatment, combined with a higher oxidation level, allows a substantial conductivity enhancement from 3600 to 5400 S cm-1. Structural properties. GIWAXS measurements give access to the structure of the PEDOT films and their crystallinity. This technique is well suited for investigating our films which are between 10 and 40 nm thick (Figure 1b). The GIWAXS intensity profiles of PEDOT:PSS doped with EG (ethylene glycol), PEDOT:OTf, PEDOT:OTf-NMP and PEDOT:Sulf-NMP, obtained with a synchrotron radiation, are presented in Figure 3a,b. Those obtained with a Smartlab diffractometer and a detailed characterization of each peak can be found in Figure S8 in the Supporting Information. Both in-plane and out-of-plane measurements revealed the structure of the as-deposited films. The different peaks were indexed according to previous works.14,21,30 In both PEDOT:PSS diffractograms, no clear diffraction peak is noticeable. However, the bump at q = 1.82 Å-1 in the in-plane scan could be associated with the (020) diffraction peak of the PEDOT orthorhombic structure. It corresponds to the Bragg diffraction of the π-π stacking of the PEDOT thiophene rings, representing the face to face oligomers stacking in the b-direction as described in Figure 3c. The broadness of the bump reveals the high degree of disorder in PEDOT:PSS and prevents the determination of the mean size of the crystallites.19,21 Nonetheless previous studies suggested a 4,5 nm crystallite size along the b axis,30 which is compatible with our measurements.
Figure 2. . Chemical analysis of the PEDOT materials using X-Ray photospectrometry. a)Survey spectra of PEDOT:OTf-
In PEDOT:OTf and its treated counterparts, the presence of smaller counter-anions leads to a better crystallinity, a more densely packed crystalline structure, and hence to clear sharp peaks in the diffractograms. In the in-plane scans recorded for each of the three samples the (020) diffraction peak is visible. In the out-of-plane diffractograms in Figure 3b, a first peak at q = 0.46 Å-1 (d = 13.65 Å) represents the Bragg diffraction of the lamellar packing of PEDOT chains in the a-direction (see Figure 3c). The other following peaks correspond to (h00) higher order peaks. These different peaks suggest a preferential orientation in which PEDOT chains are stacked along the baxis, edges on the substrate. The crystallites’ size obtained from in-plane measurements, using Scherrer equation, are 4, 5 and 5.5 +/- 0.5 nm for PEDOT:OTf, PEDOT:OTfNMP and PEDOT:Sulf-NMP respectively; out-of-plane diffractograms give 7.5, 9 and 10 +/- 1 nm. Interestingly for
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PEDOT:OTf-NMP and PEDOT:Sulf-NMP the film thickness is very close to the crystallite size deduced from outof plane measurements, suggesting that the film is one crystallite high.
Figure 3. Structural characteristics of the PEDOT materials. a) In-plane and b) out-of plane synchrotron GIWAXS diffractograms of PEDOT:PSS, PEDOT:OTf, PEDOT:OTf-NMP and PEDOT:Sulf-NMP. c) Scheme of stacking in the crystallites. d) HRTEM image of PEDOT:OTf-NMP. Inset image is a magnification of the outlined square.
These GIWAXS characteristic lengths and distances are corroborated by HRTEM images. Figure 3d and Figure S10 in the Supporting Information show HRTEM images of PEDOT:OTf-NMP deposited on a copper grid coated with graphene in order to support the PEDOT film. The images sustain the presence of crystalline domains surrounded by amorphous regions. In the crystallites, characteristic π-π stacking distances can be measured at 3.46 Å, consistent with GIWAXS data. These HRTEM images provide also clear information about the length of PEDOT chains forming the crystallites, i.e. around 6 nm for the crystallite in Figure 3c, which is the same order of magnitude as the crystallite size along the b-axis, that is around 10 nm. Given the EDOT monomer size which is about 2.72 Å
(Figure 3d),31 the number of repeating units in the PEDOT chains in the crystallite in Figure 3c can be approximated to 23 EDOT units. It seems reasonable to assume that the increase of crystallites’ mean size noticed between PEDOT:OTf and PEDOT:OTf-NMP together with the more ordered film obtained might be a reason for the dramatic improvement of the conductivity from 1200 to 3600 S cm-1 since both films depict roughly the same oxidation level. To find out the role of the co-solvent, absorption spectra were recorded with a UV-Vis-NIR spectrometer on both solutions of PEDOT:OTf and PEDOT:OTf-NMP before and after the addition of EDOT at different times (Figure S3 in the Supporting Information). The absorbance meas-
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Chemistry of Materials
urement saturates once the polymerization of EDOT leads to high PEDOT concentration. Saturation was reached after 100 min for PEDOT:OTf-NMP solution and after only 34 min for PEDOT:OTf. This is a clear evidence
of the rate slowdown of the polymerization after NMP addition,
Figure 4. Transport properties of the PEDOT materials. a) Temperature dependence of electrical conductivity (symbols) and heterogeneous model of conduction (solid lines), b) Low temperature dependence of electrical conductivity and c) Reduced activation energy (W) of PEDOT materials vs ln(T).
which is responsible for an enhanced structure in the polymer film, larger crystallite size and consequently better transport properties. Transport properties. Transport mechanisms in conducting polymer films remain highly impacted by the degree of disorder inherent to these materials.19,32 The degree of disorder and the transport properties in conducting polymers are generally assessed by the resistivity ratio (a lower ρr corresponds to a more or
dered film) and the temperature dependence of conductivity.21,33 PEDOT:PSS typically follows a variable range hopping (VRH) transport with overlapping of the carriers wave functions according to the grain sizes and the disorder.34 In the case of PEDOT:OTf and its treated counterparts, a VRH model is not applicable (see hereafter). Figure 4a,b shows the temperature dependence of conductiv-
ity of PEDOT:OTf, PEDOT:Sulf, PEDOT:OTf-NMP and PEDOT:Sulf-NMP. Compared to PEDOT:PSS with a highly disordered structure ( 20000), PEDOT:OTf and its treated counter-parts appear very ordered ( 1.72 , 1.49, 1.54 and 1.28 respectively) suggesting that the addition of a co-solvent induces higher order in the films by slowing down the polymerization rate. This is consistent with the appearance of higher order diffraction peaks in the GIWAXS diffractograms (Figure 3 and Figure S8 in the Supporting Information). This structural order improvement is further enhanced by sulfuric acid treatment. For all four samples, non-zero conductivity for T 0 K suggests the presence of charges at the Fermi level allowing conduction without thermal activation, and hence sweeps out the hypothesis of VRH conduction. The samples appear to be in the metallic side of the metal-insulator transition as further proved by the positive slope of the re-
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duced activation energy 32,35
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T < 5 K (Figure 4b). Such metallic behavior is not present in other samples at low temperature, although a slight inflection is noticeable
plotted in Figure
4c. Interestingly, PEDOT:Sulf depicts a metallic behavior with a positive temperature coefficient of resistivity for
Table 1. Detailed conduction contributions deduced from the fitting parameters. These values are given for room temperature. Materials
Metallicity [%]
Amorphous region [S cm-1] Quasi 1D-metallic [S cm-1]
Shenglike
Disordered metal
Sheng barrier T1 [K]
PEDOT:OTf
2.7
37224
707
319
76
PEDOT:OTf-NMP
4.0
88371
1846
895
46
PEDOT:Sulf
3.0
40473
439
1249
81
PEDOT:Sulf-NMP
5.5
81187
1362
3384
60
in PEDOT:Sulf-NMP around 3 K. All samples show a metallic sign for their conductivity temperature dependence (dσ/dT
