Stability of highly conductive poly-3,4-ethylene-dioxythiophene

REACTIVE & FUNCTIONAL POLYMERS Reactive & Functional Polymers 66 (2006) 479–483 www.elsevier.com/locate/react Stability of highly conductive poly-3,4-ethylene-dioxythiophene Bjørn Winther-Jensen *, Keld West The Danish Polymer Centre, Risø National Laboratory, POL-124, Frederiksborgvej 399, DK-4000 Roskilde, Denmark Received 13 January 2005; received in revised form 2 August 2005; accepted 10 August 2005 Available online 24 March 2006 Abstract In its doped state, the conjugated polymer poly-3,4-ethylene-dioxythiophene (PEDT) is an exceptionally stable organic electronic conductor that can withstand long time immersion in aqueous solutions and a wide range of pH without loosing conductivity. The conductivity is indirectly influenced by the pH of the surrounding medium because of the link between pH and oxidising power of oxygen, but these changes are largely reversible. Properly prepared, PEDT doped with tosylate is also stable under conditions where high current densities, exceeding 6000 A/cm2, are passed through the material over extended time periods. At current densities around 10000 A/cm2 an irreversible break down mechanism is initiated, resulting in a fast decrease in conductivity and colouring of PEDT to a bluish-black hue. A model for the break down mechanism is proposed.  2005 Elsevier B.V. All rights reserved. Keywords: Conducting polymers; Stability of PEDT; Polymer materials 1. Introduction Long-term stability and stability in ambient environments has always been an inherent problem for doped conjugated polymers, and this lack of stability is often the factor limiting practical use of these materials as electronic conductors. For instance, doped polyacetylene is so sensitive to oxygen and moisture that it can only be handled under carefully controlled inert conditions, and polyacetylene has found no practical use despite its very high conductivity. There are, however, a few conjugated polymers that are rather stable in the doped state, and * Corresponding author. Tel.: +45 4677 4707; fax: +45 4677 4791. E-mail address: [email protected] (B. WintherJensen). among these poly-3,4-ethylene-dioxythiophene (PEDT) is recognised for its good thermal stability and stability over time [1,2]. There are only few reports on the stability of PEDT in water and in media of varying pH. Yamamoto et al. [3] reported the influence of pH on the doping level of PEDTPSS films in the pH range from 1 to 7 and showed that pH changes are reflected in changes in the UV–Vis spectra. In the same work it was suggested that PEDT at low pH interacts with H+ leading to a new type of electronic state in the polymer. Previously, methods have been presented for obtaining very high conductivities in p-toluene sulfonate doped PEDT by controlled acidity during the polymerisation [4,5], and in the present paper the stability of these highly conducting PEDT films are examined under various conditions. Also the ability of the conductor to carry substantial 1381-5148/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.08.007 electronic currents examined. B. Winther-Jensen, K. West / Reactive & Functional Polymers 66 (2006) 479–483 for extended periods is 2. Experimental The 3,4-ethylene-dioxythiophene (EDT) monomer, Baytron M, and a 40% solution of ferric toluene sulfonate (Fe(III) tosylate) in butanol, Baytron C, was used as received from Bayer AG. p-Toluene sulfonic acid (pTSA), sodium p-toluene sulfonate (Na-pTS), pyridine and sodium hydroxide were used as obtained from Aldrich. PEDT coatings and composite coatings of PEDT and Loctite3311 on glass slides or PET foils were prepared as reported earlier [5]. For long-term storage tests of PEDT, MilliQ water with a pH of 6.8 was used. The conductivity of the samples stored in water and air was measured on dried samples. The wet samples were dried by blowing off liquid water with dry air followed by heating for 1 min at 60 C on a hot plate. Surface conductivities were measured with a four-point probe having four spring loaded tungsten carbide needles spaced 1 mm apart (Jandel Engineering Ltd.) connected to a Keithly 2400 Source Meter. The bulk conductivity of the films was calculated from the surface conductivity using the film thickness measured by a DekTak profilometer. Typical thicknesses of the PEDT films on glass and PET foil were 200– 250 nm. For tests with high current densities another four-point-probe was used consisting of four parallel gold electrodes, 10 mm long, 1 mm wide, and with 1 mm spacing between the lines. A constant DC current was applied through the outer electrodes and the voltage was measured between the inner electrodes. For storage tests of PEDT at pH values between 0 and 14, pTSA and NaOH was added to MiliQ water until the desired pH was reached. No effort was made to reach the same ionic strength in solutions of different pH values. UV–Vis spectroscopy was preformed using a Shimadzu UV1700 spectrometer, and polymerisation of EDT at the interface between to immiscible liquids was tried with a solution of 0.32 M EDT in trichloroethane (CH3CCl3) as the monomer containing organic phase and ammonium peroxydisulfate (APDS, 0.16 M) in a 1 M aqueous solution of pTSA acid as the oxidation agent. A visible blue colouring of the aqueous phase starts after approximately 30 min. During the next 24 h the colour of the aqueous phase changes over green to brownish red. No colouring of the organic phase occurs. 3. Results and discussion 3.1. Long-term stability of PEDT in air and aqueous solutions – the influence of pH on conductivity The conductivity of pure PEDT-pTS coatings on glass stored in water and in air, was followed over a one and a half year period – see Fig. 1. For the samples stored in water the conductivity rapidly decreased to a level that stayed almost constant for the remaining part of the test period. The conductivity of the samples stored in air showed a slower decrease. This gave rise to the question: Why is a stable conductivity level reached much faster when the sample is stored in water compared to the slower asymptotic decrease for samples kept in air? A clue to the answer is found in Fig. 2 showing the surface resistivity of PEDT measured on oneyear-old coatings on PET foils after storage for 20 days in solutions having different pH. The conductivity of PEDT apparently depends on the pH level, with the highest conductivities at low pH. The change is not dramatic except for pH value exceeding 11. The fact that PEDT is stable and conducting at pH 0 and below means it can be used in environments where e.g., copper would corrode. When PEDT is polymerized according to the base-inhibited scheme [4] the acidity in the product will be equivalent to pH 1. It is thus not surprising that the conductivity of the sample from the longterm stability test changes when exposed to water at pH  7, whereas the samples exposed to air needs longer time to reach an equilibrium with CO2 in the 1000 750 σ (S/cm) 480 500 250 0 1-Feb-03 1-Aug-03 1-Feb-04 1-Aug-04 Date Fig. 1. Long-term stability of PEDT on glass, in air (grey dashed line) and water (two separate experiments, black lines). Filled symbols are values after treatment in 1 M PTSa. 481 B. Winther-Jensen, K. West / Reactive & Functional Polymers 66 (2006) 479–483 400 10000 350 OHM/Sqr Ω/sqr 300 1000 250 200 150 100 50 0 Start 1. Base 1. Acid 2. Base 2. Acid 3. Base 3. Acid 100 0 2 4 6 8 10 12 14 pH Fig. 3. Reversibility of resistivity in PEDT film on PET-foil, when shifted between pH 1 and 13. Fig. 2. pH dependency of resistivity in PEDT film on PET-foil. 1.2 1 In [8], we describe a method for covalent bonding of PEDT to surfaces. This method was not used in these tests. 0.8 ABS surrounding air, leading to the slower decrease in conductivity. After a sample from the stability test in water was washed in 1 M pTSA (pH  1) for 3 h, the conductivity recovered to 94% of the original value (see Fig. 1). Samples stored in air showed the same behaviour after wash in 1 M pTSA (Fig. 1), but it was not possible to complete the test because the film easily peels off the glass substrate on cycling between the dry and the wet states due to inefficient bonding.1 Further investigation of this behaviour was made by alternating exposure of a fresh PEDT coating on PET to 1 M NaOH (pH 13) for 4 h and then to 1 M pTSA (pH 1) overnight. After three cycles the conductivity was virtually unchanged, see Fig. 3. This reversible behaviour strongly indicates that the backbone of PEDT is undamaged and that the change in conductivity is due to a change in the doping level. UV–Vis spectra of the samples stored at different pH (Fig. 4) showed different peaks corresponding to the three conductivity ‘‘levels’’ seen in Fig. 2: at pH 0–2 a peak above 1100 nm; at pH 4– 10 a peak in the range 850–950 nm; and at pH 12– 14 a peak in the range 540–590 nm. This is consistent with earlier studies. The absorption at 570 nm is attributed the p–p* transition characteristic of the neutral phase [3,6,7]. Often UV–Vis absorption around 900 nm is attributed to the bipolaron formation [3,6,7]. The absence of this band at low pH may be due to formation of a metallic state [1]. 0.4 0 300 pH 0 pH 5 pH 12 pH 14 500 700 900 1100 nm Fig. 4. UV–Vis spectra of PEDT on PET-foil in water at different pH levels. Control tests with a constant molarity of pTS (1 M) at pH  9 (1M Na-pTS) and pH  13 (1 M in Na-pTS and NaOH) were preformed to exclude the influence of the concentration of counter ion. The UV–Vis spectra of these samples were similar to the spectra from the first test, maybe with a tendency to be equivalent to samples with a slightly lower pH value. The same was seen with resistance measurements; also here the tendency was the same as seen in the first test, but again the resistance was equal to values approximately a half pH unit lower than expected from the first measurement series. The changes in absorption following a pH change are similar to the changes induced by oxidizing or reducing PEDT electrochemically [1,6,7]. In this context the spectrum of the sample equilibrated at pH 0 is equivalent to the spectrum of a sample kept potentiostatic at 0.4 V vs. AgjAgCl, pH 5 is equivalent to 0.2 V vs. AgjAgCl, pH 12 to 0.0 V vs. AgjAgCl and pH 14 to  0.2 V vs. AgjAgCl. B. Winther-Jensen, K. West / Reactive & Functional Polymers 66 (2006) 479–483 2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 300 500 850 Composites of PEDT and Loctite 3311 with a 30–70% PEDT loading have earlier showed unusual low activation energies for electronic conductivity – around 1/3 of neat PEDT – and at the same time very high conductivities are observed [5]. These promising properties make this material a candidate for ‘‘polymer wires’’, as long as this property could be retained also when large currents are passed through the composite. A 24 h test was made with a 5 lm thick and 25 mm · 80 mm wide composite film on glass. A constant current of 1 A was passed through the sample resulting in a voltage drop of approximately 0.3 V between the test electrodes. During the first hour of the test, the resistance decreased slightly from 3.08 to 2.83 X per square and remained at this value for the rest of the test. This decrease might be due to an increase in the sample temperature caused by the dissipation of 1 W in the PEDT film. Based on the sample geometry the conductivity of the composite is calculated to 714 S/cm. Taking only the fraction of the conducting polymer in the composite into consideration the conductivity of PEDT in the composite is 2400 S/cm. Similarly, the current density can be calculated to 2000 A/cm2 for the composite or 6500 A/cm2 for the PEDT fraction. It can be illustrative to relate the amount of charge passed through the sample in this test to the amount of charge carriers present. Assuming that the doping degree is 0.25 (one charge per four monomers) [9] and that the density is unity, the charge carrier density is 165 C/cm3. The average charge carrier velocity is then 39 cm/s, giving a total electron turnover 660 3.2. The ability of PEDT to withstand high current densities in the PEDT sample of 100 s 1 or 8.6 million times in the duration of the experiment. Other tests were carried out with higher current densities. When the current density in PEDT was exceeding 10,000 A/cm2 the resistance immediately raised and the film became darker blue between the electrodes. The UV–Vis spectrum of this dark blue area is shown in Fig. 5 compared with the original PEDT film and a partially reduced PEDT film (at pH 13). It is seen that a new absorption peak at 660 nm is dominating the spectrum of the film after the high current break down, a peak that is not seen in neither oxidized nor in reduced PEDT. The colouring of the ‘‘high current’’ area is not reversible over time or by exposing the sample to an acidic environment. The absorption peak at 660 nm has earlier been reported [4] for EDT polymerized by vapour phase polymerization and was then attributed the product from an undesired side-reaction where ether cleavage in the dioxy-ring occurs. In a supplementary study, where EDT polymerisation was attempted at an organic/water interphase, similar results were observed – see Fig. 6. During the first 4 h an absorption peak at 660 nm builds up in the water phase without any sign of peaks from PEDT at 550 or 1150 nm. Later typical poly-thiophene absorption peaks at 400 and 460 nm are forming together with a smaller absorption peak at 550 nm and at the same time the 660 nm peak decreases. According to our interpretation PEDT is not formed at all in this experiment. The structure that gives rise to the 660 nm peak is an oligo- or polythiophene (hereafter named ‘‘A-Th’’) similar to PEDT, but where the dixoy-ring 590 Measuring the open circuit voltage of the pH equilibrated samples relative to a Ag|AgCl reference electrode yielded similar results. From the measurements outlined above it is clear that the doping level of PEDT in aqueous electrolytes is dependent on the pH of the electrolyte. Because the electrolytes in these investigations are in equilibrium with the ambient atmosphere it is suggested that this effect is caused by the oxidising power of dissolved oxygen being dependent on pH. In more acidic electrolytes oxygen is a stronger oxidant. This causes the doping level of PEDT to increase. As the charge carrier concentration follows the doping level this also causes an increase in the conductivity. Abs 482 700 900 1100 nm Fig. 5. UV–Vis spectra of PEDT after high current (full black line) compared to the PEDT before high current (grey dashed line) and reduced PEDT (pH 13, full grey line). B. Winther-Jensen, K. West / Reactive & Functional Polymers 66 (2006) 479–483 3 A: 2 h B: 3 h C: 4 h D: 5 h E: 7 h F: 9 h G: 22 h G 2 ABS F E 1 D C 0 300 A B 400 500 600 700 800 900 1000 1100 nm Fig. 6. UV–Vis spectra of the water-phase during the inter-phase reaction of EDT and ammonium-peroxidisulfate (APDS) in 1 M PTSa. Start conditions: Solvent phase, 0.32 M EDT in trichloroethane (CH3CCl3). Aqueous phase, 0.16 M APDS in a 1 M PTSa. is partly detached from the thiophene-ring. The poly-thiophene like peaks at 400 and 460 nm stem from a structure that is the reaction product with A-Th as the starting material. We have previously [4] seen that material containing the A-Th structure cannot be converted to PEDT, so it is not surprising that the damage caused by high-current were irreversible. The process leading to the darker blue colour with an absorption peak at 660 nm is thus likely to be due to an attack of the oxy-ring possible due to over oxidation of PEDT. The explanation for this reaction must be found in the fact that the transport number of the counter ions has a finite value, although close to zero. In the high fields present at extreme current densities the drift of the counter ions exceeds the diffusion velocity and thus causes the formation of a gradient in counter ion concentration, and consequently a gradient in doping degree. This can lead to avalanche effects because deviation from the composition giving the highest conductivity will lead to a higher local field. Eventually the doping level may reach values leading to the breakdown of the polymer structure. A strategy for overcoming this problem would be to use a polymer counter ion or by immobilising the counter-ions by other means. 4. Conclusion In the present work, we have demonstrated that PEDT is an exceptionally stable conjugated poly- 483 mer that can withstand long time immersion in aqueous solutions and a wide range of pH without loosing its conductivity. This is of special interest after the development of processes leading to highly conductive PEDT [4], opening possibilities for the use of conjugated polymers for charge transport (e.g., EMC shielding, intelligent paper, organic electronics) rather than the indirect use of the electronic properties for charge injecting layers and band-gap engineering. The conductivity is indirectly influenced by the pH of the surrounding medium because of the link between pH and oxidising power of oxygen, but these changes are largely reversible. PEDT is also known to be stable against moderate oxidation and can withstand cycling between different doping levels without degradation or changes in properties [10]. Although PEDT is a hole conductor it forms ohmic contacts with metals and can carry large currents for prolonged periods of time without degradation. At very high current densities, however, an irreversible degradation of PEDT may occur. This degradation is characterised by a dark bluish black colouring of the polymer and loss of electronic conductivity. Acknowledgements Helpful suggestions from Niels B. Larsen, fruitful discussions with numerous colleagues and the help from Jun Chen (IPRI, UoW) in preparing an analysing the inter-phase polymerisation attempt of EDT is gratefully acknowledged. References [1] C. Carlberg, X. Chen, O. Inganäs, Solid State Ionics 85 (1996) 73–78. [2] L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 7 (2000) 481–494. [3] T. Yamamoto, T. Shimizu, E. Kurokawa, React. Funct. Polym. 43 (2000) 79–84. [4] B. Winther-Jensen, K. West, Macromolecules 37 (2004) 5438–5443. [5] B. Winther-Jensen, D.W. Breiby, K. West, ISCM2004, Synth. Met. 152 (2005) 1–4. [6] M. Lapkowski, A. Prón, Synth. Met. 110 (2000) 79–83. [7] S. Garreau, J.L. Duvail, G. Louarn, Synth. Met. 125 (2002) 325–329. [8] B. Winther-Jensen, K. Norrman, P. Kingshott, K. West, Plasma Process Polym. 2 (2005) 319–327. [9] L.A.A. Pettersson, F. Carlsson, O. Inganäs, H. Arwin, Thin Solid Films 313–314 (1998) 356–361. [10] S. Garreau, G. Louarn, J.P. Bruisson, S. Lefrant, Macromolecules 32 (1999) 6807–6812.