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ESR spectroelectrochemistry of poly(3,4-ethylenedioxythiophene) (PEDOT)
Electrochemistry Communications 5 (2003) 603–608 www.elsevier.com/locate/elecom
ESR spectroelectrochemistry of poly(3,4-ethylenedioxythiophene) (PEDOT) A. Zykwinska a, W. Domagala a, M. Lapkowski a
a,b,*
Department of Chemistry, Silesian University of Technology, ul. M. Strzody 9, Gliwice 44-101, Poland Institute of Coal Chemistry, Polish Academy of Sciences, ul. Sowinskiego 5, Gliwice 44-121, Poland
b
Received 29 April 2003; received in revised form 22 May 2003; accepted 22 May 2003 Published online: 21 June 2003
Abstract In situ ESR spectroelectrochemical studies of poly(3,4-ethylenedioxythiophene) (PEDOT) have been performed, in an attempt to take a closer look at species responsible for the conductivity of the polymer in the doped state. A series of ESR spectra at progressively changed potentials applied to the polymer film in the oxidation and subsequently, reduction half-cycles were recorded. The results reveal distinct ESR lines with a noteworthy concentration of spins in the reduced state of the polymer and marked changes in both the intensities and DBpp widths of the ESR signal across the studied potential range, indicating non-trivial changes in the character of charge carriers with changing potential. Also, interesting phenomena like the potential hysteresis of the spin concentration and of DBpp linewidths between the oxidation and reduction cycles of the polymer are observed. The presence of residual spins in the polymer in the reduced state may indicate that at least to some partial extent, PEDOT chains exist in the quinoid rather than benzoid configuration in the dedoped state. Hysteresis of spectroscopic parameters may imply that certain hindrance factors like slow anion expulsion speed accompany the dedoping process of the polymer. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Electrochemistry; PEDOT; ESR spectroscopy; Polarons; Bipolarons
1. Introduction The beginning of PEDOTÕs history dates back to the late 1980s when the monomer EDOT was first synthesised by research scientists at Bayer [1]. Since then, especially in the last few years, this polymer has been an issue of interest in numerous studies. The reason for it are its much sought for properties like high conductivity, interesting electrical and spectrochemical properties associated with its low band gap, as well as electrochromic and antistatic properties which have already found practical applications [2–4]. These properties stem primarily from the high regularity of the polymer backbone which is purely a–a0 coupled thanks to the presence of the cyclic 3,4-dioxy substituent. Similarly to the beforeknown poly(3,4-dimethoxythiophene) the two oxygen *
Corresponding author. Tel.: +48-32-237-17-43; fax: +48-32-2371722. E-mail address: [email protected] (M. Lapkowski).
atoms bonded directly to the thiophene backbone enrich the conjugated p-bond with electrons lowering the oxidation potential of the polymer. In addition the cyclic character of the ethylenedioxy substituent minimises the disorder in the relative arrangement of the polymer chains that linear side groups can introduce. Many aspects of PEDOTÕs chemistry and of its derivatives together with their hitherto applications have been given and discussed in a reference-rich and up-to-date review by Groenendaal et al. [5]. Some of the current interests of application of PEDOT include photovoltaic cells for the conversion of sunlight [6,7], as an electrode material in solid electrolyte capacitors [8] and fabrication of novel photodiodes [9] to name a few. Perspective applications in the biomedical field are also under investigation [10]. The electronic applications utilise the possibility of tuning the electronic properties of the polymer through control of its doping level. Depending upon the doping level, different charge carrying moieties can prevail, i.e.,
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00132-2
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polarons and bipolarons both of which can propagate the electrical current along the polymer chain [11–13]. Polarons carry a magnetic moment and can therefore be observed and distinguished from diamagnetic bipolarons by means of ESR spectroscopy. However bipolarons are not indifferent toward the chemical environment they exist in. Since at intermediate doping levels they coexist alongside polarons and interact with them [14], the spectroscopic response of polarons will be influenced by the presence and properties of bipolarons as well as other paramagnetic species that could be present in the polymer material. Recent studies looked at the influence of the doping level on such properties of PEDOT as conductivity and carrier mobility [15]. However to date no detailed ESR studies of the PEDOT polymer could be found in the literature. This is why we have decided to undertake research in this field as understanding the interactions of different charge carriers both mutual and with the polymer matrix is of profound importance since these determine the bulk properties of the material.
which came out to be still enough to avoid signal saturation. Alongside the ESR spectra, acquisition microwave resonance frequency was also being recorded. The dependence of relative concentrations of paramagnetic centres in the studied polymer upon the potential applied to the polymer film was determined. Concentration of paramagnetic centres in the sample is proportional to the area under the absorption curve, thus double integration of the first-derivative ESR spectra was performed.
3. Results and discussion The course of electropolymerisation of EDOT showing the continuous growth of the polymer film upon successive potential cycling employing a technique of cyclic voltammetry is presented in Fig. 1. In the first cycles the polymer displays an oxidation peak at 0.15 V. For a thin film this is the only oxidation peak observed. Later in the course of the film growth a pre-peak at ca. )0.4 V emerges. In the reduction half-cycle the first re-
2. Experimental Cyclic voltammetry (CV) experiments were performed on AUTOLAB potentiostat–galvanostat model PGSTAT20 (EcoChemie) driven by a computer. Platinum wire (£ ¼ 1 mm) was used as working electrode. After sealing the excess area using a Teflon tape, its geometric area was ca. 0.10 cm2 . Ag wire was used as a pseudoreference electrode. All potentials in this work are given directly versus this electrode. Platinum coil wound around the two electrodes served as an auxiliary electrode. Before experiments the whole cell was deaerated with argon. Poly(3,4-ethylenedioxythiophene) (PEDOT) was synthesised through electropolymerisation on platinum electrode form a 0.01 M solution of monomer (Bayer A.G.) in 0.1 M (Bu)4 NPF6 in CH3 CN by means of cyclic voltammetry with controlled maximum monomer oxidation current. ESR spectroelectrochemical measurements were done on ELPAN SE/X-2543 series, X-band (9.3 GHz) reflective type ESR spectrometer (RadioPAN, Poland) with magnetic modulation of 100 kHz, coupled together with ELPAN EP-21 type potentiostat driven by ELPAN EG-20 potential generator. The spectroelectrochemical cell was a thin cylindrical glass tube narrowed at the bottom (3 mm in diameter). The same set of electrodes was used as described above. The electrolyte was 0.1 M (Bu)4 NPF6 in CH3 CN the same as for polymerisation. PEDOT films were studied directly on the electrode they were synthesised on. The first-derivative ESR spectra were recorded with attenuation of microwave power of 3 dB (15 mW)
Fig. 1. Electropolymerisation of EDOT from 0.01 M solution in 0.1 M Bu4 NPF6 in CH3 CN at Pt electrode at 100 mV s1 – progressive CVs of growing PEDOT film: (a) initial, (b) intermediate and final stages. Total electropolymerisation time: 90 min. For clarity, only selected cycles are shown.
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duction peak shows up at )0.45 V but is quickly overshadowed by the second reduction peak at 0.15 V. In nearly whole potential range the film displays a high background current. After synthesis, cyclic voltammogram in monomer free electrolyte does not differ much from the one at the end of the film synthesis. The shapes of the recorded CVs are similar to the ones reported in one of the first studies of PEDOT [16] by Ingan€ as et. al. This may point to close similarities between the films obtained by us, and the ones obtained in the cited work using the galvanostatic method. The advantage of the CV method is the constant control of the synthesised polymer through evaluation of the evolving CV while the disadvantage is the slow speed of the polymerisation process. Next, in situ ESR spectra of PEDOT film at progressively changed potentials were recorded for both oxidation and reduction half-cycles (Fig. 2). Starting form the minimum reduction potential we observe a distinct narrow ESR line indicating that even at such low potentials, unpaired electrons are present in the
Fig. 2. ESR spectra of PEDOT as a function of the potential applied to the polymer film in the (a) oxidation )0.7 to 1.0 V and (b) reduction 1.0 to )0.7 V half-cycles.
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polymer. With increasing potential we observe a gradual increase of the signal amplitude, which after traversing a peak at 0.05 V drops suddenly and at the same time, broadens significantly. In the reduction half-cycle this ESR potential dependence is markedly different. There the low and broad signal holds up until ca. 0 V after which it increases but only to reach a twofold smaller maximum at )0.2 V. Comparing that with the oxidation half-cycle this maximum as well as the onset of reappearance of ESR signal intensity is shifted negative by about 250 mV. This shift is observed for other spectroscopic parameters as well as will be presented later on in the text. At the end of the reduction half-cycle the amplitude of the ESR line is similar to the one at the start of the experiment. In Fig. 3 selected ESR spectra in both oxidation and reduction half-cycles are plotted one over another in order to allow for a more quantitative comparison of spectra at different potentials to be made. It follows that the decrease in the signal intensity past or before its maximum (depending upon which half-cycle we discuss) is accompanied by broadening of the ESR signal. This fact reflects itself in the number of spins present in the studied sample.
Fig. 3. Selected ESR spectra of PEDOT at given potentials in the (a) oxidation, (b) reduction half-cycles.
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Double integration of the recorded spectra gives the relative concentrations of spins present in the polymer at different potentials (Fig. 4). In the oxidation half-cycle three distinct regions may be distinguished on the plot. In the first one between )0.7 and )0.2 V the concentration of spins increases slowly. Then, in the second region a sharp rise up to a maximum at 0.2 V takes place. In the third region at progressively higher potentials, the high concentration of spins keeps up decreasing only a little. Comparing this with the CV of the polymer it can be seen that the rapid increase in the number of spins coincides with the onset of the oxidation peak of the polymer. The decrease in the number of spins at high positive oxidation potentials may be an indication that polarons start to pair generating spinless bipolarons. In the reduction half-cycle the region of decrease of the number of spins past the maximum concentration can be further split into two due to an interesting feature – a concentration of spinsÕ hysteresis, being observed. The decrease of spins around )0.1 V slows down and continues later as if postponed by about 250 mV levelling again with the oxidation curve at about )0.5 V when the third region of low concentration of spins begins. Interestingly the two humps on the concentration curve can be reflected in the two reduction peaks on the CV of the polymer. This may imply that the reduction (dedoping) process of the polymer when
the diminution of polarons takes place, is a stepwise process. This may mean that the expulsion of counterbalancing anions is more difficult than their take-up during oxidation. This could be explained by considering the course of the reduction process, which commences at the electrode surface and propagates into the distant layers of the film. If the film is coherent enough then the anions that have to be expelled from the neutral reduced film may become trapped in it, retarding further reduction of the rest of the film. The non-zero concentration of spins in the reduced state of the polymer suggests that unpaired electrons are present in dedoped PEDOT. Considering the bond configuration of two possible electronic structures of the polymer chains – benzoid and quinoid, this observation may be explained by assuming that at least some of the PEDOT chains exist in a quinoid form with two unpaired electrons delocalised along the polymer chain (Fig. 5). In terms of molecular orbitals, this is equivalent to PEDOT having its LUMO orbital occupied with one electron promoted there across the band gap. The quinoid form of PEDOT chain has been already proposed to explain the Raman spectra of this polymer in its undoped state [17]. Another thing that came up during the calculations of the concentration of spins was the Dysonian character of the spectra at potentials when the slope of the concentration of spinsÕ dependence is
Fig. 4. Total concentration of paramagnetic centres in PEDOT film as a function of the potential during oxidation and reduction half-cycles. Above, a CV of PEDOT in 0.1 M Bu4 NPF6 in CH3 CN.
Fig. 5. Proposed quinoid structure of a PEDOT chain in the reduced state – a diradical with two unpaired electrons delocalised along the conjugated p-bond.
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Fig. 6. Changes of signal linewidths (DBpp ) of PEDOT with potential in the oxidation and reduction half-cycles.
steepest. Its presence may be an indication that at intermediate doping levels when polarons form or disappear, PEDOT film acquires certain metallic properties most probably associated with the type of conductivity it displays in this state [5]. Analysing the peak-to-peak widths (DBpp ) dependence with potential (Fig. 6) we observe a similar behaviour to that of the concentration of spins. In the oxidation cycle DBpp values stay constant up to ca. 0.05 V after which they increase sharply and markedly. A broad and small peak forms around 0.75 V after which a small decrease is observed. Changes in the DBpp of the ESR signal reflect changes in the number of spins in the polymer. With an increase in the concentration of polarons, dipolar interactions of their unpaired magnetic moments due to their increasing population increase, which causes the ESR lines to broaden. Emerging exchange interactions also cause broadening. Above 0.25 V the concentration of polarons starts to decrease. This has been explained in terms of a process of pairing of polarons, in which bipolarons are formed. An echo of this may be the narrowing of ESR lines that starts to take place above 0.75 V due to weakening dipolar interactions [18]. On reversing the potential run, up to 0.45 V the DBpp width varies little after which a small decrease is observed followed once again by a postponed decrease – the hysteresis feature appearing once again. Below )0.3 V the widths level up with the values recorded at the start of the experiment. These similarities with the concentration of spins dependence suggest that the mutual interactions between the charge carriers in the polymer strongly depend upon their concentration. Also the observed significant changes in the magnitude of the measured ESR lines may be a sign that they comprise more than one component. We are currently investigating this issue. Linewidths of ESR lines depend on the type of magnetic interactions between paramagnetic centres in
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the sample [19]. When the increasing population of spins shows statistical distribution of their magnetic fields, broadening of ESR lines takes place. The individual paramagnetic centre is located not only at the external magnetic field, but it is also located at local fields of the neighbouring magnetic dipoles. Such statistical distribution of magnetic moments is characteristic for less mobile unpaired electrons. Lines, which could be attributed to this type of spins, were observed in the doped state of PEDOT when polarons predominate. This would thus imply that weak interactions exist between these polarons which could also explain why their concentration in the doped state decreases so negligibly. Exchange or superexchange interactions of mobile unpaired electrons average statistical distribution of magnetic fields in the sample and the ESR lines become narrowed. Paramagnetic centres with averaged magnetic fields are responsible for the narrow lines. These lines were observed in the dedoped state of PEDOT. This group of paramagnetic centres may be formed by highly mobile spins. The high mobility of this type of spins is responsible for the small linewidths of the ESR lines. Such highly mobile spins could be associated with the postulated unpaired electrons present in the quinoid configuration of the polymer in the reduced state. Their mobility could be a result of the dynamic equilibrium between generation and recombination upon promotion of the electron to the LUMO orbital and its return to the ground HOMO state. The exchange interactions however would require these spins to cluster together rather than have the freedom to be randomly located throughout the polymer matrix.
4. Conclusions The results of ESR spectroelectrochemical studies of PEDOT provide some interesting facts about the paramagnetic species present in the polymer in the doped and what was surprising, also in the dedoped state. During electrochemical oxidation the concentration of paramagnetic species increases markedly indicative of formation of polarons in the process of oxidative doping. In the reduced state of the polymer however, a nonzero concentration of spins is preserved which may point to the presence of neutral spins. It is proposed, following earlier reports [17], that these spins are a result of some PEDOT chains taking up the quinoid configuration of the conjugated p-bond. Together with increase of the concentration of polarons the type of mutual interactions of paramagnetic species change. An increase in the DBpp of ESR lines taking place upon doping indicates a transition from highly mobile groups of spins to less mobile and weaker interacting ones. The course of the reduction process of the polymer is different compared to oxidation. During diminution of
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paramagnetic centres upon reduction a hysteresis of the concentration of spins is observed that can be correlated with the two reduction peaks seen on the CV of the polymer. This suggests that the reduction process is stepwise. Its stages may be related to the way of movement of counterbalancing anions out of the polymer film. A similar hysteresis feature is also observed in the potential dependence of peak-to-peak width of the ESR signal suggesting that the type of interactions between paramagnetic centres are closely related to their concentration in the polymer.
References [1] [2] [3] [4]
A.G. Bayer, Eur. Patent 339340 (1988). F. Jonas, L. Schrader, Synth. Met. 41–43 (1991) 831. G. Heywang, F. Jonas, Adv. Mater. 4 (1992) 116. I. Winter, C. Reece, J. Horms, G. Heywang, F. Jonas, Chem. Phys. 194 (1995) 207. [5] L. Bert Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2002) 481–494.
[6] T. Yohannes, O. Ingan€as, Sol. Energy Mater. Sol. Cells 51 (1998) 193. [7] F. Zhang, M. Johansson, M.R. Andersson, J.C. Hummelen, O. Ingan€as, Adv. Mater. 14 (2002) 662–665. [8] S. Ghosh, O. Ingan€as, Adv. Mater. 11 (1999) 1214. [9] A.C. Arias, M. Granstr€ om, K. Petritsch, R.H. Friend, Synth. Met. 102 (1999) 953. [10] H. Yamato, M. Ohwa, W. Wernet, J. Electroanal. Chem. 397 (1995) 163. [11] J. Chen, A.J. Heeger, F. Wudl, Solid State Commun. 58 (1986) 251. [12] J.A. Van Haare, E.E. Havinga, J.L.J. Van Dongen, R.A.J. Janssen, J. Cornil, J.L. Bredas, Chem. Eur. J. 4 (1998) 1509. [13] Y. Harima, T. Eguchi, K. Yamashita, K. Kojama, M. Shiotani, Synth. Met. 105 (1999) 121–128. [14] Y. Harima, Y. Kunugi, H. Tang, K. Yamashita, M. Shiotani, J. Ohshita, A. Kunai, Synth. Met. 113 (2000) 173–183. [15] G. Zotti, S. Zecchin, G. Schiavon, Chem. Mater. 12 (2000) 2996. [16] Q. Pei, G. Zuccarello, M. Ahlskog, O. Ingan€as, Polymer 35 (1994) 1347–1351. [17] M. Lapkowski, A. Pron, Synth. Met. 110 (2000) 79–83. [18] E. Houze, M. Nechtschein, A. Pron, Synth. Met. 84 (1997) 981. [19] J.E. Wertz, J.R. Bolton, Electron Spin Resonance. Elementary Theory and Practical Applications, Academic Press, London, New York, 1986.
ESR spectroelectrochemistry of poly(3,4-ethylenedioxythiophene) (PEDOT) A. Zykwinska a, W. Domagala a, M. Lapkowski a
a,b,*
Department of Chemistry, Silesian University of Technology, ul. M. Strzody 9, Gliwice 44-101, Poland Institute of Coal Chemistry, Polish Academy of Sciences, ul. Sowinskiego 5, Gliwice 44-121, Poland
b
Received 29 April 2003; received in revised form 22 May 2003; accepted 22 May 2003 Published online: 21 June 2003
Abstract In situ ESR spectroelectrochemical studies of poly(3,4-ethylenedioxythiophene) (PEDOT) have been performed, in an attempt to take a closer look at species responsible for the conductivity of the polymer in the doped state. A series of ESR spectra at progressively changed potentials applied to the polymer film in the oxidation and subsequently, reduction half-cycles were recorded. The results reveal distinct ESR lines with a noteworthy concentration of spins in the reduced state of the polymer and marked changes in both the intensities and DBpp widths of the ESR signal across the studied potential range, indicating non-trivial changes in the character of charge carriers with changing potential. Also, interesting phenomena like the potential hysteresis of the spin concentration and of DBpp linewidths between the oxidation and reduction cycles of the polymer are observed. The presence of residual spins in the polymer in the reduced state may indicate that at least to some partial extent, PEDOT chains exist in the quinoid rather than benzoid configuration in the dedoped state. Hysteresis of spectroscopic parameters may imply that certain hindrance factors like slow anion expulsion speed accompany the dedoping process of the polymer. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Electrochemistry; PEDOT; ESR spectroscopy; Polarons; Bipolarons
1. Introduction The beginning of PEDOTÕs history dates back to the late 1980s when the monomer EDOT was first synthesised by research scientists at Bayer [1]. Since then, especially in the last few years, this polymer has been an issue of interest in numerous studies. The reason for it are its much sought for properties like high conductivity, interesting electrical and spectrochemical properties associated with its low band gap, as well as electrochromic and antistatic properties which have already found practical applications [2–4]. These properties stem primarily from the high regularity of the polymer backbone which is purely a–a0 coupled thanks to the presence of the cyclic 3,4-dioxy substituent. Similarly to the beforeknown poly(3,4-dimethoxythiophene) the two oxygen *
Corresponding author. Tel.: +48-32-237-17-43; fax: +48-32-2371722. E-mail address: [email protected] (M. Lapkowski).
atoms bonded directly to the thiophene backbone enrich the conjugated p-bond with electrons lowering the oxidation potential of the polymer. In addition the cyclic character of the ethylenedioxy substituent minimises the disorder in the relative arrangement of the polymer chains that linear side groups can introduce. Many aspects of PEDOTÕs chemistry and of its derivatives together with their hitherto applications have been given and discussed in a reference-rich and up-to-date review by Groenendaal et al. [5]. Some of the current interests of application of PEDOT include photovoltaic cells for the conversion of sunlight [6,7], as an electrode material in solid electrolyte capacitors [8] and fabrication of novel photodiodes [9] to name a few. Perspective applications in the biomedical field are also under investigation [10]. The electronic applications utilise the possibility of tuning the electronic properties of the polymer through control of its doping level. Depending upon the doping level, different charge carrying moieties can prevail, i.e.,
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00132-2
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polarons and bipolarons both of which can propagate the electrical current along the polymer chain [11–13]. Polarons carry a magnetic moment and can therefore be observed and distinguished from diamagnetic bipolarons by means of ESR spectroscopy. However bipolarons are not indifferent toward the chemical environment they exist in. Since at intermediate doping levels they coexist alongside polarons and interact with them [14], the spectroscopic response of polarons will be influenced by the presence and properties of bipolarons as well as other paramagnetic species that could be present in the polymer material. Recent studies looked at the influence of the doping level on such properties of PEDOT as conductivity and carrier mobility [15]. However to date no detailed ESR studies of the PEDOT polymer could be found in the literature. This is why we have decided to undertake research in this field as understanding the interactions of different charge carriers both mutual and with the polymer matrix is of profound importance since these determine the bulk properties of the material.
which came out to be still enough to avoid signal saturation. Alongside the ESR spectra, acquisition microwave resonance frequency was also being recorded. The dependence of relative concentrations of paramagnetic centres in the studied polymer upon the potential applied to the polymer film was determined. Concentration of paramagnetic centres in the sample is proportional to the area under the absorption curve, thus double integration of the first-derivative ESR spectra was performed.
3. Results and discussion The course of electropolymerisation of EDOT showing the continuous growth of the polymer film upon successive potential cycling employing a technique of cyclic voltammetry is presented in Fig. 1. In the first cycles the polymer displays an oxidation peak at 0.15 V. For a thin film this is the only oxidation peak observed. Later in the course of the film growth a pre-peak at ca. )0.4 V emerges. In the reduction half-cycle the first re-
2. Experimental Cyclic voltammetry (CV) experiments were performed on AUTOLAB potentiostat–galvanostat model PGSTAT20 (EcoChemie) driven by a computer. Platinum wire (£ ¼ 1 mm) was used as working electrode. After sealing the excess area using a Teflon tape, its geometric area was ca. 0.10 cm2 . Ag wire was used as a pseudoreference electrode. All potentials in this work are given directly versus this electrode. Platinum coil wound around the two electrodes served as an auxiliary electrode. Before experiments the whole cell was deaerated with argon. Poly(3,4-ethylenedioxythiophene) (PEDOT) was synthesised through electropolymerisation on platinum electrode form a 0.01 M solution of monomer (Bayer A.G.) in 0.1 M (Bu)4 NPF6 in CH3 CN by means of cyclic voltammetry with controlled maximum monomer oxidation current. ESR spectroelectrochemical measurements were done on ELPAN SE/X-2543 series, X-band (9.3 GHz) reflective type ESR spectrometer (RadioPAN, Poland) with magnetic modulation of 100 kHz, coupled together with ELPAN EP-21 type potentiostat driven by ELPAN EG-20 potential generator. The spectroelectrochemical cell was a thin cylindrical glass tube narrowed at the bottom (3 mm in diameter). The same set of electrodes was used as described above. The electrolyte was 0.1 M (Bu)4 NPF6 in CH3 CN the same as for polymerisation. PEDOT films were studied directly on the electrode they were synthesised on. The first-derivative ESR spectra were recorded with attenuation of microwave power of 3 dB (15 mW)
Fig. 1. Electropolymerisation of EDOT from 0.01 M solution in 0.1 M Bu4 NPF6 in CH3 CN at Pt electrode at 100 mV s1 – progressive CVs of growing PEDOT film: (a) initial, (b) intermediate and final stages. Total electropolymerisation time: 90 min. For clarity, only selected cycles are shown.
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duction peak shows up at )0.45 V but is quickly overshadowed by the second reduction peak at 0.15 V. In nearly whole potential range the film displays a high background current. After synthesis, cyclic voltammogram in monomer free electrolyte does not differ much from the one at the end of the film synthesis. The shapes of the recorded CVs are similar to the ones reported in one of the first studies of PEDOT [16] by Ingan€ as et. al. This may point to close similarities between the films obtained by us, and the ones obtained in the cited work using the galvanostatic method. The advantage of the CV method is the constant control of the synthesised polymer through evaluation of the evolving CV while the disadvantage is the slow speed of the polymerisation process. Next, in situ ESR spectra of PEDOT film at progressively changed potentials were recorded for both oxidation and reduction half-cycles (Fig. 2). Starting form the minimum reduction potential we observe a distinct narrow ESR line indicating that even at such low potentials, unpaired electrons are present in the
Fig. 2. ESR spectra of PEDOT as a function of the potential applied to the polymer film in the (a) oxidation )0.7 to 1.0 V and (b) reduction 1.0 to )0.7 V half-cycles.
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polymer. With increasing potential we observe a gradual increase of the signal amplitude, which after traversing a peak at 0.05 V drops suddenly and at the same time, broadens significantly. In the reduction half-cycle this ESR potential dependence is markedly different. There the low and broad signal holds up until ca. 0 V after which it increases but only to reach a twofold smaller maximum at )0.2 V. Comparing that with the oxidation half-cycle this maximum as well as the onset of reappearance of ESR signal intensity is shifted negative by about 250 mV. This shift is observed for other spectroscopic parameters as well as will be presented later on in the text. At the end of the reduction half-cycle the amplitude of the ESR line is similar to the one at the start of the experiment. In Fig. 3 selected ESR spectra in both oxidation and reduction half-cycles are plotted one over another in order to allow for a more quantitative comparison of spectra at different potentials to be made. It follows that the decrease in the signal intensity past or before its maximum (depending upon which half-cycle we discuss) is accompanied by broadening of the ESR signal. This fact reflects itself in the number of spins present in the studied sample.
Fig. 3. Selected ESR spectra of PEDOT at given potentials in the (a) oxidation, (b) reduction half-cycles.
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Double integration of the recorded spectra gives the relative concentrations of spins present in the polymer at different potentials (Fig. 4). In the oxidation half-cycle three distinct regions may be distinguished on the plot. In the first one between )0.7 and )0.2 V the concentration of spins increases slowly. Then, in the second region a sharp rise up to a maximum at 0.2 V takes place. In the third region at progressively higher potentials, the high concentration of spins keeps up decreasing only a little. Comparing this with the CV of the polymer it can be seen that the rapid increase in the number of spins coincides with the onset of the oxidation peak of the polymer. The decrease in the number of spins at high positive oxidation potentials may be an indication that polarons start to pair generating spinless bipolarons. In the reduction half-cycle the region of decrease of the number of spins past the maximum concentration can be further split into two due to an interesting feature – a concentration of spinsÕ hysteresis, being observed. The decrease of spins around )0.1 V slows down and continues later as if postponed by about 250 mV levelling again with the oxidation curve at about )0.5 V when the third region of low concentration of spins begins. Interestingly the two humps on the concentration curve can be reflected in the two reduction peaks on the CV of the polymer. This may imply that the reduction (dedoping) process of the polymer when
the diminution of polarons takes place, is a stepwise process. This may mean that the expulsion of counterbalancing anions is more difficult than their take-up during oxidation. This could be explained by considering the course of the reduction process, which commences at the electrode surface and propagates into the distant layers of the film. If the film is coherent enough then the anions that have to be expelled from the neutral reduced film may become trapped in it, retarding further reduction of the rest of the film. The non-zero concentration of spins in the reduced state of the polymer suggests that unpaired electrons are present in dedoped PEDOT. Considering the bond configuration of two possible electronic structures of the polymer chains – benzoid and quinoid, this observation may be explained by assuming that at least some of the PEDOT chains exist in a quinoid form with two unpaired electrons delocalised along the polymer chain (Fig. 5). In terms of molecular orbitals, this is equivalent to PEDOT having its LUMO orbital occupied with one electron promoted there across the band gap. The quinoid form of PEDOT chain has been already proposed to explain the Raman spectra of this polymer in its undoped state [17]. Another thing that came up during the calculations of the concentration of spins was the Dysonian character of the spectra at potentials when the slope of the concentration of spinsÕ dependence is
Fig. 4. Total concentration of paramagnetic centres in PEDOT film as a function of the potential during oxidation and reduction half-cycles. Above, a CV of PEDOT in 0.1 M Bu4 NPF6 in CH3 CN.
Fig. 5. Proposed quinoid structure of a PEDOT chain in the reduced state – a diradical with two unpaired electrons delocalised along the conjugated p-bond.
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Fig. 6. Changes of signal linewidths (DBpp ) of PEDOT with potential in the oxidation and reduction half-cycles.
steepest. Its presence may be an indication that at intermediate doping levels when polarons form or disappear, PEDOT film acquires certain metallic properties most probably associated with the type of conductivity it displays in this state [5]. Analysing the peak-to-peak widths (DBpp ) dependence with potential (Fig. 6) we observe a similar behaviour to that of the concentration of spins. In the oxidation cycle DBpp values stay constant up to ca. 0.05 V after which they increase sharply and markedly. A broad and small peak forms around 0.75 V after which a small decrease is observed. Changes in the DBpp of the ESR signal reflect changes in the number of spins in the polymer. With an increase in the concentration of polarons, dipolar interactions of their unpaired magnetic moments due to their increasing population increase, which causes the ESR lines to broaden. Emerging exchange interactions also cause broadening. Above 0.25 V the concentration of polarons starts to decrease. This has been explained in terms of a process of pairing of polarons, in which bipolarons are formed. An echo of this may be the narrowing of ESR lines that starts to take place above 0.75 V due to weakening dipolar interactions [18]. On reversing the potential run, up to 0.45 V the DBpp width varies little after which a small decrease is observed followed once again by a postponed decrease – the hysteresis feature appearing once again. Below )0.3 V the widths level up with the values recorded at the start of the experiment. These similarities with the concentration of spins dependence suggest that the mutual interactions between the charge carriers in the polymer strongly depend upon their concentration. Also the observed significant changes in the magnitude of the measured ESR lines may be a sign that they comprise more than one component. We are currently investigating this issue. Linewidths of ESR lines depend on the type of magnetic interactions between paramagnetic centres in
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the sample [19]. When the increasing population of spins shows statistical distribution of their magnetic fields, broadening of ESR lines takes place. The individual paramagnetic centre is located not only at the external magnetic field, but it is also located at local fields of the neighbouring magnetic dipoles. Such statistical distribution of magnetic moments is characteristic for less mobile unpaired electrons. Lines, which could be attributed to this type of spins, were observed in the doped state of PEDOT when polarons predominate. This would thus imply that weak interactions exist between these polarons which could also explain why their concentration in the doped state decreases so negligibly. Exchange or superexchange interactions of mobile unpaired electrons average statistical distribution of magnetic fields in the sample and the ESR lines become narrowed. Paramagnetic centres with averaged magnetic fields are responsible for the narrow lines. These lines were observed in the dedoped state of PEDOT. This group of paramagnetic centres may be formed by highly mobile spins. The high mobility of this type of spins is responsible for the small linewidths of the ESR lines. Such highly mobile spins could be associated with the postulated unpaired electrons present in the quinoid configuration of the polymer in the reduced state. Their mobility could be a result of the dynamic equilibrium between generation and recombination upon promotion of the electron to the LUMO orbital and its return to the ground HOMO state. The exchange interactions however would require these spins to cluster together rather than have the freedom to be randomly located throughout the polymer matrix.
4. Conclusions The results of ESR spectroelectrochemical studies of PEDOT provide some interesting facts about the paramagnetic species present in the polymer in the doped and what was surprising, also in the dedoped state. During electrochemical oxidation the concentration of paramagnetic species increases markedly indicative of formation of polarons in the process of oxidative doping. In the reduced state of the polymer however, a nonzero concentration of spins is preserved which may point to the presence of neutral spins. It is proposed, following earlier reports [17], that these spins are a result of some PEDOT chains taking up the quinoid configuration of the conjugated p-bond. Together with increase of the concentration of polarons the type of mutual interactions of paramagnetic species change. An increase in the DBpp of ESR lines taking place upon doping indicates a transition from highly mobile groups of spins to less mobile and weaker interacting ones. The course of the reduction process of the polymer is different compared to oxidation. During diminution of
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paramagnetic centres upon reduction a hysteresis of the concentration of spins is observed that can be correlated with the two reduction peaks seen on the CV of the polymer. This suggests that the reduction process is stepwise. Its stages may be related to the way of movement of counterbalancing anions out of the polymer film. A similar hysteresis feature is also observed in the potential dependence of peak-to-peak width of the ESR signal suggesting that the type of interactions between paramagnetic centres are closely related to their concentration in the polymer.
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