J. Phys. Chem. B 1997, 101, 11037-11039
11037
Thermal and Electromagnetic Behavior of Doped Poly(3,4-ethylenedioxythiophene) Films R. Kiebooms, A. Aleshin, K. Hutchison, and F. Wudl* Institute for Polymers and Organic Solids, UniVersity of California at Santa Barbara, Santa Barbara, California 93106-5090 ReceiVed: June 19, 1997; In Final Form: October 8, 1997X
Structure and electrophysical properties of poly(3,4-ethylenedioxythiophene) (PEDOT) films electrochemically doped with PF6-, BF4-, and CF3SO3- have been studied. According to SEM and XRD studies, doped PEDOT films have amorphous fibrillar structure. Thermal studies show that continuous degradation occurs above 150 °C and complete decomposition above 390 °C. Temperature dependence of the dc conductivity of best doped PEDOT samples was very weak and characteristic resistivity ratio Fr ) F(1.4 K)/F(291 K) was less than 2. For these most conducting PEDOT samples the temperature coefficient of resistivity (TCR ) ∆F/ F∆Τ) changes sign below 10 K from negative to positive which is characteristic for normal metals. SQUID data are in good agreement with the conductivity results.
1. Introduction Despite considerable innovative developments in the field of conducting polymers, only very few low band gap polymers (Eg < 1.5 eV) with high conductivity are known. Research in this domain and therefore application of these materials are apparently not being pursued vigorously mainly due to the challenges posed by synthesis and stability of the final polymer. The first polymer of this class of materials is poly(isothianaphthene) (PITN)1,2 Although important progress has been made in the synthesis of PITN,3 processibility and stability of the doped polymer remain the major bottlenecks. If the absorption maximum of a low band gap polymer is in the middle of the visible range, as for example PITN, one can expect the absorption maximum to shift to the near infrared after doping. As a consequence, the material becomes transparent in the visible region. This property is a basic requirement if the material is to be incorporated as a semitransparent electrode in light-emitting devices in order to obtain an all-polymer device. Another polymer on which interesting properties have been reported is poly(3,4-ethylenedioxythiophene) (PEDOT).4-6 Although possessing a band gap of 1.5-1.6 eV, this polymer cannot be considered as a real low band gap material. Nevertheless, the contrast ratio between the doped and undoped forms is comparable to commonly used materials such as polyaniline and polypyrrole. The major advantage over PITN is that it not only has improved chemical stability but also the conducting properties appear to remain almost unaltered under aging in environmental conditions.4 Furthermore, due to the nature of the monomer, competing polymerizations through the 3- and 4-positions as in polythiophene are avoided. Therefore, since only the 2,5-couplings of 3,4-ethylenedioxythiophene (EDOT) are expected, polymerization should yield a polymer with fewer defects and thus better properties compared to its thiophene analogs. PEDOT must be thoroughly characterized and evaluated as a candidate for use in electronic devices. We therefore investigated the physicochemical properties of freestanding polymer films doped with PF6-, BF4-, and CF3SO3-. 2. Experimental Section PEDOT-PF6 films were prepared by anodic oxidation of 3,4ethylenedioxythiophene (EDOT). An electrochemical cell X
Abstract published in AdVance ACS Abstracts, November 15, 1997.
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containing a solution of 0.06 M of EDOT, and 0.06 M of tetrabutylammonium hexafluorophosphate in propylene carbonate (PC), was thoroughly purged with dry argon prior to use. PC and EDOT were freshly distilled prior to use. Tetrabutylammonium hexafluorophosphate was crystallized from ethanol and dried under vacuum at boiling toluene temperature for three days. A glassy carbon electrode and platinum foil were used for the working and counter electrodes, respectively. A constant current was applied (0.01-0.06 mA/cm2) and the polymerization temperature maintained at -30 °C. Black lustrous films with a thickness from 10 to 300 µm were peeled off the electrode, washed twice in acetonitrile, and dried in Vacuo for 72 h at room temperature. The same procedure and the same concentrations were used to produce PEDOT-BF4 and PEDOT-CF3SO3 films of comparable quality. Generally the synthesis of the free-standing films was carried out in a classical setup as used on a regular basis to produce other conducting polymers such as polypyrrole.7 Due to the low oxidation potential of 3,4-ethylenedioxythiophene, much lower current densities were required as compared to polypyrrole. The DSC and TGA measurements were carried out with a Perkin-Elmer DSC 7 and TAC 7/DX. For TGA and DSC, the temperature was increased at a rate of 10 °C/min. The TGA measurements were performed under nitrogen gas flow. The four terminal technique was used for the conductivity measurements. Electrical contacts were made in planar geometry with conducting graphite adhesive. A computer-controlled measuring system, including a helium cryostat with a superconducting magnet (8 T), was used to obtain the dc conductivity data. To avoid any sample heating, the power dissipated in the samples at low temperatures was less than 1 µW. The temperature was measured with a calibrated Cernox resistor (T ) 300-1.3 K). All susceptibility measurements were done using a Quantum Design SQUID magnetometer. The polymer sample was placed inside a gelcap and packed tight with cotton, which was placed in the center of a straw and suspended from the end of the SQUID magnetometer sample holder. All samples were fieldcooled and measurements were taken from 1.7 to 300 K at 1000 G. The background diamagnetic susceptibility of the cotton, gelcap, and straw were measured and subtracted from the polymer susceptibility data. © 1997 American Chemical Society
11038 J. Phys. Chem. B, Vol. 101, No. 51, 1997
Figure 1. TGA data of BF4-, CF3SO3-, and PF6--doped PEDOT showing the percentage of weight loss when the temperature is increased at a rate of 10 °C/min under nitrogen gas flow.
Kiebooms et al.
Figure 4. The results of the susceptibility measurements calculated using a value of -8.8 × 10-5 emu/mol EDOT for Pascal’s corrections10 for diamagnetism.
TABLE 1: Summary of Results of SQUID Magnetometry Measurements on PEDOT dopant
mass Nc N(EF) χp (mg) (spins/mol) (states/eV EDOT) (emu/mol EDOT)
6.2 BF4CF3SO3- 3.7 PF612.0
Figure 2. DSC data of BF4-, CF3SO3-, and PF6--doped PEDOT showing the amount of heat flow (mW) when the temperature is increased at a rate of 10 °C/min.
Figure 3. Temperature dependence of the dc conductivity σ(T) for metallic PEDOT samples doped with BF4-, CF3SO3-, and PF6-.
3. Results and Discussion TGA shows that doped PEDOT is stable up to the temperature of 150 °C. (Figure 1) From 150 °C on, a continuous degradation occurs until major decomposition occurs in the region between 390 and 450 °C. DSC data of the same batches only reveals possible loss of remaining traces of solvent and dopant around 150 °C (Figure 2). A maximum exotherm can be observed at 300-350 °C, coinciding with the beginning of decomposition. Figure 3 shows the temperature dependence of the dc conductivity σ(T) for typical metallic PEDOT samples doped with PF6-, BF4-, and CF3SO3-. It can be seen that the σ(T) dependences of PEDOT films are very weak for all dopants. The characteristic resistivity ratio Fr ) F(1.4 K)/F(291 K) was less than 2 for the best samples. It was found that the conductivity of all doped metallic PEDOT samples decreases
1 × 1021 6 × 1020 1 × 1021
10 9.2 3.5
3.2 × 10-4 3.0 × 10-4 1.1 × 10-4
monotonically when the temperature is decreased from 300 to 10 K. However, below 10 K the temperature coefficient of resistivity (TCR) of most metallic PEDOT samples with Fr < 2.1 changed sign from negative (as in semiconductors and dirty metals) to positive (as in normal metals), whereas for samples with Fr > 2.1 the sign of TCR remains negative in the entire temperature region. The increase in conductivity below the transition temperature was about 2-3% of the room temperature conductivity of doped PEDOT film. The observed lowtemperature conductivity anomaly in PEDOT films in the metallic regime can, to a certain extent, be compared to those observed in PF6--doped polypyrrole7 and poly(3-methylthiophene) films.8 More details on low-temperature transport in metallic PEDOT-PF6 films are reported elsewhere.9 We are the first to report on the magnetic susceptibility of PEDOT. The various PEDOT polymer molecular weights and doping levels were unknown, and thus susceptibility is reported in terms of emu/mol EDOT (polymer repeat unit). The results of the susceptibility measurements calculated using a value of -8.8 × 10-5 emu/mol EDOT for Pascal’s corrections10 for diamagnetism are shown in Figure 4. The χ(T) of amorphous conducting polymer films is the sum of the Curie (χc) and Pauli (χp) susceptibilities.11 Figure 4 shows all three polymer samples exhibited Curie-like susceptibility at T < 75 K and Pauli-like susceptibility at T > 75 K. Thus the SQUID data were fit12 using the following equation:
χ(T) ) χc + χp ) (NcµB2/kBT) + N(EF)µB2 where Nc is concentration of localized spins/mol, µB is the Bohr magneton, kB is the Boltzmann constant, and N(EF) is the density of states at Fermi level. The results of the fit are listed in Table 1. The Curie susceptibility indicates the presence of localized unpaired spins in the polymer films,11 i.e., impurities or defects in the solid state. The Pauli susceptibility at T > 75 K demonstrates the existence of delocalized charge carriers and is further evidence the PEDOT polymers are metallic.11 The PEDOT-PF6 which exhibited the highest conductivity had the lowest susceptibility; likewise PEDOT-BF4 which had the lowest conductivity had the highest susceptibilty. Hence the susceptibility and the conductivity data are in qualitative agreement.
Doped Poly(3,4-ethylenedioxythiophene) Films
J. Phys. Chem. B, Vol. 101, No. 51, 1997 11039
Figure 5. Transversal SEM picture of a PF6--doped PEDOT film.
The morphology of the film surface was studied by means of SEM (Figure 5) Before synthesis the glassy carbon electrode was polished to 0.1 mm. This polymer side facing the glassy carbon electrode was used in the conductivity measurements to fix the contacts. The SEM pictures reveal for all dopants a very smooth surface with some remaining linear traces originating from polishing. A transversal picture shows that the PF6doped film has a very homogeneous composition. At higher magnification fibril-like structures can be observed. Transversal pictures of the BF4-- and CF3SO3--doped samples reveal a less thick homogeneous film with a smooth surface and the typical granular texture of the solution side become already visible. XRD study of both sides confirms a totally amorphous structure. 4. Conclusion For the first time, free-standing PEDOT films were obtained by anodic oxidation of 3,4-ethylenedioxythiophene. The films were doped with PF6-, BF4-, and CF3SO3- using the appropriate electrolyte. Thermal studies show that a continuous degradation occurs above 150 °C and complete decomposition above 390 °C. Dc conductivity measurements show that the best samples have a resistivity ratio rr < 2. For these samples the temperature coefficient of resistivity changes sign below 10 K from negative to positive which is characteristic for normal metals. The increase in conductivity below the transition temperature was about 2-3%. SQUID data are in good agreement with the conductivity studies. SEM revealed that side of the film facing the glassy carbon electrode has a smooth morphology. The PF6doped film showed a more homogenous film with fibrillar structures. In all cases the side facing the solution showed a granular aspect. XRD indicated that the polymer is amorphous structure.
Acknowledgment. This research was supported by the MRL program of the National Science Foundation through Grant Nr. NSF-DMR 96-32716. We thank Mr. J. Morrison of the Bayer Corp. for providing a generous sample of 3,4-ethylenedioxythiophene, and Prof. A. J. Heeger and Sean Williams for their continued interest and involvement in the PEDOT project. R.K. is indebted to the National Science Foundation of Belgium (NFWO) and to NATO for a post-doctoral fellowship. References and Notes (1) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382. (2) Kobayashi, M.; Colaneri, N.; Boysel, M.; Wudl, F.; Heeger, A. J. J. Chem. Phys. 1985, 82(12), 5717. (3) Vanasselt, R.; Hoogmartens, I.; Vanderzande, D.; Gelan, J.; Froehling, P. E.; Aussems, M.; Aagaard, O.; Schellekens, R. Synth. Met. 1995, 74, 65. (4) Heywang, G.; Jonas, F. AdV. Mater. 1992, 4(2), 116. (5) Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem. 1994, 369, 87. (6) Pei, Q. B.; Zuccarello, G.; Ahlskog, M.; Inganas, O. Polymer 1994, 35, 1347. (7) Yoon, C. O.; Menon, R.; Moses, D.; Heeger, A. J. Phys. ReV. B 1994, 49(16), 10851. (8) Masubuchi, S.; Kazama, S. Synth. Met. 1995, 69, 315. (9) Aleshin A. N.; Kiebooms, R.; Reghu Menon; Wudl, F.; Heeger, A. J. Phys. ReV. B 1997, 56, 3659. (10) Ko¨nig, E. In Landolt-Bornstein New Series; Hellwege, K. H., Ed.; Springer: Berlin, 1966; Vol. II. (11) Menon, R. In ConductiVe Organic Molecules and Polymers; Nalwa, H. S., Ed.; Wiley: New York, 1996. (12) Nogami, Y.; Kaneko, H.; Ishiguro, T.; Takahashi, A.; Tsukamoto, J.; Hosoito, N. Solid State Commun. 1990, 76, 585.
