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Impedance spectroscopy study of conducting polymer blends of PEDOT:PSS and PVA
Synthetic Metals 206 (2015) 106–114
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Impedance spectroscopy study of conducting polymer blends of PEDOT:PSS and PVA Chang-hsiu Chen a,b, * , Allen Kine b , Richard D. Nelson c , John C. LaRue b a b c
Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697, United States Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92697, United States Department of Electrical Engineering and Computer Science, University of California, Irvine, CA 92697, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 January 2015 Received in revised form 30 April 2015 Accepted 7 May 2015 Available online xxx
Blends of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) with polyvinyl alcohol (PVA) have many potential applications including microelectromechanical systems (MEMS) and other electronic devices. Specifically, PEDOT:PSS acts as the electrical conductor, while the PVA provides ductility and other mechanical enhancements. Concentrations of PEDOT:PSS at 20, 30 and 40 weight percentage (wt%) in PEDOT:PSS/PVA mixtures have been found to provide optimal mechanical and electrical properties. PEDOT:PSS/PVA films are prepared using a casting method and electrochemical impedance spectroscopy is used to investigate the electrical properties. The extrapolated DC conductivities are obtained in a dry nitrogen environment at room temperature and determined to be 3.917 108, 2.383 107 and 8.369 106 S/cm, respectively, for 20, 30 and 40 wt%. In addition, as a means to better explore the conduction as a function of temperature, impedance spectroscopy measurements for the 30 wt% PEDOT:PSS are determined over a range of temperatures from 24.5 C to 79.5 C. The corresponding extrapolated DC conductivities are found to increase by a factor of about four with increase in temperature from 4.617 108 S/cm at 24.5 C to 2.083 107 S/cm at 79.5 C. Published by Elsevier B.V.
Keywords: PEDOT PEDT PVA Polyvinyl alcohol Conducting polymers Impedance spectroscopy Permittivity Dielectric modulus
1. Introduction Impedance spectroscopy, described in detail in Refs. [1–3], is used to determine the frequency dependent electrical properties of a material. For example, impedance spectroscopy can be used to determine conduction as a function of material composition, to investigate charge injection and transport, and to determine the role of an interfacial layer and the effects of trapping states in semiconductor devices containing a conducting polymer as the active layer [4–6]. The dielectric properties of blended polymer play an important role in device applications such as high performance capacitors, electrical cable insulation, electronic packaging and components [7]. From a fundamental point of view, electrical impedance spectroscopy has been widely used to analyze the microscopic and dynamical relaxation process in complicated systems. Relaxation, in general, is a delay in a response to a changing
* Corresponding author at: Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697, United States. Tel.: +1 949 824 2431. Email: [email protected]. E-mail address: [email protected] (C.-h. Chen). http://dx.doi.org/10.1016/j.synthmet.2015.05.003 0379-6779/ Published by Elsevier B.V.
stimulus in a linear system. The physical origin of the time lag between the applied electric field and the polarization is the irreversible loss of free energy to heat. Also, a larger lag implies a slower process with increased loss [7]. In dielectric spectroscopy, large frequency dependent contributions to the dielectric response, especially at low frequencies, may come from the buildup of free charges. The distribution of the dielectric regions/particles within the body of a dielectric creates localized dipoles with magnitude dependent on their relative permittivity. There are two possible ways in which space charge polarization [8] occurs. For example, in a dielectric material, localized charges (ions and vacancies, or electrons and holes) can hop from one site to neighboring sites, creating so-called hopping polarization [8]. These charges are capable of moving freely from one site to another site for a short time, and then become trapped in localized states. Occasionally, these charges make a jump surmounting a potential barrier to other sites. The movement of ions or vacancies in ionic crystals and the movement of electrons and holes in glasses and amorphous semiconductors are essentially due to the hopping process. Depending on the width and the height of the potential barrier, a charged particle on one site may hop or tunnel to the other site. The other space charge polarization is called interfacial polarization or Maxwell–Wagner–Sillars’ polarization [9–11]. It is produced by the separation of mobile
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positively and negatively charged particles under an applied field, and forms positive and negative space charges in the bulk of the material, or at the interfaces between materials with different permittivity. These space charges, in turn, modify the field distribution. In addition, the charges are often separated over a considerable distance relative to atomic and molecular scales and the contribution to the dielectric loss can be orders of magnitude larger than the dielectric response due to molecular fluctuations. Specifically, it has been noted that the interfacial polarization normally occurs in blends or phase separated polymers [12]. The differences in conductivity mechanisms between inorganic semiconductors and conducting polymers may be a direct result of the interplay of electronic properties and structural crystallinity [13]. Doping inorganic semiconductors creates charge carriers (electrons or holes) by injection or ejection of electrons in the valence or conduction bands, respectively. Doping of conducting polymers seems to create a different conducting path whereby accessible energy levels in the middle of the band gap between the highest occupied and the lowest unoccupied energy levels are created, while maintaining valence and conduction bands which remain full and empty, respectively [13]. The microstructure of conducting polymer films suggests that charge carriers do not move in a continuum carrier path as would be the case in crystalline inorganic semiconductors. This leads to intra-chain transport and charge recombination along the chains. These views further support the conductivity mechanism by hole/electron hopping. The hopping mechanism can also be envisioned as a series of redox processes taking place within the polymer film where redox applies to the transient addition or subtraction of electrons [13]. Both because of their significant fundamental and technological importance, the impedance of polymer blends has been extensively studied. The good mechanical properties and interesting electronic properties of these materials suggest that they can be used in many device applications [14–17]. PEDOT is a conducting polymer used in OLEDs and organic transistors. It has a high electrical conductivity (up to 500 S/cm) and good thermal and chemical stability [18]. However, PEDOT alone is insoluble and hence is difficult to process. Blended with polystyrene sulfonic acid, (PSS), which serves as the charge-balancing dopant during polymerization, the PEDOT:PSS blend is found to form a highly stable dispersion in water with an improved ease of processing. In this complex, oligomeric PEDOT segments are tightly attached to PSS chains by means of electrostatic forces [17]. The combination results in a water-dispersed polyelectrolyte system with reasonably good film forming properties, high conductivity (up to about 1 S/cm) and excellent thermal stability. For example, films of PEDOT:PSS can be held in air at 100 C for over 1000 h with only a minimal change in conductivity [15]. However, pure PEDOT:PSS is mechanically brittle and for this reason must be blended to increase ductility. For example, PEDOT:PSS can be blended with polyvinyl alcohol (PVA) to enhance its mechanical properties and improve processing [19]. PVA (polyvinyl alcohol), a water soluble synthetic polymer, has excellent film forming, emulsifying, and adhesive properties [20]. However, addition of PVA will decrease the conductivity. Therefore, a conductivity enhancement, N-methyl-2-pyrrolidinone (NMP) is introduced which can increase conductivity by two orders of magnitude [21]. One application is in MEMS where low Young’s modulus and reasonably high conductivity are required [22].
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(CPP105D using Clevios P v4071) [16] is provided by the manufacturer and is used in this study as well as in the previous related study reported in Ref. [23]. PVA (98–99% hydrolyzed, Alfa Aesar, average M.W. 88,000–97,000) is dissolved in de-ionized water at 90 C for 4 h to obtain a 9 wt% aqueous solution. The PEDOT:PSS is filtered through a 5 mm filter prior to addition to the PVA solution. The PEDOT:PSS dispersion (CPP105D formulation) and the PVA water solution are blended and stirred at room temperature with a magnetically driven paddle for 24 h. The PEDOT:PSS/PVA mixture is not filtered and is mixed at solid weight fractions of PEDOT:PSS to the sum of PEDOT:PSS and PVA at 20, 30 and 40 wt%. 2.2. Film preparation Casting is utilized to form polymer films for all the PEDOT:PSS/ PVA blends used in this study. The blended polymers corresponding to the various wt% of PEDOT:PSS are poured into an open metal 6 cm by 10 cm rectangular mold as shown schematically in Fig. 1. After 24 h in air at a room temperature that varies from 23 C to 26 C with a relative humidity of about 35%, the liquid has evaporated and the sheet is cut into 2 cm by 2 cm square samples. The samples have a thickness of 0.2 mm as measured with a caliper and their positions on the sheet are noted in Fig. 1. Silver epoxy (MG Chemicals, 0.38 V cm) is then applied through a stencil to the films to form 1 cm2 contacts on the top and bottom of the PEDOT: PSS/PVA samples as shown in Fig. 2. It is well known that the impedance of PEDOT:PSS/PVA blends is a function of temperature and humidity [17]. For all the measurements reported herein, the samples are placed in a chamber with dry nitrogen flowing through the chamber and where the temperature can be controlled and monitored by means of a thermocouple mounted inside the chamber close to the sample which is placed near the bottom of the chamber. The chamber is a thick walled aluminum cylinder which can be sealed when the nitrogen flow is stopped and when measurements are made. The temperature is controlled by placing the cylinder on a hot plate and then adjusting the hot plate temperature to yield the desired sample temperature. The flow of dry nitrogen is maintained for at least four days prior to any measurement. In addition, the chamber functions as a Faraday shield leading to a reduction in electronic noise. Two sets of results are presented in this paper. The first set corresponds to impedance spectroscopy measurements for PEDOT:PSS/PVA blends of 20, 30 and 40 wt% at a fixed temperature of 24.5 C. The second set corresponds to impedance spectroscopy measurements for the 30 wt% blend over a temperature range from 24.5 C to 79.5 C. All the data presented in this study are the average of three sequential measurements in specific wt% or temperature. The impedance spectroscopy measurements at room temperature in dry nitrogen are found to be repeatable to within 1% after four days in flowing dry nitrogen. However, temperature cycling over a period of several weeks is required to obtain stability of the impedance spectroscopy measurements of the 30 wt% blend in a
2. Experimental 2.1. Materials and synthesis PEDOT doped with PSS is commercially available in the form of an aqueous dispersion (Clevios P v4071) from H.C. Starck GmbH and has a solid content of 1.2%. A high conductivity formulation
Fig. 1. Sample labels on cast sheet.
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Fig. 2. Sketch of silver epoxy on both sides of polymer films.
dry nitrogen environment. The first step in the thermal cycling protocol is to heat the sample to 95 C and hold the sample at that temperature for seven days. The samples are next allowed to slowly cool during a three-day period to 79.5 C. Impedance spectroscopy measurements are then made. After the data are collected, the sample is allowed to cool in increments of 10–24.5 C and after a period of one to three days, impedance spectroscopy measurements are taken at the lower temperature. The sample is then reheated to higher temperatures generally in 10 C increments and after one to three days, impedance spectroscopy measurements are taken. The process of heating or cooling and collecting impedance spectroscopy measurement is repeated until the impedance measured at 10 Hz agrees to within 2.5%. This level of agreement is found to occur for the second and subsequent heating and cooling cycles. 2.3. Impedance spectroscopy measurement A three electrode Gamry Femtostat model PC4/300 FAS1, with Gamry Framework software version 4.05 is used for the impedance spectroscopy measurements. The measurement frequencies are logarithmic, and 20 frequencies per decade are used. For the twocontact measurement, the working electrode is connected to either of the contacts on the polymer films. The counter and reference electrodes are connected to the other contact. The measurements are performed from 1 Hz to 100 kHz with a root mean square (rms) excitation of 6 mV AC and a zero DC offset. (During a preliminary study, it has been noticed that DC offsets or rms voltages higher than 10 mV lead to extended recovery times which depend on the applied voltage). A constant amplitude voltage across the sample is maintained by the difference in voltage between the reference electrode connected to the counter electrode, and the working electrode. The results are presented in the form of Nyquist plots as a function of wt%, the real and imaginary impedances, the impedance magnitude and the impedance phase vs frequency. The permittivity and dielectric modulus is also calculated from the impedance spectroscopy data. 3. Results 3.1. Evaluation of cast samples (in ambient air at room temperature) It is found that the impedance spectroscopy results are sensitive to the location of the sample in the casting sheet. Specifically, the samples at the edge of the sheet are found to vary in thickness. In contrast, the center samples of the sheet (G, H, I as indicated in Fig. 1) are found to be of uniform thickness. Therefore only the center samples are used in this study.
Table 1 Extrapolated DC resistance and error (%) of G, H, I samples in each wt%. Sample
20 wt% DC (V)
G H I Average
Error (%)
7.307 105 6.30 6.648 105 3.29 5 6.666 10 3.03 6.874 105
30 wt%
40 wt%
DC (V)
Error (%) DC (V)
0.967 105 10.62 1.144 105 5.73 1.133 105 4.71 1.082 105
Error (%)
2.369 103 0.88 2.366 103 1.00 2.434 103 1.88 2.390 103
Table 1 shows the preliminary results of center pieces of samples (G, H, I as indicated in Fig. 1) for 20, 30, 40 wt% of PEDOT: PSS in PEDOT:PSS/PVA in ambient air at room temperature. The extrapolated DC resistances (the averaged real part of the impedance from 1 Hz to 10 Hz) and their percentage error based on the average of G, H and I in each wt% are shown in Fig. 1. As mentioned above, three measurements are sequentially obtained for each wt% and are averaged to obtain the reported data. For 20 and 30 wt%, the largest extrapolated DC resistances spread from average of about 6.3% to 10.62%, respectively. For 40 wt%, the difference is smaller (less than 2%) among the three samples. Please note these data are for room temperature ambient air and values in dry nitrogen in Section 3.2 will be different. Based on the air measurements, because of the similarity in the results above, only one sample of each wt% was then placed in dry nitrogen and allowed to stabilize prior to heating. Measurements were performed on only 30 wt% as indicated in Section 3.3. 3.2. Results for 20, 30 and 40 wt% in unheated dry nitrogen at room temperature As mentioned above, three measurements are sequentially obtained for each wt% and are averaged to obtain the reported data. Fig. 3(a) shows the magnitude of average impedances for 20, 30 and 40 wt% of PEDOT:PSS/PVA. The variation in DC impedance in each set of three measurements is about 1%. The extrapolated DC resistance (the averaged real part of the impedance from 1 Hz to 10 Hz) for the 20, 30 and 40 wt% of PEDOT:PSS/PVA is 5.106 105, 8.391 104 and 2.390 103 V, respectively which corresponds to DC conductivities for the 20, 30 and 40 wt% of, respectively, 3.917 108, 2.383 107 and 8.369 106 S/cm. Fig. 3(b) shows the phase angle. For 20 and 30 wt%, there are small peaks at frequencies higher than 10 kHz which indicate that there would be two time constants in a circuit model. Fig. 3(c) and (d) shows the real and imaginary parts of the impedances, respectively. From Fig. 3(c), again for the 20 and 30 wt%, it is observed that the decrease of impedance with frequency is not monotonic. This observation is also consistent with the observation that there is more than one time constant in a circuit model. From Fig. 3(d), it can be seen that the frequency that corresponds to the maximum value of the imaginary impedance increases with increasing PEDOT:PSS wt%. The primary relaxation peaks occur at frequencies of 80, 335 and 30900 Hz for, respectively, 20, 30 and 40 wt%. Fig. 4 shows the Nyquist plots for all wt% of PEDOT:PSS. The instrument maximum frequency is 100 kHz, and, for that reason, the 40 wt% data are not complete. It is observed that all the Nyquist plots are nearly but not perfect semi-circles. This indicates that the PEDOT:PSS/PVA films contain a constant phase element (CPE). 3.3. Impedance for 30 wt% as a function of temperature Fig. 5(a) and (b) shows the magnitude and imaginary impedance for 30 wt% from room temperature to 79.5 C. For this portion of the study, only one sample is used. The heating of the PEDOT:PSS/PVA results in thermal degradation of the conductivity as compared to the unheated dry nitrogen measurements. Applicable detailed thermal degradation mechanisms are described in Refs. [24–28]. It can be also seen that the magnitude
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Fig. 3. (a) Impedance magnitude versus frequency, (b) phase angle versus frequency, (c) real impedance versus frequency and (d) imaginary impedance versus frequency for 20, 30 and 40 wt% PEDOT:PSS.
ofthe impedance decreases as temperature increases. Three measurements at the same temperature are taken sequentially and the spectra are averaged to reduce the noise. After stabilization is achieved, all the averaged measurements at each temperature are averaged again. The extrapolated DC resistance spread is found to vary by about 2.5% for the 15 measurements made after stabilization. At room temperature, the magnitude of the impedance is about 4.119 105 V and at 79.5 C, the magnitude is about 9.6 104 V. In addition, as shown in Fig. 5(b), the peak of the imaginary impedance decreases with increasing temperature and shifts from 110 Hz at room temperature to 354 Hz at 79.5 C. This shift indicates that the time constant decreases as temperature increases. Table 2 shows the values of the extrapolated DC resistance, the DC conductivity and the value of the peak frequency of the imaginary impedance as a function of temperature. The variation of the DC resistance and peak frequency as a function of temperature are shown in Fig. 6. The variation is monotonic but not linear with temperature.
shown in Fig. 3(d) are 80, 335 and 30900 Hz at 24.5 C for, respectively, 20, 30, and 40 wt%. The relaxation time, t , for the 20, 30 and 40 wt% of PEDOT:PSS can be computed using the values of the peak frequency and Eq. (1) and are shown in Table 3 and plotted as a function of temperature in Fig. 7 as expected, the higher wt% samples with the higher conductivity are associated with the shorter time constants.
t¼
1 2pf peak
4.2. Permittivity The real, e0 and imaginary e00 part of the complex permittivity can be computed using the impedance measurement as shown in Fig. 3(a), the real part of the impedance, Z0 , as shown in Fig. 3(c) and the imaginary part of the impedance, Z00 , as shown in Fig. 3(d) along with Eqs. (2) and (3).
4. Discussion 4.1. Relaxation time constant for unheated dry nitrogen As noted in Section 3.2, the peak frequencies, fpeak, which correspond to the maximum value of the imaginary impedance as
(1)
e0 ¼
Z 00 jZj e0 v Gf 2
(2)
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Fig. 4. Nyquist plot of (a) 20 wt%, (b) 30 wt% and (c) 40 wt% of PEDOT:PSS.
Fig. 5. (a) Plot of impedance magnitude versus frequency for 30 wt% PEDOT:PSS/PVA and (b) plot of imaginary impedance versus frequency for 30 wt% PEDO:PSS. The plots are from room temperature to 79.5 C. curves labeled a, b, c, d, e, f and g correspond, respectively to temperatures of 24.5 C, 30.5 C, 39.5 C, 49 C, 58.6 C, 68 C, and 79.5 C.
Table 2 DC resistance, DC conductivity and frequency of peak in imaginary impedance for 30 wt% as a function of temperature. Temperature ( C)
DC resistance (V)
DC conductivity (S/cm)
fpeak (Hz)
24.5 30.5 39.5 49.0 58.6 68.0 79.5
4.119 105 3.678 105 3.075 105 2.476 105 1.867 105 1.463 105 9.600 104
4.855 108 5.438 108 6.504 108 8.078 108 1.071 107 1.367 107 2.083 107
110 120 142 170 212 257 354
e00 ¼
Z0 jZj e0 v Gf 2
(3)
where e0 is permittivity in vacuum (8.85 1014 F/cm); Gf is the geometry factor which has a value of 50 (Gf is defined as the area of the contact which is 1 cm2 divided by the thickness of the film which is 0.2 cm), and the radial frequency v. Fig. 8(a) and (b) shows the real and imaginary part of the permittivity, respectively. It is observed that the real part of the permittivity of 20 wt% at 1 Hz frequency is approximately 1000–2000 while the real part of the
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Fig. 6. (a) DC resistance versus temperature and (b) frequency of peak in value of imaginary impedance peak versus temperature for 30 wt% PEDOT:PSS from 24.5 C to 79.5 C. Table 3 Relaxation time constant for 20 wt%, 30 wt% and 40 wt% of PEDOT: PSS at room temperature. wt% of PEDOT:PSS
Time constant (s)
20 30 40
1.99 103 4.75 104 5.15 106
permittivity of 40 wt% at 1 Hz frequency is approximately 7000–8000. The permittivity decreases with increasing frequencies. From Fig. 8(b), it is observed that the imaginary part of the permittivity exhibits a peak at 25 kHz for the 20 wt%. This again indicates that there would be two time constants in a circuit model. The real part of permittivity indicates energy storage, and the imaginary part of the permittivity indicates energy loss. The permittivity decreases with increasing frequency as the polarization cannot respond at higher frequencies. The high permittivity is due to the interfacial polarization [29]. Relaxations in the permittivity plots can be masked by high space charge. It is believed that, for PEDOT:PSS/PVA, the high interfacial polarization results in the masking space charge [29]. 4.3. Electric modulus In the case of high space charge, relaxations are not well displayed in permittivity plots. However, the relaxations can be observed in plots of the imaginary electric modulus as a function of frequency [28]. The electric modulus is defined as the inverse of the complex permittivity and is calculated from the impedance measurements using Eqs. (4) and (5) which show the real part, M', and imaginary part, M00 , of the electric modulus, respectively. M0 ¼ Z 00 e0 v Gf
Fig. 7. Relaxation time constant as a function of conductivity at room temperature.
Fig. 8. (a) Real part of the complex permittivity and (b) imaginary part of the complex permittivity.
(4)
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M00 ¼ Z 0 e0 v Gf
(5)
Fig. 9(a) and (b) shows the real and imaginary part of the electric modulus as a function of frequency. For 20 and 30 wt% of PEDOT:PSS, the plot of M00 exhibits two peaks which correspond to two time constants in a circuit model. Fig. 10 shows the electric modulus Nyquist plots. The Nyquist plot for 20 and 30 wt% also indicates that there are two time constants. 4.4. Temperature dependence Values of the DC resistance and peak frequency corresponding to the maximum value of the imaginary impedance are tabulated in Table 2 as a function of temperature for the 30 wt% sample. Fig. 11 shows the plots of the time constant in seconds, and the DC resistance in ohms as a function of temperature. As shown in the plot, both the normalized extrapolated DC resistance and the normalized time constant decrease with the increasing temperature. PEDOT is a p-type wide band gap semiconductor [17] with a negative temperature coefficient of resistance (TCR) [12] associated with semiconductors. The resistivity of PEDOT:PSS is dependent on the PEDOT:PSS grain size (determined by the manufacturing process), percentage of PSS in PEDOT:PSS, solvent/doping additives, percentage of PEDOT:PSS in films, film thickness, thermal history, post deposition treatments, relative humidity/film moisture (PEDOT:PSS is hydroscopic), blends with polymers, and anisotropic conduction [17]. Depending on the film processing and composition the resistivity can range from very low conduction to metallic behavior. Thus because of the difference in the PEDOT:PSS used in Refs. [12,17], in comparison to that of the current study, values of conductivity from those and the current study are not directly comparable. The conductivity of 30 wt% PEDOT:PSS places the conductivity in the transition range between an insulator and a conductor. A pure PEDOT:PSS film has the metallic conductivity of a conductor. Although the DC resistance decreases with temperature increases, it does not have an Arrenhius temperature characteristic. With increasing temperature, the time constant decreases which implies that mobility is increased with increasing temperature [30].
Fig. 10. Nyquist plot of the electric modulus for 20, 30 and 40 wt% PEDOT:PSS.
4.5. Power law fit Fig. 12(a) and (b) shows the frequency dependence of the normalized conductivity for various temperatures. The conductivity at each temperature is normalized by the DC value of conductivity at that temperature. The dots are the measured data points and the solid lines are the power law fit curves. The fits are obtained from the normalized Jonscher’s power law, which can be written as [31]: 1 þ avs
(6)
where a is the hopping frequency constant, v is angular frequency and s is the frequency exponent parameter ranges between 0 and 1. Values not equal to 1 indicate deviation from Debye behavior [32,33]. Values for the hopping frequency constant, the frequency exponent parameter along with the Chi squared measure of the goodness of the fit are shown in Table 4 for frequency ranges from 1 Hz to 100 kHz and 1 Hz to 31 kHz. For the larger frequency range, the value of the frequency exponent parameter is about 0.8 for all temperatures and for the lower frequency range is about 0.93. Thus, the system deviates from the Debye behavior.
Fig. 9. (a) Real electric modulus versus frequency and (b) imaginary electric modulus versus frequency for 20, 30 and 40 wt% PEDOT:PSS.
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contributes to the non-Debye behavior. Also, as noted in Section 3.2, there is some evidence that a circuit model of this system would have to include a constant phase element which is consistent with the observation of non-Debye behavior. n 1X s measured s fitted 2 Chi square ðX 2 Þ ¼ n i¼n s measured The chi square (X2) is the measure of the variability of the measurement relative to the curve fit. The standard deviation of the difference between the measured and fitted data for 1–31 kHz is 0.9331 at 24.5 C and 0.2964 at 79.5 C. For 1–100 kHz, the standard deviation is 5.276 at 24.5 C and 1.462 at 79.5 C. 5. Summary The impedance of cast PEDOT:PSS/PVA films of 20, 30 and 40 wt % is determined. DC conductivities measured perpendicular to the film surfaces are found to be 3.917 108, 2.383 107 and 8.369 106 S/cm under dry nitrogen, respectively for 20, 30 and 40 wt%. Different formalisms such as complex permittivity, complex dielectric modulus, and complex impedance are explored to interpret the dielectric spectra. The results from dielectric modulus show the blended films have two relaxation time constants and that the space charge effect is suppressed for the imaginary part of the dielectric modulus. Temperature dependence results of 30 wt% PEDOT:PSS/PVA are also presented and show the resistivity decreases as temperature increases. The relaxation process is also found to be non-Debye for all temperatures which is consistent with the inhomogeneous distribution of the PEDOT:PSS in the PVA.
Fig. 11. Plot of normalized time constant (solid line) and normalized extrapolated DC resistance verus temperature (dashed line) for 30 wt% PEDOT:PSS. The time constant is based on peak frequency data as shown in Table 2 and computed using Eq. (1). The time constants are normalized by the value of 1.447 ms at 24.5 C and DC resistance as shown in Table 2 are normalized value of 411.9 kV at 24.5 C.
A non-homogenous distribution of conducting properties can be shown to have a power of exponent of other than one, that is, a non-Debye behavior [31–33]. In a previous study [19], SEM photos of the cast PEDOT:PSS/PVA are seen to exhibit a non-homogeneous distribution of the PEDOT:PSS in the PVA. The SEM photos also indicate that the PEDOT:PSS exist as colloids and aggregates through out the PVA. It is this non-homogeneous distribution that
Fig. 12. Power law fit for normalized resistance for various temperatures over the frequency range from (a) 1 Hz to 100 kHz and (b) 1 Hz to 31 kHz. The top curve in each figure corresponds to 24.5 C and the other curves in descending or downward order correspond to temperatures of, respectively 30.5 C, 39.5 C, 49 C, 58.6 C, 68 C, and 79.5 C.
Table 4 Power law fit parameters. Fit 1 Hz to 100 kHz
24.5 C 30.5 C 39.5 C 49 C 58.6 C 68 C 79.5 C
Fit 1 Hz to 31 kHz
a
s
X2
a
s
X2
0.01324 0.01154 0.00931 0.00730 0.00569 0.00463 0.00327
0.79084 0.79474 0.79942 0.80438 0.80433 0.80429 0.80323
0.403 0.376 0.313 0.263 0.213 0.168 0.102
0.00280 0.00238 0.00197 0.00155 0.00120 0.00096 0.00070
0.92415 0.93018 0.93253 0.93700 0.93745 0.93889 0.93413
0.0556 0.0556 0.0441 0.0384 0.0320 0.0239 0.0133
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Acknowledgements The authors acknowledge Professor James Earthman for his generous loan of the Gamry Femtostat used in this study, Dr. Sandra Tran for her help in the collection of the data, and Ms. Kim Nguyen for her help in the design and construction of the dry nitrogen chamber. References [1] J.R. Macdonald, W.B. Johnson, Impedance Spectroscopy: Theory, Experiment, and Applications, in: E. Barsoukov, J.R. Macdonald (Eds.), 2nd edition, Wiley, Hoboken, NJ, 2005, pp. 1–26 (chapter 1). [2] F. Huet, J. Power Sources 70 (1998) 59–69. [3] X. Wu, E.S. Yang, H.L. Evans, J. Appl. Phys. 68 (1990) 2845–2848. [4] H.C.F. Martens, H.B. Brom, Phys. Rev. B 60 (1999) R8489–R8492. [5] J. Werner, A.F.J. Levi, R.T. Tung, M. Anzlowar, M. Pinto, Phys. Rev. Lett. 60 (1988) 53–56. [6] C. Melzer, E.J. Koop, D. Mihailetchi, P.W.M. Blom, Adv. Funct. Mater. 14 (2004) 865–870. [7] P. Dutta, S. Biswas, S.K. De, Mater. Res. Bull. 37 (2002) 193–200. [8] K.C. Kao, Dielectric Phenomena in Solids, Elsevier, San Diego, CA, 2004, pp. 41– 114 (chapter 2). [9] J.C. Maxwell, Electricity and Magnetism, Clarendon Press, Oxford, England, 1892 452 pp. [10] K.W. Wagner, Arch. Electrotech. 2 (1914) 371–387. [11] R.W. Sillars, J. Inst. Electr. Eng. 80 (1937) 389–394. [12] A. Schonhals, B. Kremer, Broadband Dielectric Spectroscopy, in: B. Kremer, A. Schonhals (Eds.), Springer, Berlin, Germany, 2003, pp. 87–93 (chapter 3.4). [13] J.F. Rubinson, Y.P. Kayinamura, Chem. Soc. Rev. 38 (2009) 3339–3347. [14] S. Kirchmeyer, K. Reuter, J. Mater. Chem. 15 (2005) 2077–2088.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Impedance spectroscopy study of conducting polymer blends of PEDOT:PSS and PVA Chang-hsiu Chen a,b, * , Allen Kine b , Richard D. Nelson c , John C. LaRue b a b c
Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697, United States Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92697, United States Department of Electrical Engineering and Computer Science, University of California, Irvine, CA 92697, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 January 2015 Received in revised form 30 April 2015 Accepted 7 May 2015 Available online xxx
Blends of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) with polyvinyl alcohol (PVA) have many potential applications including microelectromechanical systems (MEMS) and other electronic devices. Specifically, PEDOT:PSS acts as the electrical conductor, while the PVA provides ductility and other mechanical enhancements. Concentrations of PEDOT:PSS at 20, 30 and 40 weight percentage (wt%) in PEDOT:PSS/PVA mixtures have been found to provide optimal mechanical and electrical properties. PEDOT:PSS/PVA films are prepared using a casting method and electrochemical impedance spectroscopy is used to investigate the electrical properties. The extrapolated DC conductivities are obtained in a dry nitrogen environment at room temperature and determined to be 3.917 108, 2.383 107 and 8.369 106 S/cm, respectively, for 20, 30 and 40 wt%. In addition, as a means to better explore the conduction as a function of temperature, impedance spectroscopy measurements for the 30 wt% PEDOT:PSS are determined over a range of temperatures from 24.5 C to 79.5 C. The corresponding extrapolated DC conductivities are found to increase by a factor of about four with increase in temperature from 4.617 108 S/cm at 24.5 C to 2.083 107 S/cm at 79.5 C. Published by Elsevier B.V.
Keywords: PEDOT PEDT PVA Polyvinyl alcohol Conducting polymers Impedance spectroscopy Permittivity Dielectric modulus
1. Introduction Impedance spectroscopy, described in detail in Refs. [1–3], is used to determine the frequency dependent electrical properties of a material. For example, impedance spectroscopy can be used to determine conduction as a function of material composition, to investigate charge injection and transport, and to determine the role of an interfacial layer and the effects of trapping states in semiconductor devices containing a conducting polymer as the active layer [4–6]. The dielectric properties of blended polymer play an important role in device applications such as high performance capacitors, electrical cable insulation, electronic packaging and components [7]. From a fundamental point of view, electrical impedance spectroscopy has been widely used to analyze the microscopic and dynamical relaxation process in complicated systems. Relaxation, in general, is a delay in a response to a changing
* Corresponding author at: Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697, United States. Tel.: +1 949 824 2431. Email: [email protected]. E-mail address: [email protected] (C.-h. Chen). http://dx.doi.org/10.1016/j.synthmet.2015.05.003 0379-6779/ Published by Elsevier B.V.
stimulus in a linear system. The physical origin of the time lag between the applied electric field and the polarization is the irreversible loss of free energy to heat. Also, a larger lag implies a slower process with increased loss [7]. In dielectric spectroscopy, large frequency dependent contributions to the dielectric response, especially at low frequencies, may come from the buildup of free charges. The distribution of the dielectric regions/particles within the body of a dielectric creates localized dipoles with magnitude dependent on their relative permittivity. There are two possible ways in which space charge polarization [8] occurs. For example, in a dielectric material, localized charges (ions and vacancies, or electrons and holes) can hop from one site to neighboring sites, creating so-called hopping polarization [8]. These charges are capable of moving freely from one site to another site for a short time, and then become trapped in localized states. Occasionally, these charges make a jump surmounting a potential barrier to other sites. The movement of ions or vacancies in ionic crystals and the movement of electrons and holes in glasses and amorphous semiconductors are essentially due to the hopping process. Depending on the width and the height of the potential barrier, a charged particle on one site may hop or tunnel to the other site. The other space charge polarization is called interfacial polarization or Maxwell–Wagner–Sillars’ polarization [9–11]. It is produced by the separation of mobile
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positively and negatively charged particles under an applied field, and forms positive and negative space charges in the bulk of the material, or at the interfaces between materials with different permittivity. These space charges, in turn, modify the field distribution. In addition, the charges are often separated over a considerable distance relative to atomic and molecular scales and the contribution to the dielectric loss can be orders of magnitude larger than the dielectric response due to molecular fluctuations. Specifically, it has been noted that the interfacial polarization normally occurs in blends or phase separated polymers [12]. The differences in conductivity mechanisms between inorganic semiconductors and conducting polymers may be a direct result of the interplay of electronic properties and structural crystallinity [13]. Doping inorganic semiconductors creates charge carriers (electrons or holes) by injection or ejection of electrons in the valence or conduction bands, respectively. Doping of conducting polymers seems to create a different conducting path whereby accessible energy levels in the middle of the band gap between the highest occupied and the lowest unoccupied energy levels are created, while maintaining valence and conduction bands which remain full and empty, respectively [13]. The microstructure of conducting polymer films suggests that charge carriers do not move in a continuum carrier path as would be the case in crystalline inorganic semiconductors. This leads to intra-chain transport and charge recombination along the chains. These views further support the conductivity mechanism by hole/electron hopping. The hopping mechanism can also be envisioned as a series of redox processes taking place within the polymer film where redox applies to the transient addition or subtraction of electrons [13]. Both because of their significant fundamental and technological importance, the impedance of polymer blends has been extensively studied. The good mechanical properties and interesting electronic properties of these materials suggest that they can be used in many device applications [14–17]. PEDOT is a conducting polymer used in OLEDs and organic transistors. It has a high electrical conductivity (up to 500 S/cm) and good thermal and chemical stability [18]. However, PEDOT alone is insoluble and hence is difficult to process. Blended with polystyrene sulfonic acid, (PSS), which serves as the charge-balancing dopant during polymerization, the PEDOT:PSS blend is found to form a highly stable dispersion in water with an improved ease of processing. In this complex, oligomeric PEDOT segments are tightly attached to PSS chains by means of electrostatic forces [17]. The combination results in a water-dispersed polyelectrolyte system with reasonably good film forming properties, high conductivity (up to about 1 S/cm) and excellent thermal stability. For example, films of PEDOT:PSS can be held in air at 100 C for over 1000 h with only a minimal change in conductivity [15]. However, pure PEDOT:PSS is mechanically brittle and for this reason must be blended to increase ductility. For example, PEDOT:PSS can be blended with polyvinyl alcohol (PVA) to enhance its mechanical properties and improve processing [19]. PVA (polyvinyl alcohol), a water soluble synthetic polymer, has excellent film forming, emulsifying, and adhesive properties [20]. However, addition of PVA will decrease the conductivity. Therefore, a conductivity enhancement, N-methyl-2-pyrrolidinone (NMP) is introduced which can increase conductivity by two orders of magnitude [21]. One application is in MEMS where low Young’s modulus and reasonably high conductivity are required [22].
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(CPP105D using Clevios P v4071) [16] is provided by the manufacturer and is used in this study as well as in the previous related study reported in Ref. [23]. PVA (98–99% hydrolyzed, Alfa Aesar, average M.W. 88,000–97,000) is dissolved in de-ionized water at 90 C for 4 h to obtain a 9 wt% aqueous solution. The PEDOT:PSS is filtered through a 5 mm filter prior to addition to the PVA solution. The PEDOT:PSS dispersion (CPP105D formulation) and the PVA water solution are blended and stirred at room temperature with a magnetically driven paddle for 24 h. The PEDOT:PSS/PVA mixture is not filtered and is mixed at solid weight fractions of PEDOT:PSS to the sum of PEDOT:PSS and PVA at 20, 30 and 40 wt%. 2.2. Film preparation Casting is utilized to form polymer films for all the PEDOT:PSS/ PVA blends used in this study. The blended polymers corresponding to the various wt% of PEDOT:PSS are poured into an open metal 6 cm by 10 cm rectangular mold as shown schematically in Fig. 1. After 24 h in air at a room temperature that varies from 23 C to 26 C with a relative humidity of about 35%, the liquid has evaporated and the sheet is cut into 2 cm by 2 cm square samples. The samples have a thickness of 0.2 mm as measured with a caliper and their positions on the sheet are noted in Fig. 1. Silver epoxy (MG Chemicals, 0.38 V cm) is then applied through a stencil to the films to form 1 cm2 contacts on the top and bottom of the PEDOT: PSS/PVA samples as shown in Fig. 2. It is well known that the impedance of PEDOT:PSS/PVA blends is a function of temperature and humidity [17]. For all the measurements reported herein, the samples are placed in a chamber with dry nitrogen flowing through the chamber and where the temperature can be controlled and monitored by means of a thermocouple mounted inside the chamber close to the sample which is placed near the bottom of the chamber. The chamber is a thick walled aluminum cylinder which can be sealed when the nitrogen flow is stopped and when measurements are made. The temperature is controlled by placing the cylinder on a hot plate and then adjusting the hot plate temperature to yield the desired sample temperature. The flow of dry nitrogen is maintained for at least four days prior to any measurement. In addition, the chamber functions as a Faraday shield leading to a reduction in electronic noise. Two sets of results are presented in this paper. The first set corresponds to impedance spectroscopy measurements for PEDOT:PSS/PVA blends of 20, 30 and 40 wt% at a fixed temperature of 24.5 C. The second set corresponds to impedance spectroscopy measurements for the 30 wt% blend over a temperature range from 24.5 C to 79.5 C. All the data presented in this study are the average of three sequential measurements in specific wt% or temperature. The impedance spectroscopy measurements at room temperature in dry nitrogen are found to be repeatable to within 1% after four days in flowing dry nitrogen. However, temperature cycling over a period of several weeks is required to obtain stability of the impedance spectroscopy measurements of the 30 wt% blend in a
2. Experimental 2.1. Materials and synthesis PEDOT doped with PSS is commercially available in the form of an aqueous dispersion (Clevios P v4071) from H.C. Starck GmbH and has a solid content of 1.2%. A high conductivity formulation
Fig. 1. Sample labels on cast sheet.
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Fig. 2. Sketch of silver epoxy on both sides of polymer films.
dry nitrogen environment. The first step in the thermal cycling protocol is to heat the sample to 95 C and hold the sample at that temperature for seven days. The samples are next allowed to slowly cool during a three-day period to 79.5 C. Impedance spectroscopy measurements are then made. After the data are collected, the sample is allowed to cool in increments of 10–24.5 C and after a period of one to three days, impedance spectroscopy measurements are taken at the lower temperature. The sample is then reheated to higher temperatures generally in 10 C increments and after one to three days, impedance spectroscopy measurements are taken. The process of heating or cooling and collecting impedance spectroscopy measurement is repeated until the impedance measured at 10 Hz agrees to within 2.5%. This level of agreement is found to occur for the second and subsequent heating and cooling cycles. 2.3. Impedance spectroscopy measurement A three electrode Gamry Femtostat model PC4/300 FAS1, with Gamry Framework software version 4.05 is used for the impedance spectroscopy measurements. The measurement frequencies are logarithmic, and 20 frequencies per decade are used. For the twocontact measurement, the working electrode is connected to either of the contacts on the polymer films. The counter and reference electrodes are connected to the other contact. The measurements are performed from 1 Hz to 100 kHz with a root mean square (rms) excitation of 6 mV AC and a zero DC offset. (During a preliminary study, it has been noticed that DC offsets or rms voltages higher than 10 mV lead to extended recovery times which depend on the applied voltage). A constant amplitude voltage across the sample is maintained by the difference in voltage between the reference electrode connected to the counter electrode, and the working electrode. The results are presented in the form of Nyquist plots as a function of wt%, the real and imaginary impedances, the impedance magnitude and the impedance phase vs frequency. The permittivity and dielectric modulus is also calculated from the impedance spectroscopy data. 3. Results 3.1. Evaluation of cast samples (in ambient air at room temperature) It is found that the impedance spectroscopy results are sensitive to the location of the sample in the casting sheet. Specifically, the samples at the edge of the sheet are found to vary in thickness. In contrast, the center samples of the sheet (G, H, I as indicated in Fig. 1) are found to be of uniform thickness. Therefore only the center samples are used in this study.
Table 1 Extrapolated DC resistance and error (%) of G, H, I samples in each wt%. Sample
20 wt% DC (V)
G H I Average
Error (%)
7.307 105 6.30 6.648 105 3.29 5 6.666 10 3.03 6.874 105
30 wt%
40 wt%
DC (V)
Error (%) DC (V)
0.967 105 10.62 1.144 105 5.73 1.133 105 4.71 1.082 105
Error (%)
2.369 103 0.88 2.366 103 1.00 2.434 103 1.88 2.390 103
Table 1 shows the preliminary results of center pieces of samples (G, H, I as indicated in Fig. 1) for 20, 30, 40 wt% of PEDOT: PSS in PEDOT:PSS/PVA in ambient air at room temperature. The extrapolated DC resistances (the averaged real part of the impedance from 1 Hz to 10 Hz) and their percentage error based on the average of G, H and I in each wt% are shown in Fig. 1. As mentioned above, three measurements are sequentially obtained for each wt% and are averaged to obtain the reported data. For 20 and 30 wt%, the largest extrapolated DC resistances spread from average of about 6.3% to 10.62%, respectively. For 40 wt%, the difference is smaller (less than 2%) among the three samples. Please note these data are for room temperature ambient air and values in dry nitrogen in Section 3.2 will be different. Based on the air measurements, because of the similarity in the results above, only one sample of each wt% was then placed in dry nitrogen and allowed to stabilize prior to heating. Measurements were performed on only 30 wt% as indicated in Section 3.3. 3.2. Results for 20, 30 and 40 wt% in unheated dry nitrogen at room temperature As mentioned above, three measurements are sequentially obtained for each wt% and are averaged to obtain the reported data. Fig. 3(a) shows the magnitude of average impedances for 20, 30 and 40 wt% of PEDOT:PSS/PVA. The variation in DC impedance in each set of three measurements is about 1%. The extrapolated DC resistance (the averaged real part of the impedance from 1 Hz to 10 Hz) for the 20, 30 and 40 wt% of PEDOT:PSS/PVA is 5.106 105, 8.391 104 and 2.390 103 V, respectively which corresponds to DC conductivities for the 20, 30 and 40 wt% of, respectively, 3.917 108, 2.383 107 and 8.369 106 S/cm. Fig. 3(b) shows the phase angle. For 20 and 30 wt%, there are small peaks at frequencies higher than 10 kHz which indicate that there would be two time constants in a circuit model. Fig. 3(c) and (d) shows the real and imaginary parts of the impedances, respectively. From Fig. 3(c), again for the 20 and 30 wt%, it is observed that the decrease of impedance with frequency is not monotonic. This observation is also consistent with the observation that there is more than one time constant in a circuit model. From Fig. 3(d), it can be seen that the frequency that corresponds to the maximum value of the imaginary impedance increases with increasing PEDOT:PSS wt%. The primary relaxation peaks occur at frequencies of 80, 335 and 30900 Hz for, respectively, 20, 30 and 40 wt%. Fig. 4 shows the Nyquist plots for all wt% of PEDOT:PSS. The instrument maximum frequency is 100 kHz, and, for that reason, the 40 wt% data are not complete. It is observed that all the Nyquist plots are nearly but not perfect semi-circles. This indicates that the PEDOT:PSS/PVA films contain a constant phase element (CPE). 3.3. Impedance for 30 wt% as a function of temperature Fig. 5(a) and (b) shows the magnitude and imaginary impedance for 30 wt% from room temperature to 79.5 C. For this portion of the study, only one sample is used. The heating of the PEDOT:PSS/PVA results in thermal degradation of the conductivity as compared to the unheated dry nitrogen measurements. Applicable detailed thermal degradation mechanisms are described in Refs. [24–28]. It can be also seen that the magnitude
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Fig. 3. (a) Impedance magnitude versus frequency, (b) phase angle versus frequency, (c) real impedance versus frequency and (d) imaginary impedance versus frequency for 20, 30 and 40 wt% PEDOT:PSS.
ofthe impedance decreases as temperature increases. Three measurements at the same temperature are taken sequentially and the spectra are averaged to reduce the noise. After stabilization is achieved, all the averaged measurements at each temperature are averaged again. The extrapolated DC resistance spread is found to vary by about 2.5% for the 15 measurements made after stabilization. At room temperature, the magnitude of the impedance is about 4.119 105 V and at 79.5 C, the magnitude is about 9.6 104 V. In addition, as shown in Fig. 5(b), the peak of the imaginary impedance decreases with increasing temperature and shifts from 110 Hz at room temperature to 354 Hz at 79.5 C. This shift indicates that the time constant decreases as temperature increases. Table 2 shows the values of the extrapolated DC resistance, the DC conductivity and the value of the peak frequency of the imaginary impedance as a function of temperature. The variation of the DC resistance and peak frequency as a function of temperature are shown in Fig. 6. The variation is monotonic but not linear with temperature.
shown in Fig. 3(d) are 80, 335 and 30900 Hz at 24.5 C for, respectively, 20, 30, and 40 wt%. The relaxation time, t , for the 20, 30 and 40 wt% of PEDOT:PSS can be computed using the values of the peak frequency and Eq. (1) and are shown in Table 3 and plotted as a function of temperature in Fig. 7 as expected, the higher wt% samples with the higher conductivity are associated with the shorter time constants.
t¼
1 2pf peak
4.2. Permittivity The real, e0 and imaginary e00 part of the complex permittivity can be computed using the impedance measurement as shown in Fig. 3(a), the real part of the impedance, Z0 , as shown in Fig. 3(c) and the imaginary part of the impedance, Z00 , as shown in Fig. 3(d) along with Eqs. (2) and (3).
4. Discussion 4.1. Relaxation time constant for unheated dry nitrogen As noted in Section 3.2, the peak frequencies, fpeak, which correspond to the maximum value of the imaginary impedance as
(1)
e0 ¼
Z 00 jZj e0 v Gf 2
(2)
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Fig. 4. Nyquist plot of (a) 20 wt%, (b) 30 wt% and (c) 40 wt% of PEDOT:PSS.
Fig. 5. (a) Plot of impedance magnitude versus frequency for 30 wt% PEDOT:PSS/PVA and (b) plot of imaginary impedance versus frequency for 30 wt% PEDO:PSS. The plots are from room temperature to 79.5 C. curves labeled a, b, c, d, e, f and g correspond, respectively to temperatures of 24.5 C, 30.5 C, 39.5 C, 49 C, 58.6 C, 68 C, and 79.5 C.
Table 2 DC resistance, DC conductivity and frequency of peak in imaginary impedance for 30 wt% as a function of temperature. Temperature ( C)
DC resistance (V)
DC conductivity (S/cm)
fpeak (Hz)
24.5 30.5 39.5 49.0 58.6 68.0 79.5
4.119 105 3.678 105 3.075 105 2.476 105 1.867 105 1.463 105 9.600 104
4.855 108 5.438 108 6.504 108 8.078 108 1.071 107 1.367 107 2.083 107
110 120 142 170 212 257 354
e00 ¼
Z0 jZj e0 v Gf 2
(3)
where e0 is permittivity in vacuum (8.85 1014 F/cm); Gf is the geometry factor which has a value of 50 (Gf is defined as the area of the contact which is 1 cm2 divided by the thickness of the film which is 0.2 cm), and the radial frequency v. Fig. 8(a) and (b) shows the real and imaginary part of the permittivity, respectively. It is observed that the real part of the permittivity of 20 wt% at 1 Hz frequency is approximately 1000–2000 while the real part of the
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Fig. 6. (a) DC resistance versus temperature and (b) frequency of peak in value of imaginary impedance peak versus temperature for 30 wt% PEDOT:PSS from 24.5 C to 79.5 C. Table 3 Relaxation time constant for 20 wt%, 30 wt% and 40 wt% of PEDOT: PSS at room temperature. wt% of PEDOT:PSS
Time constant (s)
20 30 40
1.99 103 4.75 104 5.15 106
permittivity of 40 wt% at 1 Hz frequency is approximately 7000–8000. The permittivity decreases with increasing frequencies. From Fig. 8(b), it is observed that the imaginary part of the permittivity exhibits a peak at 25 kHz for the 20 wt%. This again indicates that there would be two time constants in a circuit model. The real part of permittivity indicates energy storage, and the imaginary part of the permittivity indicates energy loss. The permittivity decreases with increasing frequency as the polarization cannot respond at higher frequencies. The high permittivity is due to the interfacial polarization [29]. Relaxations in the permittivity plots can be masked by high space charge. It is believed that, for PEDOT:PSS/PVA, the high interfacial polarization results in the masking space charge [29]. 4.3. Electric modulus In the case of high space charge, relaxations are not well displayed in permittivity plots. However, the relaxations can be observed in plots of the imaginary electric modulus as a function of frequency [28]. The electric modulus is defined as the inverse of the complex permittivity and is calculated from the impedance measurements using Eqs. (4) and (5) which show the real part, M', and imaginary part, M00 , of the electric modulus, respectively. M0 ¼ Z 00 e0 v Gf
Fig. 7. Relaxation time constant as a function of conductivity at room temperature.
Fig. 8. (a) Real part of the complex permittivity and (b) imaginary part of the complex permittivity.
(4)
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M00 ¼ Z 0 e0 v Gf
(5)
Fig. 9(a) and (b) shows the real and imaginary part of the electric modulus as a function of frequency. For 20 and 30 wt% of PEDOT:PSS, the plot of M00 exhibits two peaks which correspond to two time constants in a circuit model. Fig. 10 shows the electric modulus Nyquist plots. The Nyquist plot for 20 and 30 wt% also indicates that there are two time constants. 4.4. Temperature dependence Values of the DC resistance and peak frequency corresponding to the maximum value of the imaginary impedance are tabulated in Table 2 as a function of temperature for the 30 wt% sample. Fig. 11 shows the plots of the time constant in seconds, and the DC resistance in ohms as a function of temperature. As shown in the plot, both the normalized extrapolated DC resistance and the normalized time constant decrease with the increasing temperature. PEDOT is a p-type wide band gap semiconductor [17] with a negative temperature coefficient of resistance (TCR) [12] associated with semiconductors. The resistivity of PEDOT:PSS is dependent on the PEDOT:PSS grain size (determined by the manufacturing process), percentage of PSS in PEDOT:PSS, solvent/doping additives, percentage of PEDOT:PSS in films, film thickness, thermal history, post deposition treatments, relative humidity/film moisture (PEDOT:PSS is hydroscopic), blends with polymers, and anisotropic conduction [17]. Depending on the film processing and composition the resistivity can range from very low conduction to metallic behavior. Thus because of the difference in the PEDOT:PSS used in Refs. [12,17], in comparison to that of the current study, values of conductivity from those and the current study are not directly comparable. The conductivity of 30 wt% PEDOT:PSS places the conductivity in the transition range between an insulator and a conductor. A pure PEDOT:PSS film has the metallic conductivity of a conductor. Although the DC resistance decreases with temperature increases, it does not have an Arrenhius temperature characteristic. With increasing temperature, the time constant decreases which implies that mobility is increased with increasing temperature [30].
Fig. 10. Nyquist plot of the electric modulus for 20, 30 and 40 wt% PEDOT:PSS.
4.5. Power law fit Fig. 12(a) and (b) shows the frequency dependence of the normalized conductivity for various temperatures. The conductivity at each temperature is normalized by the DC value of conductivity at that temperature. The dots are the measured data points and the solid lines are the power law fit curves. The fits are obtained from the normalized Jonscher’s power law, which can be written as [31]: 1 þ avs
(6)
where a is the hopping frequency constant, v is angular frequency and s is the frequency exponent parameter ranges between 0 and 1. Values not equal to 1 indicate deviation from Debye behavior [32,33]. Values for the hopping frequency constant, the frequency exponent parameter along with the Chi squared measure of the goodness of the fit are shown in Table 4 for frequency ranges from 1 Hz to 100 kHz and 1 Hz to 31 kHz. For the larger frequency range, the value of the frequency exponent parameter is about 0.8 for all temperatures and for the lower frequency range is about 0.93. Thus, the system deviates from the Debye behavior.
Fig. 9. (a) Real electric modulus versus frequency and (b) imaginary electric modulus versus frequency for 20, 30 and 40 wt% PEDOT:PSS.
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contributes to the non-Debye behavior. Also, as noted in Section 3.2, there is some evidence that a circuit model of this system would have to include a constant phase element which is consistent with the observation of non-Debye behavior. n 1X s measured s fitted 2 Chi square ðX 2 Þ ¼ n i¼n s measured The chi square (X2) is the measure of the variability of the measurement relative to the curve fit. The standard deviation of the difference between the measured and fitted data for 1–31 kHz is 0.9331 at 24.5 C and 0.2964 at 79.5 C. For 1–100 kHz, the standard deviation is 5.276 at 24.5 C and 1.462 at 79.5 C. 5. Summary The impedance of cast PEDOT:PSS/PVA films of 20, 30 and 40 wt % is determined. DC conductivities measured perpendicular to the film surfaces are found to be 3.917 108, 2.383 107 and 8.369 106 S/cm under dry nitrogen, respectively for 20, 30 and 40 wt%. Different formalisms such as complex permittivity, complex dielectric modulus, and complex impedance are explored to interpret the dielectric spectra. The results from dielectric modulus show the blended films have two relaxation time constants and that the space charge effect is suppressed for the imaginary part of the dielectric modulus. Temperature dependence results of 30 wt% PEDOT:PSS/PVA are also presented and show the resistivity decreases as temperature increases. The relaxation process is also found to be non-Debye for all temperatures which is consistent with the inhomogeneous distribution of the PEDOT:PSS in the PVA.
Fig. 11. Plot of normalized time constant (solid line) and normalized extrapolated DC resistance verus temperature (dashed line) for 30 wt% PEDOT:PSS. The time constant is based on peak frequency data as shown in Table 2 and computed using Eq. (1). The time constants are normalized by the value of 1.447 ms at 24.5 C and DC resistance as shown in Table 2 are normalized value of 411.9 kV at 24.5 C.
A non-homogenous distribution of conducting properties can be shown to have a power of exponent of other than one, that is, a non-Debye behavior [31–33]. In a previous study [19], SEM photos of the cast PEDOT:PSS/PVA are seen to exhibit a non-homogeneous distribution of the PEDOT:PSS in the PVA. The SEM photos also indicate that the PEDOT:PSS exist as colloids and aggregates through out the PVA. It is this non-homogeneous distribution that
Fig. 12. Power law fit for normalized resistance for various temperatures over the frequency range from (a) 1 Hz to 100 kHz and (b) 1 Hz to 31 kHz. The top curve in each figure corresponds to 24.5 C and the other curves in descending or downward order correspond to temperatures of, respectively 30.5 C, 39.5 C, 49 C, 58.6 C, 68 C, and 79.5 C.
Table 4 Power law fit parameters. Fit 1 Hz to 100 kHz
24.5 C 30.5 C 39.5 C 49 C 58.6 C 68 C 79.5 C
Fit 1 Hz to 31 kHz
a
s
X2
a
s
X2
0.01324 0.01154 0.00931 0.00730 0.00569 0.00463 0.00327
0.79084 0.79474 0.79942 0.80438 0.80433 0.80429 0.80323
0.403 0.376 0.313 0.263 0.213 0.168 0.102
0.00280 0.00238 0.00197 0.00155 0.00120 0.00096 0.00070
0.92415 0.93018 0.93253 0.93700 0.93745 0.93889 0.93413
0.0556 0.0556 0.0441 0.0384 0.0320 0.0239 0.0133
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