The effect of humidity on the electrical conductivity of mesoporous polythiophene

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Sensors and Actuators B 130 (2008) 661–667

The effect of humidity on the electrical conductivity of mesoporous polythiophene W.M. Sears Physics Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada Received 16 July 2007; received in revised form 18 October 2007; accepted 19 October 2007 Available online 26 October 2007

Abstract Commercial samples of bromine terminated polythiophene powder have been compacted to a mesoporous state in order to measure their electrical properties at different relative humidities by impedance spectroscopy, in this case the complex admittance versus frequency. The sample powders were compacted between the electrodes of a co-axial tube capacitor arrangement and connected to a standard impedance bridge with a frequency range of 20–1 MHz. The low humidity conductivity shows a dc limit at about 4 × 10−6 S/m. When the conductivity measurements were recalculated by subtracting the low humidity response, assumed dry, from the medium or high humidity response a peak was seen at values ranging from 20 to 60 kHz with a sharp fall off at about 100 kHz. Plots of the change in susceptibility did not show peaks, but the excess response vanished at a lower frequency than seen for the conductivity. The log–log plots of the data showed good low frequency fits to a straight line, implying power law dependencies similar to those seen in ionic dielectric materials. I conclude that in humid air there is no significant conduction through the adsorbed water layer itself, but that the presence of surface water molecules affects the polythiophene conduction that occurs through the overlap of adjacent ␲-bonds. In this model the presence of surface water enhances the ␲-bond overlap, but has a strong frequency dependence. © 2007 Elsevier B.V. All rights reserved. Keywords: Polythiophene; Conductivity; Susceptibility; Humidity

1. Introduction In previous papers a number of approaches have been used in the continuing study of the mechanisms of electrical conduction through water layers adsorbed in sintered porous pellets or mesoporous compacted powders. The effect of oxygen stoichiometry (vacancies) was examined for bismuth iron molybdate pellets, where the vacancies were produced by chemical reduction in a methanol atmosphere at elevated temperatures [1]. It was concluded that the Grotthuss chain reaction was the dominant mechanism on an oxidized surface, but for the reduced surfaces, Fermi level shifts were increasingly important. In follow-up work on the time dependence of dc bias polarization [2] the results confirmed the dominance of the Grotthuss chain reaction for electrical conduction, even at low humidity. It was shown that it is possible to determine the isosteric heat of water vapor adsorption by measuring the temperature variation of the electrical conduction as a function of humidity [3] by using the

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constant conductance data as a proxy for water layer thickness. This method of iso-conductance gave a measure of latent heat consistent with other techniques. It was concluded that the oxidized bismuth iron molybdate surface is hydrophobic and that at high humidity, water vapor adsorbs in clusters that display bulk properties. Following up on the phobic or philic nature of water adsorption and moving to ac admittance measurements, mesoporous compacted powders of various silicates were studied [4]. It was seen that there is an optimal balance between the hydrophobic versus the hydrophilic nature of the surface, as regards its use as a sensitive humidity sensor. The frequency dependence of the complex susceptibility tended to follow the “universal” dielectric response of solid materials as developed by Jonscher [5,6]. In this paper I expand the use of ac admittance measurements to study the humidity dependence of compacted polythiophene powder. Since this material is significantly conducting in the dry state, we will be concerned with separating the contributions due to the semiconducting polymer versus the adsorbed water. Following previous work we will review the most common mechanisms used to explain electrical conductance due to water

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adsorption [7,8]. Water molecules will irreversibly chemisorb onto available cation sites as hydroxyl groups when the surface is dry, and thus remain stable to any further changes of humidity at low to moderate temperature. This forms the base for the reversible physisorption of water molecules, which is a strong function of humidity. Electrical conduction can only occur through the water layers by the electrical migration of either protons or hydronium ions. A single monolayer of chemisorbed hydroxyl groups can conduct by proton hopping, due to hydroxyl dissociation under the influence of an applied electrical field, but if physisorbed water is present then hydronium ions can diffuse or migrate as a unit. At a high concentration of physisorbed water only some of the water molecules will be ionized as H3 O+ and the extra proton will hop from molecule to molecule, by a process called the Grotthuss chain reaction [9]. Finally, at extremely high humidity, water will condense to fill a spherical mesopore if its radius is not too large, due to a surface tension effect attributed to Kelvin [7,10]. In the bulk liquid water thus produced both the Grotthuss process and hydronium ion diffusion may occur. This succession of mechanisms naturally gives a rapid increase of conductance with increased humidity [7,9,11]. The surface adsorption of water vapor is often not uniform. A hydrophobic surface will promote cluster growth of water molecules at lower relative humidity than a hydrophilic surface will. A hydrophilic surface tends to favor multi-layer growth, whereas clustering occurs near 50% coverage of OH sites on a hydrophobic surface [12]. The process is reversible with an adsorption energy near the latent heat of condensation of free water (0.45 eV). Polythiophene has seen application in the field of organic semiconductors as the central component of field effect transistors, light emitting diodes, photovoltaic cells, and so on [13]. The conductivity mechanism for an organic semiconducting polymer is somewhat different than that of a covalent solid such as silicon, and in this case is a result of the ␲-bonding of the p-orbitals that make up each five membered aromatic thiophene ring. When these rings line up in the same plane the orbital overlap can be said to extend over many rings, with the conjugation length defined as the extent of this overlap. If the coplanar backbone is twisted the conjugation length is greatly reduced. Thus, the conjugation length determines the separation between adjacent energy levels, which I will call the bandgap, with undisturbed ␲-bonding representing a filled valence band and empty ␲* -antibonding orbitals representing the conduction band. Therefore, to produce electrical conduction in the polymer an electron must be excited from the valence to the conduction band by disturbing the ␲-bonds. This can be done through doping, that is oxidization or reduction with an appropriate counterion [14–17], or physical stress that disrupts the conjugation and produces donor or acceptor states of the resultant defects. The mobile charge carrier often takes the form of a polaron or bipolaron [14] with p-type doping the more common [18–20]. The polaron carrier readily moves as a unit along the thiophene chain but, in order to produce macroscopic conduction, charge must move between molecular units. This is facilitated

by the packing of planar molecules in a layered fashion [21–24] with carriers hopping perpendicular [18,21] to the molecular axis. This orientation can be achieved by vacuum evaporation unto a suitable substrate [21] where self-assembly of conjugated thiophene oligomers occurs. It can also occur by the application of a shear stress, as we will see later. The conductivity of polythiophene has been measured to lie in the range of 10−5 to 10−4 S/m with a mobility of 10−9 to 10−8 m2 /V/s [18]. In another reference [19,20] the reduced or neutral form of polythiophene, seen as red in color, had a conductivity of 1.8 × 10−6 S/m, whereas the oxidized form was gray with a conductivity of 1.5 × 10−5 S/m. In both cases the polymer was p-type and stated to be stable to moisture and oxygen. Of course, the conductivity of the polymer will be greatly dependent on the dopant used. An indirect bandgap of 1.8 eV is claimed [20] but another reference gives a value of 1.95 eV for a direct gap transition [15]. 2. Experimental details As fully described in a previous work [4] an enclosed and electrically shielded chamber was custom designed for the purpose of measuring the impedance spectroscopy of powdered samples. The powder is poured between stainless steel co-axial electrodes into a space with a volume of 43.2 mm3 , and with the electrodes set into an insulating Teflon base. The powder is added using the funnel shape of the electrode tops and tapped into place with a Teflon spatula. Gold wires, held in place with silver dag, are used to connect the electrode sample space to contacts that connect through the sealed 182 cm3 Plexiglas chamber, which is designed to hold a small glass vial below the sample mount. The vial contains a concentrated salt solution that is used to control the equilibrium humidity of the chamber, once it is sealed with thumb screws. The chamber and salt vials were carefully designed in accordance with criteria set forth in the 1998 Annual Book of ASTM Standards [25] (i.e. 25 cm3 chamber volume per cm2 of solution surface area). Three salt solutions were used for a low, medium, and high humidity range. These are LiCl (11.3%), K2 CO3 (43.2%), and NaCl (75.3%) relative humidities at 25 ◦ C [25,26]. The experiments were performed at room temperature (≈22 ◦ C) and the samples were allowed to equilibriate overnight at each humidity. Steel rods at the top of the Plexiglas chamber are connected with simple clamps to the 16047A test fixture of a Hewlett Packard 4284A precision LCR meter. The chamber is surrounded with a grounded Faraday cage and the LCR meter is configured to measure the complex admittance of the powder capacitor. The absolute precision of the instrument is 10 pS with acceptable accuracy down to 1 nS. Ignoring edge effects, the open air capacitance Co of the coaxial cylinder was calculated to be 1.52 pF. The measured value was higher (≈3.4 pF) as it included stray or parasitic capacitance due to the leads and construction of the chamber. There was only a slight dependence on frequency. The powdered sample will not exhibit edge effects, as it does not extend outside the co-axial electrodes the way the air or Teflon holder does. This means that both stray capacitance and edge effects can be taken care

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of by subtracting the open air admittance from the sample data. After dismantling the sample the powder was weighed on an electronic scale. The polythiophene powder studied here was obtained from Sigma–Aldrich and is bromine terminated. It is a dark red loose powder that shows silver colored metallic streaks when scrapped into a dense packing. This seems to be a shearing effect as the silver streaks are not there to begin with, although there are some small particles of silver specks in the original powder. The steaks appear only at the sheared surface of the packing. The same effect is seen when polythiophene is ground with a mortar and pestle as happens when the powder is packed into the co-axial sample chamber with a Teflon spatula. The silvery surface shows long-term stability and a sample has been sitting in a mortar bowl for about a year, with no change in appearance. The density of packing or shearing is not the same from measurement to measurement and this is reflected in the two sample results, labeled Poly1 and Poly2, to follow, where the packing density of Poly2 was measured to be 1.67 times that of Poly1. It takes about 2 h to reach equilibrium after a change in relative humidity. This appears to be the time required for the air in the humidity chamber to regain equilibrium after a switch of salt solution. It does not represent the response time of the polythiophene powder, which is unknown. 3. Results 3.1. Data analysis procedure We start with the measured admittance data in the form Ym (f, Gm , Bm ) as obtained from the frequency scans as well as the open air offset data Yo (f, Go , Bo ) as measured for the three humidity ranges, which we will refer to as low (11.3%), medium (43.2%), and high (75.3%). The data sets were converted to the conductivity and the susceptibility for each humidity range by the following transformations σ=

εo (Gm − Go ) = 5.80 m−1 (Gm − Go ) Co

χ = χ =

Bm − Bo ωCo

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and the susceptibility as  χ = χ∞ +

f fo

n−1 sin

 nπ  2

(5)

where fo is a characteristic frequency and σ o = fo /2πεo . We see that σ ∝ fn but that χ ∝ fn−1 at low frequency and approaches a constant χ∞ at high frequency. 3.2. Admittance spectroscopy Fig. 1 shows the conductivity spectra of the Poly1 sample at low, medium, and high humidity. There is a strong differential response to humidity at low frequency but this vanishes at high frequency. I will discuss possible reasons for this in the next section. The low humidity curve implies a dc limit at about 4 × 10−6 S/m, which is typical for the reduced or neutral form of polythiophene as discussed in the introduction [19,20]. The susceptibility spectra in Fig. 2 shows a similar humidity effect, although with less response at intermediate frequencies. We see that the low frequency susceptibilities are very large, of the order of 1000. Taking the data from Fig. 1, in Fig. 3 the low humidity response, assumed dry, is subtracted from the medium and high humidity responses to produce two new curves. The resultant conductivity, due to the adsorbed water, peaks at about 22 or 45 kHz with a sharp fall off at about 100 kHz as summarized in Table 1. The same thing was done for polymer sample Poly2, as is shown in Fig. 4 and Table 1. The conductivity values are higher and the peak response has shifted up to about 60 kHz, which is a result of the denser packing.

(1) (2)

where ω = 2πf and Co = 1.52 pF. In interpreting the following results we will use the nonDebye model developed by Jonscher [5,6] in which the admittance is expected to have the following form Y = jωC + An (jω)n ,

with n < 1

(3)

where in circuit terms the capacitor C is parallel to a constant phase element (CPE). √ An is a constant dependent only on the exponent n and j = −1. The conductivity can then be written as  n  nπ  f cos (4) σ = σo fo 2

Fig. 1. Conductivity spectra of polythiophene sample Poly1 at different relative humidities. In this and the succeeding figures only every second data point is shown.

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Fig. 2. Susceptibility spectra of polythiophene sample Poly1 at different relative humidities.

Fig. 4. Differential conductivity spectra of Poly2 for medium and high relative humidity.

In the same fashion Fig. 5 displays a plot of the change in susceptibility for sample Poly1 as calculated from the data of Fig. 2. There are no peaks here but the excess response vanishes at a lower frequency than seen for the conductivity. The power law slopes (n) as defined in Eqs. (4) and (5) are calculated for the low frequency limits and summarized in Table 2. Fig. 6 shows the same calculation for the change in susceptibility of sample Poly2. The low frequency slopes are better defined here as shown in Table 2.

Fig. 3. Differential conductivity spectra of Poly1 for medium and high relative humidity. Table 1 Humidity effect conductivity peaks Sample

Frequency (kHz)

Magnitude (␮S/m)

Poly-1 (Fig. 3) Medium–low High–low

45 22

5.94 13.04

Poly-2 (Fig. 4) Medium–low High–low

60 60

8.43 25.3

Water (Fig. 7)

600

1890

Fig. 5. Differential susceptibility spectra of Poly1 for medium and high relative humidity.

W.M. Sears / Sensors and Actuators B 130 (2008) 661–667

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Table 2 Humidity effect power law slopes at low frequency Sample

n (σ)

n − 1 (χ)

Poly-1 (Figs. 3 and 5) Medium–low High–low

0.358 0.426

−0.791 −0.992

Poly-2 (Figs. 4 and 6) Medium–low High–low

0.323 0.203

−0.980 −1.039

Water (Figs. 7 and 8)

0.165

−1.070

Fig. 8. Susceptibility spectrum of distilled water.

Fig. 6. Differential susceptibility spectra of Poly2 for medium and high relative humidity.

Fig. 7 is a plot of the conductivity of distilled water measured in the same sample chamber. This was a difficult measurement because of the high evaporation rate of water in contact with a metal surface with a high thermal conductivity. The chamber was sufficiently leak tight so that moderate overfilling of the electrode space and a rapid frequency scan solved the problem. The figure shows the beginnings of a peak in conductivity near 600 kHz. Fig. 8 is a plot of the susceptibility of the same distilled water. It shows a χ∞ limit at high frequency of about 80 as expected. The low frequency slope is also typical as summarized in Tables 1 and 2. 4. Discussion

Fig. 7. Conductivity spectrum of distilled water.

The main point of discussion is the interpretation of the enhancement of conductivity seen in polythiophene exposed to humid air. There would seem to be only two possible explanations. The first is the standard model of the Grotthuss chain reaction which involves the movement of protons between water molecules and hydronium ions physisorbed on the mesoporous surface. Although the magnitude of the conductivity enhancement is similar to that seen in other studies of compacted powders [4,27] the abrupt loss of response at about 100 kHz is unusual. The peak in the change in conductivity due to humid air is usually not apparent for most materials, but this may be due to the greater difference between wet and dry condition conductivity, than occurs here for polythiophene. We should also note that if the conductivity is converted to the dielectric loss (i.e. χ = σ/2πεo f) the peak is no longer seen in the frequency spectrum. The second explanation is that there is no significant conduction through the adsorbed water layer itself, but that the presence of surface water molecules affects the polythiophene conduction that occurs through the overlap of adjacent ␲-bonds. This

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is similar to the Fermi level shift effect occasionally seen in other materials [1]. In this model the presence of surface water enhances the ␲-bond overlap, but has a strong frequency dependence. The likely hydrophobic nature of polythiophene makes this more probable, since there would be less water adsorption. As shown by Eq. (4) the conductivity is expected to increase with frequency as a power law function and this is certainly seen in the raw data of Fig. 1 and the low frequency regions in Figs. 3 and 4, as well as in Table 2. Therefore, the conductivity peaks in Figs. 3 and 4 represent a significant departure from Jonscher’s universal dielectric response [5,6], which is another argument for a Fermi level shift like effect. The susceptibility is expected to show a sharp decline with frequency as stated by Eq. (5) when n < 1, and is seen in Figs. 2, 5 and 6. This occurs in general because it is difficult for the dielectric to follow the increasingly more rapid changes in polarization at higher frequency. Therefore, our results might be explained either by the restricted motion of adsorbed water molecules, or a water enhanced ␲-bonding polarization. But again the abrupt loss of response at a moderate frequency of about 20 kHz is not consistent with conduction through adsorbed water layers. The flattening of the susceptibility of distilled water at about this frequency in Fig. 8 seems to be a similar limitation, but the question arises as to why the χ∞ limit would be the same for wet and dry polythiophene. As previously mentioned it was observed that polythiophene undergoes a color change, from red to silver, when subjected to shear stress. This is also accompanied by an increased conductivity, which is most likely due to an enhancement of ␲-bond orbital overlap as the different polymer strands are forced to line up. The planar nature of the thiophene rings makes this stacking between molecules fairly easy. This color change is similar to observations that polythiophene changes from red in a reduced or neutral condition to gray in an oxidized state [17]. The two samples used differ in packing density and thus the degree of orbital overlap, with the denser sample (Poly2) showing the greater response to humidity. Examining Table 1 in more detail we see that the peaks are at frequencies an order of magnitude lower than for free water and that the peak magnitudes for the high humidity case are about 1% of that for water. The relative humidity ratio of high to medium is 1.75 compared to the peak magnitude ratios of 2.20 for Poly1 and 3.00 for Poly2. In Table 2 we have the power law slopes for the differential humidity response of the two polymers. The n values for the conductivity curves give the expected range of values [5,6], although somewhat on the lossy side, as expected for water. The n − 1 values for the susceptibility curves give a common low frequency limit near −1 which represents an inverse frequency dependence (i.e. 1/f). The fact that this is also true for distilled water may mean that hydronium ion conduction dominates over Fermi level effects. But since this is a common observation that may be associated with space charge effects arising at the electrodes [5] we should not read too much into this. The conductivity and susceptibility curves give different n values which suggest a more complicated behavior than Jonscher’s universal response [6].

If one were to design a humidity sensor using polythiophene the optimum sensitivity is, of course, at the peak conductivity frequencies given in Table 1. But if used in a capacitive circuit, a low frequency would have the advantage of a higher dielectric constant. 5. Conclusion I conclude that in humid air there is no significant conduction through the adsorbed water layer itself, but that the presence of surface water molecules affects the polythiophene conduction that occurs through the overlap of adjacent ␲-bonds. In this model the presence of surface water enhances the ␲-bond overlap, but has a strong frequency dependence. Acknowledgment I would like to thank my daughter, Wendy Ann Sears, for getting me interested in the field of organic semiconductors. References [1] W.M. Sears, The effect of oxygen stoichiometry on the humidity sensing characteristics of bismuth iron molybdate, Sens. Actuators B 67 (2000) 161–172. [2] W.M. Sears, The effect of DC polarization on the humidity sensing characteristics of bismuth iron molybdate, Sens. Actuators B 107 (2005) 623– 631. [3] W.M. Sears, Isosteric heat of adsorption of water vapor on bismuth iron molybdate measured by the method of constant surface conductance, Langmuir 17 (2001) 5237–5244. [4] W.M. Sears, J.T. Banks, K.L. Hardy, S.M. Hartzke, The effect of humidity on the electrical susceptibility of mesoporous silicates formed from organosilane substitution on octylamine micelles, Sens. Actuators B 102 (2004) 86–96. [5] A.K. Jonscher, Analysis of the alternating current properties of ionic conductors, J. Mater. Sci. 13 (1978) 553–562. [6] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983. [7] E. Traversa, Ceramic sensors for humidity detection: the state-of-the-art and future developments, Sens. Actuators B 23 (1995) 135–156. [8] W.J. Fleming, A physical understanding of solid state humidity sensors, in: Proc. Int. Automotive Meet, Paper no. 810432, SAE, Detroit, USA, 1981, pp. 51–62. [9] J.H. Anderson, G.A. Parks, The electrical conductivity of silica gel in the presence of adsorbed water, J. Phys. Chem. 72 (1968) 3662– 3668. [10] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1967. [11] G. Gusmano, G. Montesperelli, E. Traversa, G. Mattogno, Microstructure and electrical properties of MgAl2 O4 thin films for humidity sensing, J. Am. Ceram. Soc. 76 (1993) 743–750. [12] B. Fubini, V. Bolis, M. Bailes, F.S. Stone, The reactivity of oxides with water vapor, Solid State Ionics 32/33 (1989) 258–272. [13] Denis Fichou (Ed.), Handbook of Oligo- and Polythiophenes, Wiley, New York, 1999. [14] G. Horowitz, Field-effect transistors based on short organic molecules, J. Mater. Chem. 9 (1999) 2021–2026. [15] Q.T. Vu, M. Pavlik, N. Hebestreit, U. Rammelt, W. Plieth, J. Pfleger, Nanocomposites based on titanium dioxide and polythiophene: structure and properties, React. Funct. Polym. 65 (2005) 69–77. [16] S. Geetha, D.C. Trivedi, A new route to synthesize high degree polythiophene in a room temperature melt medium, Synth. Met. 155 (2005) 232–239.

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Biography W.M. Sears received a BSc in physics from Acadia University in Nova Scotia (1972) and a PhD from McMaster University in Ontario (1978). He is presently professor of physics at Lakehead University with interests in surface science, gas sensors, and organic semiconductors.