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High-pressure low-temperature electrical properties of polypyrrole doped with p-toluenesulfonate
ELSEVIER
Synthetic Metals 81 (1996) 59-63
High-pressure low-temperature electrical properties of polypyrrole doped with p-toluenesulfonate I. Qrgzall a, B. Lorenz b, L. Dunsch ‘, A. Bard’, S.T. Ting b, P.-H. Hor b, H.D. Hochheimer d a Hochdrucklabor bei der Universitiit Potsdam, Telegrafenberg A43, D-14473 Potsdam, Germany b Texas Centerfor Superconductivity, University of Houston, Houston Science Center, Houston, TX 77204-5932, c Institutjiir Festktirperund Werkstofforschung eV, HelmholtzstraJe 20, D-01 I71 Dresden, Germany a Department of Physics, Colorado State University, Fort Collins, CO 80523, USA
USA
Received 16January1996;revised11April 1996;accepted 24April 1996
Abstract Theelectricalpropertiesof polypyrroledopedwith p-toluenesulfonate areinvestigatedat temperatures from 300to 1.2K andpressures up to 2 GPa.A lineartemperature dependence of thethermopower below150K supports theintrinsicmetallicnatureof thesamples. Theelectrical conductivity showsthe crossoverfrom nearest-neighbour hoppingto variablerangehoppingat low temperatures. Application of pressure resultsin a remarkabledecrease of thelocalizationstrengthof the chargecarriersanda weakertemperature dependence of theconductivity. Keywords:
Polypyrrole; Conductivity;Highpressure; Doping
1. Introduction
Polypyrrole is one of the conducting polymers with great promise for applications. The electrochemical synthesis and characterization of polypyrrole have been of great interest over the last years. The variety of processing parameters, e.g. the nature and concentration of counter ions, temperature, applied electrochemical potential and mode of synthesis, has a great influence on the properties of the polymer films. The stability of polypyrrole films depends on the choice of the counterion and on the electrochemical polymerization parameters [ 11. It has been shown that a microscopic twodimensional structure is responsible for the high stability and high conductivity of these films. Various structures for the two-dimensional metallic islands have been proposed in [ 21. The effect of the supporting electrolyte is important in two ways with respect to the properties of polypyrrole. Due to the possibility of applying higher anodic potentials the current density during the polymerization can be increased. Thus, the high polymerization rate causes the formation of two-dimensional structures. Furthermore, chemical side reactions can be avoided by the choice of the supporting electrolyte. Therefore, only the anions incorporated into the polymer matrix during the formation will influence the physical structure of the films. It has been demonstrated that polypyrrole films containing organic anions like tosylate or dodecylsulfate show better 0379-6779/96/$15.00 0 1996ElsevierScience S.A.All rightsreserved DrrPn970 6770,cG\02714 I\
mechanical and chemical stabilities and higher electronic conductivity in comparison with inorganic anions such as Cl- or ClO,- [ 31. X-ray induced photoelectron spectroscopy shows evidence for instabilities in perchlorate-doped polypyrrole films, whereas tosylate-doped samples are chemically stable. Furthermore, the current density during the polymerization process is also a crucial parameter in determining the properties of polypyrrole. The formation of two types of polypyrrole was identified. At low current densities (i< 0.3 mA/ cm2) one-dimensional chains are formed but at high current densities (i > 3 mA/cm’) two-dimensional microscopic structures grow. Ultraviolet and X-ray induced photoelectron spectra provided chemical and structural evidence for the existenceof two microscopic structure types. Since polypyrrole films have often been synthesized at medium current densities, a transition behaviour between these two structures has been observed. This may explain some of the discrepancies which can be found in the literature. The influence of synthesis conditions on the structure of electrochemically prepared conducting polymers has also been investigated using different methods: X-ray scattering, photoluminescence and electron spin resonance spectroscopy [ 1,2], Concerning the electrical conductivity much work has been done to understand the conduction mechanism (see Refs. in [4]).
60
I. Orgzall
et al. /Synthetic
In order to get more insight into the physical processes of conduction in these polymers we have performed detailed measurements at low temperatures (T> 1.2 K) and at high pressures (P<2 GPa). Two samples are synthesized with different current densities well above the 3 mA/cm* threshold in order to investigate the influence of the current density on the formation of the two-dimensional metallic islands and the electrical properties of the polymer. 2. Experimental 2.1. Sample synthesis
Polypyrrole films with predominantly two-dimensional microstructure were prepared by electrochemical polymerization on a platinum foil in acetonitrile in a glove box operated at water vapour and oxygen partial pressures below 1 ppm, respectively. Pyrrole was distilled prior to polymerization. Tosylate was used as counterion and dopant. The electrochemical potentials were kept at values above 1 V. Black homogeneous films were obtained at the electrode surface by electropolymerization, typically within several minutes. Two polypyrrole samples of different thickness were prepared using different average current densities. Sample Pl with a thickness of 3.5 p,rn was synthesized with a current density decreasing from 12 mA/cm2 at the beginning of the synthesis to 6 mA/cm’ at the end of the process. A second sample P2 was produced by keeping the current density at the constant value of 12 mA/cm* resulting in a film of 17 pm thickness. The room-temperature conductivity of Pl was 90 S/cm, almost twice as large as that of P2 (50 S/cm). 2.2. Experimental setup
Strips of size 3 X 7 mm wefe cut from the polymer foil for the thermopower and electrical conductivity measurements.
Metals
81 (1996)
59-63
Four-probe conductivity measurements were made by sputtering four gold contacts (600 nm thickness) on the sample surface and attaching 0.03 mm diameter Pt wires using silver paint. The film resistance was measured using an a-c. (16 Hz) resistance bridge LR400 (Linear Research Corporation). The maximum excitation voltage was 2 mV. A K-type thermocouple (300 to 40 K) and a germanium resistor (below 40 K) were used for temperature measurements. For thermopower measurements two heaters were pasted to the opposite edges of the sample. Two thermocouples were used to monitor the temperatures and to measure the voltage. The sample was cooled to about 20 K in a liquid helium Dewar. High-pressure experiments were performed using a beryllium-copper high-pressure clamp cell [ 51 which allowed us to generate hydrostatic pressures up to 2 GPa. The sample was prepared in a Teflon container. Fluorinert, FC-77 (3M), a completely fluorinated liquid organic compound, was used as a pressure transmitting medium. The pressure was measured at low temperatures by observing the superconducting transition temperature of a lead manometer enclosed in the sample chamber. The high-pressure cell fitted into a helium cryostat, allowing conductivity measurements from 300 to 1.2 K.
3, Thermopower pressure
and electrical conductivity
The temperature dependence of the thermopower of both samples is shown in Fig. 1 from room temperature to about 20 K. As indicated by the dashed lines the thermopower below 150 K shows typical ‘metallic’ behaviour, as frequently observed for highly conducting polypyrrole samples [ 61. This is in accordance with the metallic island model
10
, / , , ,
100
150
200
250
PI
300
Temperature [ K ] Fig. 1. Thermopower
of polypyrrole
at ambient
samples Pl and P2.. The linear temperature
dependence
is indicated
by the dashed lines.
1. Orgzall
et al. /Synthetic Metals 81 (1996) 59-63
proposed for this class of materials [4]. In contrast to the conductivity, the thermopower is expected to reAect mainly the properties of the metallic regions in the polymer explaining the observed linear temperature dependence. However, at higher temperatures, a systematic deviation from straightline behaviour is observed. This is an indication of temperature-induced changes of the metailic density of states by the introduction of disorder and the reduction of the coherence length of the conducting states. Comparing both samples (Pl and P2) it becomes obvious that there is a correlation between room-temperature conductivi ty and thermopower. The better conducting sample (PI ) shows the higher thermopower. Within the metallic island model we suggest that PI is characterized by a larger portion of metallic regions due to an increased number or size of the two-dimensional islands. The smaller average current density for the Pl synthesis obviously yields better conditions for the formation of the two-dimensionally ordered structures. Hence, the increase of current densities does not necessarily result in an improvement of the electrical and structural properties of the polypyrrole samples.
61
The electrical conductivity was determined from room temperature to 1.2 K. Fig. 2(a) and (b) (upper curves for P = 0) shows plots of the resistance in logarithmic scale versus TL1” (Mott plot, [7] ) for samples Pl and P2, respectively. Below 10 K the typical variable range hopping behaviour with the Mott exponent l/2 is indicated by the linear portion of the curves (dashed lines in Fig. 2). Despite the different room-temperature conductivities the temperature dependences of both samples Pl and P2 are almost identical. The conductivity is basically determined by the conduction paths between the metallic islands but not by conduction processes within the islands. The charge carrier transfer from island to island as the rate limiting process for conduction follows the same physical mechanism for samples Pl and P2. In order to get more insight into the nature of conduction at low as well as at high temperatures, we used the method proposed by Hill [ 81. Assuming a resistivity of the form: R=ATMb exp[ (To/T)p]
(1)
the activation energy AE = d( In R) /d( 1/kT) is given by
1.3
GPa
1.9GPa
0.0
0.2
0.4
0.6
0.8
1.0
T -112
10000
0.2 GPa
1000 rx $
l.OGPa 100
1.5 GPa
l10
0.0
0.2
0.4
0.6
0.8
1.0
T -112
Fig. 2. Temperature dependence of the electrical resistance of polypyrrole at different pressures (unlabelled: zero pressure): (a) sample PI; (b) sample P2.
62
AE= bkT+ TzpkT’ -p
I. Orgzall
et al. /Synthetic
(2)
At low enough temperatures the exponential in Eq. (1) represents the dominating temperature dependence compared to the exponential dependence TBb, and the first term in Eq. (2) can be neglected. Then, a double logarithmic plot of AE versus T allows for the determination of the exponent p. At high temperatures, however, the linear temperature term in Eq. (2) represents the dominant contribution to AE. For small enough To this high temperature limit may be valid even below room temperature. In the following we discuss the corresponding Hill’s representation of the data of sample P2 since the results for PI are almost identical. In Fig. 3, Hill’s curve for P2 shows two linear portions which are well distinguished by their slopes. The high temperature region extends from 300 to about 40 K with slope unity. This indicates that the temperature dependence of the resistivity (Eq. ( 1) ) is dominated by the prefactor T-b. This is also supported by the small value of To characteristic for highly conducting polymers. The plot of AE versus T yields a strictly linear dependence from room temperature to 40 K. This indicates that the second term in Eq. (2) does not depend on temperature, i.e.p = 1. This value ofp is characteristic for nearest-neighbour hopping and is expected as the high temperature limit of localized charge carrier transport [ 71. The parameters b and T,, are estimated from the A E versus T plot as b =0.74 and To= 3.5 K. The small value of To in the nearest-neighbour hopping regime is consistent with the weak localization of charge carriers and the high conductivity of the polypyrrole samples. In the low temperature limit the slope of the curve in Fig. 3 is l/2, indicating that variable range hopping is the dominant mechanism for conduction. This is consistent with the linear behaviour seen in the Mott plot of Fig. 2. The exponent p=1/2 can b e d ue to a one-dimensional conduction path connecting different metallic”islands as is expected if the length of the path is larger than the average hopping distance. On the other hand, the role of Coulomb interactions has to be taken into account. As shown by Efros and Shklovskii [9]
Metals
81 (1996)
59-63
the exponent l/2 is also obtained for a three-dimensional variable range hopping process if, at low temperatures, a gap in the density of states at the Fermi level is introduced by the Coulomb repulsion of the charge carriers. Since the T-l” dependence is only observed below 10 K the latter mechanism cannot be ruled out. The Mott temperatures are estimated as T,,= 100 K and To= 115 K for samples PI and P2, respectively. This is another indication that both samples are similar with respect to their conductive properties.
4. Electrical conductivity
at high pressures
The influence of pressure on the temperature dependence of the resistivity is also shown in Fig. 2. The lower curves represent the temperature dependence of R for different pressures up to 2 GPa. Compared with the zero-pressure data the overall characteristics of these curves are not changed by application of pressure. However, the conductivity shows a considerably weaker temperature dependence under pressure. For example, the resistance ratio R( 4.2 K) lR(300 K) for sample Pl decreases from 80 at ambient pressure to only 5 at about 2 GPa. The ambient pressure value is in good agreement with previous results 141. The main effect of pressure is the change of the Mott temperature Toin the variable range hopping regime. Fig. 4 shows a rapid decrease of T,, for small pressure, which tends to saturate at higher pressure. The data for both samples fit to the same curve as indicated by the dashed line in the figure. From the decrease of Toit is concluded that the pressure leads to a delocalization of the charge carriers along the conduction paths connecting the metallic islands. Accordingly, the tem-
r;
0
PI
,
100
E p
50
0
Fig. 3. Hill plot of the conductivity activation energy (Eq. (2)) for sample P2. The linear dependences at high and low temperatures are characterized by slopes 1 and l/2, respectively.
-
Pressure [GPa] Fig. 4. Pressuredependence of the Mott temperature 7’, in the variable range hopping regime: (0) sample Pl; (0) sample P2.
I. Orgzall
et al. /Synthetic
perature below which the Mott variable range hopping behaviour is observed shifts to lower values with increasing pressure (e.g. from 8 to 4 K for sample P2). It is worth noting that the extrapolation of the linear dependence in the variable range hopping regime to infinite temperature yields approximately the same value of A (Eq. ( 1) ) for all pressures. This shows that the prefactor A in Eq. ( 1) is independent of pressure.
Metals
81 (1996)
59-63
63
more detailed investigation of the optimal electrochemical conditions and current densities. The application of hydrostatic pressure mainly results in a decrease of the Mott temperature ?“, and a delocalization of the localized charge carriers. With increasing pressure the temperature dependence of the conductivity becomes appreciably weaker than at ambient pressure.
Acknowledgements 5. Conclusions Polypyrrole samples with a typical two-dimensionalmetallit island structure have been synthesized at different current densities and doped withp-toluenesulfonate. The temperature dependence of the electrical conductivity of these samples is described by the Mott variable range hopping model with a Mott exponent of l/2 at low temperatures. Above 40 K a crossover to the nearest-neighbour hopping regime is observed. The temperature dependence in this regime can be explained taking into account the pre-exponential temperature dependence in the hopping model. The linear behaviour of the thermopower below 150 K supports the validity of the metallic island model. The room-temperature values of the conductivity and thermopower depend on the electrochemical conditions of the synthesis, especially on the current density. In the high density region (i > 10 mA/cm2) a smaller or decreasing current density may even lead to a larger amount of two-dimensional metallic islands. The synthesis of polypyrrole with high electrical conductivity requires a
This work was supported by a grant from the Deutsche Forschungsgemeinschaft.
References [ 1] D. Schmeiaer, H. Naarmann and W. G&l, Synth. Met., 59 (1993) 211. [2] A. Bartl, L. Dunsch, H. Naarmann, D. Schmeiker and W. GGpel, Synth. Met., 61 (1993) 167. [3] H. Naarmann, in J.L. BrCdas and R.R. Chance (eds.), Conjugated Polymeric Materials, Kluwer, Dordrecht, 1990. [4] G. Paasch, D. Schme%er, A. Bard, H. Naarmann, L. Dunsch and W. Gijpel, Synth. Met., 66 (1994) 135. [5] C.W. Chu, T.F. Smith and W.E. Gardener, Phys. Rev. Mt., 20 (1968) 198. [6] CO. Yoon, M. Reghu, D. Moses and A.J. Heeger, Phys. Rev. B, 49 (1994) 10 851. [7] N.F. Mott and E.A. Davis, Electronic Processes in Non-crystalline Materials, Clarendon Press,Oxford, 2nd edn., 1979. [S] R.M. Hill, Phys. Status Solidi (a), 34 (1976) 601; R.M. Hill, Phys. Status Solidi (a), 35 (1976) K29. [9] A.L. Efros and B.I. Shklovskii, J. Phys. C: Solid State Phys., 8 (1975) L49.
Synthetic Metals 81 (1996) 59-63
High-pressure low-temperature electrical properties of polypyrrole doped with p-toluenesulfonate I. Qrgzall a, B. Lorenz b, L. Dunsch ‘, A. Bard’, S.T. Ting b, P.-H. Hor b, H.D. Hochheimer d a Hochdrucklabor bei der Universitiit Potsdam, Telegrafenberg A43, D-14473 Potsdam, Germany b Texas Centerfor Superconductivity, University of Houston, Houston Science Center, Houston, TX 77204-5932, c Institutjiir Festktirperund Werkstofforschung eV, HelmholtzstraJe 20, D-01 I71 Dresden, Germany a Department of Physics, Colorado State University, Fort Collins, CO 80523, USA
USA
Received 16January1996;revised11April 1996;accepted 24April 1996
Abstract Theelectricalpropertiesof polypyrroledopedwith p-toluenesulfonate areinvestigatedat temperatures from 300to 1.2K andpressures up to 2 GPa.A lineartemperature dependence of thethermopower below150K supports theintrinsicmetallicnatureof thesamples. Theelectrical conductivity showsthe crossoverfrom nearest-neighbour hoppingto variablerangehoppingat low temperatures. Application of pressure resultsin a remarkabledecrease of thelocalizationstrengthof the chargecarriersanda weakertemperature dependence of theconductivity. Keywords:
Polypyrrole; Conductivity;Highpressure; Doping
1. Introduction
Polypyrrole is one of the conducting polymers with great promise for applications. The electrochemical synthesis and characterization of polypyrrole have been of great interest over the last years. The variety of processing parameters, e.g. the nature and concentration of counter ions, temperature, applied electrochemical potential and mode of synthesis, has a great influence on the properties of the polymer films. The stability of polypyrrole films depends on the choice of the counterion and on the electrochemical polymerization parameters [ 11. It has been shown that a microscopic twodimensional structure is responsible for the high stability and high conductivity of these films. Various structures for the two-dimensional metallic islands have been proposed in [ 21. The effect of the supporting electrolyte is important in two ways with respect to the properties of polypyrrole. Due to the possibility of applying higher anodic potentials the current density during the polymerization can be increased. Thus, the high polymerization rate causes the formation of two-dimensional structures. Furthermore, chemical side reactions can be avoided by the choice of the supporting electrolyte. Therefore, only the anions incorporated into the polymer matrix during the formation will influence the physical structure of the films. It has been demonstrated that polypyrrole films containing organic anions like tosylate or dodecylsulfate show better 0379-6779/96/$15.00 0 1996ElsevierScience S.A.All rightsreserved DrrPn970 6770,cG\02714 I\
mechanical and chemical stabilities and higher electronic conductivity in comparison with inorganic anions such as Cl- or ClO,- [ 31. X-ray induced photoelectron spectroscopy shows evidence for instabilities in perchlorate-doped polypyrrole films, whereas tosylate-doped samples are chemically stable. Furthermore, the current density during the polymerization process is also a crucial parameter in determining the properties of polypyrrole. The formation of two types of polypyrrole was identified. At low current densities (i< 0.3 mA/ cm2) one-dimensional chains are formed but at high current densities (i > 3 mA/cm’) two-dimensional microscopic structures grow. Ultraviolet and X-ray induced photoelectron spectra provided chemical and structural evidence for the existenceof two microscopic structure types. Since polypyrrole films have often been synthesized at medium current densities, a transition behaviour between these two structures has been observed. This may explain some of the discrepancies which can be found in the literature. The influence of synthesis conditions on the structure of electrochemically prepared conducting polymers has also been investigated using different methods: X-ray scattering, photoluminescence and electron spin resonance spectroscopy [ 1,2], Concerning the electrical conductivity much work has been done to understand the conduction mechanism (see Refs. in [4]).
60
I. Orgzall
et al. /Synthetic
In order to get more insight into the physical processes of conduction in these polymers we have performed detailed measurements at low temperatures (T> 1.2 K) and at high pressures (P<2 GPa). Two samples are synthesized with different current densities well above the 3 mA/cm* threshold in order to investigate the influence of the current density on the formation of the two-dimensional metallic islands and the electrical properties of the polymer. 2. Experimental 2.1. Sample synthesis
Polypyrrole films with predominantly two-dimensional microstructure were prepared by electrochemical polymerization on a platinum foil in acetonitrile in a glove box operated at water vapour and oxygen partial pressures below 1 ppm, respectively. Pyrrole was distilled prior to polymerization. Tosylate was used as counterion and dopant. The electrochemical potentials were kept at values above 1 V. Black homogeneous films were obtained at the electrode surface by electropolymerization, typically within several minutes. Two polypyrrole samples of different thickness were prepared using different average current densities. Sample Pl with a thickness of 3.5 p,rn was synthesized with a current density decreasing from 12 mA/cm2 at the beginning of the synthesis to 6 mA/cm’ at the end of the process. A second sample P2 was produced by keeping the current density at the constant value of 12 mA/cm* resulting in a film of 17 pm thickness. The room-temperature conductivity of Pl was 90 S/cm, almost twice as large as that of P2 (50 S/cm). 2.2. Experimental setup
Strips of size 3 X 7 mm wefe cut from the polymer foil for the thermopower and electrical conductivity measurements.
Metals
81 (1996)
59-63
Four-probe conductivity measurements were made by sputtering four gold contacts (600 nm thickness) on the sample surface and attaching 0.03 mm diameter Pt wires using silver paint. The film resistance was measured using an a-c. (16 Hz) resistance bridge LR400 (Linear Research Corporation). The maximum excitation voltage was 2 mV. A K-type thermocouple (300 to 40 K) and a germanium resistor (below 40 K) were used for temperature measurements. For thermopower measurements two heaters were pasted to the opposite edges of the sample. Two thermocouples were used to monitor the temperatures and to measure the voltage. The sample was cooled to about 20 K in a liquid helium Dewar. High-pressure experiments were performed using a beryllium-copper high-pressure clamp cell [ 51 which allowed us to generate hydrostatic pressures up to 2 GPa. The sample was prepared in a Teflon container. Fluorinert, FC-77 (3M), a completely fluorinated liquid organic compound, was used as a pressure transmitting medium. The pressure was measured at low temperatures by observing the superconducting transition temperature of a lead manometer enclosed in the sample chamber. The high-pressure cell fitted into a helium cryostat, allowing conductivity measurements from 300 to 1.2 K.
3, Thermopower pressure
and electrical conductivity
The temperature dependence of the thermopower of both samples is shown in Fig. 1 from room temperature to about 20 K. As indicated by the dashed lines the thermopower below 150 K shows typical ‘metallic’ behaviour, as frequently observed for highly conducting polypyrrole samples [ 61. This is in accordance with the metallic island model
10
, / , , ,
100
150
200
250
PI
300
Temperature [ K ] Fig. 1. Thermopower
of polypyrrole
at ambient
samples Pl and P2.. The linear temperature
dependence
is indicated
by the dashed lines.
1. Orgzall
et al. /Synthetic Metals 81 (1996) 59-63
proposed for this class of materials [4]. In contrast to the conductivity, the thermopower is expected to reAect mainly the properties of the metallic regions in the polymer explaining the observed linear temperature dependence. However, at higher temperatures, a systematic deviation from straightline behaviour is observed. This is an indication of temperature-induced changes of the metailic density of states by the introduction of disorder and the reduction of the coherence length of the conducting states. Comparing both samples (Pl and P2) it becomes obvious that there is a correlation between room-temperature conductivi ty and thermopower. The better conducting sample (PI ) shows the higher thermopower. Within the metallic island model we suggest that PI is characterized by a larger portion of metallic regions due to an increased number or size of the two-dimensional islands. The smaller average current density for the Pl synthesis obviously yields better conditions for the formation of the two-dimensionally ordered structures. Hence, the increase of current densities does not necessarily result in an improvement of the electrical and structural properties of the polypyrrole samples.
61
The electrical conductivity was determined from room temperature to 1.2 K. Fig. 2(a) and (b) (upper curves for P = 0) shows plots of the resistance in logarithmic scale versus TL1” (Mott plot, [7] ) for samples Pl and P2, respectively. Below 10 K the typical variable range hopping behaviour with the Mott exponent l/2 is indicated by the linear portion of the curves (dashed lines in Fig. 2). Despite the different room-temperature conductivities the temperature dependences of both samples Pl and P2 are almost identical. The conductivity is basically determined by the conduction paths between the metallic islands but not by conduction processes within the islands. The charge carrier transfer from island to island as the rate limiting process for conduction follows the same physical mechanism for samples Pl and P2. In order to get more insight into the nature of conduction at low as well as at high temperatures, we used the method proposed by Hill [ 81. Assuming a resistivity of the form: R=ATMb exp[ (To/T)p]
(1)
the activation energy AE = d( In R) /d( 1/kT) is given by
1.3
GPa
1.9GPa
0.0
0.2
0.4
0.6
0.8
1.0
T -112
10000
0.2 GPa
1000 rx $
l.OGPa 100
1.5 GPa
l10
0.0
0.2
0.4
0.6
0.8
1.0
T -112
Fig. 2. Temperature dependence of the electrical resistance of polypyrrole at different pressures (unlabelled: zero pressure): (a) sample PI; (b) sample P2.
62
AE= bkT+ TzpkT’ -p
I. Orgzall
et al. /Synthetic
(2)
At low enough temperatures the exponential in Eq. (1) represents the dominating temperature dependence compared to the exponential dependence TBb, and the first term in Eq. (2) can be neglected. Then, a double logarithmic plot of AE versus T allows for the determination of the exponent p. At high temperatures, however, the linear temperature term in Eq. (2) represents the dominant contribution to AE. For small enough To this high temperature limit may be valid even below room temperature. In the following we discuss the corresponding Hill’s representation of the data of sample P2 since the results for PI are almost identical. In Fig. 3, Hill’s curve for P2 shows two linear portions which are well distinguished by their slopes. The high temperature region extends from 300 to about 40 K with slope unity. This indicates that the temperature dependence of the resistivity (Eq. ( 1) ) is dominated by the prefactor T-b. This is also supported by the small value of To characteristic for highly conducting polymers. The plot of AE versus T yields a strictly linear dependence from room temperature to 40 K. This indicates that the second term in Eq. (2) does not depend on temperature, i.e.p = 1. This value ofp is characteristic for nearest-neighbour hopping and is expected as the high temperature limit of localized charge carrier transport [ 71. The parameters b and T,, are estimated from the A E versus T plot as b =0.74 and To= 3.5 K. The small value of To in the nearest-neighbour hopping regime is consistent with the weak localization of charge carriers and the high conductivity of the polypyrrole samples. In the low temperature limit the slope of the curve in Fig. 3 is l/2, indicating that variable range hopping is the dominant mechanism for conduction. This is consistent with the linear behaviour seen in the Mott plot of Fig. 2. The exponent p=1/2 can b e d ue to a one-dimensional conduction path connecting different metallic”islands as is expected if the length of the path is larger than the average hopping distance. On the other hand, the role of Coulomb interactions has to be taken into account. As shown by Efros and Shklovskii [9]
Metals
81 (1996)
59-63
the exponent l/2 is also obtained for a three-dimensional variable range hopping process if, at low temperatures, a gap in the density of states at the Fermi level is introduced by the Coulomb repulsion of the charge carriers. Since the T-l” dependence is only observed below 10 K the latter mechanism cannot be ruled out. The Mott temperatures are estimated as T,,= 100 K and To= 115 K for samples PI and P2, respectively. This is another indication that both samples are similar with respect to their conductive properties.
4. Electrical conductivity
at high pressures
The influence of pressure on the temperature dependence of the resistivity is also shown in Fig. 2. The lower curves represent the temperature dependence of R for different pressures up to 2 GPa. Compared with the zero-pressure data the overall characteristics of these curves are not changed by application of pressure. However, the conductivity shows a considerably weaker temperature dependence under pressure. For example, the resistance ratio R( 4.2 K) lR(300 K) for sample Pl decreases from 80 at ambient pressure to only 5 at about 2 GPa. The ambient pressure value is in good agreement with previous results 141. The main effect of pressure is the change of the Mott temperature Toin the variable range hopping regime. Fig. 4 shows a rapid decrease of T,, for small pressure, which tends to saturate at higher pressure. The data for both samples fit to the same curve as indicated by the dashed line in the figure. From the decrease of Toit is concluded that the pressure leads to a delocalization of the charge carriers along the conduction paths connecting the metallic islands. Accordingly, the tem-
r;
0
PI
,
100
E p
50
0
Fig. 3. Hill plot of the conductivity activation energy (Eq. (2)) for sample P2. The linear dependences at high and low temperatures are characterized by slopes 1 and l/2, respectively.
-
Pressure [GPa] Fig. 4. Pressuredependence of the Mott temperature 7’, in the variable range hopping regime: (0) sample Pl; (0) sample P2.
I. Orgzall
et al. /Synthetic
perature below which the Mott variable range hopping behaviour is observed shifts to lower values with increasing pressure (e.g. from 8 to 4 K for sample P2). It is worth noting that the extrapolation of the linear dependence in the variable range hopping regime to infinite temperature yields approximately the same value of A (Eq. ( 1) ) for all pressures. This shows that the prefactor A in Eq. ( 1) is independent of pressure.
Metals
81 (1996)
59-63
63
more detailed investigation of the optimal electrochemical conditions and current densities. The application of hydrostatic pressure mainly results in a decrease of the Mott temperature ?“, and a delocalization of the localized charge carriers. With increasing pressure the temperature dependence of the conductivity becomes appreciably weaker than at ambient pressure.
Acknowledgements 5. Conclusions Polypyrrole samples with a typical two-dimensionalmetallit island structure have been synthesized at different current densities and doped withp-toluenesulfonate. The temperature dependence of the electrical conductivity of these samples is described by the Mott variable range hopping model with a Mott exponent of l/2 at low temperatures. Above 40 K a crossover to the nearest-neighbour hopping regime is observed. The temperature dependence in this regime can be explained taking into account the pre-exponential temperature dependence in the hopping model. The linear behaviour of the thermopower below 150 K supports the validity of the metallic island model. The room-temperature values of the conductivity and thermopower depend on the electrochemical conditions of the synthesis, especially on the current density. In the high density region (i > 10 mA/cm2) a smaller or decreasing current density may even lead to a larger amount of two-dimensional metallic islands. The synthesis of polypyrrole with high electrical conductivity requires a
This work was supported by a grant from the Deutsche Forschungsgemeinschaft.
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