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D.c. electrical transport in a new conducting polymer: oxidized poly(N-vinylpyrrole)
S y n t h e t i c Metals, 46 (1992) 127-131
127
Short Communication
D.c. electrical t r a ns por t in a n e w c o n d u c t i n g polymer: oxidized poly(N-vinylpyrrole) R. C a g n o l a t i , M. L u c c h e s i a n d P. A. R o l l a D i p a r t i m e n t o d i F i s i c a e Consorzio I n t e r u n i v e r s i t a r i o d i F i s i c a d e l l a M a t e r i a , p i a z z a TorriceUi 2, 56100 P i s a (Italy)
V. C a s t e l v e t r o * , F. C i a r d e l l i a n d A. C o l l i g i a n i * * Dipartimento di Chimica e Chimica Ind~triale,
U n i v e r s i t d d i Pisa, P i s a (Italy)
(Received May 8, 1991; in revised form July 28, 1991; accepted August 8, 1991)
Abstract D.c. electrical transport properties of pellets of oxidized poly(N-vinylpyrrole), a new conducting ladder polymer, are studied. D.c. conductivity data are coherent with a threedimensional variable range hopping transport model. Relevant microscopic parameters of the model are inferred from data and are briefly discussed.
Introduction R e s e a r c h o n o l i g o m e r s a n d p o l y m e r s o f p y r r o l e h a s b e e n c o n t i n u o u s in r e c e n t y e a r s : in t h i s f i e l d o u r a t t e n t i o n h a s b e e n d e v o t e d t o t h e s t u d y o f e l e c t r i c a l t r a n s p o r t p h e n o m e n a in a p y r r o l e p o l y m e r , o x i d i z e d p o l y ( N - v i n y l p y r r o l e ) , t h e s t r u c t u r e o f w h i c h is s h o w n in F i g . 1. T h e s a m p l e s t h a t w e
CH
I
CH
CH
I
Fig. 1. Molecular structure of oxidized poly(N-vinylpyrrole). *Also belongs to: ENI/Scuola Normale Superiore, Pisa, Italy. **Also belongs to: Dipartimento di Chimica Industriale, Universit~ di Messina, Italy.
0379-6779/92/$5.00
© 1 9 9 2 - Elsevier Sequoia. All rights reserved
128 characterized are semiconductors with a r o o m t em perat ure conductivity of 1 0 - 3 - 1 0 -4 S/cm; we made measurements from 100 K to r o o m t em perat ure and found a conductivity increasing with temperature, as reported in the literature on pyrrole polymers [1, 2]. These data are discussed in terms of the variable range hopping model, which fits well the electrical behaviour of this conducting macromolecule.
Experimental
Sample preparation Oxidized poly(N-vinylpyrrole) is a ladder polymer, the conducting chain of which is formed by oxidative polymerization of the side groups (pyrrole rings) of the starting polymeric chain (saturated aliphatic chain). Oxidative polymerization of the pyrrole rings was obtained by stirring an aqueous solution of FeCl3 in HCI (Fea+/pyrrole r a t i o = 4 : l ) and a mixture of poly(N-vinylpyrrole) in acetonitrile at 70 °C for 12 h: at the end C1- was the doping counterion; detailed information is given elsewhere [3-5]. The obtained black precipitate was filtered and washed with water and acetone, dried under vacuum, finely powdered and pressed at 104 kg/cm 2 to 5 m m diameter pellets under a controlled environment. The doping level, resulting from the C1/N ratio determined by elemental analysis, c o r r e s p o n d e d to one charge for three pyrrole rings.
Measurements The d.c. conductivity of oxidized poly(N-vinylpyrrole) was measured as a function of temperature using a simple two-electrode cell in the form of a plane parallel-plate capacitor suspended in a glass vessel, which can be immersed in a liquid nitrogen bath [6]: samples were pressed between the parallel plates and kept at a low pressure under argon. Contacts were assured by means of indium foils between the pellet and electrode plates. Resistance versus t em pe r at ur e m e a s urem ent s were p e r f o r m e d so that the linear behaviour of the I - V characteristics of the sample was verified in the whole temp e r at ur e range. The current was supplied by a stabilized current generator and voltage m e a s ur e m ent s were perform ed with a Schlumberger 7081 precision digital voltmeter. Temperature was measured by a Fluke 2190A digital thermometer, using E thermocouples. Absolute resistivity at r o o m temp er atu r e was m e a s ur e d by a four-point probe (Jandel Engineering Ltd.). Data acquisition was p e r f o r m e d via an IEEE-488 bus by an IBM PC.
Results and comments Resistances of oxidized poly(N-vinylpyrrole) pellets were m easured in the temp er atu r e range 1 0 0 - 3 0 0 K. In Fig. 2 the log of the sample resistance,
129
:F 5: 0a r,.,
4~-
o
0 1.35
1.4.
1.4,5
1.5
1.55
1.6
1.65
1.7
1.75
1.8
0000/r(K))~0.~ Fig. 2. Arrhenius plot of the normalized resistance poly(N-vinylpyrrole) vs. (1000/T(K)) TM.
R(T)/R(295 K)
of a s a m p l e of oxidized
normalized according to the value m e a s ured at 295 K, is plotted versus (1000/T) TM for a typical sample. Experimental data are fitted by the following equation, according to Mott's variable range hopping model in three dimensions [7]: P = Po exp[(To/T) TM]
(1)
The characteristic t e m p e r a t u r e To can be written: 16a s To = - KN(E~)
(2)
where K is the Boltzmann constant, N(EF) is the density of states at the Fermi level and the localization of states a-~ can be expressed in terms of the hopping distance d by a - ~= 81rd4N(EF)KT
(3)
9 The definition of Po in eqn. (1) is given as 1 po = e 2 N ( E f ) d 2 u
°
(4)
where e is the electron charge and Uo is a frequency factor that can be related to the transition rate P according to
130 TABLE 1 Resistivity and microscopic parameters of a pellet of oxidized poly(N-vinylpyrrole) estimated according to the three-dimensional variable range hopping model Resistivity p (300 IO Localization of states aHopping distance d (300 K) Density of states at Fermi level N(EF) Hopping energy W (300 IO
0.8)< 104 l~ cm 2.~ 20 ~, 3 × 102° states/(eV cm3) 0.14 eV
w h e r e W i n d i c a t e s t h e e n e r g y difference b e t w e e n the h o p p i n g localized s t a t e s i n v o l v e d in t h e p r o c e s s . T h e c h a r a c t e r i s t i c t e m p e r a t u r e To with its s t a n d a r d deviation, d e t e r m i n e d f r o m t h e r e s u l t s o b t a i n e d f r o m fitting p r o c e d u r e s ( m i n i m u m 22 ) p e r f o r m e d o n s e v e n different e x p e r i m e n t a l d a t a sets, is ( 8 . 6 _ + 0 . 6 ) × 1 0 7 K. In o r d e r to calculate the m i c r o s c o p i c p a r a m e t e r s o f the c o n d u c t i n g p o l y m e r i c s a m p l e , an e s t i m a t e o f the f r e q u e n c y f a c t o r 9o is n e c e s s a r y . A c c o r d i n g to c a l c u l a t i o n s r e p o r t e d e l s e w h e r e [8] for a m o r p h o u s s y s t e m s w h e r e t r a n s p o r t m o d e l l i n g is b a s e d o n localized states, we h a v e a s s u m e d an e s t i m a t e o f 9o o f 1018 s - ~ to b e reliable. T h e e s t i m a t e s of m i c r o s c o p i c p a r a m e t e r s d e t e r m i n e d a c c o r d i n g to eqns. ( 1 ) - ( 4 ) are g i v e n in T a b l e 1.
Conclusions Electrical t r a n s p o r t b e h a v i o u r o f oxidized p o l y ( N - v i n y l p y r r o l e ) c a n b e r e l a t e d to t h e b e h a v i o u r o f a m o r p h o u s s e m i c o n d u c t i n g m a t e r i a l s , w h e r e the localization o f s t a t e s p l a y s a r e l e v a n t role in t r a n s p o r t p r o c e s s e s [7]. In p a r t i c u l a r it h a s b e e n s h o w n t h a t the o b s e r v e d v a r i a t i o n o f resistivity v e r s u s t e m p e r a t u r e c a n b e e x p l a i n e d in t e r m s o f t h e t h r e e - d i m e n s i o n a l v a r i a b l e range hopping model. The microscopic parameters determined from experimental data according t o t h e v a r i a b l e r a n g e h o p p i n g m o d e l are c o n s i s t e n t with d a t a r e p o r t e d in t h e l i t e r a t u r e f o r b o t h p o l y m e r i c a n d inorganic a m o r p h o u s s e m i c o n d u c t o r s . No e v i d e n c e h a s b e e n o b t a i n e d a b o u t t h e p e c u l i a r kind o f localized states; t h e i r c h a r a c t e r i s t i c s c a n b e r e l a t e d to t h e t h e o r y o f S u e t al. [9, 10]. This m e a n s t h a t l o c a l i z e d s t a t e s c a n b e s k e t c h e d a s p o l a r o n s or b i p o l a r o n s , due to t h e n o n - d e g e n e r a t e g r o u n d state of t h e p o l y m e r i c c h a i n o f oxidized p o l y ( N - v i n y l p y r r o l e ) , a n d c h a r g e c a r r i e r s h o p b e t w e e n t h e m in t h e c o n d u c t i o n p r o c e s s , m a i n l y in a n i n t e r c h a i n m e c h a n i s m [ 11, 12 ]. Effects o f s t r u c t u r a l disorder, p o i n t e d o u t b y p r e v i o u s c h a r a c t e r i z a t i o n [3], a n d s u r f a c e states, d u e to t h e p o w d e r y f o r m o f s a m p l e s , m a y also h a v e a n influence o n t h e t r a n s p o r t p r o c e s s [11, 12].
131
Acknowledgement This research has been Fisica della Materia (INFM).
supported
by
Consorzio
Interuniversitario
di
References 1 G. Wegner, W. W e m e t , D. T. Glatzhofer, J. Ulaiiski, Ch. Kroehnke and M. Mohammadi, Synth. Met., 18 (1987) 1-6. 2 D. S. Maddison, J. Unsworth and R. B. Roberts, Synth. Met., 26 (1988) 9 9 - 1 0 8 . 3 V. Castelvetro, A. Colligiani, F. Ciardelli, G. Ruggeri and M. Giordano, New Polym. Mater., 2 (1990) 93. 4 A. Priola, G. Gatti and S. Cesca, Makromol. Chem., 180 (1979) 1. 5 B. A. Trofimov, A. I. Mikhaleva and L. V. Morozova, Russ. Chem. Rev., 54 (1985) 609. 6 B. J. Jones, D. Tidy and G. Williams, J. Phys. E: Sci. Instrum., 9 (1976) 693. 7 N. F. Mott and E. A. Davies, Electronic Processes in Non-crystalline Materials, Clarendon Press, Oxford, 1971. 8 D. Wuertz and P. Thomas, Phys. Status Solidi Co), 88 (1978) K73. 9 W.-P. Su, J. R. Schrieffer and A. J. Heeger, Phys. Rev. Lett., 42 (1979) 1698. 10 W.-P. Su, J. R. Schrieffer and A. J. Heeger, Phys. Rev. B, 22 (1980) 2099. 11 V. Enkelmann, J. Rfihe and G. Wegner, Synth. Met., 37 (1990) 7 9 - 8 9 . 12 F. Kremer, J. Rtihe and W. H. Meyer, Makromol. Chem., Macromol. Symp., 37 (1990) 115.
127
Short Communication
D.c. electrical t r a ns por t in a n e w c o n d u c t i n g polymer: oxidized poly(N-vinylpyrrole) R. C a g n o l a t i , M. L u c c h e s i a n d P. A. R o l l a D i p a r t i m e n t o d i F i s i c a e Consorzio I n t e r u n i v e r s i t a r i o d i F i s i c a d e l l a M a t e r i a , p i a z z a TorriceUi 2, 56100 P i s a (Italy)
V. C a s t e l v e t r o * , F. C i a r d e l l i a n d A. C o l l i g i a n i * * Dipartimento di Chimica e Chimica Ind~triale,
U n i v e r s i t d d i Pisa, P i s a (Italy)
(Received May 8, 1991; in revised form July 28, 1991; accepted August 8, 1991)
Abstract D.c. electrical transport properties of pellets of oxidized poly(N-vinylpyrrole), a new conducting ladder polymer, are studied. D.c. conductivity data are coherent with a threedimensional variable range hopping transport model. Relevant microscopic parameters of the model are inferred from data and are briefly discussed.
Introduction R e s e a r c h o n o l i g o m e r s a n d p o l y m e r s o f p y r r o l e h a s b e e n c o n t i n u o u s in r e c e n t y e a r s : in t h i s f i e l d o u r a t t e n t i o n h a s b e e n d e v o t e d t o t h e s t u d y o f e l e c t r i c a l t r a n s p o r t p h e n o m e n a in a p y r r o l e p o l y m e r , o x i d i z e d p o l y ( N - v i n y l p y r r o l e ) , t h e s t r u c t u r e o f w h i c h is s h o w n in F i g . 1. T h e s a m p l e s t h a t w e
CH
I
CH
CH
I
Fig. 1. Molecular structure of oxidized poly(N-vinylpyrrole). *Also belongs to: ENI/Scuola Normale Superiore, Pisa, Italy. **Also belongs to: Dipartimento di Chimica Industriale, Universit~ di Messina, Italy.
0379-6779/92/$5.00
© 1 9 9 2 - Elsevier Sequoia. All rights reserved
128 characterized are semiconductors with a r o o m t em perat ure conductivity of 1 0 - 3 - 1 0 -4 S/cm; we made measurements from 100 K to r o o m t em perat ure and found a conductivity increasing with temperature, as reported in the literature on pyrrole polymers [1, 2]. These data are discussed in terms of the variable range hopping model, which fits well the electrical behaviour of this conducting macromolecule.
Experimental
Sample preparation Oxidized poly(N-vinylpyrrole) is a ladder polymer, the conducting chain of which is formed by oxidative polymerization of the side groups (pyrrole rings) of the starting polymeric chain (saturated aliphatic chain). Oxidative polymerization of the pyrrole rings was obtained by stirring an aqueous solution of FeCl3 in HCI (Fea+/pyrrole r a t i o = 4 : l ) and a mixture of poly(N-vinylpyrrole) in acetonitrile at 70 °C for 12 h: at the end C1- was the doping counterion; detailed information is given elsewhere [3-5]. The obtained black precipitate was filtered and washed with water and acetone, dried under vacuum, finely powdered and pressed at 104 kg/cm 2 to 5 m m diameter pellets under a controlled environment. The doping level, resulting from the C1/N ratio determined by elemental analysis, c o r r e s p o n d e d to one charge for three pyrrole rings.
Measurements The d.c. conductivity of oxidized poly(N-vinylpyrrole) was measured as a function of temperature using a simple two-electrode cell in the form of a plane parallel-plate capacitor suspended in a glass vessel, which can be immersed in a liquid nitrogen bath [6]: samples were pressed between the parallel plates and kept at a low pressure under argon. Contacts were assured by means of indium foils between the pellet and electrode plates. Resistance versus t em pe r at ur e m e a s urem ent s were p e r f o r m e d so that the linear behaviour of the I - V characteristics of the sample was verified in the whole temp e r at ur e range. The current was supplied by a stabilized current generator and voltage m e a s ur e m ent s were perform ed with a Schlumberger 7081 precision digital voltmeter. Temperature was measured by a Fluke 2190A digital thermometer, using E thermocouples. Absolute resistivity at r o o m temp er atu r e was m e a s ur e d by a four-point probe (Jandel Engineering Ltd.). Data acquisition was p e r f o r m e d via an IEEE-488 bus by an IBM PC.
Results and comments Resistances of oxidized poly(N-vinylpyrrole) pellets were m easured in the temp er atu r e range 1 0 0 - 3 0 0 K. In Fig. 2 the log of the sample resistance,
129
:F 5: 0a r,.,
4~-
o
0 1.35
1.4.
1.4,5
1.5
1.55
1.6
1.65
1.7
1.75
1.8
0000/r(K))~0.~ Fig. 2. Arrhenius plot of the normalized resistance poly(N-vinylpyrrole) vs. (1000/T(K)) TM.
R(T)/R(295 K)
of a s a m p l e of oxidized
normalized according to the value m e a s ured at 295 K, is plotted versus (1000/T) TM for a typical sample. Experimental data are fitted by the following equation, according to Mott's variable range hopping model in three dimensions [7]: P = Po exp[(To/T) TM]
(1)
The characteristic t e m p e r a t u r e To can be written: 16a s To = - KN(E~)
(2)
where K is the Boltzmann constant, N(EF) is the density of states at the Fermi level and the localization of states a-~ can be expressed in terms of the hopping distance d by a - ~= 81rd4N(EF)KT
(3)
9 The definition of Po in eqn. (1) is given as 1 po = e 2 N ( E f ) d 2 u
°
(4)
where e is the electron charge and Uo is a frequency factor that can be related to the transition rate P according to
130 TABLE 1 Resistivity and microscopic parameters of a pellet of oxidized poly(N-vinylpyrrole) estimated according to the three-dimensional variable range hopping model Resistivity p (300 IO Localization of states aHopping distance d (300 K) Density of states at Fermi level N(EF) Hopping energy W (300 IO
0.8)< 104 l~ cm 2.~ 20 ~, 3 × 102° states/(eV cm3) 0.14 eV
w h e r e W i n d i c a t e s t h e e n e r g y difference b e t w e e n the h o p p i n g localized s t a t e s i n v o l v e d in t h e p r o c e s s . T h e c h a r a c t e r i s t i c t e m p e r a t u r e To with its s t a n d a r d deviation, d e t e r m i n e d f r o m t h e r e s u l t s o b t a i n e d f r o m fitting p r o c e d u r e s ( m i n i m u m 22 ) p e r f o r m e d o n s e v e n different e x p e r i m e n t a l d a t a sets, is ( 8 . 6 _ + 0 . 6 ) × 1 0 7 K. In o r d e r to calculate the m i c r o s c o p i c p a r a m e t e r s o f the c o n d u c t i n g p o l y m e r i c s a m p l e , an e s t i m a t e o f the f r e q u e n c y f a c t o r 9o is n e c e s s a r y . A c c o r d i n g to c a l c u l a t i o n s r e p o r t e d e l s e w h e r e [8] for a m o r p h o u s s y s t e m s w h e r e t r a n s p o r t m o d e l l i n g is b a s e d o n localized states, we h a v e a s s u m e d an e s t i m a t e o f 9o o f 1018 s - ~ to b e reliable. T h e e s t i m a t e s of m i c r o s c o p i c p a r a m e t e r s d e t e r m i n e d a c c o r d i n g to eqns. ( 1 ) - ( 4 ) are g i v e n in T a b l e 1.
Conclusions Electrical t r a n s p o r t b e h a v i o u r o f oxidized p o l y ( N - v i n y l p y r r o l e ) c a n b e r e l a t e d to t h e b e h a v i o u r o f a m o r p h o u s s e m i c o n d u c t i n g m a t e r i a l s , w h e r e the localization o f s t a t e s p l a y s a r e l e v a n t role in t r a n s p o r t p r o c e s s e s [7]. In p a r t i c u l a r it h a s b e e n s h o w n t h a t the o b s e r v e d v a r i a t i o n o f resistivity v e r s u s t e m p e r a t u r e c a n b e e x p l a i n e d in t e r m s o f t h e t h r e e - d i m e n s i o n a l v a r i a b l e range hopping model. The microscopic parameters determined from experimental data according t o t h e v a r i a b l e r a n g e h o p p i n g m o d e l are c o n s i s t e n t with d a t a r e p o r t e d in t h e l i t e r a t u r e f o r b o t h p o l y m e r i c a n d inorganic a m o r p h o u s s e m i c o n d u c t o r s . No e v i d e n c e h a s b e e n o b t a i n e d a b o u t t h e p e c u l i a r kind o f localized states; t h e i r c h a r a c t e r i s t i c s c a n b e r e l a t e d to t h e t h e o r y o f S u e t al. [9, 10]. This m e a n s t h a t l o c a l i z e d s t a t e s c a n b e s k e t c h e d a s p o l a r o n s or b i p o l a r o n s , due to t h e n o n - d e g e n e r a t e g r o u n d state of t h e p o l y m e r i c c h a i n o f oxidized p o l y ( N - v i n y l p y r r o l e ) , a n d c h a r g e c a r r i e r s h o p b e t w e e n t h e m in t h e c o n d u c t i o n p r o c e s s , m a i n l y in a n i n t e r c h a i n m e c h a n i s m [ 11, 12 ]. Effects o f s t r u c t u r a l disorder, p o i n t e d o u t b y p r e v i o u s c h a r a c t e r i z a t i o n [3], a n d s u r f a c e states, d u e to t h e p o w d e r y f o r m o f s a m p l e s , m a y also h a v e a n influence o n t h e t r a n s p o r t p r o c e s s [11, 12].
131
Acknowledgement This research has been Fisica della Materia (INFM).
supported
by
Consorzio
Interuniversitario
di
References 1 G. Wegner, W. W e m e t , D. T. Glatzhofer, J. Ulaiiski, Ch. Kroehnke and M. Mohammadi, Synth. Met., 18 (1987) 1-6. 2 D. S. Maddison, J. Unsworth and R. B. Roberts, Synth. Met., 26 (1988) 9 9 - 1 0 8 . 3 V. Castelvetro, A. Colligiani, F. Ciardelli, G. Ruggeri and M. Giordano, New Polym. Mater., 2 (1990) 93. 4 A. Priola, G. Gatti and S. Cesca, Makromol. Chem., 180 (1979) 1. 5 B. A. Trofimov, A. I. Mikhaleva and L. V. Morozova, Russ. Chem. Rev., 54 (1985) 609. 6 B. J. Jones, D. Tidy and G. Williams, J. Phys. E: Sci. Instrum., 9 (1976) 693. 7 N. F. Mott and E. A. Davies, Electronic Processes in Non-crystalline Materials, Clarendon Press, Oxford, 1971. 8 D. Wuertz and P. Thomas, Phys. Status Solidi Co), 88 (1978) K73. 9 W.-P. Su, J. R. Schrieffer and A. J. Heeger, Phys. Rev. Lett., 42 (1979) 1698. 10 W.-P. Su, J. R. Schrieffer and A. J. Heeger, Phys. Rev. B, 22 (1980) 2099. 11 V. Enkelmann, J. Rfihe and G. Wegner, Synth. Met., 37 (1990) 7 9 - 8 9 . 12 F. Kremer, J. Rtihe and W. H. Meyer, Makromol. Chem., Macromol. Symp., 37 (1990) 115.