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A highly conductive form of the S+Et2Me(TCNQ)2 complex in reticulate doped polymer films
Synthetic Metals, 37 (1990) 189 - 192
189
A HIGHLY CONDUCTIVE FORM OF THE S+EtzMe(TCNQ)2 COMPLEX IN RETICULATE DOPED POLYMER FILMS J. Pl~CHERZa, L. FIRLEJb, J. K. JESZKA%J. ULANSKIc and M. KRYSZEWSKIa a Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 LSd5 (Poland) Institute for Molecular Physics, Polish Academy of Sciences, 60-179 Poznaft (Poland) " Polymer Institute, Technical University of L6d~, 90-924 LSd~ (Poland)
Abstract S ÷ Et2Me(TCNQ) 2 is an organic semiconductor exhibiting the electrical conductivity of 5 × 10 -2 S/cm at room temperature. We found t h a t the films of a copolymer of vinylidene fluoride with C2F3H reticulate doped with this salt show a = 10- 3 S/cm for the additive concentration 1 to 1.5 wt.% (i.e. at least one order of magnitude higher than expected). At low temperatures the conductivity of the doped films is even higher than that of the pure additive because the activation energy for reticulate doped films is considerably lower compared to that of single crystals of the additive. It is concluded that a different, highly conductive crystalline form of this complex salt crystallizes during the reticulate doping process.
Introduction Reticulate doped polymers are heterogeneous systems containing a network of low molecular weight additive crystallites penetrating the polymer matrix [1- 3]. Such materials can be obtained by crystallization of the additive in situ during film casting and, because of controlled crystallization, conductive films can be obtained for extremely low additive concentrations (percolation threshold below 0.003) [3]. During preparation of conductive polymeric materials by reticulate doping, the conductive CT complex crystallizes in unusual conditions. Because of fast, diffusion-controlled growth, different crystalline forms can grow. In general, microcrystallites formed under such conditions have more defects so their conductivity is expected to be lower than that of equilibrium good quality single crystals. However, it has been found that in some cases (e.g. in n-propylphthalazinum-(TCNQ)2) the microcrystallites forming a conductive network in reticulate doped polymers show a conductivity remarkably higher than t h a t reported for single crystals [4]. 0379-6779/90/$3.50
~ Elsevier Sequoia/Printed in The Netherlands
190 In this communication we present the second example of the CT complex showing this interesting effect. In reticulate doping of a vinylidene fluoride copolymer we use as a conductive additive the TCNQ salt of diethyl methyl sulfonimn cation. The properties of single crystals have been investigated [5] and their comparison with those found for reticulate doped films suggests that a new highly conductive form is obtained in reticulate doped films.
Experimental The tetracyanoquinodimethane (TCNQ) complex salt of diethyl methyl sulfonium cation (S+Et2Me) was obtained from S+Et2Me iodide in a one-step reaction and its composition was confirmed by elemental analysis and UV spectroscopy. Reticulate doped films were obtained by the standard technique of casting a solution containing the proper amounts of the additive and vinylidene fluoride-C2F3H (6:4) copolymer (Atochem, France). Acetone was used as a solvent and the casting temperature was 15- 40 °C. Electrical properties were measured using the four-probe method; electrodes were made with silver paint.
Results and discussion S+Et2Me(TCNQ)2 has been found to be an organic semiconductor. Investigations of conductivity and the Seebeck effect carried out for single crystals in the temperature range 77- 330 K show that it is an intrinsic semiconductor above 225 °C with an activation energy of 0.30 eV and an n-type extrinsic semiconductor below this temperature (activation energy, 0.22 eV). Its d.c. conductivity can be described by a one-electron approximation model with localized impurity states [5]. The electrical conductivity at room temperature is 5 x 10 -2 S/cm. This would imply that the polymer films reticulate doped with 1 wt.% of this complex will show a conductivity of the order of 10 -4 S/cm or lower according to the Clausius-Mossotti approximation. The temperature dependence of the electrical conductivity of a copolymer film reticulate doped with S+Et2Me(TCNQ)~ is compared with that of a single crystal of S+Et2Me(TCNQ)2 in Fig. 1. One can see that the temperature dependence of the conductivity of the film is much weaker than that of the single crystal of the additive. This unusual relationship combined with the relatively high conductivity of the film at room temperature gives the surprising effect of crossing of the two temperature dependences. At low temperature the electrical conductivity of the film containing only 1 wt.% of the additive is much higher than that of the single crystal of the additive. This implies that the conductivity of the microcrystallites forming the con-
191
_,t _2! \ ~-4
~
-5
x x × XX×x ×××
%
×
X%
-6
'~'
b
7
oj
X X X
i
1'1 J
1000/T
Fig, 1. Temperature dependence of the electrical conductivity of a P V D F copolymer film reticulate doped with l wt.% of S+Et2Me(TCNQ)2 ( x ) compared with that of a single crystal of S+Et2Me(TCNQ)2 ([2).
ductive network is higher by several orders of magnitude than that of single crystals obtained in equilibrium conditions reported in ref. 5. Therefore, a different crystalline form with higher conductivity is presumed to grow under non-equilibrium conditions. It is worth noting that the specific conductivity of the investigated reticulate doped films is only one order of magnitude lower than that of polymers reticulate doped with molecular metals like TTF-TCNQ in spite of the four orders of magnitude difference in conductivities of the single crystals between TTF-TCNQ and S+Et2Me(TCNQ)2.
30
26 22 1B 14 WAVENUMBER [ 103 cm-1]
Fig. 2. Absorption spectra of S+ Et2Me(TCNQ)2 powder (curve 1), of a polymer film reticulate doped with 1 wt.% of the complex (curve 2) and of the TCNQ salt in acetone solution (curve 3).
192
Figure 2 presents the absorption spectrum of a polymer film reticulate doped with 1 wt.% of S+Et,Me(TCNQ), compared with the spectrum of the TCNQ salt in solution and the powder spectrum of the crystalline salt. The observed absorption is entirely due to TCNQ and TCNQ- because the cation does not absorb in this range. One can see that the spectrum of the reticulate doped film does not correspond to the spectrum of the salt in solution (which was observed in the polycarbonate + tetrathiotetracene-TCNQ system [3] and others) but rather to the powder spectrum of the TCNQ salt. This is due to the very low solubility of the salt in the copolymer used as the matrix and to the partial crystallinity of the matrix. No significant shift of the maxima can be observed in this range, indicative of the formation of the other crystalline form of the investigated TCNQ salt in the reticulate doped film (the absorption around 20.5 x lo3 cm-l is probably due to degradation of TCNQ). This suggests that the difference between the two forms is not very important. Probably the structure of the TCNQ stacks is different and overlapping of the neighboring molecules is better or dimerization is suppressed in the highly conductive form. However, since the crystal structures of the two forms have not been determined, the reason for the large difference in conductivity is not clear.
Conclusions
The investigated TCNQ salt with the S+Et,Me cation is a second example showing that a highly conductive form of a semiconducting complex salt can grow in non-equilibrium conditions during the reticulate doping process. We show that the electrical conductivity of this form is several orders of magnitude higher than that measured in equilibrium single crystals, while its activation energy is much lower.
Acknowledgement
This work was supported by the Polish Academy of Sciences under Projects CPBP 01.12 and 01.14.
References 1 J. K. Jeszka, J. Ulariski and M. Kryszewski, Nature (London), 298 (1981) 390. 2 M. Kryszewski, J. K. Jeszka, J. Ulaliski and A. Tram, Pure Appl. Own., 56 (1984) 355. 3 A. Tram, J. K. Jeszka, E. El Shafee, J. Ulariski and M. Kryszewski, J. Phys. D: Appl. Phys., 19 (1986) 1047. 4 A. Tracz, J. K. Jeszka, M. Kryszewski, J. Ulaliski, G. Boiteux, L. Firlej and A. Graja, Synth. Met. 24 (1988) 107. 5 J. Pqcherz, M. Kryszewski, L. Firlej and A. Graja, Synth Met., 35 (1990) 39.
189
A HIGHLY CONDUCTIVE FORM OF THE S+EtzMe(TCNQ)2 COMPLEX IN RETICULATE DOPED POLYMER FILMS J. Pl~CHERZa, L. FIRLEJb, J. K. JESZKA%J. ULANSKIc and M. KRYSZEWSKIa a Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 LSd5 (Poland) Institute for Molecular Physics, Polish Academy of Sciences, 60-179 Poznaft (Poland) " Polymer Institute, Technical University of L6d~, 90-924 LSd~ (Poland)
Abstract S ÷ Et2Me(TCNQ) 2 is an organic semiconductor exhibiting the electrical conductivity of 5 × 10 -2 S/cm at room temperature. We found t h a t the films of a copolymer of vinylidene fluoride with C2F3H reticulate doped with this salt show a = 10- 3 S/cm for the additive concentration 1 to 1.5 wt.% (i.e. at least one order of magnitude higher than expected). At low temperatures the conductivity of the doped films is even higher than that of the pure additive because the activation energy for reticulate doped films is considerably lower compared to that of single crystals of the additive. It is concluded that a different, highly conductive crystalline form of this complex salt crystallizes during the reticulate doping process.
Introduction Reticulate doped polymers are heterogeneous systems containing a network of low molecular weight additive crystallites penetrating the polymer matrix [1- 3]. Such materials can be obtained by crystallization of the additive in situ during film casting and, because of controlled crystallization, conductive films can be obtained for extremely low additive concentrations (percolation threshold below 0.003) [3]. During preparation of conductive polymeric materials by reticulate doping, the conductive CT complex crystallizes in unusual conditions. Because of fast, diffusion-controlled growth, different crystalline forms can grow. In general, microcrystallites formed under such conditions have more defects so their conductivity is expected to be lower than that of equilibrium good quality single crystals. However, it has been found that in some cases (e.g. in n-propylphthalazinum-(TCNQ)2) the microcrystallites forming a conductive network in reticulate doped polymers show a conductivity remarkably higher than t h a t reported for single crystals [4]. 0379-6779/90/$3.50
~ Elsevier Sequoia/Printed in The Netherlands
190 In this communication we present the second example of the CT complex showing this interesting effect. In reticulate doping of a vinylidene fluoride copolymer we use as a conductive additive the TCNQ salt of diethyl methyl sulfonimn cation. The properties of single crystals have been investigated [5] and their comparison with those found for reticulate doped films suggests that a new highly conductive form is obtained in reticulate doped films.
Experimental The tetracyanoquinodimethane (TCNQ) complex salt of diethyl methyl sulfonium cation (S+Et2Me) was obtained from S+Et2Me iodide in a one-step reaction and its composition was confirmed by elemental analysis and UV spectroscopy. Reticulate doped films were obtained by the standard technique of casting a solution containing the proper amounts of the additive and vinylidene fluoride-C2F3H (6:4) copolymer (Atochem, France). Acetone was used as a solvent and the casting temperature was 15- 40 °C. Electrical properties were measured using the four-probe method; electrodes were made with silver paint.
Results and discussion S+Et2Me(TCNQ)2 has been found to be an organic semiconductor. Investigations of conductivity and the Seebeck effect carried out for single crystals in the temperature range 77- 330 K show that it is an intrinsic semiconductor above 225 °C with an activation energy of 0.30 eV and an n-type extrinsic semiconductor below this temperature (activation energy, 0.22 eV). Its d.c. conductivity can be described by a one-electron approximation model with localized impurity states [5]. The electrical conductivity at room temperature is 5 x 10 -2 S/cm. This would imply that the polymer films reticulate doped with 1 wt.% of this complex will show a conductivity of the order of 10 -4 S/cm or lower according to the Clausius-Mossotti approximation. The temperature dependence of the electrical conductivity of a copolymer film reticulate doped with S+Et2Me(TCNQ)~ is compared with that of a single crystal of S+Et2Me(TCNQ)2 in Fig. 1. One can see that the temperature dependence of the conductivity of the film is much weaker than that of the single crystal of the additive. This unusual relationship combined with the relatively high conductivity of the film at room temperature gives the surprising effect of crossing of the two temperature dependences. At low temperature the electrical conductivity of the film containing only 1 wt.% of the additive is much higher than that of the single crystal of the additive. This implies that the conductivity of the microcrystallites forming the con-
191
_,t _2! \ ~-4
~
-5
x x × XX×x ×××
%
×
X%
-6
'~'
b
7
oj
X X X
i
1'1 J
1000/T
Fig, 1. Temperature dependence of the electrical conductivity of a P V D F copolymer film reticulate doped with l wt.% of S+Et2Me(TCNQ)2 ( x ) compared with that of a single crystal of S+Et2Me(TCNQ)2 ([2).
ductive network is higher by several orders of magnitude than that of single crystals obtained in equilibrium conditions reported in ref. 5. Therefore, a different crystalline form with higher conductivity is presumed to grow under non-equilibrium conditions. It is worth noting that the specific conductivity of the investigated reticulate doped films is only one order of magnitude lower than that of polymers reticulate doped with molecular metals like TTF-TCNQ in spite of the four orders of magnitude difference in conductivities of the single crystals between TTF-TCNQ and S+Et2Me(TCNQ)2.
30
26 22 1B 14 WAVENUMBER [ 103 cm-1]
Fig. 2. Absorption spectra of S+ Et2Me(TCNQ)2 powder (curve 1), of a polymer film reticulate doped with 1 wt.% of the complex (curve 2) and of the TCNQ salt in acetone solution (curve 3).
192
Figure 2 presents the absorption spectrum of a polymer film reticulate doped with 1 wt.% of S+Et,Me(TCNQ), compared with the spectrum of the TCNQ salt in solution and the powder spectrum of the crystalline salt. The observed absorption is entirely due to TCNQ and TCNQ- because the cation does not absorb in this range. One can see that the spectrum of the reticulate doped film does not correspond to the spectrum of the salt in solution (which was observed in the polycarbonate + tetrathiotetracene-TCNQ system [3] and others) but rather to the powder spectrum of the TCNQ salt. This is due to the very low solubility of the salt in the copolymer used as the matrix and to the partial crystallinity of the matrix. No significant shift of the maxima can be observed in this range, indicative of the formation of the other crystalline form of the investigated TCNQ salt in the reticulate doped film (the absorption around 20.5 x lo3 cm-l is probably due to degradation of TCNQ). This suggests that the difference between the two forms is not very important. Probably the structure of the TCNQ stacks is different and overlapping of the neighboring molecules is better or dimerization is suppressed in the highly conductive form. However, since the crystal structures of the two forms have not been determined, the reason for the large difference in conductivity is not clear.
Conclusions
The investigated TCNQ salt with the S+Et,Me cation is a second example showing that a highly conductive form of a semiconducting complex salt can grow in non-equilibrium conditions during the reticulate doping process. We show that the electrical conductivity of this form is several orders of magnitude higher than that measured in equilibrium single crystals, while its activation energy is much lower.
Acknowledgement
This work was supported by the Polish Academy of Sciences under Projects CPBP 01.12 and 01.14.
References 1 J. K. Jeszka, J. Ulariski and M. Kryszewski, Nature (London), 298 (1981) 390. 2 M. Kryszewski, J. K. Jeszka, J. Ulaliski and A. Tram, Pure Appl. Own., 56 (1984) 355. 3 A. Tram, J. K. Jeszka, E. El Shafee, J. Ulariski and M. Kryszewski, J. Phys. D: Appl. Phys., 19 (1986) 1047. 4 A. Tracz, J. K. Jeszka, M. Kryszewski, J. Ulaliski, G. Boiteux, L. Firlej and A. Graja, Synth. Met. 24 (1988) 107. 5 J. Pqcherz, M. Kryszewski, L. Firlej and A. Graja, Synth Met., 35 (1990) 39.