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On the influence of p-quinquephenyl doping on some electrical properties of 1,4-cis-polybutadiene
Synthetic Metals 109 Ž2000. 255–258 www.elsevier.comrlocatersynmet
On the influence of p-quinquephenyl doping on some electrical properties of 1,4-cis-polybutadiene ´ ˛tek S.W. Tkaczyk ) , J. Swia Institute of Physics, Pedagogical UniÕersity, Al. Armii Krajowej 13 r 15 PL, 42-200 Cze˛stochowa, Poland Received 26 June 1999; received in revised form 24 July 1999; accepted 10 September 1999
Abstract This paper contains results on the electrical conductivity of 1,4-cis-polybutadiene doped with p-quinquephenyl. The aim of these measurements was to study conduction mechanisms. Methods of introducing charge carriers into the bulk of the material and transport properties were taken into consideration. X-ray diffraction measurements were carried out to study the structure of the material as well as the phase transitions. As this work presents only part of our investigations, they are restricted to the results obtained for a layer thickness of d s 2.506 mm with aluminium and gold electrodes. Measurements were performed in the temperature range from 16 to 364 K and electric fields up to 10 9 Vrm. 1,4-cis-Polybutadiene consisting of 96% cis- and 4% trans- form was purchased from Philips Petroleum. Quinquephenyl was produced by Tokyo Kasei Kogyo. DC conductivity was controlled by Poole–Frenkel effect. The energy of activation was also determined. q 2000 Elsevier Science S.A. All rights reserved. Keywords: 1,4-cis-Polybutadiene; p-Quinquephenyl; Thin films; DC conductivity; Hopping; Poole–Frenkel effect
1. Introduction The application of organic materials in microelectronics depends on their mechanical and electrical properties. The electrical properties of polymers are determined by surface states, contact phenomena, structure of the polymer. The density of the traps and their energetical distribution have great influence on the shape of the current–voltage characteristics. At low temperatures, high electric fields and with thin layers hopping and tunneling are very often the predominant mechanisms of conductivity. For thicker layers, the Poole–Frenkel mechanism and space charge limited currents have to be taken into account w1–5x.
This polymer was used as a matrix with p-quinquephenyl as a dopant. The influence on the dopant on electrical properties of the polymeric layer was studied w3,5x. The studies were performed for the ‘‘sandwich type’’ samples with Au and Al as electrodes. We know that for organic materials gold is a hole injecting electrode whereas the
2. Experimental The material under study was 1,4-cis-polybutadiene with the content of 96% of cis- and 4% of trans- form.
)
Corresponding author. E-mail: [email protected]
Fig. 1. Molecular structure of studied materials. ŽA. 1,4-cis-polybutadiene Žshaded area shows the monomeric unit.. ŽB. p-Quinquephenyl.
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 2 9 - 5
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´ ˛ tek r Synthetic Metals 109 (2000) 255–258 S.W. Tkaczyk, J. Swia
Fig. 2. Roentgenographs of studied materials for the temperature range 123–293 K. Ža. 1,4-cis-Polybutadiene. Žb. 1,4-cis-Polybutadiene doped with p-quinquephenyl.
Fig. 3. Typical current–voltage characteristics for pure 1,4-cis-polybutadiene and doped polymer at different temperatures. a. Current-voltage characteristics for pure 1,4-cis-polybutadiene, ŽAu–Al, AuŽy., temperature T s 16 K. and 1,4-cis-polybutadiene doped p-quinqephenyl Ž500:1., ŽAu–Al, AuŽy., temperature T s 18 K.. b. Current-voltage characteristics for pure 1,4-cis-polybutadiene, ŽAu–Al, AuŽy., temperature T s 320 K. and 1,4-cis-polybutadiene doped p-quinqephenyl Ž500:1., ŽAu–Al, AuŽy., temperature T s 320 K..
´ ˛ tek r Synthetic Metals 109 (2000) 255–258 S.W. Tkaczyk, J. Swia
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using the ratio: one p-quinquephenyl molecule to 500 monomeric units of the polymer Ž1:500.. The molecular structures of the polymer and the dopant are shown in Fig. 1. Fig. 2 shows roentgenograms obtained for pure polymeric matrix and 1,4-cis-polybutadiene doped with pquinquephenyl. After cooling the sample to 123 K, the studies were repeated at various temperatures up to room temperature. It was observed that doped polybutadiene is not in a pure amorphous state at temperatures above melting temperatures for crystallites ŽFig. 2a.. Structural changes and the activation energy for a phase transition was studied using differential scanning calorymetry ŽDSC. and thermally stimulated depolarisation ŽTSD. methods. The results of the measurements are in preparation for publication.
Fig. 4. The dependence Ea s f ŽU . for 1,4-cis-polybutadiene doped with p-quinquephenyl Ž1:500. with Au–Al electrodes, with AuŽq..
aluminium one forms a blocking contact. Thin layers of the materials Žthickness of from 1 to 10 mm. were prepared by spinning from a solution Žtoluene was used as a solvent.. The current as a function of electric strength was measured for different temperatures. The temperature was ranging from 16 to 325 K. The samples were prepared
Fig. 5. The dependence ln I s f ŽU 1r 2 . for 1,4-cis-PB, BR-150 doped with p-quinquephenyl with AuŽq. electrode measured for two different temperatures: T s128 K and T s 364 K.
3. Results and discussion The results obtained enable us to draw a conclusion that the mechanism of the conductivity depends on the electric field and temperature. The shape the of current–voltage characteristics ŽFig. 3. suggest a non-ohmic mechanism of conductivity.
Fig. 6. The dependence ln I s f ŽTy1 r3 . for 1,4-cis-PB, BR-150 doped with p-quinquephenyl with Au–Al electrodes and AuŽq.; ds 2.5 mm.
258
´ ˛ tek r Synthetic Metals 109 (2000) 255–258 S.W. Tkaczyk, J. Swia
place by hopping between localised states. The results presented in Fig. 6 suggest us two-dimensional hopping w5,6x. Charge carriers injection into the bulk of the material is caused mainly by thermionic emission with addition of field emission. Fowler–Nordheim ŽF.–N.. characteristics show us that for temperature below 200 K, thermionic emission plays a predominant role. The typical F.–N. diagram is shown in Fig. 7 where logŽ Id 2rU 2 . s f Ž drU . dependence is shown.
4. Conclusions 1. We can see from the presented results that in the material under study, it is impossible to identify a single conduction mechanism. 2. Charge carriers transport is taking place by hopping between localised states. Charge carriers jump trough potential barriers the height of which depends on the electric field according to Poole–Frenkel effect. 3. The electrical conductivity of doped polymer is higher than that of the pure polymeric matrix. 4. Charge carriers injection into the bulk of the material is taking place by thermionic emission. Fig. 7. The Fowler–Nordheim plot for 1,4-cis-PB, BR-150 doped with p-quinquephenyl with Au–Al electrodes; AuŽy.. Temperature T s 20 K.
References The shape of the curves was almost independent of the electrode bias. The activation energy of the conductivity was determined from the ln I s f Ž1rT . dependence. Its value is rather small and changes from 0.03 to 0.28 eV and decreases with increasing electrical field ŽFig. 4.. It may be explained by the Poole–Frenkel effect ŽFig. 5.. Charge carriers transport through the bulk of the sample is taking
w1x w2x w3x w4x
G.A. Niklason, K. Brantervik, J. Appl. Phys. 59 Ž1986. 3. R.H. Hill, Philos. Mag. 25 Ž1972. 130. S.W. Tkaczyk, Electron Technology 29 Ž2–3. Ž1996. 232–235. ´ ˛tek, Adv. Mater. Opt. Electron. 7 S.W. Tkaczyk, T. Galinski, J. Swia ´ Ž1997. 87–91. ´ ˛tek, A. Kwiatkowska, Prog. Colloid Polym. Sci. w5x S. Tkaczyk, J. Swia 78 Ž1988. 136–138. w6x S.W. Tkaczyk, Electron Technology 31 Ž3–4. Ž1998. 461–467.
On the influence of p-quinquephenyl doping on some electrical properties of 1,4-cis-polybutadiene ´ ˛tek S.W. Tkaczyk ) , J. Swia Institute of Physics, Pedagogical UniÕersity, Al. Armii Krajowej 13 r 15 PL, 42-200 Cze˛stochowa, Poland Received 26 June 1999; received in revised form 24 July 1999; accepted 10 September 1999
Abstract This paper contains results on the electrical conductivity of 1,4-cis-polybutadiene doped with p-quinquephenyl. The aim of these measurements was to study conduction mechanisms. Methods of introducing charge carriers into the bulk of the material and transport properties were taken into consideration. X-ray diffraction measurements were carried out to study the structure of the material as well as the phase transitions. As this work presents only part of our investigations, they are restricted to the results obtained for a layer thickness of d s 2.506 mm with aluminium and gold electrodes. Measurements were performed in the temperature range from 16 to 364 K and electric fields up to 10 9 Vrm. 1,4-cis-Polybutadiene consisting of 96% cis- and 4% trans- form was purchased from Philips Petroleum. Quinquephenyl was produced by Tokyo Kasei Kogyo. DC conductivity was controlled by Poole–Frenkel effect. The energy of activation was also determined. q 2000 Elsevier Science S.A. All rights reserved. Keywords: 1,4-cis-Polybutadiene; p-Quinquephenyl; Thin films; DC conductivity; Hopping; Poole–Frenkel effect
1. Introduction The application of organic materials in microelectronics depends on their mechanical and electrical properties. The electrical properties of polymers are determined by surface states, contact phenomena, structure of the polymer. The density of the traps and their energetical distribution have great influence on the shape of the current–voltage characteristics. At low temperatures, high electric fields and with thin layers hopping and tunneling are very often the predominant mechanisms of conductivity. For thicker layers, the Poole–Frenkel mechanism and space charge limited currents have to be taken into account w1–5x.
This polymer was used as a matrix with p-quinquephenyl as a dopant. The influence on the dopant on electrical properties of the polymeric layer was studied w3,5x. The studies were performed for the ‘‘sandwich type’’ samples with Au and Al as electrodes. We know that for organic materials gold is a hole injecting electrode whereas the
2. Experimental The material under study was 1,4-cis-polybutadiene with the content of 96% of cis- and 4% of trans- form.
)
Corresponding author. E-mail: [email protected]
Fig. 1. Molecular structure of studied materials. ŽA. 1,4-cis-polybutadiene Žshaded area shows the monomeric unit.. ŽB. p-Quinquephenyl.
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 2 9 - 5
256
´ ˛ tek r Synthetic Metals 109 (2000) 255–258 S.W. Tkaczyk, J. Swia
Fig. 2. Roentgenographs of studied materials for the temperature range 123–293 K. Ža. 1,4-cis-Polybutadiene. Žb. 1,4-cis-Polybutadiene doped with p-quinquephenyl.
Fig. 3. Typical current–voltage characteristics for pure 1,4-cis-polybutadiene and doped polymer at different temperatures. a. Current-voltage characteristics for pure 1,4-cis-polybutadiene, ŽAu–Al, AuŽy., temperature T s 16 K. and 1,4-cis-polybutadiene doped p-quinqephenyl Ž500:1., ŽAu–Al, AuŽy., temperature T s 18 K.. b. Current-voltage characteristics for pure 1,4-cis-polybutadiene, ŽAu–Al, AuŽy., temperature T s 320 K. and 1,4-cis-polybutadiene doped p-quinqephenyl Ž500:1., ŽAu–Al, AuŽy., temperature T s 320 K..
´ ˛ tek r Synthetic Metals 109 (2000) 255–258 S.W. Tkaczyk, J. Swia
257
using the ratio: one p-quinquephenyl molecule to 500 monomeric units of the polymer Ž1:500.. The molecular structures of the polymer and the dopant are shown in Fig. 1. Fig. 2 shows roentgenograms obtained for pure polymeric matrix and 1,4-cis-polybutadiene doped with pquinquephenyl. After cooling the sample to 123 K, the studies were repeated at various temperatures up to room temperature. It was observed that doped polybutadiene is not in a pure amorphous state at temperatures above melting temperatures for crystallites ŽFig. 2a.. Structural changes and the activation energy for a phase transition was studied using differential scanning calorymetry ŽDSC. and thermally stimulated depolarisation ŽTSD. methods. The results of the measurements are in preparation for publication.
Fig. 4. The dependence Ea s f ŽU . for 1,4-cis-polybutadiene doped with p-quinquephenyl Ž1:500. with Au–Al electrodes, with AuŽq..
aluminium one forms a blocking contact. Thin layers of the materials Žthickness of from 1 to 10 mm. were prepared by spinning from a solution Žtoluene was used as a solvent.. The current as a function of electric strength was measured for different temperatures. The temperature was ranging from 16 to 325 K. The samples were prepared
Fig. 5. The dependence ln I s f ŽU 1r 2 . for 1,4-cis-PB, BR-150 doped with p-quinquephenyl with AuŽq. electrode measured for two different temperatures: T s128 K and T s 364 K.
3. Results and discussion The results obtained enable us to draw a conclusion that the mechanism of the conductivity depends on the electric field and temperature. The shape the of current–voltage characteristics ŽFig. 3. suggest a non-ohmic mechanism of conductivity.
Fig. 6. The dependence ln I s f ŽTy1 r3 . for 1,4-cis-PB, BR-150 doped with p-quinquephenyl with Au–Al electrodes and AuŽq.; ds 2.5 mm.
258
´ ˛ tek r Synthetic Metals 109 (2000) 255–258 S.W. Tkaczyk, J. Swia
place by hopping between localised states. The results presented in Fig. 6 suggest us two-dimensional hopping w5,6x. Charge carriers injection into the bulk of the material is caused mainly by thermionic emission with addition of field emission. Fowler–Nordheim ŽF.–N.. characteristics show us that for temperature below 200 K, thermionic emission plays a predominant role. The typical F.–N. diagram is shown in Fig. 7 where logŽ Id 2rU 2 . s f Ž drU . dependence is shown.
4. Conclusions 1. We can see from the presented results that in the material under study, it is impossible to identify a single conduction mechanism. 2. Charge carriers transport is taking place by hopping between localised states. Charge carriers jump trough potential barriers the height of which depends on the electric field according to Poole–Frenkel effect. 3. The electrical conductivity of doped polymer is higher than that of the pure polymeric matrix. 4. Charge carriers injection into the bulk of the material is taking place by thermionic emission. Fig. 7. The Fowler–Nordheim plot for 1,4-cis-PB, BR-150 doped with p-quinquephenyl with Au–Al electrodes; AuŽy.. Temperature T s 20 K.
References The shape of the curves was almost independent of the electrode bias. The activation energy of the conductivity was determined from the ln I s f Ž1rT . dependence. Its value is rather small and changes from 0.03 to 0.28 eV and decreases with increasing electrical field ŽFig. 4.. It may be explained by the Poole–Frenkel effect ŽFig. 5.. Charge carriers transport through the bulk of the sample is taking
w1x w2x w3x w4x
G.A. Niklason, K. Brantervik, J. Appl. Phys. 59 Ž1986. 3. R.H. Hill, Philos. Mag. 25 Ž1972. 130. S.W. Tkaczyk, Electron Technology 29 Ž2–3. Ž1996. 232–235. ´ ˛tek, Adv. Mater. Opt. Electron. 7 S.W. Tkaczyk, T. Galinski, J. Swia ´ Ž1997. 87–91. ´ ˛tek, A. Kwiatkowska, Prog. Colloid Polym. Sci. w5x S. Tkaczyk, J. Swia 78 Ž1988. 136–138. w6x S.W. Tkaczyk, Electron Technology 31 Ž3–4. Ž1998. 461–467.