1. Science Update Programme Conductive Polymers: From Research to Products
Education Bureau, HKSAR
Department of Chemistry
University of Hong Kong
May 2002

2. Course Coordinator Dr. Wai Kin Chan, Department of Chemistry, HKU
Content
Introduction to Electrical Conductivity
Electrical Conductivity of Polymers
Polyacetylene: First example of conducting polymer
Conduction Process in Conjugate Polymers
Examples of other conducting polymers and their syntheses
Applications of Conducting Polymers

3. Conductive Polymers
In 2000, The Nobel Prize in Chemistry was awarded to
A. J. Heeger, A. G. MacDiarmid, and H. Shirakawa
“for the discovery and development of electrically conductive polymers”

4. References
“Handbook of Conducting Polymers” First Edition, T. A. Skotheim Ed., Marcel Dekker, New York, 1986.
“Functional Monomers and Polymers” K. Takemoto, R. M. Ottenbrite, M. Kamachi Eds., Marcel Dekker, New York, 1997.
“Handbook of Organic Conductive Molecules and Polymers” Volume 2, H. S. Nalwa Ed., Wiley, West Sussex, 1997.
“Handbook of Conducting Polymers” Second Edition, T. A. Skotheim, R. L. Elsenbaumer, J. R. Reynolds Eds., Marcel Dekker, New York, 1998.
“Electronic Materials: The Oligomer Approach” K. Müllen and G. Wegner Eds., Wiley-VCH, Weinheim, 1998.
“Semiconducting Polymers: Chemistry, Physics and Engineering” G. Hadziioannou and P. F. van Hutten Eds., Wiley-VCH, Weinheim, 2000.
Other research articles in Nature, Science, Advanced Materials, Applied Physics Letters, Journal of Applied Physics, Macromolecules, Chemistry of Materials, Synthetic Metals etc.

5. Electrical Conductivity of Materials
Insulators
s ~ 10-7 S cm-1
Semiconductors
s ~ 10-7 to 102 S cm-1
Metals
s > 102 S cm-1
Units are expressed as resistivity (W cm) or conductivity (W-1 cm-1 or S cm-1)

6. Quartz: s = S cm-1
Silver/copper: s = 106 S cm-1

7. Interpretation of electrical conductivity in terms of simple energy band diagram
For a current to flow, an applied electric field must impart kinetic energy to electrons by promoting them to higher energy band states
Conductivity depends on the density of available filled and empty states

8. Bulk electrical conductivity s where
s = enµn
where
e is the electronic charge of the carrier
n is the carrier density (no. of carriers per unit volume)
µn is the carrier mobility
Charge transport: negative (n-type) or positive (p-type) carriers

9. The Energy Band Gap Insulators Semiconductor
Band Gap of several electron volts (eV)
1 eV = cm-1 or  J
The empty states are inaccessible by either electric field or thermal excitation
Semiconductor
Energy gaps < 2 eV
Thermal excitation across the gap is possible
Pure silicon: s ~ 10-5 S cm-1

10. Number of carriers increases rapidly as the energy gap decreases
Dopants can also increase the conductivity. They produce states in the energy gap that lie close to either the conduction or the valence band
Conduction can be either due to electrons (n-type) or holes (p-type): s ~ 10-1 S cm-1
Conductivity decreases at low temperature (Boltzmann distribution of states)

11. Metals Conductivity of materials increases as temperature decreases
Thermal motion of the lattice and scattering processes are reduced
True metallic behavior results when the energy gap between filled and empty states disappears
What are the advantageous of using conducting polymers compared to metallic materials?

12. Electrical Conductivity of Polymers
Typical insulator
Polyethylene s ~ S cm-1
Polytetrafluoroethylene (PTFE) s ~ S cm-1
Polystyrene s ~ S cm-1
For saturated chemical structures, all the valence electronic form strongly localized chemical-bonds and the energy gap is large (PE: 8 eV)

13. “Metallic Polymers”
By pyrolysis of insulating or semiconducting polymers
Precursor
polymers °C
Product
Precursor: polyesters, polyamides, PVC, phenolic resins
Product: graphite type carbons (intermediate structures are not well defined)

14. Materials with s ~ 0.1 to 800 S cm-1
These polymers are not soluble in any solvent, and are not well-characterized °C
PAN
(spin-coated film)
Materials with s ~ 0.1 to 800 S cm-1
Without doping

15. Conductor-Filled Polymers
A physical mixture of conducting fillers with insulating polymer matrix
Fillers: carbon, metal powders (Ni, Cu, Ag, Al, Fe)
Both the electrical and thermal conductivity are enhanced
The concentration of fillers has to reach a threshold value in order to form conductive paths

16. Example: silver-loaded epoxy adhesives Applications:
s: up to 103 S cm-1
Example: silver-loaded epoxy adhesives
Applications:
Self-regulating heaters: the filled polymers expands as temperature increases, leading to a sharp decrease in conductivity
Antistatic components
Resistors

17. Semiconducting Polymers
Some are characterized by their photoconductivity when exposed to light
Example: polymer with active pendant group
Poly(N-vinylcarbazole) (PVK)
Pure PVK is a hole conductor with dark conductivity of ~ S cm-1
It is a very common photoconducting materials

18. Polymers with Unsaturated (Conjugated) backbone structure
A conjugated main chain with alternating single and double bond
First example of conjugate polymer:
Polyacetylene
Pure polyacetylene: s ~ 10-9 (cis) and 10-5 (trans) S cm-1
High electrical conductivity was observed when the polymer was “doped” with oxidizing or reducing agents

19. Synthesis of Polyacetylene
By Ziegler-Natta Catalyst
Effect of Temperature:
At -78 °C or below: all-cis PA
At 180 °C or higher: all-trans PA

20. Polymerization by Other catalysts
Acetylene PA
Catalysts:
WCl6/(C6H5)4Sn
WCl6/n-BuLi
MoCl5/ (C6H5)4Sn
Note: These conjugate polymers are usually insoluble in organic solvents of have very low solubility

21. Durham Method
Isomerization of Polybenzvalene

22. The Conduction Process in Conjugate Polymers
Soliton: a defect in which the change in bond alternation is extended over 5 to 9 repeating units
The charge and spin of the defect will depend on the occupancy of the state
Chemical doping will create such defects in the polymer chain (e.g. by iodine I2, which abstract an electron from the polymer and forms I3- counteranion )

23. Positively charged Negatively charged Soliton S+ Neutral soliton S0
Conduction band
Valence band

24. Isolated solitons are not stable in polymers, charge exchange will lead to the formation of S0-S+ (or S0-S-) pairs, which will be strongly localized to form a polaron
The polaron is mobile along the polymer chain

25. Two polarons may collapse to form a bipolaron, which has zero spin but with charges
Q = +2e
S = 0
The two positive charges of bipolaron are not independent, but move as a pair.
The spins of the bipolarons sum to S = 0.

26. In chemical terms:

27. The mobility of a polaron along the polyacetylene chain can be high and charge is carried along the backbone. However, the counteranion I3- is not very mobile, a high concentration of counteranion is required so that the polaron can move close to the counteranion.
Hence, high dopant concentration is necessary.
The charge can “hop” from one polymer molecule to another--”hopping conductivity”
Dopants:
Oxidative: AsF5, I2, Br2, AlCl3, MoCl5 (p-type doping)
Reductive: Na, K, lithium naphthalides (n-type doping)

28. Dopant Conditions Conductivity (S cm-1)
Electrical Conductivity of Some Doped Polyacetylenes
Dopant I2
IBr
HBr
AsF5
SeF6
FeCl3
MoCl5
WCl6
Conditions
Vapor
CH3NO2
Toluene
Anisole
Conductivity (S cm-1)
360
120
7 x 10-4
560
180
897
9.0
563
356
365
8.48
Note: PA is insoluble and labile to atmospheric oxygen

29. Other Conjugated Polymers
Poly(1,4-phenylene) or Poly(p-phenylene) (PPP)
A conjugated polymer based on aromatic units on the main chain
Synthesis
n ~ 5-15

30. Pure PPP is not soluble neither
Pure PPP is not soluble neither. The solubility can be enhanced by attaching flexible groups to the polymer chain.
R = C6H13

31. Soliton, polaron, and bipolaron in poly(p-phenylene)

32. Conductivity of PPP Dopant s (S cm-1) AsF5 FeCl3 SbCl5 I2 SO3 AlCl3
Napht-K+
Napht-Li+
s (S cm-1)
500
0.30
< 10-3
< 10-4
10-1 to 10-4
8.0 50 5

33. Polypyrrole
A conjugated polymer based on heterocyclic aromatic units on the main chain
Synthesized by chemical or electrochemical polymerization from pyrrole
Mechanism: oxidative coupling reaction

34. Proposed mechanism for the electrochemical polymerization

35. Polythiophene Environmental stable and highly resistant to heat
Synthesized by the electrochemical polymerization of thiophene
Can also be obtained by various types of metal catalyzed coupling reaction

36. The solubility and processibility can be enhanced by attaching substitution groups at the 3 position
However, the coupling can be either head-to-head (HH), head-to-tail (HT), or tail-to-tail (TT)

37. Synthesis of Regioregular Polythiophene
Copolymers with aromatic compounds or vinylene group

38. Conductivity of polythiophenes and polypyrroles doped under different conditions
Material
Polythiophene
Poly(3-methylthiophene)
Poly(3-ethylthiophene)
Poly(3-buthylthiophene)
Poly(3-hexylthiophene)
Polypyrrole
Dopant
SO3CF3-
PF6- I2
FeCl3
Br2
Cl2
s (S cm-1)
10-20
510
270 4 30 11
3-200
2-8 5
0.5

39. Synthesis of Conjugate Polymers by Precursor Approach
To prepare a processible/soluble precursor polymer, which can subsequently be processed into the final form

40. Polyaniline
A conducting polymer that can be grown by using aqueous and non-aqueous route
Can be obtained by electrochemical synthesis or oxidative coupling of aniline
Doping achieved by adding protonic acid
Several forms: leucoemaraldine, emaraldine, emaraldine salt, pernigraniline

42. Electrical Conductivity
Medium
5-Sulfosalicyclic acid
Benzene sulfonic acid
p-Toluene sulfonic acid
Sulfamic acid
Sulfuric acid
s (S cm-1)
2.0
5.0
1.2

43. Applications of Conducting Polymers
Rechargeable Batteries
Higher energy and power densities (light weight) than conventional ones using lead-acid or Ni-Cd
e.g. polyaniline used in 3V coin-shaped batteries (Poly. Adv. Tech. 1990, 1, 33)
Rechargeable -> reversible doping

44. Electromagnetic shielding, corrosion inhibitor (polyaniline)
Antistatic materials
Poly(ethylenedioxythiophene) doped with acid
Also used as conducting layer in light emitting devices
Polyaniline used as antistatic layer in computer disk by Hitachi-Maxwell (Synth. Metals 1993, 57, 3696)

45. Other Applications Emission Properties
Polymers with extended p-conjugated systems usually absorb strongly in the visible region
Many of them also emit light after absorbing a photon (photoexcitation)

46. Some Possible Electronic Transitions After Absorption of Photons

47. LED Display
Light emission resulted from the recombination of holes and electrons in a semiconductor

48. When hole and electron recombine:
Hole Electron-
Excited states
Ground states
Light emission

49. Organic Light Emitting Polymer
First reported in 1990 (Nature 1990, 347, 539)
Based on poly(p-phenylenevinylene) (PPV), with a bandgap of 2.2 eV
ITO: Indium-tin-oxide -A transparent electrical conductor

50. Threshold for charge injection (turn-on voltage): 14 V (E-field = 2 x 106 V/cm
Quantum efficiency = 0.05 %
Emission color: Green
Processible ? No!!
Polymer is obtained by precursor approach. It cannot be redissolved once the polymer is synthesized

51. Other PPV Derivatives MEH-PPV
More processible, can be dissolved in common organic solvents (due to the presence of alkoxy side chains)
Fabrication of Flexible light-emitting diodes (Nature 1992, 357, 477)

52. Substrate: poly(ethylene terephthlate) (PET)
Anode: polyaniline doped with acid-a flexible and transparent conducting polymer
EL Quantum efficiency: 1 %
Turn-on voltage: 2-3 V

53. Other Examples of Light Emitting Polymers
Poly(p-phenylene) (PPP)
BLUE light
emission
Poly(9,9-dialkyl fluorene)
CN-PPV: RED light emission
Nature 1993, 365, 628
Polythiophene derivatives
A blend of these polymers produced variable colors, depending on the composition
Nature 1994, 372, 443

54. Applications
Flat Panel Displays: thinner than liquid crystals displays or plasma displays (the display can be less than 2 mm thick)
Flexible Display Devices for mobile phones, PDA, watches, etc.
Multicolor displays can also be made by combining materials with different emitting colors.

55. Photodiode For an Electroluminescence process: Electrons Photons
Can we reverse the process?
Photons
Electrons
YES!
Photodiode
Production of electrons and holes in a semiconductor device under illumination of light, and their subsequent collection at opposite electrodes.
Light absorption creates electron-hole pairs (excitons). The electron is accepted by the materials with larger electron affinity, and the hole by the materials with lower ionization potential.

56. A Two-Layer Photovoltaic Devices
Conversion of photos into electrons
Solar cells (Science 1995, 270, 1789; Appl. Phys. Lett. 1996, 68, 3120)
(Appl. Phys. Lett. 1996, 68, 3120)
490 nm
Max. quantum efficiency: ~ 9 %
Open circuit voltage Voc: 0.8 V

57. MEH-PPV e- h+ C60 Another example: Science 1995, 270, 1789.
ITO/MEH-PPV:C60/Ca
Active materials: MEH-PPV blended with a C60 derivative
light
MEH-PPV
ITO/MEH-PPV:C60/Ca
dark e- h+
light
C60
ITO/MEH-PPV/Ca
dark

58. A Photodiode fabricated from polymer blend
(Nature 1995, 376, 498)
Device illuminated at 550 nm (0.15 mW/cm2)
Open circuit voltage (Voc): 0.6 V
Quantum yield: 0.04 %

59. Field Effect Transistors (FET)
Using poly(3-hexylthiophene) as the active layer
“All Plastics” integrated circuits (Appl. Phys. Lett. 1996, 69, 4108; recent review: Adv. Mater. 1998, 10, 365)

60. More Recent Development
Use of self-assembled monolayer organic field-effect transistors
Possibility of using “single molecule” for electronic devices
(Nature 2001, 413, 713)