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Science Update Programme Conductive Polymers: From Research to Products. Education Bureau, HKSAR Department of Chemistry University of Hong Kong. May 2002. Course Coordinator Dr. Wai Kin Chan, Department of Chemistry, HKU. Content Introduction to Electrical Conductivity

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science update programme conductive polymers from research to products

Science Update ProgrammeConductive Polymers: From Research to Products

Education Bureau, HKSAR

Department of Chemistry

University of Hong Kong

May 2002

course coordinator dr wai kin chan department of chemistry hku
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
conductive polymers

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”

http://www.nobel.se

references
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.
electrical conductivity of materials
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 (Wcm) or conductivity (W-1cm-1 orS cm-1)
slide6

Quartz: s = 10-18 S cm-1

Silver/copper: s = 106 S cm-1

interpretation of electrical conductivity in terms of simple energy band diagram
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

slide8
Bulk electrical conductivity s

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
the energy band gap
The Energy Band Gap
  • Insulators
    • Band Gap of several electron volts (eV)
    • 1 eV = 8065.7 cm-1 or 1.602  10-19 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
slide10
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)
slide11
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?

electrical conductivity of polymers
Electrical Conductivity of Polymers
  • Typical insulator
    • Polyethylenes ~ 10-15 S cm-1
    • Polytetrafluoroethylene (PTFE)s ~ 10-16 S cm-1
    • Polystyrenes ~ 10-15 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)
metallic polymers
“Metallic Polymers”
  • By pyrolysis of insulating or semiconducting polymers

Precursor

polymers

1000-2000 °C

Product

Precursor: polyesters, polyamides, PVC, phenolic resins

Product: graphite type carbons (intermediate structures are not well defined)

slide14

These polymers are not soluble in any solvent, and are not well-characterized

600-1200 °C

PAN

(spin-coated film)

Materials with s ~ 0.1 to 800 S cm-1

Without doping

conductor filled polymers
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
slide16
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
semiconducting polymers
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 ~ 10-14 S cm-1

It is a very common photoconducting materials

polymers with unsaturated conjugated backbone structure
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

synthesis of polyacetylene
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

slide20

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

slide21

Durham Method

Isomerization of Polybenzvalene

the conduction process in conjugate polymers
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 )

slide23

Positively charged

Soliton S+

Negatively charged

Soliton S-

Neutral soliton S0

Conduction band

Valence band

slide24

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

slide25

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.

slide27

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)

slide28

Electrical Conductivity of Some Doped Polyacetylenes

Dopant

I2

IBr

HBr

AsF5

SeF6

FeCl3

MoCl5

WCl6

Conditions

Vapor

Vapor

Vapor

Vapor

Vapor

CH3NO2

Toluene

Anisole

Toluene

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

other conjugated polymers
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

slide30

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

R = C6H13

conductivity of ppp
Conductivity of PPP

s (S cm-1)

500

0.30

< 10-3

< 10-4

10-1 to 10-4

8.0

50

5

Dopant

AsF5

FeCl3

SbCl5

I2

SO3

AlCl3

Napht-K+

Napht-Li+

polypyrrole
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
polythiophene
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
slide36

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)

synthesis of regioregular polythiophene
Synthesis of Regioregular Polythiophene

Copolymers with aromatic compounds or vinylene group

conductivity of polythiophenes and polypyrroles doped under different conditions
Conductivity of polythiophenes and polypyrrolesdoped under different conditions

Material

Polythiophene

Poly(3-methylthiophene)

Poly(3-ethylthiophene)

Poly(3-buthylthiophene)

Poly(3-hexylthiophene)

Poly(3-hexylthiophene)

Polypyrrole

Polypyrrole

Polypyrrole

Polypyrrole

Dopant

SO3CF3-

PF6-

PF6-

I2

PF6-

I2

FeCl3

I2

Br2

Cl2

s (S cm-1)

10-20

510

270

4

30

11

3-200

2-8

5

0.5

synthesis of conjugate polymers by precursor approach
Synthesis of Conjugate Polymers by Precursor Approach

To prepare a processible/soluble precursor polymer, which can subsequently be processed into the final form

polyaniline
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
electrical conductivity
Electrical Conductivity

s (S cm-1)

0.2-1.0

2.0

5.0

2.0

1.2

Medium

5-Sulfosalicyclic acid

Benzene sulfonic acid

p-Toluene sulfonic acid

Sulfamic acid

Sulfuric acid

applications of conducting polymers
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
slide44
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. Metals1993, 57, 3696)
other applications
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)
slide47

LED Display

  • Light emission resulted from the recombination of holes and electrons in a semiconductor
slide48

When hole and electron recombine:

Hole+ + Electron-

Excited states

Ground states

Light emission

organic light emitting polymer
Organic Light Emitting Polymer
  • First reported in 1990 (Nature1990, 347, 539)
  • Based on poly(p-phenylenevinylene) (PPV), with a bandgap of 2.2 eV

ITO: Indium-tin-oxide-A transparent electrical conductor

slide50
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
other ppv derivatives
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(Nature1992, 357, 477)
slide52

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

other examples of light emitting polymers
Other Examples of Light Emitting Polymers

Poly(p-phenylene) (PPP)

BLUElight

emission

Poly(9,9-dialkyl fluorene)

CN-PPV: RED light emission

Nature1993, 365, 628

Polythiophene derivatives

A blend of these polymers produced variable colors, depending on the composition

Nature1994, 372, 443

applications
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.
slide55

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.

slide56

A Two-Layer Photovoltaic Devices

  • Conversion of photos into electrons
  • Solar cells (Science1995, 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

slide57

Another example: Science1995, 270, 1789.

ITO/MEH-PPV:C60/Ca

Active materials: MEH-PPV blended with a C60 derivative

light

ITO/MEH-PPV:C60/Ca

MEH-PPV

dark

e-

h+

light

C60

ITO/MEH-PPV/Ca

dark

slide58
A Photodiode fabricated from polymer blend

(Nature1995, 376, 498)

Device illuminated at 550 nm (0.15 mW/cm2)

Open circuit voltage (Voc): 0.6 V

Quantum yield: 0.04 %

slide59
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)
more recent development
More Recent Development
  • Use of self-assembled monolayer organic field-effect transistors
  • Possibility of using “single molecule” for electronic devices

(Nature2001, 413, 713)