Download Science Update Programme Introduction to Conducting Polymers

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
• 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
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
1. “Handbook of Conducting Polymers” First Edition, T. A. Skotheim Ed.,
Marcel Dekker, New York, 1986.
2. “Functional Monomers and Polymers” K. Takemoto, R. M. Ottenbrite,
M. Kamachi Eds., Marcel Dekker, New York, 1997.
3. “Handbook of Organic Conductive Molecules and Polymers” Volume 2,
H. S. Nalwa Ed., Wiley, West Sussex, 1997.
4. “Handbook of Conducting Polymers” Second Edition, T. A. Skotheim,
R. L. Elsenbaumer, J. R. Reynolds Eds., Marcel Dekker, New York,
1998.
5. “Electronic Materials: The Oligomer Approach” K. Müllen and G.
Wegner Eds., Wiley-VCH, Weinheim, 1998.
6. “Semiconducting Polymers: Chemistry, Physics and Engineering” G.
Hadziioannou and P. F. van Hutten Eds., Wiley-VCH, Weinheim, 2000.
7. 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
• 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)
Conductivity (W-1 cm-1)
Insulators
10
-20
10
-16
10
Semiconductors Metals
-12
10
10 -4
-8
10 4
copper
germanium
silicon
glass
nylon
Teflon
quartz
Quartz: s = 10-18 S cm-1
Silver/copper: s = 106 S cm-1
10
0
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
• 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
• 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
– 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)
• 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
• Typical insulator
– Polyethylene s ~ 10-15 S cm-1
– Polytetrafluoroethylene (PTFE) s ~ 10-16 S cm-1
– Polystyrene s ~ 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”
• 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)
polyac rylonitrile (PAN)
CN
CN
CN
CN
CN
CN
CN
ox idation at 200-300 °C
N
N
N
N
N
N
N
N
These polymers are
not soluble in any
solvent, and are not
well-characterized
Ladder Polymer
Doped with Br 2 or I2
-1
s = 10 S c m
PAN
600-1200 °C
(spin-coated film) Without doping
Materials with s ~ 0.1 to 800 S cm-1
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
• 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
• 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
• 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
By Ziegler-Natta Catalyst
Effect of Temperature:
At -78 °C or below: all-cis PA
At 180 °C or higher: all-trans PA
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
Durham Method
Isomerization of Polybenzvalene
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 )
Positively charged
Soliton S+
Neutral soliton S0
Negatively charged
Soliton S-
Conduction band
Valence band
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
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.
In chemical terms:
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)
Electrical Conductivity of Some Doped Polyacetylenes
Dopant
I2
IBr
HBr
AsF5
SeF6
FeCl3
MoCl5
WCl6
Conditions Conductivity (S cm-1)
Vapor
Vapor
Vapor
Vapor
Vapor
CH3NO2
Toluene
Anisole
Toluene
Toluene
Anisole
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
Poly(1,4-phenylene) or Poly(p-phenylene) (PPP)
A conjugated polymer based on aromatic units on the
main chain
Synthesis
n ~ 5-15
Pure PPP is not soluble neither. The solubility can be
enhanced by attaching flexible groups to the polymer chain.
R = C6H13
Soliton, polaron, and bipolaron in poly(p-phenylene)
Conductivity of PPP
Dopant
s (S cm-1)
AsF5
FeCl3
SbCl5
I2
SO3
AlCl3
Napht-K+
Napht-Li+
500
0.30
< 10-3
< 10-4
10-1 to 10-4
8.0
50
5
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
Proposed mechanism for the electrochemical polymerization
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
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), headto-tail (HT), or tail-to-tail (TT)
Synthesis of Regioregular Polythiophene
Copolymers with aromatic compounds or vinylene group
Conductivity of polythiophenes and
polypyrroles doped under different conditions
Material
Dopant
s (S cm-1)
Polythiophene
Poly(3-methylthiophene)
Poly(3-ethylthiophene)
Poly(3-buthylthiophene)
Poly(3-hexylthiophene)
Poly(3-hexylthiophene)
SO3CF3PF6PF6I2
PF6I2
10-20
510
270
4
30
11
Polypyrrole
Polypyrrole
Polypyrrole
Polypyrrole
FeCl3
I2
Br2
Cl2
3-200
2-8
5
0.5
Synthesis of Conjugate Polymers by Precursor
Approach
To prepare a processible/soluble precursor polymer, which can
subsequently be processed into the final form
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
Medium
s (S cm-1)
5-Sulfosalicyclic acid
Benzene sulfonic acid
p-Toluene sulfonic acid
Sulfamic acid
Sulfuric acid
0.2-1.0
2.0
5.0
2.0
1.2
Applications of Conducting Polymers
• Rechargeable Batteries
– Higher energy and power densities (light
weight) than conventional ones using leadacid or Ni-Cd
– e.g. polyaniline used in 3V coin-shaped
batteries
(Poly. Adv. Tech. 1990, 1, 33)
– Rechargeable -> reversible doping
• 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)
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)
Some Possible Electronic Transitions After
Absorption of Photons
LED Display
• Light emission resulted from the
recombination of holes and electrons in a
semiconductor
When hole and electron
recombine:
Hole+ + ElectronExcited
states
Ground
states
Light emission
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
• 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
• 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)
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
Poly(p-phenylene) (PPP)
Poly(9,9-dialkyl fluorene)
BLUE light
emission
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
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.
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.
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
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
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 %
• 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
• Use of self-assembled monolayer
organic field-effect transistors
• Possibility of using “single molecule” for
electronic devices
(Nature 2001, 413, 713)