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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
Education Bureau, HKSAR
Department of Chemistry
University of Hong Kong
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”
Silver/copper: s = 106 S cm-1
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
s = enµn
e is the electronic charge of the carrier
n is the carrier density (no. of carriers per unit volume)
µn is the carrier mobility
What are the advantageous of using conducting polymers compared to metallic materials?
Precursor: polyesters, polyamides, PVC, phenolic resins
Product: graphite type carbons (intermediate structures are not well defined)
These polymers are not soluble in any solvent, and are not well-characterized
Materials with s ~ 0.1 to 800 S cm-1
Pure PVK is a hole conductor with dark conductivity of ~ 10-14 S cm-1
It is a very common photoconducting materials
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
By Ziegler-Natta Catalyst
Effect of Temperature:
At -78 °C or below: all-cis PA
At 180 °C or higher: all-trans PA
Note: These conjugate polymers are usually insoluble in organic solvents of have very low solubility
Isomerization of Polybenzvalene
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 )
Neutral soliton S0
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.
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”
Oxidative: AsF5, I2, Br2, AlCl3, MoCl5 (p-type doping)
Reductive: Na, K, lithium naphthalides (n-type doping)
Conductivity (S cm-1)
7 x 10-4
Note: PA is insoluble and labile to atmospheric oxygen
Poly(1,4-phenylene) or Poly(p-phenylene) (PPP)
A conjugated polymer based on aromatic units on the main chain
n ~ 5-15
Pure PPP is not soluble neither. The solubility can be enhanced by attaching flexible groups to the polymer chain.
R = C6H13
s (S cm-1)
10-1 to 10-4
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)
Copolymers with aromatic compounds or vinylene group
s (S cm-1)
To prepare a processible/soluble precursor polymer, which can subsequently be processed into the final form
s (S cm-1)
Benzene sulfonic acid
p-Toluene sulfonic acid
Hole+ + Electron-
ITO: Indium-tin-oxide-A transparent electrical conductor
Anode: polyaniline doped with acid-a flexible and transparent conducting polymer
EL Quantum efficiency: 1 %
Turn-on voltage: 2-3 V
CN-PPV: RED light emission
Nature1993, 365, 628
A blend of these polymers produced variable colors, depending on the composition
Nature1994, 372, 443
Can we reverse the process?
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.
(Appl. Phys. Lett. 1996, 68, 3120)
Max. quantum efficiency: ~ 9 %
Open circuit voltage Voc: 0.8 V
Active materials: MEH-PPV blended with a C60 derivative
(Nature1995, 376, 498)
Device illuminated at 550 nm (0.15 mW/cm2)
Open circuit voltage (Voc): 0.6 V
Quantum yield: 0.04 %
(Nature2001, 413, 713)