1

Conductive Polymers

Haiping LinStudent seminar in TU, Berlin

23rd June 2005

Outline

• Nobel prize in Chemistry 2000• Electronic structure of conjugated polymers• Intrinsic conductivity of conjugated polymers• Mechanisms of doping• Charge transport• Applications

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Story of the Noble prize

CC

CC

CC

CH

H

H

H

H

H

H

Polyacetylene (PA)

I2σ = 10-9 S/cm σ = 38 S/cm

CC

CC

CC

CH

H

H

H

H

H

H

H H H

H H H H

Polyethylene ”Plastic wrap”

A transparent Insulator

Polyacetylene

A silver-metallic SemiconductorC

CC

CC

CC

H

H

H

H

H

H

H

Remove one hydrogen per carbon!

Only conjugated polymers are conducting

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SP2 Bonding

π• In orbitals, electrons can be delocalized.

• In the language of chemistry -‘resonance’.

• The overlap between orbitals largely

determine the electronic properties of conjugated polymers

+

SP2 Pz

Sigma bond

Sigma bond

Pi bond

Pi bond

π

Polyacetylene• PA is the simplest conjugated polymer• Two forms

• One dimensional metal?

• A moderate insulator• Why?

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One dimensional chain of identical atoms

• Using π electron approximation (ignore sigma bonds)

• Treating all carbon atoms equally, irrespective of their local environment

• Assuming all carbon atoms interact only with their immediate neighbours

• Each carbon atom form bond with only one unpaired electron in Pz orbital.

H =

α β 0 0 0β α β 0 00 β α β 00 0 β α β0 0 0 β α

⎛

⎝

⎜⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟⎟

i H j =α if i = j

β if i = j ±10 otherwise

⎧

⎨⎪

⎩⎪

H Ψ = E Ψ Ψ = cj jj=1

N

∑

cj H jj=1

N

∑ = E cj jj=1

N

∑ project onto p⎯ →⎯⎯⎯⎯

cj p H jj=1

N

∑ = E cj p jj=1

N

∑ = Ecp

This can be written in matrix form, just like the 2-atom case!

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α − E β 0 0 0β α − E β 0 00 β α − E β 00 0 β α − E β0 0 0 β α − E

⎛

⎝

⎜⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟⎟

c1

⋅ ⋅ ⋅cj

⋅ ⋅ ⋅cN

⎛

⎝

⎜⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟⎟

= 0

cj p H jj=1

N

∑ = Ecp

One dimensional chain of identical atoms

With large value of number N, the band-gap is also predicted to be vanished.

This model fails

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Need more complicated models

• The sigma bonds cannot be ignored• Bond length are not identical in PA• Pi electron need to be approximated with

more exchange, resonance and overlap integrals

• How to explain the different bond length in Polyacetylene?

Electron-phonon interaction-Peierlsdistortion

• There always exists a distortion of the lattice that lowers the total energy while lowering the symmetry and removing the orbital degeneracy

• Breaks the regular one-dimensional structure to give a bond alternation, also called Peiers Dimerization

• Opens an energy gap at the femi level at absolute zero of temperature

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Peierls distortation

CC

CC

CCCHHH

H H H H

(k)

(E)

EF

π/aπ/2aa{ Half-filled band!

(k)

(E)

EF

π/aπ/2a

CC

CC

CCCHHH

H H H H

2a

Eg}

Filled band!

Electron-electron Interaction-Hubbard’s Distortion

• Coulomb repulsion U between two electrons at the same lattice site.

• If the band is half-filled, there will be one electron at each site

• Adding an additional electron will require the energy U to overcome electron-electron repulsion

• Creation of a coulomb gap in a half-filled band.

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Degenerate ground states• Why trans-polyacetylene has higher electric conductivity

than cis-polyacetylene ?

• Trans-PA has two degenerated ground states

• Cis-PA has non-degenerated ground states

’Bonding order A’ ’Bonding order B’Same energy

Soliton

• Combination of conjugation sequence creates “misfit”

• When bond alternation interrupted by two single bonds, a dangling bond forms a radical

-

-

-

-

-

-

misfit

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Solition’Bonding order A’ ’Bonding order B’

Same energyS

Geometric distortion

E

E C

V

Soliton:• Spin but no charge!

Non-degenerated ground states......

......

Switch single/boublebond order

”quinoid” rings has a higherenergy as comparedto benzene rings

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Minimization of bond length alternation

• Polythiophene has a wide band gap (~2eV)• Small contribution from quinoid structure • Significant single bond character of the thiophene-

thiophene linkages• Large bond length alternation• Copolymerization of Aromatic and Quinoid

heterocycles

more stableless stable

Donor-Accepter copolymerization

Donor-Acceptor Concept (1993)

• Donor - High lying energy levels

• Acceptor – Low lying energy levels

• Narrow band gap• Increase of

conductivity of 2-5 orders of magnitude

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Doping in polymer

• Doping of polymers can yield an increase in conductivity of several orders of magnitude (from10-10-10-5S/cm to ~1-104S/cm)

• A number of doping methods available • Doping level can be well controlled

Concept of Doping• The doping of all conducting polymers are

accomplished by partial addition (reduction) or removal (oxidation) of electron to/from the π system of the polymer backbone

The doped polymer is thus a salt. However it is not the counter ions but the charges that are the mobile charge carriers

Reductive doping

Oxidative doping[CH]n + 3x/2 I2 [CH]nx+ + xI3

-

[CH]n + xNa [CH]nx- + xNa +

I3-+

+I3-

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Solitions and Polarons

LUMO

HOMO

- - - - - -

Positive Solition

One charge 0 spin

Neutral Soliton Negative Soliton

Polymers with degenerated ground states

0 charge ½ spin One charge 0 spin

Doping mechanismOxidative doping[CH]n + 3x/2 I2 [CH]n

x+ + xI3-

• Low mobility of counterions

• Coulomb attraction

• The redical cation is localised

• High concentration of dopantsis needed so that the polaroncan move in the field of close counterions

++

I3-

I3-

++

I3-

I3-

I2

++

I3-

I3-

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Change in absorption spectrum

The optical absorption ofpolyacetylene with increasingdopant density.

The π π* transition (@1.7eV)reduced in strength

A midgap state (@0.7eV)appear and grow at the expense of the others

Origin of new transitions• Electrons are removed from HOMO• Structural relaxation occurs• Levels are “pulled into the band-gap”• Additional transitions grow at the expense of others

I2

Idoine “strips” electronfrom HOMO

Structure relaxationof the polymer

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Charge transfer between different polymer chains

Intersoliton hoping mechanism

Charged solitions (bottom) are trapped by dopant couterions

Neutral solitions (top) are free to move

A neutral solition interact with the charged solition

Electron hops from one defect to the other

Doping methods• Chemical doping (e.g. trans-PA in iodine vapor)

• Electrochemical doping (e.g. immersing a trans-PA film in solution of LiClO4, and anodic oxidation)trans-[CH]x + (xy)(ClO4)- → [(CH)+y(ClO4)y-]x + (xy)e-

• Charge-inject doping carried out using a metal/insulator/semiconductor system

• Photodoping

Oxidative doping[CH]n + 3x/2 I2 [CH]nx+ + xI3

-

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Temperature dependant

Applications

• Plastic wires• Organic light emission displayer (OLED)• Solar cell• Heterogeneous Catalysts• Potential modified electrodes• Porous films

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Schematic of LED in operation

Emissive devices with 180o view angleFast response: few µs for displayUltra thin materialsColour tuning via chemistry

Low drive voltage < 5VLow drive currentHigh brightnessLarge display area