conductive polymers - max planck society
TRANSCRIPT
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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
2
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
3
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?
4
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