1 Conductive Polymers Haiping Lin Student seminar in TU, Berlin 23 rd 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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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
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!
5
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
6
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
• High concentration of dopantsis needed so that the polaroncan move in the field of close counterions
++
I3-
I3-
++
I3-
I3-
I2
++
I3-
I3-
13
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
14
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
-
15
Temperature dependant
Applications
• Plastic wires• Organic light emission displayer (OLED)• Solar cell• Heterogeneous Catalysts• Potential modified electrodes• Porous films
16
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