Conductive Polymers Outline

Haiping Lin Student 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

1 Story of the Noble prize

H H H C C C C C C C H H H H

Polyacetylene (PA)

I2 σ = 10-9 S/cm σ = 38 S/cm

Only conjugated polymers are conducting Polyethylene ”Plastic wrap” H H H H H H C C C C C C C A transparent Insulator H H H H HHH H Remove one hydrogen per carbon!

Polyacetylene H H H C C C A silver-metallic C C C C Semiconductor H H H H

2 SP2 Bonding SP2 Pz • In π orbitals, + electrons can be delocalized. • In the language of Sigma bond chemistry -

Sigma bond ‘resonance’. • The overlap between π orbitals largely determine the Pi bond electronic properties Pi bond of conjugated polymers

Polyacetylene

• PA is the simplest conjugated polymer • Two forms

• One dimensional metal?

• A moderate insulator •Why?

3 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.

⎛α β 000⎞ ⎧ α if i = j ⎜ ⎟ ⎪ βαβ 00 i Hjˆ = ⎨β if i = j ±1 ⎜ ⎟ Hˆ = ⎜ 0 βαβ 0⎟ ⎪ 0 otherwise ⎩ ⎜ 00βαβ⎟ N ⎜ ⎟ ˆ ⎝ 000βα⎠ H Ψ = E Ψ Ψ = ∑cj j j=1

N N ˆ project onto p ∑cj Hj= Ec∑ j j ⎯⎯⎯⎯⎯→ j=1 j=1 N N ˆ ∑cj p Hj= Ec∑ j pj = Ecp j=1 j=1

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

4 One dimensional chain of identical atoms

N ˆ ∑cj p Hj= Ecp j=1

⎛α − E β 000⎞ ⎛ c1 ⎞ ⎜ βα− E β 00⎟ ⎜ ⋅⋅⋅⎟ ⎜ ⎟ ⎜ ⎟ ⎜ 0 βα− E β 0 ⎟ ⎜ cj ⎟ = 0 ⎜ 00βα− E β ⎟ ⎜ ⋅⋅⋅⎟ ⎜ ⎟ ⎜ ⎟ ⎝ 000βα− E⎠ ⎝ cN ⎠

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

This model fails

5 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-Peierls distortion

• 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

6 Peierls distortation (E) H H H C C C C C C C E HHHH F π/2a π/a (k)

{ Half-filled band! a

(E) H H H C C C C C C C E }E HHHH F g π/2a π/a (k) 2a 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.

7 Degenerate ground states

• Why trans-polyacetylene has higher electric conductivity than cis-polyacetylene ?

• Trans-PA has two degenerated ground states

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

• Cis-PA has non-degenerated ground states

Soliton

• Combination of conjugation sequence creates “misfit” • When bond alternation interrupted by two single bonds, a dangling bond forms a radical

- - misfit

- -

8 Solition ’Bonding order A’ ’Bonding order B’

Same energy S

Geometric distortion

E C Soliton: • Spin but no charge! E V

Non-degenerated ground states

...... Switch single/bouble bond order

......

”quinoid” rings has a higher energy as compared to benzene rings

9 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

less stable more 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

10 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

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

x- + Reductive doping [CH]n + xNa [CH]n + xNa - + I 3 - + I 3 The doped polymer is thus a salt. However it is not the counter ions but the charges that are the mobile charge carriers

11 Solitions and Polarons

Polymers with degenerated ground states

LUMO

HOMO

------

Positive Solition Neutral Soliton Negative Soliton

One charge 0 spin 0 charge ½ spin One charge 0 spin

Doping mechanism

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

I2 • Low mobility of counterions I - + 3 • Coulomb attraction + I - 3 • The redical cation is localised

- + I3 • High concentration of dopants is needed so that the polaron I - + 3 can move in the field of close counterions I - + 3 - + I3

12 Change in absorption spectrum

The optical absorption of polyacetylene with increasing dopant density.

The ππ* 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

Idoine “strips” electron Structure relaxation from HOMO of the polymer

13 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)

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

• Electrochemical doping (e.g. immersing a trans-PA film

in solution of LiClO4, and anodic oxidation) - +y y- - trans-[CH]x + (xy)(ClO4) → [(CH) (ClO4) ]x + (xy)e

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

• Photodoping

14 Temperature dependant

Applications

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

15 Schematic of LED in operation

Emissive devices with 180o view angle Low drive voltage < 5V Fast response: few µs for display Low drive current Ultra thin materials High brightness Colour tuning via chemistry Large display area