8. Optical processes in conjugated materials

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8.1. Electron-Phonon Coupling

E

Q

Absorption

Relaxation effects

Emission

Ground state

Lowest excitation state

Excitations

1.2

1.0

0.8

0.6

0.4

0.2

0.0

PL

Int

ens

ity (

arb

. uni

ts)

3.0

2.5

2.0

1.5

1.0

0.5

(x 1

05 cm

-1)1.2

1.0

0.8

0.6

0.4

0.2

0.0

PL

Int

ens

ity (

arb

. uni

ts)

4.03.53.02.52.0Photon Energy (eV)

2.5

2.0

1.5

1.0

0.5

(x 1

05 cm

-1)n

8.1.1. Fluorescence

Polyfluorene (F8)

Carlos Silva, University of Cambridge

- Weak self-absorption

- Vibronic structure

8.1.2. Intrachain Exciton

1

2 3

4 5

6

3 2

3 46 2

64

4 4

Site number

Lowest excitedstate

INDO/SCI

Exciton size Binding energy 0.3 eV

Exciton=electron-hole pair

Probability to find the e- and h+ at one site

n8.2.1. Optical transition versus chain size

The “conjugation length” is the length of the oligomer emitting the same luminescence spectrum as the polymer. While the polymer may easily be 10-100 times longer than a conjugation length, the chain is effectively operating as a sequence of conjugation lengths along a common string. This description is valid for the behaviour of absorption processes; where emission is relevant, the excited state is often more localized.

Cornil, J. et al. Chem. Phys. Lett. (1997), 278, 139

1/m (m=number of bonds)

absorption

emissionabsorption

emission

8.2. Conjugation length

• Switch between different structures by applying mechanical force while monitoring the Langmuir monolayer's optical spectra. The figure shows the chemical structures, conformations and spatial arrangements at the air–water interface of the polymer.

8.2.1. Conformation changes

• Compression causes a transition from the face-on to the zipper structure, which breaks the conjugation, i.e. decreases the π-conjugation length and generates a large blue shift (34-nm).

Kim et al. Nature 411, 1030 - 1034 (2001)

SEKUNDÄR EFFEKTVridning av ring minskar pz-pz överlapp

w < W

EN

ER

GI

k

R

R

PRIMÄR EFFEKT:Tillför (tar bort) laddning

EN

ER

GI

kR

R

Bandgap och Dispersionvia Sidogrupper

Bandgap och Dispersionvia Sidogrupper

R

R

PRIMÄR EFFEKT

SEKUNDÄR EFFEKT

Gult område:pz densitet

8.3. Influence of Electroactive Substituents

Al

Need for small energy barriers to optimize hole/electron injection

+ 0.27 eV

+ 0.08 eVE

- 1.17 eV

- 0.99 eV

Molecular engineering to modulate the energy of the band edges

n

OCH3

CH3O n

CN

CN n

• Donor:

• σ-donor (electronegativity): symmetric destabilization • π-donor: asymmetric destabilization

• Acceptor:

• σ-acceptor : symmetric stabilization• π-acceptor : asymmetric stabilization

• Note that ”-O-CH3” acts as a globally as a donor. This is the results of a competition between its π-donor and σ-acceptor characters.

8.4. Modulation of the Optical Properties

Sn

nn

Red Yellow-Green Blue

- Molecular backbone

- Chain size

K. Müllen and co

8.4.1 Structure of the conjugated chain

8.4.2 Optical properties and Doping

E

H

L

Polaron Bipolaron

S

S

S

S

S

S

Electrochromism

Neutral

Singly charged

Doubly charged

Electrochromism in a substituted polythiophene, under electrochemical doping in contact with an electrolyte. The suppression of bandgap absorption in the polymer (with a maximum at 500 nm) due to doping is highly visible; formation of polarons is hardly visible, but the two optical transitions due to bipolarons are found, one peaking at 800 nm and another below 1200 nm. From Peter Åsberg, work in progress, Biorgel, IFM, LiU

Electrochromic Displays on Papers

Wavelength (nm)

300 400 500 600 700 800 900

Abs

0.0

0.1

0.2

0.3

0.4

0.5

Reduced PEDOTOxidized PEDOT

Prof. M. Berggren, Norrköping

8.5.1. Transition Dipole Moment

Atomic transition densities

1Ag 1BuN = 20

INDO/SCI

+-

qK rK = K

1Ag 1Bu

qK = 0 K

8.5. Solid State Effect: Exciton Splitting

1

2

E

G

E

G

Cofacial dimer

+ -

+-

2E1

E2+ -+ -

tot = 0

tot = 2

8.5.2. H-Aggregate

8.5.3. J-Aggregate

E

G

E

G

+ -+-

2E1

E2

+ -

+ -

tot = 0

tot = 2

8.6. Charge and energy transfer in conjugated polymers

Organic Solar Cells

Glass ITO

e

h

Energy transfer

LUMO

HOMOHOMO

LUMO

Charge transfer

8.6.1.Photoinduced Charge Transfer

LUMO

HOMOHOMO

LUMO

E

Photoinduced ELECTRON transfer

LUMO

HOMOHOMO

LUMO

E

Photoinduced HOLE transfer

Chemical Sensors

H

L

TNTPolymer

CH3

NO2NO2

NO2

TNT

Land-mine detector

(Detection limit : 10-15 g)

Photoinduced Electron Transfer

Tim Swager and co, MIT

8.6.2. Polymer / Polymer Interfaces

DMOS-PPV

Si

C8H17

O

O

C2H5

C4H9

MEH-PPV

OC6H13

C6H13O

OC6H13

C6H13O

CN

CN

CN-PPV

0.55 eV

0.17 eVH

L0.63 eV

0.44 eVH

L

MEH-PPV / CN-PPV Blend

0

5

10

15

20

25

30

35

0

1

2

3

4

5

0.0 0.2 0.4 0.6 0.8 1.0

Ph

oto

lum

ines

cen

ce e

ffic

ien

cy (

%)

Qu

antu

m yield

(%)

Weight fraction of CN-PPV

8.6.3. Charge transfer8.6.3. Charge transfer

J.J.M. Halls, J. Cornil, et al., Phys. Rev. B 60, 5721 (1999)

0.63 eV

0.44 eVH

L

MEH-PPV CN-PPV

DMOS-PPV / CN-PPV Blend

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.6 1.8 2.0 2.2 2.4

No

rmal

ised

PL

inte

nsi

ty

Energy (eV)

Blend

CN-PPV

DMOS-PPV

8.6.4. Energy transfer8.6.4. Energy transfer

DMOS-PPV CN-PPV

0.55 eV

0.17 eVH

L

8.6.5. Charge versus Energy Transfer

Penalty to pay to dissociate an exciton on the order of 0.35 eV

0.63 eV

0.44 eVH

L

MEH-PPV CN-PPV

INTRA

INTRA

INTER

One-electron levels Excited states

GROUND STATE

INTRA CN-PPV

INTER

INTRA MEH-PPV0.28 eV 0.19 eV

Charge transfer at the polymer/polymer interface

MEH-PPV / CN-PPV Blend : Charge transfer

Penalty to pay to dissociate an exciton on the order of 0.35 eV

0.55 eV

0.17 eVH

L

DMOS-PPV CN-PPV

INTRA

INTRAINTER

One-electron levels Excited states

GROUND STATE

INTRA CN-PPV

INTER

INTRA DMOS-PPV

0.38 eV

0.20 eV

Energy transfer towards the CN-PPV chains

DMOS-PPV / CN-PPV Blend : Energy transfer