Transcript 8_optical properties of conjugated polymers

8. Optical processes in conjugated materials
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8.1. Electron-Phonon Coupling
E
Excitations
Lowest
excitation
state
Relaxation
effects
Absorption
Emission
Ground
state
Q
1.0
2.5
0.8
2.0
0.6
1.5
0.4
1.0
0.2
0.5
Polyfluorene (F8)
-1
3.0
5
1.2
a (x 10 cm )
PL Intensity (arb. units)
8.1.1. Fluorescence
0.0
n
2.5
1.0
- Weak self-absorption
0.8
2.0
- Vibronic structure
0.4
1.0
Carlos Silva, University of Cambridge
-1
1.5
5
0.6
a (x 10 cm )
ntensity (arb. units)
1.2
8.1.2. Intrachain Exciton
2
3
32
4
5
62
34
64
1
6
4
INDO/SCI
4
Lowest excited
state
Exciton=electron-hole pair
Exciton size
Site number
Binding energy  0.3 eV
Probability to find the e- and h+ at one site
8.2. Conjugation length
8.2.1. Optical transition versus chain size
emission
n
absorption
absorption
emission
1/m (m=number of bonds)
Cornil, J. et al. Chem. Phys. Lett. (1997), 278, 139
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.
8.2.1. Conformation changes
•
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.
• 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)
Bandgap och Dispersion
via Sidogrupper
PRIMÄR EFFEKT
R
Gult område:
pz densitet
R
R
ENERGI
R
PRIMÄR EFFEKT:
Tillför (tar bort) laddning
k
R
ENERGI
R
SEKUNDÄR EFFEKT
Vridning av ring minskar pz-pz överlapp
w<W
k
SEKUNDÄR EFFEKT
8.3. Influence of Electroactive Substituents
Need for small energy barriers to optimize
hole/electron injection
Al

Molecular engineering to modulate
the energy of the band edges
E
+ 0.08 eV
- 1.17 eV
+ 0.27 eV
- 0.99 eV
OCH3
CH3O
CN
n
n
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
8.4.1 Structure of the conjugated chain
- Molecular backbone
S
Red
n
n
n
Yellow-Green
Blue
- Chain size
K. Müllen and co
8.4.2 Optical properties and Doping
S
S
S
S
S
S
E
L
H
Polaron
Bipolaron
Electrochromism
Doubly charged
Intensity (arb. units)
100
Neutral
80
60
Singly charged
40
20
0
0.0
0.5
1.0
1.5
Energy (eV)
2.0
2.5
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
0.5
Reduced PEDOT
Oxidized PEDOT
0.4
Abs
0.3
0.2
0.1
0.0
300
400
500
600
700
Wavelength (nm)
800
900
Prof. M. Berggren, Norrköping
8.5. Solid State Effect: Exciton Splitting
8.5.1. Transition Dipole Moment
-
+
1Ag 1Bu
N = 20
0.15
INDO/SCI
Transition density (|e|)
0.10
Atomic transition densities
0.05
 qK = 0
0.00
K
-0.05
 qK rK =  1A
-0.10
K
-0.15
0
2
4
6
8
10
12
Site number
14
16
18
20
g 1Bu
8.5.2. H-Aggregate
1
Cofacial dimer
2
G
+
G
E1
-

tot = 0
2
E2
+
+
E
tot = 2 
-
E
-
+
8.5.3. J-Aggregate
-
+
G
+
G
E1
-

tot = 2 
2
+
-
+
E
E2
-
E
tot = 0
8.6. Charge and energy transfer in conjugated polymers
Glass ITO
e
h
Organic Solar Cells
LUMO
LUMO
HOMO
HOMO
Energy transfer
Charge transfer
8.6.1.Photoinduced Charge Transfer
E
LUMO
LUMO
Photoinduced ELECTRON
transfer
HOMO
HOMO
E
LUMO
LUMO
Photoinduced HOLE
transfer
HOMO
HOMO
Chemical Sensors
CH3
NO2
NO2
TNT
NO2
Photoinduced Electron Transfer
Land-mine detector
(Detection limit :
10-15
L
g)
H
Polymer
Tim Swager and co, MIT
TNT
8.6.2. Polymer / Polymer Interfaces
C4H9
C 2H 5
C8H17
Si
OC6H13
OC6H13
CN
C6H13O
DMOS-PPV
CN
C6H13O
O
CN-PPV
MEH-PPV
L
L
0.63 eV
0.55 eV
H
O
0.17 eV
H
0.44 eV
8.6.3. Charge transfer
5
35
30
4
25
3
20
15
2
10
1
MEH-PPV
CN-PPV
L
0.63 eV
H
5
0
0.0
Quantum yield (%)
Photoluminescence efficiency (%)
MEH-PPV / CN-PPV Blend
0.44 eV
0
0.2
0.4
0.6
0.8
1.0
Weight fraction of CN-PPV
J.J.M. Halls, J. Cornil, et al., Phys. Rev. B 60, 5721 (1999)
8.6.4. Energy transfer
DMOS-PPV / CN-PPV Blend
Normalised PL intensity
1.2
1.0
CN-PPV
0.8
DMOS-PPV
CN-PPV
Blend
L
0.55 eV
0.6
DMOS-PPV
0.4
H
0.2
0.0
1.6
1.8
2.0
2.2
Energy (eV)
2.4
0.17 eV
8.6.5. Charge versus Energy Transfer
MEH-PPV / CN-PPV Blend : Charge transfer
Penalty to pay to dissociate an exciton on the order of 0.35 eV
Excited states
One-electron levels
L
0.63 eV
INTRA MEH-PPV
0.28 eV
INTRA
0.19 eV
INTRA CN-PPV
INTER
INTER
INTRA
H
0.44 eV
MEH-PPV
CN-PPV
GROUND STATE
Charge transfer at the polymer/polymer interface
DMOS-PPV / CN-PPV Blend : Energy transfer
Penalty to pay to dissociate an exciton on the order of 0.35 eV
Excited states
One-electron levels
L
0.55 eV
INTRA
INTRA DMOS-PPV
0.20 eV
INTER
0.38 eV
INTER
INTRA CN-PPV
INTRA
H
0.17 eV
DMOS-PPV
CN-PPV
GROUND STATE
Energy transfer towards the CN-PPV chains