Introduction to Plastic Electronics

1
Introduction to Plastic Electronics

• Dr. Jérôme Cornil

Laboratory for Chemistry of Novel
Materials University of Mons-Hainaut
2
Conjugated Polymers
- Traditional plastic Polyethylene
Ethylene
- Conjugated polymer Trans-polyacetylene
Acetylene
3
Molecular versus Band Models
M. Lögdlund et al.
E
n
E (eV)
Electron affinity
LUMO
Bandgap
HOMO
Ionization potential
4
Electronic Structure
Localized (l) level
E
Delocalized (d) level
- Key role in determining the optical properties
5
Ultraviolet Photoelectron Spectroscopy (UPS)
N. Armstrong and co
6
Optical Absorption Spectra
PPV11
R.H. Friend and co
I
IV
II
III
Intensity (arb. units)
Energy (eV)
7
Excited-State Characterization
INDO/SCI
Lowest excited
state
Exciton size
Site number
8
Excited-state Wavefunction
Band I
Band III
Band IV
Site number
9
Electron-Phonon Coupling
Excitation
Doping
E
E
LES
1
Relaxation effects
Relaxation effects
Absorption
Ionization
Emission
GS
GS
Q
Q
10
Geometry Relaxation
LUMO
AM1(CI)
HOMO
? Polaron / Radical-ion
LUMO
HOMO
? Polaron-exciton
11
Fluorescence
Polyfluorene (F8)
- Weak self-absorption
- Vibronic structure
Carlos Silva, University of Cambridge
12
Conducting Polymers
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Nobel Prize in Chemistry 2000
For the Discovery and Development of Conductive
Polymers
Hideki Shirakawa University of Tsukuba
Alan MacDiarmid University of Pennsylvania
Alan Heeger University of California
at Santa Barbara
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Conjugated Polymers
Trans-polyacetylene (t-PA)
Polythiophene (PT)
Polypyrrole (PPY)
Poly(p-phenylene) (PPP)
Poly(p-phenylenevinylene) (PPV)
y
1-y
Polyaniline (PAN)
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Conducting Polymers
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Advantages
Combination of properties
Metals
Plastics
High conductivity
Lightness
Ease of processing (spin coating)
Low cost
Tailored synthesis
- Low oxidation potential (doping by I2, AsF5,
SbF5, )
- High electron affinity (doping by Li, Na, K, )
- Stability of the backbone not affected upon
doping
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Chemical Doping of Conjugated Polymers
CB
VB
Radical-cation / Polaron
?
Dication / Bipolaron

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Electronic Structure
E
L
L
L
POL2
BIP2
BIP1
POL1
H
H
H
Polaron
Bipolaron
Neutral
J. Cornil et al., Adv. Mater. 8, 447 (1996)
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Optical Properties
Bipolaron
Neutral
Polaron
Energy (nm)
J. Guay et al.
? Electrochromic windows, sensors
Chem. Mater. 4, 1097 (1992)
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Applications
- Batteries
- Electrostatic coatings
Baytron P (Bayer) / Orgacon (AGFA)
PEDOT
PSS
- Electromagnetic shielding
- Electrochromic windows
- Actuators (Artificial muscles)
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Organic Light-Emitting Diodes
R.H. Friend et al., Nature 397, 121 (1990)
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Electroluminescence
Injection
1
F
Migration
2
-
-
2
EF
Recombination
3
1
Al
3
V
Excitons
L
L
EF

ITO

PPV
H
H
Singlet
Triplet
- Spin statistics 25
23
Molecular Engineering
EF
? Need for small energy barriers to optimize
hole/electron injection
E
L
0.08 eV
- 1.17 eV
H
0.27 eV
- 0.99 eV
n
n
n
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Electroluminescence Quantum Yield
?EL n1 n2 n3 n4
Number of excitons
n1
? 100
Number of charges
Number of singlet excitons
n2
? 25-50
Number of triplet excitons
?EL 1/10000
Number of photons emitted
n3
Number of singlet excitons
Number of photons escaping
n4
8/100
Number of photons emitted
25
Color Tuning
S0?S1
INDO/SCI
3.08 eV
2.42 eV
2.24 eV
FEDOT
F
FBT
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Interchain Interactions
INDO/SCI
Absorption blue shift
Emission red shift
R
Interchain distance (Å)
27
Influence of Positional Disorder
z
x
y
J. Cornil et al., J. Am. Chem. Soc. 120, 1289
(1998)
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Influence of the relative positions
?
S0?S2
S0?S2
Transfer of intensity
S0?S1
S0?S1
- Perpendicular orientation of the chain axes
Spiro molecules
(Salbeck, Salaneck)
29
H versus J Aggregate
INDO/SCI
S3
S2
S1
J.Ph. Calbert et al.
30
Cambridge Display Technology
Full color display
- Active matrix
- 2 inch diagonal
- 200 x 150 Pixels
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Ink-jet Printing
CDT
Philips
32
Commercial Products
Norelco Spectra 8894 XL
Thickness 2.8 mm Brightness 100 cd/m2
Philips / CDT
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Large Area Displays
34
Organic Field-Effect Transistors
Organic layer
S
D
Dielectric (SiO2)
G
E
L
EF
H
Source
Organic layer
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Electron versus Hole Injection

-
S
D
S
D
G
G
-

E
EF
E
L
L
H
H
EF
Source
Organic layer
Source
Organic layer
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Organic Field-Effect Transistors
VDS
S
D
G
Electrical characteristics
Saturation
IDS
VG
VG
Linear
VG 0
VDS
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Electronic Circuits
Philips
Active matrix in LCD
Integrated circuit
(electronic tags)
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Transport Properties
Effective bandwidth (Weff)
Polaron relaxation energy (Erel)
T
Band regime
Hopping regime
Weff gtgt Erel
Weff ltlt Erel
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Mobility versus Order
C.D. Dimitrakopoulos and D.J. Mascaro, IBM J.
Res. Dev. 45, 11 (2001)
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Hopping Regime
Marcus theory (semi-classical limit)
M1 M2
M1 M2
Reorganization energy ( ? )
Transfer integral ( t )
? ?i ?e
E
1
LUMO
2 tL
?i2
z
GS
HOMO
2 tH
?i1
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Reorganization Energy
Devos et al., Phys. Rev. B 58, 8236 (1998)
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Reorganization Energy in Pentacene
Gas-phase Ultraviolet Photoelectron Spectra
Exp.
?exp 118 meV
?th 98 meV
DFT-B3LYP (6-31G)
Th.
N.E. Gruhn, J.L. Brédas and co, JACS, 124, 7918
(2002)
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Reorganization Energy in Oligoacenes
Electrons
Holes
S. Coropceanu et al., Phys. Rev. Lett. 89, 275503
(2002)
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Discotic Liquid Crystals
Hexabenzocoronene (HBC)
- High degree of organization
- Self-healing
ZOA V2.0
- Mesophases at room temperature
- One-dimensional pathway for carriers
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Reorganization Energies in Discotics
T
HAT
0.30 / 0.40
P / P-
(R H)
0.18 / 0.26
0.30 / 0.27
HBC
TNA
HATNA
0.10 / 0.14
0.06 / 0.08
0.14 / 0.10
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Impact of Substitution
Positive polarons
(? in eV)
Negative polarons
(? in eV)
- Large modulation induced by the substituents
- Significant increase upon alkoxy substitution
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Transfer Integrals
Ethylene
Interchain transfer integral
INDO
4 Å
E
L1
L
HOMO
H
LUMO
H-1
J.L. Brédas et al., Proc. Nat. Acad. Sci. USA 99,
5804 (2002)
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Influence of the Relative Position
HOMO
d4.0 Å
LUMO
LUMO
HOMO
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Rotations
HOMO
Splitting (eV)
Distance 3.5 Å
LUMO
Rotational angle (degrees)
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Rotational Degrees of Freedom
Splitting (eV)
Rotational angle (degrees)
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Supramolecular Organization
PCFF
R SC2H5
Boltzmann statistics
? ti2 exp(-Ei / kT)
ltt2gt
? exp(-Ei / kT)
V. Lemaur et al., JACS 126, 3271 (2004)
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Organic Solar Cells
Glass
ITO
Donor
Acceptor
PPV
C60
E
L
H
Donor
Acceptor
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Absorption
Donor
Acceptor
Glass
ITO
e
h
E
-
L
L

H
H
Donor
Binding energy ? 0.4 eV
Donor
Acceptor
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Energy Migration
Donor
Acceptor
Glass
ITO
e
Diffusion length ? 10 nm
E
L
H
Key role of the morphology !
Donor
Acceptor
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Photoinduced Charge Generation
Donor
Acceptor
Glass
ITO
E
L
H
Donor
Acceptor
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Photoinduced Charge Transfer
E
LUMO
LUMO
Photoinduced ELECTRON transfer
HOMO
HOMO
E
LUMO
LUMO
Photoinduced HOLE transfer
HOMO
HOMO
57
Organic Solar Cells
University of Linz
10 x 15 cm Active area 80 cm2
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Optical Scanners
Al
Polymer/C60
ITO
Glass
102 strips of ITO (Width 185 ?m
Spacing 450 ?m)
Correction for human eyes
C AR R AG G AB B
UNIAX
59
Chemical Sensors
TNT
Photoinduced Electron Transfer
Land-mine detector
L
(Detection limit 10-15 g)
H
TNT
Polymer
Tim Swager and co, MIT
60
Photoinduced Charge Transfer
E
LUMO
LUMO
Photoinduced ELECTRON transfer
HOMO
HOMO
E
LUMO
LUMO
Photoinduced HOLE transfer
HOMO
HOMO
61
Polymer / Polymer Interfaces
DMOS-PPV
MEH-PPV
CN-PPV
L
L
0.63 eV
0.55 eV
H
H
0.17 eV
0.44 eV
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MEH-PPV / CN-PPV Blend
Charge transfer
63
DMOS-PPV / CN-PPV Blend
Energy transfer
J.J.M. Halls, J. Cornil, et al., Phys. Rev. B 60,
5721 (1999)
64
Charge versus 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 DMOS-PPV
0.20 eV
INTER
INTRA
INTER
0.38 eV
INTRA CN-PPV
INTRA
H
0.17 eV
GROUND STATE
DMOS-PPV
CN-PPV
Energy transfer towards the CN-PPV chains
65
Charge versus Energy 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
0.19 eV
INTRA CN-PPV
INTRA
INTER
INTER
INTRA
H
0.44 eV
GROUND STATE
MEH-PPV
CN-PPV
Charge transfer at the polymer/polymer interface
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Dynamical Aspects
Photoinduced Charge Transfer
Electron Hopping
Absorption
Charge Recombination
Hole Hopping
Need to estimate charge transfer rates !!