Electronic and Optoelectronic Polymers

1
Electronic and Optoelectronic Polymers
Wen-Chang Chen Department of Chemical
Engineering Institute of Polymer Science and
Engineering National Taiwan University
2
Outlines

• History of Conjugated Polymers
• Electronic Structures of Conjugated Polymers
• Polymer Light-emitting Diodes
• Polymer-based Thin Film Transistors
• Polymer-based Photovoltaics

3
Optical Absorbance
Absorption of light and the excited states of
molecules
Beer-Lambert Law
A  2 - log10 T
A is absorbance
C is concentration
I0 is intensity of incident light
? is wavelength of light
I1 is intensity after passing through the
materials
l is path length
k is extinction coefficient
a is molar absorptivity or absorption coefficient
a is a measurement of the chromophores
oscillator strength or the probability that the
molecule will absorb a quantum of light during
its interaction with a photon
4
Photophysics Process
Jablonski Diagram
Non-Radiative Process
Internal conversion (IC) electron conversion
between states of identical multiplicity
Intersystem conversion (ISC) electron conversion
between states of different multiplicity
singlet state all electrons are paired (
)with opposite spins
Triplet state same spins pairing of electrons (
)
5
Photophysics Process
From Quantum Statistics
Triplet state (symmetric)
75
Spin unpaired, S1
1/v2
Singlet state (anit-symmetric)
25
1/v2
Spin paired, S0
6
Photophysics Process
Radiative Process
(S1 S0)
(T1 S0)
gt100ns
0.110ns
Absorption or excitation spectroscopy is used to
probe ground state electronic structure and
properties
Emission or luminescence spectroscopy is used to
probe excited state electronic structure and
properties
7
Photophysics Process
Fluorescence spontaneously emitted radiation
ceases immediately after exciting radiation is
extinguished
Phosphorescence spontaneously may persist for
long period
mirror image
8
Excitons (bounded electron-hole paies)
Excited States are produced upon light absorption
by a conjugated polymers
Charge Transfer (CT) Exciton typical of organic
materilas
Ground state
Excited state
Molecular picture
binding energy 1eV Diffusion radius 10Å
Treat excitions as chargeless particles capable
of diffusion and also view them as exited stated
of the molecules
9
Why PLEDs ?
Easy and low-cost fabrication
Solution processibility
Light and flexible
Easy color tuning
Spin coating and inject printing
10
History of Organic Light Emitting Diodes
1963
First organic electroluminescene based on
anthracene single crystal
Low quantum efficiency and high operating voltage
(gt100V)
1987
The first efficient, bright, and thin film
organic light emitting diode (OLED) was reported
by C. W. Tang et al. Appl Phys Lett 1987, 51, 913
(Kodak Research Labs, Rochester, NY)
quantum efficiency (1) and low operating
voltage (10V) 3 cd/A (green)
1990
Conjugate polymers LEDs (PPV) were first reported
by R. H. Friend and coworkers Nature 1990, 347,
539 (Univ. of Cambridge, England)
Quantum efficiency 0.05
Green yellow Light
11
Progress of Light Emitting Diodes (LEDs)
Performance
12
Geometry Mechanism of PLEDs
13
Mechanism of PLEDs
Schematic of PLED operations
14
Mechanism and Design of PLEDs
Single-layer LED Structure
Energy Level Diagram
The problem of charge injection
15
Scheme of Multilayer PLEDs
16
Fabrications of Organic Light Emitting Diodes

• Cathode
• Metal (Al, Mg, Ca) by Vacuum Evaporation

• Electron Transport Layer
• Vacuum Evaporation of Dyes/Oligomers
• Spin Coating of Polymers

• Transparent substrate
• Plastic
• Glass

• Emissive Layer
• Vacuum Evaporation of Dyes/Oligomers
• Spin Coating of Polymers
• Layer-by-layer Self-assembly

• Anode
• ITO (sputter)
• Conducting Polymer (spin coating)

• Hole Transport Layer
• Vacuum Evaporation of Dyes/Oligomers
• Spin Coating of Polymers

Emitters 50150nm
CTL 550nm
Cathode 100400 nm
ITO 100500 nm
17
Device Preparation and Growth (use thermal coater)

• Glass substrates precoated with ITO
• - 94 transparent
• - 15 O/square
• Precleaning
• Tergitol, TCE
• Acetone, 2-Propanol
• Growth
• - 5 x 10-7 Torr
• - Room T
• - 20 to 2000 Å
• layer thickness

18
Hole Transport Materials (HTM) in PLEDs
Triarylamine as functional moiety
Poly (9,9-vinlycarazole) (PVK)
IP between ITO (f4.7) and emitters
Typically IP 5.0eV
19
Electron Transport Materials (ETM) in PLEDs
EL mechanism
Energy level diagram
Exciton recombination
PLED architectures with ETM
Control charge injection, transport, and
recombination by ETM

• lower barrier for electron injection

• µe gt µh in ETM

• Larger ?IP to block hole

SA Jenekhe et al, Chem Mater 2004, 16, 4556
20
Electron Transport Materials (ETM) an Electrode
in PLEDs
Cathode Electrode
Small work function of metal
Electron transport materials
Commonly used in Cathode Materials

• Reversible high reduction potential
• Suitable EA IP for electron injection and hole
  block
• High electron mobility
• High Tg and thermal stability
• Processability (vacuum evaporation or spin
  casting)
• Amorphous morphology (prevent light scattering)

Protective layer
Nitrogen-contaning heterocyclic ring Electron
withdrawing in main backbone or substituents
Anode Electrode
Large work function (ITO, fa4.74.8 eV)
SA Jenekhe et al, Chem Mater 2004, 16, 4556
21
Electron Transport Materials in OLEDs
Benzothiadiazole Polymers
Oxadiazole Molecules and Dendrimers
Triazines
Polymeric Oxadiazole
Azobased Materials
Polybenzobisaoles
Metal Chelates
Pyridine-based Materials
SA Jenekhe et al, Chem Mater 2004, 16, 4556
22
Electron Transport Materials in OLEDs
Quinoline-based Materials
Phenanthrolines
Anthrazoline-based Materials
Siloles
Cyano-containing Materials
Perfluorinated Materials
High EA 3eV
High degree of intermolecular p- p stacking
Enhanced EQE brightness luminance yield
SA Jenekhe et al, Chem Mater 2004, 16, 4556
23
Visible Spectrum Color CIE 1931 Coordinate
24
Emissive Materials in PLEDs
Blue emitters
White emitters
436nm (0.15,0.22)
Green emitters
546 nm (0.15,0.60)
Red emitters
(0.33,0.33) cover all visible region
700nm (0.65,0.35)
25
Efficiency
Experimental setup for direct measurement of EQE
External Quantum Efficiency (EQE)
Np phonon number
Ne electron number
Definition of efficiency
26
Mechanism and Design of PLEDs
Key Process in EL Devices
Double Charge (electrons and holes) Injection (At
interface)
? injection efficiency if ohmic contact, ? 1
Charge Transport/Trapping
Excited State Generation by Charge Recombination
? singlet exction generation efficiency 0.25?
Radiative Decay of Excitons
f Fluorescence efficiency
27
Towards Improved PLEDs
Better Efficiency (gt 5)
High Luminance (gt106 cd/cm2)
Stability with Packaging (500025000 hrs)
Low operating Voltage (310V)
Charge Injection (choose suitable work function
electrode)
Charge Transport (choose high electron and hole
mobility)
28
Flexible Internet Display Screen
THE ULTIMATE HANDHELD COMMUNICATION DEVICE
UDC, Inc.
29
Cambridge Display Technology (CDT)
Full color display
- Active matrix
- 2 inch diagonal
- 200 x 150 Pixels
30
Eletrophosphorescence from Organic Materials
Excitons generated by charge recombination in
organic LEDs
2P? 2P-? 1P 3P
Singlet electroluminescence
Triplet electrophosphorescence
Spin statistics says the ratio of singlet
triplet, 1P 3P 1 3
To obtain the maximum efficiency from an organic
LED, one should harness both the singlet and
triplet excitations that result from electrical
pumping
31
Eletrophosphorescence from Organic Materials
The external quantum efficiency (?ext) is given
by
?ext ?int ?ph (? ?ex fp )?ph
?ph light out-coupling from device
?ex fraction of total excitons formed which
result in radiative transitons
(0.25 from fluoresent polymers)
? ratio of electrons to holes injected from
opposite contacts
fp intrinsic quantum efficiency for radiative
decay
If only singlets are radiative as in fluorescent
materials, ?ext is limited to 5, assuming ?ph
1/2n2 20 for a glass substrate (n1.5)
By using high efficiency phosphorescent
materials, ?int can approach 100 , in which
case we can anitcipate ?ph 20
32
High Efficiency LEDs from Eletrophosphorescence
Organometallic compounds which introduce
spin-orbit coupling due to the central heavy atom
show a relatively high ligand based
phosphorescence efficiency even at room
temperature
All emission colors possible by using appropriate
phosphorescent molecules
From S. R. Forrest Group (EE, Princeton
University)
Maximum EQE
Blue emitters
Green emitters
Red emitters
7.5 0.8
15.4 0.2
7 0.5
Nature, 2000, 403, 750
APL 2003, 82, 2422
APL, 2001, 78, 1622
33
http//www.cibasc.com/pic-ind-pc-tech-protection-l
ightstabilization2.jpg
As DCM2 acts as a filter that removes singlet
Alq3 excitons, the only possible origin of the
PtOEP luminescence is Alq3 triplet states that
have diffused through the DCM2 and intervening
Alq3 layers.