Electronic and Optoelectronic Polymers
Wen-Chang Chen
Department of Chemical Engineering
Institute of Polymer Science and Engineering
National Taiwan University
Outlines

History of Conjugated Polymers

Electronic Structures of Conjugated Polymers

Polymer Light-emitting Diodes

Polymer-based Thin Film Transistors

Polymer-based Photovoltaics
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 λ is wavelength of light
I1 is intensity after passing through the materials
l is path length
k is extinction coefficient
α is molar absorptivity or absorption coefficient
α is a measurement of the chromophore’s oscillator strength or the
probability that the molecule will absorb a quantum of light during its
interaction with a photon
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 (
)
Photophysics Process
From Quantum Statistics
Excited state
Ground state
Singlet
Triplet
Triplet state (symmetric)
75%
Spin unpaired, S=1
25%
1/√2
(
1/√2
(
+
)
-
)
Singlet state (anit-symmetric)
Spin paired, S=0
Photophysics Process
Radiative Process
(S1
S0)
0.1~10ns
(T1
S0)
>100ns
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
Photophysics Process
Fluorescence: spontaneously emitted radiation ceases immediately after
exciting radiation is extinguished
Phosphorescence: spontaneously may persist for long period
mirror
image
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
Why PLEDs ?
Easy and low-cost fabrication
Solution processibility
Light and flexible
Easy color tuning
Spin coating and inject printing
History of Organic Light Emitting Diodes
1963
First organic electroluminescene based on anthracene single crystal
Low quantum efficiency and high operating voltage (>100V)
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
Progress of Light Emitting Diodes (LEDs) Performance
Geometry & Mechanism of PLEDs
Mechanism of PLEDs
Schematic of PLED operations
Mechanism and Design of PLEDs
Single-layer LED Structure
V
e-
h+
Light
Anode
EL
Material
Cathode
Energy Level Diagram
The problem of charge injection
Vacuum Level
EA
LUMO
Φanode
Φcathode
Barrier to
electron
injection
IP
Barrier to
hole
injection
HOMO
EL
Anode Material Cathode
Scheme of Multilayer PLEDs
Fabrications of Organic Light Emitting Diodes
Cathode:

Electron Transport Layer:


Vacuum Evaporation of
Dyes/Oligomers
Metal (Al, Mg, Ca) by
Vacuum Evaporation
Cathode
Spin Coating of Polymers
V
Transparent substrate

Plastic

Glass
Electron Transport Layer
Emissive Layer
Emissive Layer:

Hole Transport Layer
Vacuum Evaporation of
Dyes/Oligomers

Spin Coating of
Polymers

Layer-by-layer Selfassembly
Anode
Anode

ITO (sputter)
Substate

Conducting Polymer
(spin coating)
Hole Transport Layer:

Vacuum Evaporation of
Dyes/Oligomers

Spin Coating of Polymers
Emitters 50~150nm
CTL 5~50nm
Cathode 100~400 nm
ITO 100~500 nm
Device Preparation and Growth (use thermal coater)
Glass substrates precoated with ITO
- 94% transparent
- 15 Ω/square
Precleaning
Tergitol, TCE
Acetone, 2-Propanol
Growth
- 5 x 10-7 Torr
- Room T
- 20 to 2000 Å
layer thickness
Hole Transport Materials (HTM) in PLEDs
Triarylamine as functional moiety
Poly (9,9-vinlycarazole) (PVK)
H2C
CH
N
IP between ITO (φ=4.7) and emitters
Typically IP~ 5.0eV
n
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 > μh in ETM
 Larger △IP to block hole
SA Jenekhe et al, Chem Mater 2004, 16, 4556
Electron Transport Materials (ETM) an Electrode in PLEDs
Cathode Electrode
Small work function of metal
Commonly
used in
Cathode
Materials
Electron transport materials
 Reversible high reduction potential
 Suitable EA & IP for electron injection and hole block
 High electron mobility
Protective
layer
 High Tg and thermal stability
 Processability (vacuum evaporation or spin casting)
 Amorphous morphology (prevent light scattering)
Nitrogen-contaning heterocyclic ring
Electron withdrawing in main backbone or substituents
Anode Electrode
Large work function (ITO, φa=4.7~4.8 eV)
SA Jenekhe et al, Chem Mater 2004, 16, 4556
Electron Transport Materials in OLEDs
Oxadiazole Molecules and Dendrimers
Benzothiadiazole Polymers
Triazines
Polymeric Oxadiazole
Azobased Materials
Polybenzobisaoles
Metal Chelates
Pyridine-based Materials
SA Jenekhe et al, Chem Mater 2004, 16, 4556
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 π- π stacking
Enhanced EQE & brightness & luminance yield
SA Jenekhe et al, Chem Mater 2004, 16, 4556
Visible Spectrum & Color & CIE 1931 Coordinate
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)
Efficiency
Experimental setup for direct
measurement of EQE
External Quantum Efficiency (EQE)
Np phonon number Ne electron number
Definition of efficiency
Mechanism and Design of PLEDs
Cathode
V
Electron Transport Layer
Emissive Layer
Hole Transport Layer
Anode
Substate
Key Process in EL Devices
Double Charge (electrons and holes) Injection (At interface)
γ = injection efficiency if ohmic contact, γ = 1
Charge Transport/Trapping
Excited State Generation by Charge Recombination
η = singlet exction generation efficiency~ 0.25?
Radiative Decay of Excitons
φ = Fluorescence efficiency
Towards Improved PLEDs
Better Efficiency (> 5%)
High Luminance (>106 cd/cm2)
Stability with Packaging (5000~25000 hrs)
Low operating Voltage (3~10V)
Charge Injection (choose suitable work function electrode)
Charge Transport (choose high electron and hole mobility)
Flexible Internet Display Screen
THE ULTIMATE HANDHELD COMMUNICATION DEVICE
UDC, Inc.
Cambridge Display Technology (CDT)
Full color display
- Active matrix
- 200 x 150 Pixels
- 2 inch diagonal
Eletrophosphorescence from Organic Materials
Excitons generated by charge recombination in organic LEDs
2P+‧
+ 2P-‧
1P*
Singlet :electroluminescence
+ 3P*
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
Eletrophosphorescence from Organic Materials
The external quantum efficiency (ηext) is given by
ηext = ηint ηph = (γ ηex φp )ηph
ηph = light out-coupling from device
ηex = fraction of total excitons formed which result in radiative transitons
(~0.25 from fluoresent polymers)
γ = ratio of electrons to holes injected from opposite contacts
φp = intrinsic quantum efficiency for radiative decay
If only singlets are radiative as in fluorescent materials, ηext is limited to
~ 5%, assuming ηph ~ 1/2n2~ 20 % for a glass substrate (n=1.5)
By using high efficiency phosphorescent materials, ηint can approach
100 %, in which case we can anitcipate ηph ~ 20 %
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
7.5 ± 0.8 %
APL 2003, 82, 2422
Green emitters
Red emitters
15.4 ± 0.2 %
7 ± 0.5%
Nature, 2000, 403, 750
APL, 2001, 78, 1622
http://www.cibasc.com/pic-ind-pc-tech-protection-lightstabilization2.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.