Electronic and Optoelectronic Polymers

Wen-Chang ChenDepartment of Chemical Engineering

Institute of Polymer Science and Engineering

National Taiwan University

History of Conjugated Polymers

Electronic Structures of Conjugated Polymers

Polymer Light-emitting Diodes

Polymer-based Thin Film Transistors

Polymer-based Photovoltaics

Outlines

Optical AbsorbanceAbsorption of light and the excited states of molecules

A is absorbance I0 is intensity of incident light

I1 is intensity after passing through the materials

l is path length

C is concentration

λ is wavelength of light

k is extinction coefficient

α is molar absorptivity or absorption coefficient

Beer-Lambert Law

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

A = 2 - log10 %T

Photophysics 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 ( )

Jablonski Diagram

Non-Radiative Process

Photophysics Process

-

+

(

( )

)

Ground stateExcited state

Singlet Triplet

1/√2

1/√2Singlet state (anit-symmetric)

Triplet state (symmetric)

Spin unpaired, S=1

Spin paired, S=0

From Quantum Statistics

25%

75%

Photophysics Process

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

Radiative Process

(S1 S0) (T1 S0)

0.1~10ns >100ns

Photophysics ProcessFluorescence: spontaneously emitted radiation ceases immediately after exciting radiation is extinguished

Phosphorescence: spontaneously may persist for long period

mirror image

Excitons (bounded electron-hole paies)

binding energy ~1eVDiffusion radius ~10Å

Charge Transfer (CT) Exciton : typical of organic materilas

Excited States are produced upon light absorption by a conjugated polymers

Molecular pictureGround state Excited state

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

First organic electroluminescene based on anthracene single crystal

1963

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)

1990Conjugate polymers LEDs (PPV) were first reported by R. H. Friend and coworkers Nature 1990, 347, 539 (Univ. of Cambridge, England)

Low quantum efficiency and high operating voltage (>100V)

Quantum efficiency ~0.05%

quantum efficiency (~1%) and low operating voltage (~10V) 3 cd/A (green)

Green yellow Light

Progress of Light Emitting Diodes (LEDs) Performance

Geometry & Mechanism of PLEDs

Schematic of PLED operations

Mechanism of PLEDs

LUMO

HOMO

Vacuum Level

ELMaterial CathodeAnode

IP

EA

Φ anode

Φ cathodeBarrier toelectroninjection

Barrier tohole

injection

Anode CathodeEL

MaterialLight

V

h+ e-

Mechanism and Design of PLEDsSingle-layer LED Structure

Energy Level Diagram

The problem of charge injection

Scheme of Multilayer PLEDs

Fabrications of Organic Light Emitting Diodes

Electron Transport Layer:

Vacuum Evaporation of Dyes/Oligomers

Spin Coating of Polymers

Emissive Layer:

Vacuum Evaporation of Dyes/Oligomers

Spin Coating of Polymers

Layer-by-layer Self-assembly

Hole Transport Layer:

Vacuum Evaporation of Dyes/Oligomers

Spin Coating of Polymers

Cathode:

Metal (Al, Mg, Ca) by Vacuum Evaporation

Transparent substrate

Plastic

Glass

Anode

ITO (sputter)

Conducting Polymer (spin coating)

Emitters 50~150nm

CTL 5~50nm

Cathode 100~400 nm

ITO 100~500 nm

Cathode

Electron Transport Layer

Hole Transport Layer

Emissive Layer

Substate

Anode

V

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

Device Preparation and Growth (use thermal coater)

Hole Transport Materials (HTM) in PLEDsTriarylamine as functional moiety

N

CHH2Cn

Poly (9,9-vinlycarazole) (PVK) IP between ITO (φ=4.7) and emitters

Typically IP~ 5.0eV

SA Jenekhe et al, Chem Mater 2004, 16, 4556

Electron Transport Materials (ETM) in PLEDsEL mechanism

Exciton recombination

PLED architectures with ETM

Energy level diagram

Control charge injection, transport, and recombination by ETM

lower barrier for electron injection μe > μh in ETM

Larger IP to block hole△

Electron Transport Materials (ETM) an Electrode in PLEDsCathode Electrode

Small work function of metal

Anode Electrode

Large work function (ITO, φa=4.7~4.8 eV)

Electron transport 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)

Nitrogen-contaning heterocyclic ring

Electron withdrawing in main backbone or substituents

Commonly used in Cathode Materials

SA Jenekhe et al, Chem Mater 2004, 16, 4556

Protective layer

Electron Transport Materials in OLEDsOxadiazole Molecules and Dendrimers

Polymeric Oxadiazole

Metal Chelates

Azobased Materials

Triazines

Polybenzobisaoles

Benzothiadiazole Polymers

Pyridine-based Materials

SA Jenekhe et al, Chem Mater 2004, 16, 4556

Quinoline-based Materials

Anthrazoline-based Materials

Phenanthrolines

SilolesCyano-containing Materials

Perfluorinated Materials

Electron Transport Materials in OLEDs

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

Green emitters

Red emitters

White emitters

~436nm (0.15,0.22)

~546 nm (0.15,0.60)

~700nm (0.65,0.35)

(0.33,0.33) cover all visible region

Efficiency

Experimental setup for direct measurement of EQE External Quantum Efficiency (EQE)

Np phonon number Ne electron number

Definition of efficiency

Cathode

Electron Transport Layer

Hole Transport Layer

Emissive Layer

Substate

Anode

V

Mechanism and Design of PLEDs

Double Charge (electrons and holes) Injection (At interface)

Charge Transport/Trapping

Excited State Generation by Charge Recombination

Radiative Decay of Excitons

γ = injection efficiency if ohmic contact, γ = 1

η = singlet exction generation efficiency~ 0.25?

φ = Fluorescence efficiency

Key Process in EL Devices

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)

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Eletrophosphorescence from Organic Materials

Excitons generated by charge recombination in organic LEDs

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

2P+‧ + 2P-‧ 1P* + 3P*

Singlet :electroluminescence Triplet: electrophosphorescence

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 10

0 %, in which case we can anitcipate ηph ~ 20 %

All emission colors possible by using appropriate phosphorescent molecules

Maximum EQE

Blue emitters Green emitters Red emitters

7.5 ± 0.8 % 15.4 ± 0.2 % 7 ± 0.5%

Nature, 2000, 403, 750APL 2003, 82, 2422 APL, 2001, 78, 1622

From S. R. Forrest Group (EE, Princeton University)

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

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.