Literature Seminar 2ndM2 Noriko Takahashi
28th May 2008

Materials for Organic Electroluminescence– What's inside the black box? –Message
You may have heard the word ''Organic Electroluminescence(OEL)" in somewhere such as TV commercials or other advertisements.
And also you may have some ideas of what it is or what it's like.
HOWEVER, do you know exactly what it is?
In modern society, technology is developing too fast for us to catch up with.It's called "black box of technology".
I've been wondering what's inside the black box.How about you?
Don't be afraid. It's not hard to understand;just organic compounds that we're familiar!!
So let's open the black box.
Contents
1. Introduction
2. Materials -the Essential of Organic Electroluminescence3. Organo Lanthanide Metal Complex
4. Futures

1. Introduction
Organic Electroluminescence(OEL)= Organic light-emitting diode(OLED)= Light emitting Polymer(LEP)• Next generation of full-color flat panel display••• thin, light, beautiful• In 2004, more than 5000 patents• Commercial use now ••• small screens for mobile phone andportable audio players, digital camerasEstimated market scaleComparsion of display technique
nameCRT(cathode-ray tube)
LCD(liquid crystal display)

the present statemajority for TV
mass productivity
majority for note PC,
mobile phonemass productivity
used in thin,large-scale TV

present & future
low cost,lasting
thick(can't be thin)
thin,flat,everlastingbacklight is necessary.

PDP
be used in large-scale display,but can't be small(plasma display)
big consumption of electric power
OEL
partially used in mobile phone high-quality
durability is task to solve.
(organic electroluminescence)
FED
trial manufacture(field emission display)consumption speed large-scale quality durability cost viewing angle brightness
LCD

passiveOEL

activeflexibility

heat-resistantA significant benefit of OLED displays over liquid crystal displays is that OLEDs don't require a backlight to function.
Thus they draw far less power and can operate longer on the same charge.
Because there's no need to distribute the backlight, an OLED display can also be much thinner than an LCD panel.
However, degradation of OLED materials has limited their use.

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Applycation of OEL –Not only display!!–• General space illumination,large-area light-emitting elements• Electronic paper• Full color printing

History
Generally organic compounds are classfied asonconductor.
But in order to make it flash, it's necessary to carry electricity.
Some organic compounds could be (semi)conductor:Usually they require over 100 V.
So it is highly challenging to make it behave as a conductor at low voltage.
1960s Study on electroluminescence of organic materials started;
not worked well.(power-conversion efficiency remained less than 0.1 %.)1977 Prof. Shirakawa succeeded to achieve high conductivity by chemical doping in !-conjugated high polymer.
(Nobel prize for Chemistry in 2000)
1987 C.W.Tang and S.A. VanSlkye succeededed to fabricate multi layers cell.
1990 R.H. Friend succeeded to fabricate polymer materials.
1993 J. Kido succeeded to device white-color Organic Electroluminescence

Very severe competitions!!!!!
2007 SONY released OEL television in Japan.

• First example of thin-film organic device (organic material issandwiched between two injecting electrodes.)• High brightness at a low voltage• Fast degradation• Broad spectrum

ITO = Indium-tin-oxide

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2. Materials – the Essential of Electroluminescence–
Structure
Recombination occurs here!!

Cathode and anode are inorganic compounds.To have good affinity between inorganic layer and organic layer,some buffers are necessary to be sandwiched;injection and transport layers.(Merit of multi-layers.)
In polymer system, single layer is used, that's why many elements are necessary to be included.
Four layers structure is the most common one.
The role of layers are following:
Electron injection layer

Small molecules
Alkali metals
Li complex

Electron transport layer*2Al complex
phenantrolines

1Hole injection layer

Aryl amines
Cu complex

5Hole transport layer*2

Aryl amines

2Emitting Layer

Al complex
3Anthorathene
Organo lanthanide complex
Ir complex

6!-conjugated•••polyphenylenevilylene
polyfluorene
coloring matterhigh luminescence efficiency

CathodeITO(Indium tin oxide)

ITO

Anode

Aluminium
Al:Li
Mg:Ag

Aluminium

high work function,transparent to visible light
low work function

Dopament

Polycyclic aromatics 4Polymers *1
BariumCalcium

Required featuresatching work function of cathode andHOMO,LUMO of transport layer
High mobility of electron
Hole-brocking

high luminescene
not cohered*1 Single-layer*2 Some emitting materials also can work as it.

Example of used materials
1 Electron transport layer

TPD has been used as material for copy machine.
TPD has relatively sharp emission peaks at 410-420 nm,purple,blue
2 Hole transport layer

NN"-NPD

3/11

3 Emitting Layer(small molecules)

MeMeNOAlO ONNNNIr
NIr(ppy)3

MeAlmq3

described in Chapter 34 Dopament
1-2 % doping of them makes luminescence effictive
color change,color mixing

PeryleneRubrene
5 Hole injection layer
pigment
heat-resistance, durability
NMeNNNCuNNNNNNN
CuPc

MeMem-MTDATAn

6 Emitting Layer(Polymer)
On!#conjugatedcoloring mattern
PPVMeONn

MEH-PPV

Conjugated chain provides gap between HOMO and LUMO small,which causes wave length long.
Only yellow and red light.blue light is difficult to obtain.
de chain makes polymer soluble in organic solvent.

PVKN ONPVOXD

Small pigments are polymerized:
same featues as small molecules.
PVK has hole-transportability.

Which is better??
-small molecules or polymers??At present, researches on small molecules are much more popular than ones on polymers.
Small molecules are popular in Japan, on the other hand,so are polymers in Europe.
The manufacture of polymer is much better than that of small molecules.
(low cost, excellent printing technique)So it's reasonable that research on polymer will be advanced.
Small molecules•••high efficiency, long lifetime, sppedy research
Polymer•••low efficiency, still not succeeded in blue-light, hard research(many function with one material)

4/11

3. Organo Lanthanide Metal Complex       
Recombination of hole and electron at emission layer makes molecules
excited.Then emission of light from excited state occurs.

LUMO

HOMO
ground state singlet excited state

tripletet excited statePL(photoluminescence)••• emission from singlet excited state,fast(10 nsec.)
PS(phosphotescence)••• emission from triplet excited state,slow(>msec.).degradation as heat,hardly to be observe.
Internal quantum efficiency was limited to 25%.
photoluminescence : phosphorescence = 1: 3

It was hard to obtain pure and sharp emission from organic materials;generally spectra show broad peaks.
Also fine tuning without affecting the EL material's physical properties was hard to realize.
Some metal complexes(ex. Ir) show highly efficient phosphorescence up to100%.

IEu3+ (4f6)H

Lanthanides
It's Lanthanides that make it true!!
They exhibit extremely sharp emission bands due to their 4f electrons.
No limitation of the internal quantum efficiency up to 100%.
Ln = Xe(5s25p6)4fn6s2

5d4fn5p65s2

4d10

4f6GD

7F

7F
6546s2
Coulomb
interaction

3210

spin-orbital
interaction

4f electrons; 4f orbitals are effectively shielded from the influence of the external forces by the overlying 5s2 and 5P6 orbitals.
Emission bands as well as absorption bands(f-f transitions) are extremely sharp.
5/11

ach ion's complex
Eu3+,Tb3+
Tb
5D4

            in    
light range.

the strongest emission at 545 nm (green color)
7F transition of Tb3+ ion
5Eu the strongest emission at 615 nm (red color)
5D
7F transition of Eu3+ ion
45Eu3+
1991 Kido
1994 Kido
1994 Takada
1994 Sano
2001 Huang

Eu(TTA)30.3 cd/m2 at 18 V, not volatile
Eu(DBM)3(Phen) 460 cd/m2, 614 nm,at 16 V, PBD as hostEu(TTA)3(Phen) microcavity,angular dependence
Eu(TTA)3(Phen) 137 cd/m2Eu(DBM)3EPBM 180 cd/m2 at 18 V

Tb3+
1990 Kido
1999 H.J. Zhang
2001 Jabbour

Tb(acac)3
7 cd/m2
Tb(MTP)3Phen thermally stable,152 cd/m2 at 24 V
oxadiazole functionalized ligand, 100 cd/m2 at 15 V

6/11

2001 Jabbour
oxadiazole ligand plays an important role in high efficiency;
good electron-transporting and hole-blocking materials
two water molecules are coordinated and H-O vibration tends toquench the fluorescence intensity
If replaced with another ligand, EL efficiency will increase.

Secret of white color -exciting work of Prof. Kido1990 Tb(acac)3 green light(545 nm)
1991 Eu(TTA)3 red light(615 nm)
1995 TPD
blue light(410-420 nm)•••polymer, emitting layer as well as hole-transporting material

At that time it's usually said that R+G+B ! Wbecause•••*R and G and B are called "Light's three promary colors ".
Mixing them makes white-light.
WBlue
B

Green
RedG

R

Energy transfer will occur to the lowest energy level.
Only red light is observed by just mixing with three colors.

One day a student obtained red-blue-white light while preparing red light by accident.
He was disappointed, but Prof. Kido took cue to realize white-light!!

BlueGreenRed

Energy transfer is in inverse proportion to (distance)6

A;blue-green-white lighta broad band ranging from 500-700 nm was observed.
B;white-light
three sharp strong peaks at around 410-420, 545, 615 nm,
corresponding to the emission from TPD,Tb(acac)3(Phen),andEu(DBM)3(Phen), respectively, are clearly seen.
7/11

Nd3+,Er3+
intra-4f transition at near-infraed emission peakuseful for fiber optical telecommunication devices
4I
Er3+ 4I13/2
15/2 (1530 nm)
3+
44I
Nd
F3/2
11/2 (1350 nm)

Nd3+

1999 Yanagida, Nd(DBM)3
Hasegawa
sharp emission peak,degraded during the measurment
1999 Klink
Nd-lissamine compex
triphenylene(excited at 350 nm), high intersystem crossing yield
2001 Slooff
Nd-lissamine compex,
890 nm emitter, blended with polymer

Er3+1999 Gillin and Curry
2000 SunErQ complex
rt(300 K), if excitation density was high, it got burning.
Er(acac)3(Phen)
blend with PVK polymer, excited by 600-350 nm

NOO
N

ErNO

8/11

Development of Lanthanide complex as secret ink –Work of Prof. Hasegawa–Role of ligand
OEL:multi-layer, stability and volitability rather than efficiency
Ink: single-layer, high absorption , energy transfer

Eu ion has been known as a useful red-light emitter.
Because of •••Eu ion is excited by ligand excitation
100-1000 times bigger than Eu ion itself.
">10 M-1cm-1

photosensitization energy transfer! < 400 nm! = 615 nm (red color)">10000 M-1cm-1• Vibrating deactivation
Emission intensity of Ln is weak.
Because energy transfer via vibration is considered as the dominant quenching process.
:Energy gap of the radiative transition in Nd3+ ion(5400 cm-1) = C-H and O-H bond vibration(5900 cm-1 and 6900 cm-1),solvent molecules with C-H and O-H bond lead to effective quenching of the Nd3+ ion excited state.

replacement of C-H bond with C-F bond and C-D bond.
••• C-F bond (1200 cm-1) and C-D(2100 cm-1), O-D bond (2500 cm-1)
deuterated solvent and deuterated HFA as ligand

1996 proposal of low vibration ligand(without aromatic rings)
2000 (hfa) ligand good durability

1650 cm-1 1200 cm-1
CF3OCOCC
CF31600 cm-1
D

2100 cm-1• Synergism
HFA(hexafluoroacetylacetone)
Ln ions are square-antiprism,coordination number = 8
two water molecules were coordinated, which made high vibration
Other organophosphine ligand are developed(TPPO).(P=O 1100 cm-1)• Dissymmetric
Reducing the symmetry enhances its photophysical properties; toleration of forbidden transition.
SAP = square-antiprism

a and d contain the symmetryaxis, mirror plane.
b and c have no symmetry axis,
mirror plane, inversion center

9/11

Eu compleax with BIPHEPO(structure b) showed the best lasing propety.

This work have succeeded in commercial application!!

Development Lanthanide Complex in Biology"on–off" switchable near-infrared luminescence(2007 Tsukube)
external anion

Highly luminescent lanthanide tags suitable for protein labelingand time–resolved luminescence imaging(2004 Ziessel)

10/11

4. Futures
OEL will become popular without doubt!!
Necessary for practical application• development of durability,cost(SONY 11 inch 200,000 yen),energy-saving(crucial for 21 century),• mass productivity with high-quality(up to mg-scale purity)
not only materials but also manufacturing methods• matters around techiniques are important; patent,strategy(company,nation) etc.

Academic point of view
• Photophysics of Ln is still under development.
As application is developed, photophysics of Ln will be developed, also opposite is the case.
• Organic chemistry can make material endlessly,
New materials will be developed in the future, so keep eyes on it.
• No materials, No progress.

SONY XEL-1 "MIRAIKAN"

Main sources (references cited therein)
images: IT media –Belive in Techenology–
CNET JAPAN –ideas for innovation–ideas:"Yuki EL no subete"(Junji Kido),"Gendai Kagaku 2008 4 p25-","Kagaku 2008 1 p47-""Toshiba Review vol.62 No.5 (2007)"
Yuki Electroluminescence -Wikipedia"Science of Rare Earths"(Gin-ya Adachi)(Special thanks to Prof. Shibasaki)
articles: Chem. Rev. 2002, 102, 2357 (Special thanks to Chen san)Chem. Mater. 2005, 17, 1933

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