highly efficient multilayer organic pure -blue - light ... : oled withneatfilm ofpmc anode cathode...

C2H5

C2H5

N

N

CH3 CH3

CH3 CH3

CH3

N

C2H5

C2H5

N

N

CH3 CH3

CH3 CH3

CH3

N

C2H5

C2H5

N

N

CH3 CH3

CH3 CH3

CH3

N

C2H5

C2H5

N

N

CH3 CH3

CH3 CH3

CH3

N

CH3 CH3

CH3 CH3

CH3

N

C2H5

C2H5

N

N

A. Fischer, S. Forget, S. Chénais, M.-C. Castex,

Lab. de Physique des Lasers, Univ. Paris Nord, France

Highly efficient multilayer organic pureHighly efficient multilayer organic pure--blueblue--light emitting diodes with substituted light emitting diodes with substituted carbazolecarbazole

compounds in the emitting layer.compounds in the emitting layer.

D. Adès, A. Siove, Lab. Biomateriaux et Polymères de Spécialité, Univ. Paris Nord, France

C. Denis, P. Maisse and B. GeffroyLab. Cellules et Composants, CEA Saclay, France

2CLEO ’06 – Long Beach (USA)

Outline

Introduction : why BLUE oleds ?

Two new carbazolic compounds : PMC

(Pentamethylcarbazole) and DEC (Dimer of N-ethylcarbazole)

Devices using neat films of PMC and DEC in

single layer and multilayer structures

Devices using doped films of PMC:DPVBi

and DEC:DPVBi

Conclusion


Introduction

Organic Light Emitting Diodes :

Ultrathin light sources, lightweight

High brightness and viewing angle > 160°

Low drive voltage (3-10 V) and low power consumption

Extremely rich diversity of materials : All visible colorsavailable (≠ inorganic LEDs), including saturated colors

Potentially flexible

Long lifetimes (> 20 000 h reported)

Low cost potential for mass production

Applications : flat-panel RGB DISPLAYS, solid-state lighting,...


needs efficient blue emitters

Why BLUE ?

Why Blue OLEDs with high efficiencies are needed ?

different approaches for multi-color emission :

RGB emitters

+ : power efficient, mature

- : different aging andoptimization


(efficient R,G already exist)

White emitters + Filters

+ : homogeneous aging

- : not efficient (filters)


to achieve bright white

Color changing media

+ : homogeneous aging

- : not efficient (photoconversion)


OLEDs materials

Requirements for an efficient blue material :

Chemical stability and Electrochemical stability

High Tg

High quantum yield of photoluminescence in the solid state

Chromaticity coordinates approaching

the spectrum locus (saturated color)

Active research for new blue-emitting organic materials(both fluorescent and phosphorescent)

CIE 1931


OLEDs materials

Carbazolic derivatives

CH3 CH3

CH3 CH3

CH3

N

CH3 CH3

CH3 CH3

CH3

N

PMC

C2H5

C2H5

N

N

DEC

Carbazole unit :

penta-methylcarbazole Dimer of N-Ethyl carbazole

• Chemically and thermally stable (up to 430 °C)

• Tg = 75°C

•Polaronic transport levels measured by cyclicvoltammetry (eV) :

- Blue emitters: Carbazole-substituted Distyrylarylenes(DSA)

- Hole Transport materials : PVK

- Host material for triplet emitters: CBP

Vacuum level

Lowest

Unoccupied

Molecular

Orbital

Highest

Occupied

Molecular Orbital

PMCDEC

5.9

2.82.5

5.6

Already used

as…

new


OLEDs structures

1st DEC-based diode : single layer

Drawbacks:

• Low ext. quantum efficiency ηηηηext. = 7.10-2 %

• High operating voltage (20 V), crystallizationduring operation (short-circuit)

DEC

ITO

Al

hνννν

V

D. Romero, A. Siove et al., Adv. Mater. 9, 1158 (1997)

This work : Use of DEC (and PMC) in a multilayer OLED structure with both neat films and doped films configurations: efficient deep-blue organic emitter

Bad performance due to recombination and quenching of excitons at Al/DEC interface, poor charge injection


Device a : OLED with NEAT film of DEC

Anode

ITO

100-150nm

Cathode

LUMO

HOMO

CuPc

10nm

ET

L

NPB

50 nm

Alq3

10nm

LiF / Al

1.2 / 100nm

HIL

HT

L

HB

L

BCP

10nm

5.7

3.0

2.92.4

6.1

4.7

5.3

3.6

2.4

5.45.6

DEC

50 nm

N

N

N

N

N

N

NN

Cu

CuPc

N N

NPB

C2H5

C2H5

N

N

holes

electrons2.4

N

O

AlO

N

O

N

CH3

N

CH3

N

2.5


Device a : OLED with neat film of DEC

Anode

Cathode

LUMO

HOMO

ET

LHIL

HT

L

HB

L

5.7

3.0

2.92.4

6.1

4.7

5.3

3.6

2.4

5.45.6

C2H5

C2H5

N

N

holes

electrons2.4

2.5

Main recombination zone

ηext = 1.5 % (optical design not optimized)

ITO

100-150nm

CuPc

10nm

NPB

50 nm

Alq3

10nm

LiF / Al

1.2 / 100nm

BCP

10nm

DEC

50 nm


Anode

Cathode

LUMO

HOMO

ET

LHIL

HT

L

HB

L

5.7

3.0

2.92.4

6.1

4.7

5.3

3.6

2.4

5.4

N

O

AlO

N

O

N

CH3

N

CH3

N

N N

NPB

5.9

2.8

CH3 CH3

CH3 CH3

CH3

Nholes

electrons

Device a : OLED with neat film of PMC

PMC OLED

ηext = 0.6 % → attributed to bad electron transport

properties of PMC / electron barrier of BCP

ITO

100-150nm

CuPc

10nm

NPB

50 nm

Alq3

10nm

LiF / Al

1.2 / 100nm

BCP

10nm

PMC

50 nm


Device a : OLED with neat film of PMC

Anode

Cathode

LUMO

HOMO

ET

LHIL

HT

L

HB

L

5.7

3.0

2.92.4

6.1

4.7

5.3

3.6

2.4

5.45.9

2.8

CH3 CH3

CH3 CH3

CH3

Nholes

electrons

PMC OLED

ηext = 0.6 % → attributed to bad electron transport

properties of PMC / electron barrier of BCP

Main recombination zone

ITO

100-150nm

CuPc

10nm

NPB

50 nm

Alq3

10nm

LiF / Al

1.2 / 100nm

BCP

10nm

PMC

50 nm


Device a (neat films) : Experimental results

Electroluminescence spectra

a

Chromaticity coordinates

PMC

DEC

Aggregates,

excimers ?

PMC : CIE x = 0.153 ; y = 0.100

DEC : CIE x = 0.192 ; y = 0.209Ext. Quantum efficiency : ηext = 0.6 % (PMC)

ηext = 1.5 % (DEC)

Brightness L = 236 cd/m2 @ 60 mA/cm2 (PMC)

Luminous efficiency ηpower = 0.2 lm/W (PMC)

→ Bright saturated blue

With PMC, but modest efficiency


Investigating emitting mixtures (« doping »)

The role of emitting mixtures (or « doping » but not in the electrical sense !)

« energy transfer » doping = diluting a low-gap guest material inside a wide-gap host : Förster (and Dexter) energy transfers possible

→ Very efficient mechanismbut not useful for blue emitters

guestguesthosthost

other types of doping : the dopant « impurities » can enhance exciton recombination by trapping charge carriers (and diffusing excitons)

guestguesthosthost

Ex : Barrier for electrons + trap for holes = improved

recombination rate


Device b : OLEDs with DPVBi doped with PMC (DEC)

CuPc 10nm

NPB 50nm

DPVBi (PMC or DEC)50nm

Alq3 10nm

LiF 1.2nm/Al 100nm

(b)

ITO glass

C2H5

C2H5

N

N

DEC

CH3 CH3

CH3 CH3

CH3

N

PMC

+ or

5% wt.

2% wt.DPVBi

4,4’-bis(2,2’-diphenylvinyl)-

1,1’-biphenyl

Vacuum

levelLowest

Unoccupied Molecular

Orbital

Highest

Occupied Molecular

OrbitalPMC

DEC

5.9

2.82.5

5.6

DPVBI

5.9

2.8

Doping by coevaporationfrom 2 resistively heated cells


OLEDs with DPVBi doped with DEC

Anode

ITO

100-150nm

Cathode

LUMO

HOMO

CuPc

10nm

ET

L

NPB

50 nm

Alq3

10nm

LiF / Al

1.2 / 100nm

HIL

HT

L

5.7

3.02.9

4.7

5.3

3.6

2.4

5.4

DEC:DPVBi

50 nm

holes

electrons2.5

5.6

5.9

2.8

2% DEC

DPVBi


OLEDs with DPVBi doped with DEC

Anode

Cathode

LUMO

HOMO ET

L

HIL

HT

L

5.7

3.02.9

4.7

5.3

3.6

2.4

5.4

holes

electrons2.5

5.6

5.9

2.8

2% DEC

DPVBi

Recombination zone

ηext = 3.3 %

ITO

100-150nm

CuPc

10nm

NPB

50 nm

Alq3

10nm

LiF / Al

1.2 / 100nm

DEC:DPVBi

50 nm


Anode

Cathode

LUMO

HOMO ET

L

HIL

HT

L5.7

3.02.9

4.7

5.3

3.6

2.4

5.4

holes

electrons

5.9

2.8

5% PMC

DPVBi

OLEDs with DPVBi doped with PMC

Recombination zone

ηext = 2.8 %

ITO

100-150nm

CuPc

10nm

NPB

50 nm

Alq3

10nm

LiF / Al

1.2 / 100nm

PMC:DPVBi

50 nm


Anode

Cathode

LUMO

HOMO ET

L

HIL

HT

L5.7

3.02.9

4.7

5.3

3.6

2.4

5.4

holes

electrons

5.9

2.8

DPVBi

Recombination zone

ηext = 2.7 %

Comparison point : OLEDs with DPVBi ALONE

ITO

100-150nm

CuPc

10nm

NPB

50 nm

Alq3

10nm

LiF / Al

1.2 / 100nm

PMC:DPVBi

50 nm


Device b (doping) : SUMMARY

PMC:DPVBiDEC:DPVBi

DPVBi

0.1120.1690.1760.2090.100C.I.E. y

0.1490.1580.1600.1920.153C.I.E. x

…28252279…236L (cd/m2) @ 60

mA/cm2

1.21.31.2…0.2ηpower

(lm/W)

2.73.32.81.50.6ηext

(%)

Device (b)

DPVBi

nondoped

Device (b)

DPVBi DEC-

doped (2%)

Device (b)

DPVBi PMC-

doped (5%)

Device (a)

DEC

Device (a)

PMC

► All spectrasimilar to DPVBiand NPB : whichmaterial isemitting light ?

►no shoulder in DEC spectra : suppression ofaggregates by dilution


Summary

We demonstrated state-of-the-art external quantum efficiency

of 3.3% with a deep-blue OLED (CIE x = 0.15 ; y = 0.17) using a

DEC:DPVBi emitting mixture

Close to the max 5% = 25% (singlet/triplet ratio) x 20% (extraction efficiency)

Efficiency of the doping approach : DEC:DPVBi better than

DPVBi alone (or DPVBI:PMC) : attributed to enhanced

trapping of charged carriers

PMC exhibits the most saturated color (x = 0.15 ; y= 0.10) :

better efficiency would be achievable with a different design

while keeping the CIE coordinates (in progress)

highly efficient multilayer organic pure -blue - light ... : oled withneatfilm ofpmc anode cathode...

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