H. Choukri, A.Fischer, S. Forget, S. Chénais, M.-C. Castex, Lab. de Physique des Lasers, Univ. Paris Nord, France Color-control (including White) in OLEDs

H. Choukri, A.Fischer, S. Forget, S. Chénais, M.-C.

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

Color-control (including White) in Color-control (including White) in OLEDs by shifting the position of OLEDs by shifting the position of an ultrathin yellow layer in a blue an ultrathin yellow layer in a blue

matrix.matrix.

Color-control (including White) in Color-control (including White) in OLEDs by shifting the position of OLEDs by shifting the position of an ultrathin yellow layer in a blue an ultrathin yellow layer in a blue

matrix.matrix.

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

N.Lemaitre, B. GeffroyLab. Cellules et Composants, CEA Saclay, France

2CLEO ’06 – Long Beach (USA)

WHITE oleds

WHITE oleds applicationsSolid-state LIGHTING : efficient and energy-saving

large areas (≠ inorganic leds) and potentially flexible devices :

new paradigm in lighting

Pixels or backlight panels for DISPLAYS

Demanding requirements to compete with fluorescent and incandescent

light bulbs : 1) 100% internal quantum efficiency (phosphorescent

materials ?)

2) Low operational voltages and power consumption ( p and n-doped

injection layers ?)

3) efficient and reliable color-mixing schemes to achieve white

http://www.univ-paris13.fr/outils/plan.html

http://www.univ-paris13.fr/outils/aide.html

http://www.univ-paris13.fr/outils/ecri.html



INTRODUCTION

Getting white from OLEDs :

Down-Conversion (Blue OLED + Phosphors) Using exciplex/excimer emission Mixing basic colors in a single layer (« doping ») Mixing colors obtained in separate regions

engineering of the recombination zone microcavity effects






INTRODUCTION : underlying concepts

Mixing two complementary colors in appropriate proportions : blue + yellow

« doping strategy » = mixing a small proportion (typ. <5%) of yellow emitter in a blue matrix → accurate control of weak doping levels difficult

Alternative : Including an ultrathin yellow layer in a blue matrix : better control of the thin film deposition

Varying different parameters (thickness, position) to finely tune the color






STRUCTURE of the OLEDs

+

Glass substrate

Anode ITO

HIL HTL

ETL

Cathode (Al+LiF)EL (blue) EL (yellow)

Thin film deposition by thermal evaporation under high vacuum (10-7

torr)

Anode

ITO

100-150nm

Cathode

+ + ++

+

- - - --

LUMO

HOMO

CuPc

10 nm

ET

L

NPB

50 nm

DPVBi : Rubrène

60-e : e nm

Alq3

10nm

LiF / Al

1.2 / 100nmH

IL

HT

L

Blu

e E

mit

ter

Blu

e E

mit

ter

Yel

low

Em

itte

r

+

2.4

2.8

2.8

5.4

5.9 5.9

5.7

3.0

2.9

4.7 5.3

3.6

3.2

5.4






350 450 550 650 750

Wavelength

Inte

nsity

(A

.U.) absorption

Photoluminescence

Photoluminescence

CHEMISTRY

Well-known materials :

CuPcAlq3

NPB

N

N

N

N

N

N

NN

Cu

N

O

AlO

N

O

N N N

DPVBi

Rubrene

Transporting layersEmitting layers

Anode

ITO

100-150nm

Cathode

+ + ++

+

- - - --

LUMO

HOMO

CuPc

10 nm

ET

L

NPB

50 nm

DPVBi : Rubrène

60-e : e nm

Alq3

10nm

LiF / Al

1.2 / 100nm

HIL

HT

L

Blu

e E

mit

ter

Blu

e E

mit

ter

Yel

low

Em

itte

r

+

Efficient Förster energy transfer

(Blue)

(Yellow)






WHITE OLEDs : principle

Anode

ITO

100-150nm

Cathode

+ + ++

+

- - - --

LUMO

HOMO

CuPc

10nm

ET

L

NPB

50 nm

DPVBi : Rubrène

60-e : e nm

Alq3

10nm

LiF / Al

1.2 / 100nm

HIL

HT

L

Blu

e E

mit

ter

Blu

e E

mit

ter

Yel

low

Em

itte

r

Exciton diffusion + Förster transferExciton formation

Yellow + Blue = White

2.4

2.8

5.4

5.9

5.4

5.7

3.0

2.9

4.7

5.3

3.6

3.6

5.3

3.2






Influence of the total thickness

Design of the OLEDs - part 1Taking into account microcavity effects = locating the recombination zone at an antinode for blue and yellow eigenmodes

ITO[150] CuPc[10] NPB[45.5] Ru[1] NPB[3.5] DPVBi[x] Alq3[10] Al[100]

Balanced white Al

mirrorITO/glass interface

blueyellow






Experimental determination of optimum thickness

ETFOS© software

0

0,2

0,4

0,6

0,8

1

1,2

0 50 100 150 200 250 300

DPVBi thickness (x, in nm)

Norm

aliz

ed

Lum

ina

nce (

A.U

.)

simulation

experimental data

60 nm 140 nm 230 nm

350 450 550 650 750

Wavelength (nm)

350 450 550 650 750

Wavelength (nm)

350 450 550 650 750

Wavelength (nm)

Etfos© software






Influence of the rubrene thickness

Design of the OLEDs - part 2Variation of the rubrene thickness e [1-10 nm]

NPB [50 nm]

Ru [e nm]

DPVBi + Ru [60 nm]

[5 nm] [(55 – e) nm]

0

0,5

1

1,5

2

2,5

3

3,5

0 2 4 6 8 10 12

Rubrene thickness (e, in nm)

Exte

rna

l Qua

ntu

m e

fficie

ncy

ηext (%

)

Optimal Rubrene thickness :

~1 nm

→ « monolayer »

17 Å

7 Å

Rubrene molecule

14 Å






Anode

ITO

100-150nm

Cathode

+ + ++

+

---

-

LUMO

HOMO

CuPc

10nm

ET

L

NPB

50 nm

DPVBi : Rubrène

60-e : e nm

Alq3

10nm

LiF / Al

1.2 / 100nm

Ene

rgy

HIL

HT

L

Blu

e E

mit

ter

Blu

e E

mit

ter

Yel

low

Em

itte

r

Possible explanations : 1) electrons are trapped when rubrene thickness e exceeds the monolayer width poor e-/holes balance at the interface

2) Concentration Quenching of excitons in neat rubrene

-

Influence of the rubrene thickness

e






Influence of the rubrene position

Design of the OLEDs - part 3Variation of the rubrene position d [-10 ► +20 Variation of the rubrene position d [-10 ► +20 nm]nm]

NPB [50 nm] DPVBi [60 nm]●d = 0 +-

Ru [1 nm]▲

Variation of the OLED color from pure yellow (Rubrene) to pure blue (DPVBi) via white







NPB [50 nm]

DPVBi [60 nm]

d = 0 +-

Ru [1 nm]▲

d = -10 nm

Color Spectrum CIE x,y Performances

ηext = 3.4 %

1.2 lm/W

2275 cd/m²

@60mA/cm²

Anode

CathodeVariation of the rubrene Variation of the rubrene position d [-10 ► +20 nm]position d [-10 ► +20 nm]







NPB [50 nm]

DPVBi [60 nm]

d = 0 +-

Ru [1 nm]▲

d = -5 nm


ηext = 1.7 %

0.9 lm/W

1700 cd/m²

Anode








NPB [50 nm]

DPVBi [60 nm]

d = 0 +-

Ru [1 nm]▲

d = -3.5 nm


ηext = 1.7 %

1.1 lm/W

1795 cd/A

Variation of the rubrene Variation of the rubrene position d [-10 ► +20 nm]position d [-10 ► +20 nm]

Bright white : CIE coordinates x= 0.32 ; y=0.33

Color Rendering Index = 73







NPB [50 nm]

DPVBi [60 nm]

d = 0 +-

Ru [1 nm]▲

d = -3 nm


ηext = 1.4 %

0.9 lm/W

1689 cd/m²

Variation of the rubrene Variation of the rubrene position d [-10 ► +20 nm]position d [-10 ► +20 nm]

Anode

Cathode







NPB [50 nm]

DPVBi [60 nm]

d = 0 +-

Ru [1 nm]▲

d = 0 nm


ηext = 1.25 %

1.2 lm/W

1600 cd/m²

Anode








NPB [50 nm]

DPVBi [60 nm]

d = 0 +-

Ru [1 nm]▲

d = 5 nm


ηext = 2.6 %

2.5 lm/W

4067 cd/m²

Anode








NPB [50 nm]

DPVBi [60 nm]

d = 0 +-

Ru [1 nm]▲

d = 10 nm


ηext = 2 %

1 lm/W

1700 cd/m²

Anode








NPB [50 nm]

DPVBi [60 nm]

d = 0 +-

Ru [1 nm]▲

d = 20 nm


ηext = 2.8 %

1.1 lm/W

1900 cd/A

Anode







Anode

Cathode

+ + +

+

+

- -

-

-

CuPc

10nm

NPB

50 nm

DPVBi : Rubrène

60-e : e nmAlq3

10nm

LiF / Al

1.2 / 100nm

-

Rubrene into NPB is better

d = +20 nmx,y = (0.174 ;

0.151)ηext = 2.8 %

d = -10 nm

x,y = (0.172 ; 0.147)ηext = 3.4

%

Anode

Cathode

+ + +

+

+

--- -

CuPc

10nm

NPB

50 nm

DPVBi : Rubrène

60-e : e nmAlq3

10nm

LiF / Al

1.2 / 100nm

-

Rubrene in NPB : No hole

trapping

Rubrene in DPVBi : electron trapping

Comparison of two BLUE diodes with identical CIE x,y :







Estimation of the exciton diffusion length

Simple exciton diffusion model :

Peak rubrene intensity exp(-d/Ld)► Width of recombination zone ~ 15 nm







Summary :NPB [50 nm] DPVBi [60 nm]●

d = 0 +-

Ru [1 nm]▲






Conclusion

We show that we can finely control the OLED color by tuning the position and thickness of a thin layer of pure Rubrene in DPVBi

We obtain very good WHITE Oled with CIE coordinates of (0.32 ; 0.33)and CRI > 70

Same kind of design with three colors (R, G, B) could pave the way toward full color control of OLED in a given chromatic gamut.






Electrical characterization

YELLOW OLED

e=1nm d=0nm (DPVBi)

BLUE OLED

e=1 d=-10 (NPB)

WHITE OLED

e=1 d=-3.5 (NPB)





Documents

H. Choukri, A.Fischer, S. Forget, S. Chénais, M.-C. Castex, Lab. de Physique des Lasers, Univ. Paris Nord, France Color-control (including White) in OLEDs