Solution Processable OLEDs -...

Solution Processable OLEDs

Merck KGaAMerck KGaAAnna Hayer

EuroDisplay 2013

16/09/2013

Content

1 Introduction

2 OLED Basics

EuroDisplay 2013 – Hayer - Merck – Solution Processable OLEDs2 16/09/2013

3 Challenges for Solution Processing

4 Current Results

5 Summary

Why OLED?

ultra-thin & light-weight high contrast / true blackwide viewing angle

fast switching

high energy efficiency


large area

flat light source

fast switching

new design opportunities

� arbitrary 2D shape

� flexible

� transparent

Why solution-processing?

already in mass productionoffer the advantages of

� greater ease of processing

� reduced manufacturing costs

evaporation-based OLEDs solution-processed OLEDs

EuroDisplay 2013 – Hayer - Merck – Solution Processable OLEDs

� reduced manufacturing costs

� very large area application

� variety of processing techniques available

4 16/09/2013

= Light Modulation

Liquid Crystal

= Light Generation

Organic Light Emitting Diode

OLED: What is the buzz about?

OLED: A fully controllable light bulb with the correct colour in every pixel.

3 – 6 V

Back Light Back Light


Content

1 Introduction

2 OLED Basics



4 Current Results

5 Summary

-LUMO

EF

Working principle of simple OLEDs

3 –

6 V


HOMO+

eU

EF

Anode CathodeEmissive Layer

7 16/09/2013

Anode CathodeEmissive Layer

Device setup: Simple stack

� Basic Processes

– Charge injection (1)

– Charge transport (2)

– Exciton formation (3)

21

3

hν


– Recombination (4)

� Drawbacks of single layer device

– Large injection barriers

– Holes und electrons not balanced

– Leakage current

Anode CathodeEmissive layer

(EML)

1 2

3

hν 4

8 16/09/2013

Device setup: Improved stack

Improvements via multilayer:

� Additional injection layer

– Lower voltage

2

1

3

hν 4


� Additional blocking layer

– Reduced leakage

� Specialization of materials

– High mobilities

– High quantum efficienciesAnode CathodeEML

12

3

4

ETLHTL

Electron

injection

layer

(EIL)

Hole

injection

layer

(HIL)

9 16/09/2013

OLED characterisation: Initial performance

apply drive voltage; increase in steps

measure current measure brightness measure spectrum

0.001

0.01

0.1

1

10

100

curr

en

t de

nsi

ty [m

A/c

m²]

0.4

0.6

0.8

1

1.2

EL

no

rm.

[arb

. un

its]

100

1000

10000

100000

lum

ina

nce

[cd

/m²]

EuroDisplay 2013 – Hayer - Merck – Solution Processable OLEDs10

calculate efficiency in

� EQE � photons per electron

� cd/A � visible light per current

� lm/W � optical power per

electrical power

calculate colour coordinatesCIEx, CIEy

0.000001

0.00001

0.0001

0 5 10

curr

en

t de

nsi

ty [m

A/c

m²]

voltage [V]

0

10

20

30

40

50

60

70

80

0 2000 4000

eff

icie

ncy

[cd

/A]

luminance [cd/m²]

460

470

480

490

520

530

540

550

560

570

580

590

600

610

630680

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.5

Y

X

0

0.2

0.4

400 500 600 700 800

EL

no

rm.

[arb

. un

its]

wavelength [nm]

1

10

0 5 10lu

min

an

ce [c

d/m

²]voltage [V]

16/09/2013

OLED characterisation: Long term stability

apply constant current

measure light output over time measure voltage increase over time

6000

8000

10000

12000

lum

ina

nce

[cd

/m²]

3

4

5

6

vo

lta

ge

[V]


0

2000

4000

6000

0 200 400 600 800

lum

ina

nce

[cd

/m²]

time [h]

0

1

2

3

0 200 400 600 800

vo

lta

ge

[V]

time [h]

Lifetime LT50 :

time until the brightness is reduced to 50%

LT80, LT95, LT97 etc. accordingly

16/09/2013

OLED Discovery

1987:

Tang & van Slyke

@Eastman Kodak

OLED from evaporated small molecules

1990:

Burroughes, Bradley, Brown, Marks,

Mackay, Friend, Burn, Holmes

@University of Cambridge

polymer OLED from solution-processed precursor


OLED preparation: Vacuum evaporation

SubstrateCathode

separator

RGB Pixels


Source Shadow Mask

13 16/09/2013

Advantages:

� already in mass production

� high performance

� easy fabrication of multilayer stacks

Drawbacks:

� relatively low material utilisation

� scaling to very large areas challenging

(fine metal masks, ...)

OLED preparation: Solution Processing

Already in mass production in LC industry:

Inkjet printing Examples: other printing methods


Already in mass production in LC industry:

� used for colour filters

� up to Gen 8 (glass size : ~ 2.2 x 2.5 m²)

� to be adapted to functional layers

gravure printing flexographic printing

slot die coating(unstructured, for wide area)

Spin-coating

� Fast and easy process� ideal for material evaluation

� unstructured � not for polychrome displays

Content

1 Introduction

2 OLED Basics



4 Current Results

5 Summary

Solubility

standard evaporable material:

not soluble

Examples for low solubility in common

organic solvents:

N

N

N

Ir

3

Ir(ppy)3

a green emitterNPB

a hole transport material


soluble OLED material

N

N

N

*

*n

16/09/2013

octyl-NPB TFB

one approach to solubilise:

long alkyl chains

often purely aromatic compounds

Film Formation

What is needed

� during coating:

� low aggregation & crystallisation

tendency

� low phase separation

Examples: What it should not look like


� low phase separation

(in a mixture)

� good wetting

� during solvent removal:

� high glass transition temperature

� low aggregation & crystallisation

tendency

16/09/2013

Materials: Polymers vs. SM

Polymers Soluble small molecules

Film formation very goodcrystallisation & aggregation

can be an issue

Reproducibilityin synthesis

challenging to reproduce

molecular weight &

polydispersity

� impact on processing &

device performance

very reproducible

Purification challengingbroad range of established

methods (including sublimation)


Multilayer from solution: ChallengeCathode

ETL

EML

HTL

ITO

EML

HTL

ITO

HTL

ITO

achieve achieve

Avoid washing off or redisolving previous layer during deposition of next layer


EML

ITO

EML/HTL

ITO

EML

ITO

avoid

avoid

ETL/EML/HTL

ITO

washed off reduced mixed mixed

Multilayer approaches: Orthogonal solvents

ETL

EML

HTL

ITO

EML

HTL

ITO

HTL

ITO


ITOITOITO

first layer

from water

second layer

from organic solvent (e.g. xylene)

third layer

from alcohol (e.g. methanol)

example:

PEDOT/PSS PFO PFON+(CH3)3I--PBD

Ma et al., Adv.Mat. 2005, 17, 274

* *n

O

N N

NN

* *n

+

+I-

I-OO

S* *n

**

n

SO3-

Multilayer approaches: Cross-linking

ETL

EML

ITO

EML

ITOdeposition of first

curing next layer can be

hν or ∆T

photoinitiator,

RT


example: Du et al., Macromol. Rapid

Commun. 2006, 27, 412:

O

Al

N

O

O

O

O

N

O

OO

O

N

OO

O

N

Al

O

N

AlO

N Al

O

O

Al

NO

N

O

N

n

m

l

for a review, see

Zuniga et al., Chem.Mat. 2011, 23, 658

ITOdeposition of first

layer from solution

curing

� polymerisation

� layer becomes insoluble

next layer can be

deposited from solution (even from same solvent)

Content

1 Introduction

2 OLED Basics



4 Current Results

5 Summary

Limitations of simple device

50

75

100

125

Lif

eti

me

[k

h]

Triplet Green

no

t accessib

le20

30

40

50

60

Lif

eti

me

[k

h]

Triplet Red

no

t accessib

le

Ba/Al

EML

simple device,no evaporated layers


Efficiency in simple device is limited� cathode quenching

� non-optimal emission zone

� non-tunable electron injection

How to overcome limitation?

0

25

0 25 50 75

efficiency [cd/A]

Lif

eti

me

[k

h]

no

t accessib

le

0

10

20

0 5 10 15

efficiency [cd/A]

Lif

eti

me

[k

h]

no

t accessib

le

EML

HTL

HIL

ITO

16/09/2013

Evaporated layers for higher efficiency

Ba/Al

EML

IL

Al

ETL

HBL

EML0

2

4

6

8

10

12

14

0 5000

EQ

E [

%]

evaporated

solution-processed

RED


IL

Buffer

ITO

EML

HTL

HIL

ITO

0 5000

luminance [cd/m²]

Evaporated layers improve efficiency by ca. 40%

solution-processed

Introduce evaporated HBL + ETL to overcome

efficiency limit

processed

0

2

4

6

8

10

12

14

16

0 5000

EQ

E [

%]

luminance [cd/m²]

GREEN

16/09/2013

Hybrid device for high efficiency

30

40

50

60

Lif

eti

me

[k

h] simple device

with ETL

Triplet GreenTriplet Red

75

100

125

150

Lif

eti

me

[k

h]

simple device

with ETL

Ba/Al

EML

IL

Buffer

ITO

solution-processed


With ETL: Efficiency limitation overcome

0

10

20

30

0 5 10 15

efficiency [cd/A]

Lif

eti

me

[k

h]

0

25

50

75

0 25 50 75

efficiency [cd/A]

Lif

eti

me

[k

h]

16/09/2013

Al

ETL

HBL

EML

HTL

HIL

ITO

evaporated

solution-processed

Al

ETL

HBL

EML

Performance progess via material and stack optimisation

150

200

250

300

350

LT

50

[k

h]@

10

00

cd

/m² with ETL

evaporated

solution-

HTL

HIL

ITO


With optimised materials: strong increase in lifetime as well as efficiency

0

50

100

150

0 25 50 75

LT

50

[k

h]@

10

00

cd

/m²

eff. [cd/A]@1000 cd/m²

simple device

� improved materials

� optimised material combinations

� adapted layer thickness

16/09/2013

solution-processed

� high

� low

amount ofh-transporting

Example: Mixture optimisation

10%

15%

20%

25%

EQ

E

LT8

0

more

h

In this case, both efficiency & lifetime increase when balance is tuned towards more holes

�� initial mix was much too electron-dominated

h-transporting component

EML mixture with 3 components:

� electron-transporting host

� co-host

� triplet emitter

Balance optimisation: how much

� electron-transporting

� hole-transporting

component is needed for best performance?

EuroDisplay 2013 – Hayer - Merck – Solution Processable OLEDsh27 16/09/2013

0%

5%

e-transporting component e-transporting component

more emore e

In this case, both efficiency & lifetime increase when balance is tuned towards more holes

�� initial mix was much too electron-dominated

0.1

1

10

100

1000

cu

rre

nt d

en

sity [

mA

/cm

²]

More h-injecting HTL for high lifetime

9000

9500

10000

lum

ina

nce

[cd

/m²]

HTL 1HTL 2HTL 3

bipolar device

HTL 1

1E-3

0.01

0.1

0 2 4 6

cu

rre

nt d

en

sity [

mA

/cm

²]

voltage [V]

8000

8500

0 200 400 600 800 1000

lum

ina

nce

[cd

/m²]

time [h]

If hole transport is so important for this mixture:

Improve best mix even further by also increasing hole injection


hole-only device

HTL 1HTL 2HTL 3

With new HTL: 300 000h lifetime achieved

>900h @ 10 000 cd/m²

� 300 000h @ 1 000 cd/m²

Content

1 Introduction

2 OLED Basics



4 Current Results

5 Summary

Conclusion

� OLEDs for ultra-thin lighting tiles and displays with wide viewing

angle, high efficiency & contrast and new design opportunities

� solution-processing for very large area applications, greater ease

of processing and reduced manufacturing costs

� key parameters for solution processing: � key parameters for solution processing:

solubility & film formation

� multilayer OLED stacks from solution:

e.g. via use of orthogonal solvents or via cross-linking

� high efficiency in solution-processed OLED achieved with

evaporated ETL

� improved materials, optimised material combinations and adapted

layer thickness for very high performance:

300 000h lifetime and 80cd/A / 21% EQE in soluble green


ETL/

EML/HTL

ITO

0

50

100

150

200

250

300

350

0 25 50 75

LT

50 [

kh

]@1000 c

d/m

²

eff. [cd/A]@1000 cd/m²

simple

device

with ETL

0

25

50

75

100

125

150

0 25 50 75

efficiency [cd/A]

Lif

eti

me

[k

h]

simple device

with ETL

Solution Processable OLEDs -...

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