Joachim Kaiser

IMID 2018 / Aug 30, 2018

A25-3

Efficiency Engineering ofBlue, Green and Yellow OLEDs

𝜂𝑒𝑥𝑡 = 𝛾 ∗ 𝜒 ∗ 𝜙 ∗ 𝜂𝑜𝑢𝑡

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Introduction

External Quantum Efficiency in OLEDs

charge

balance

radiative

exciton ratio

PL quantum

yield

light

outcoupling

external

quantum efficiencyHow to influence

by materials?

Transport materials

(e/h)

Emissivematerials

(host/dopant)

Charge balanceand confinement

Indirectly by e.g. stimulating TTA/

avoiding quenching

RefractiveIndex/

scattering

Emissivemechanism

DopantPLQY

Emissive dipoleorientation

(Mixed) host type&ratio

Exciplexforming host

A25-3, IMID 2018 | 30.08.20183

Agenda

New Fluorescent Blue Dopant Development

Highly Oriented Phosphorescent Emissive Systems

Tuning Efficiency by Transport Layers

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Blue OLED performance is strongly influenced by a phenomenological charge balance law, which typically leads to a trade-off between LT and efficiency.

Observation

Efficiency and lifetime as function of charge balance

To tune OLED performance, it is key to understand the charge balance regime a given device is operating in

Ref: E. Böhm et al, IDW’09, OLED1-2

Statistics with various ETL configuartions: Reducing e-transport by diluting ETL:

ETM diluter [%]

Blue OLED performance is strongly influenced by a phenomenological charge balance law, which typically leads to a trade-off between LT and efficiency.

De-tunedregime

De-tunedregim

e

Efficiency optimizedregime

Balancedregime

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Charge balance – Influence of transport layers

The key to tune and understand HTM/ETM performance in blue

e-rich/h-poor

Eff

icie

ncy

Only changingcharge balance

Efficiency / LT plots help to understand and categorize charge-balance induced performance changes

e-poor/h-rich

LT

Eff

icie

ncy

LT

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Example

Merck’s Electron Transport Material portfolio (selection)

ETM portfolio covering a wide range of charge balance for tuning between LT and eff.

more e-transporting

less e-transporting

ETL

Blue EML

EBL

HTL

p-doped HTL

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Example

Merck’s Electron Blocking Material portfolio for s-blue (selection)

• HTM portfolio for tuning between LT and efficiency by choice of EBL• Interaction with choice of ETL has to be considered

h-poor

h-rich

Same order of materials but different curvature

ETL

Blue EML

EBL

HTL

p-doped HTL

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Example

Interaction between HTL and EBL

• Strong interaction with HTL• Most efficient EBL in one device can be least efficient in another!

Eff

icie

ncy

LT

Reminder

HOMO

HTL EBL HTL EBL HTL EBL ETL

Blue EML

EBL

HTL

p-doped HTL

h-poor h-rich

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Is it only charge balance?

Influence of triplet-triplet annihilation (TTA)

• TTA plays a role in efficiency increase of e-rich OLED “charge balance induced TTA”

• However, other mechanisms must be involved

Measurement method: Results:

0.00

0.10

0.20

0.30

0.40

0.50

-10 0 10 20

EL [a

.u.]

t [µs]

DEL (delayed EL)

Steady-state EL

DEL/Steady-state EL

applied on

e-diluter EQE @10mA/cm²

DEL ratio@10mA/cm²

EQE withoutDEL

50% 7.3% 31% 5.6%

70% 5.6% 23% 4.6%

75% 4.6% 11% 4.1%

e-p

oor

ETL:diluter

Blue EML

EBL

HTL

p-doped HTL

ETM diluter [%]

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How about green OLED?

Merck’s Electron Blocking Material portfolio for green (selection)

• Efficiency / LT trade-off is typically also present in phosphorescent green OLEDs• Wide EBL portfolio range available for green OLED

Established generation

New generation

ETL

Green EML

EBL

HTL

p-doped HTL

„Charge Balance“ is often

dominating the LT/efficiencyperformance

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Key Messages

Tuning efficiency by transport layers

Portfolio approach is key for offeringTransport Layers.

Interplay between HTL/EBL/HBL/ETL determines overall performance.

Extensive combination screening isnecessary to optimize OLED stack.

• Actively developing materials for various OLED layers to understand full stack• Significant investments in physics / application lab resources

Merck‘sapproach

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Agenda

Tuning Efficiency by Transport Layers

Highly Oriented Phosphorescent Emissive Systems

New Fluorescent Blue Dopant Development

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In 2014, Merck introduced a novel ultra-deep blue dopant (peak <450nm)

Review

Introduction of ultra-deep blue dopant

• Ultra-deep blue dopant with excellent color and lifetime properties• However, EQE limited due to dopant PLQY of 0.8

Ref: H. Heil et al, SID 2014, 35.1

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New development

Ultra-deep blue dopant with improved efficiency

• Successor dopant with similar characteristics but 10% higher PLQY• Ideally suited for ultra-deep blue bottom emission application

95

96

97

98

99

100

101

0 50 100

rel. lu

m. [%

] @

60

mA

/cm

²

time [h]

0

2

4

6

8

10

12

0 2000 4000 6000

EQ

E [%

]

luminance [cd/m²]

0

0,2

0,4

0,6

0,8

1

1,2

400 500 600

EL n

orm

. [a

rb. u

nits]

wavelength [nm]

Al

1 nm LiQ

30 nm ETL:LiQ

20 nm Blue EML

10 nm EBL

160 HTL

20 nm HTL:p-doped

ITO

Dopant PLQY EL peak FWHM CIE x/y Voltage@10mA/cm²

Lum. Eff. @10mA/cm²

EQE@10mA/cm²

LT95 @60mA/cm²

LT95 @1000cd/m²

Old 0.80 447 nm 30 nm 0.148 / 0.080 4.0 V 6.0 cd/A 8.0 % 78 h ~400 h

New 0.88 449 nm 26 nm 0.146 / 0.078 4.0 V 6.5 cd/A 8.9 % 76 h ~450 h

Old

New

A25-3, IMID 2018 | 30.08.201815

Agenda

Tuning Efficiency by Transport Layers

New Fluorescent Blue Dopant Development

Highly Oriented Phosphorescent Emissive Systems

Measurement method: Simulation: Results:

Molecular dynamics sim.

Optical excitation of film by polarized laser of evaporation process

Measurement of PL spectra (angularly res.)

Fit optical model to obtain dopant orientation

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Dopant orientation

Preferred dipole orientation parallel to substrate

New ph.-yellow dopants with significant suppression of perpendicular orientation

Dopant Orientation simulated(p|| : p┴)

Orientation measured(p|| : p┴)

Isotropic 2:1

Perfectorientation

2:0

New Yellow 1 2:0.68 2:0.58

New Yellow 2 2:0.43 2:0.44

Ref: Appl. Phys. Lett. 96, 073302 (2010) Similar method to:

Nature Comm. 8, 791 (2017)

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Dopant orientation

Phosphorescent yellow dopants with improved efficiency

Dopant Orientation measured

EQE @ 2000 cd/m²

Ir(bzq)3 2:1(assumed)

19.6%

New Yellow 1 2:0.58 25.4%

New Yellow 2 2:0.44 29.8%

Up to x1.5

EQE

increase!

• Close to 30% EQE from a conventional bottom emission OLED!• Concept will be transferred to green dopants

Al

ETL:Liq (x%)

Yellow EML

EBL

HTL

p-doped HTL

ITO

- Ir(bzq)3

- New Yellow 1

- New Yellow 2

New ultra-deep blue dopant with increasedefficiency

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Summary

Merck offers a rich material portfolio addressing efficiency topics

Typically dominating efficiency tuning bytransport layers

New highly efficient phosphorescent yellowdopants

Charge Balance

PLQY

Emitter Orientation

Merck INNOVATION CUP 2019

Anniversary research grands▪ Top young talent from all over the world combined with experienced professionals

▪ Insight on R&D in the pharmaceutical and chemical industry and chance to develop an own idea into a convincing project plan

▪ Team with the best project plan wins the Merck Innovation Cup along with 20,000 Euro

▪ Best worked out project plans are implemented and R&D budget allocated

▪ Concept has won multiple innovation awards

Application phase starts NOVEMBER 1ST

innovationcup.merckgroup.com

See our curiosity at IMID exhibition,

8/29~31, BEXCO Hall 2B, 1F

10/24~26, COEX Hall D, 3F