Challenges in Achieving Low Cost, Spectrally Broad OLED

Outcoupling

Steve Forrest

Departments of Electrical Engineering and

Computer Science, and Physics

University of Michigan

Ann Arbor, MI 48109

OLEDs: Major Remaining Challenges for Lighting

• Getting the Light Out

• Blue PHOLED Lifetime

• Cost & Yield

– Patterning & Deposition

– Throughput

80% of Light is Trapped in the OLED

ηEQE = ηIQE (~100%) × ηExt ≈ 20%

Total internal reflection at the air substrate interface —substrate modes

Higher refractive indices of organic materials and ITO – waveguide modes

Organic metal interface –surface plasmon modes

3

Cathode metal (mirror)

Organics

ITO

Glass substrate

k||

k

θ

• Air modes: EQE first increases, then decreases with ETL thickness

• Waveguide modes: Only one waveguide mode TE0 due to thin ETL (<30nm). TM0 appears when >50nm.

• Surface plasmon polariton modes: Reduced with ETL thickness • Both waveguide and SPP modes are quantized • Total energy is the integral of Power Intensity cos(θ), so SPP

not as small as it looks

Where do all the photons go?

0 0.2 0.4 0.6 0.8 1 1.20

0.5

1

1.5

2

2.5

3

3.5x 10

-3

k||/k

pow

er

inte

nsity

10nm

30nm

50nm

70nm

90nm

ETL thickness

(20%) (20%)

(15%) (45%)

• Good solutions

Inexpensive

Viewing angle independent

Independent of OLED structure

• Among those things that have been tried

Optical gratings or photonic crystals1

Corrugations or grids embedded in OLED2

Nano-scale scattering centers3

Dipole orientation management

5

1Y .R. Do, et al, Adv. Mater. 15, 1214 (2003). 2Y. Sun and S.R. Forrest, Nat Phot. 2, 483 (2008). 3Chang, H.-W. et al. J. Appl. Phys. 113, - (2013).

Getting all the photons out

Substrate Modes: ~2X Improvement

ηext~40%

Microlens arrays Polymer hemispheres Much smaller than pixel

Möller, S. & Forrest, S. R. 2001. J. Appl. Phys., 91, 3324.

Several adequate solutions exist today

7

Metal electrode pixel

Organics

Low-index grid

ITO

Glass substrate

Side view

wLIG

worg

Waveguide Modes Embedded Low Index Grid

ηext~60% (incl. substrate modes)

Sun, Y. & Forrest, S. R. 2008. Nature Photon., 2, 483.

8

2mm

The Real Things

• OLED >> Grid size >> Wavelength • Embedded into OLED structure • May partially decouple waveguide mode from SPPs

These solutions often interfere with OLED structure

9

400 600 800

0.0

0.5

1.0

Norm

. E

L (

a.u

.)

Wavelength (nm)

Dev. 1

Dev. 4

10-6

1x10-5

1x10-4

10-3

10-2

10-1

0

10

20

30

Po

we

r E

ffic

ien

cy (

lm/W

)

Exte

rna

l Q

ua

ntu

m E

ffic

ien

cy (

%)

Current Density (mA/cm2)

Dev. 4

Dev. 3

Dev. 2

Dev. 1

0

10

20

30

40

50

60

70

80

Dev. 1 x 1.32

Dev.1x1.68

Dev. 1 x 2.3

10-3 10-2 10-1 100 101 102 10-3 10-2 10-1 100 101 102

Device Performance Using Embedded Grids + Microlens

Device 1: Conventional Device 2: LIG only Device 3: Microlenses only Device 4: LIG + Microlenses

Method is Wavelength Independent

Sun, Y. & Forrest, S. R. 2008. Nature Photon., 2, 483.

A better approach: Sub-Anode Grid

10

Cathode

nglass=1.5

nhost

norg=1.7, nITO=1.8

ngrid ngrid waveguided power + dissipation

Collect substrate mode power

Qu,Slootsky, Forrest, Nature Photonics (2015)

Multi-wavelength scale dielectric grid between glass and transparent anode (sub-anode grid)

The grid is outside of the OLED active region

Waveguided light is scattered into substrate and air modes

Emission fields

11

WITH GRID

WITHOUT GRID

VERTICAL DIPOLE

HORIZONTAL DIPOLE

Mostly in-plane (Waveguided)

Mostly out-of-plane (Glass + air)

450 500 550 600 650 700 750 8000.0

0.5

1.0

Inte

nsity (

No

rm.)

Wavelength (nm)

Optical Characteristics

12

Little or no impact on emission characteristics of the OLED

Optical Power Distribution

Thick-ETL organic structure:

340nm grid/70nm ITO/2nm MoO3/40nm TcTa/15nm CBP: Ir(ppy)3/10nm TPBi/230nm Bphen:Li/Al

2nm MoO3/40nm CBP/15nm CBP:Ir(ppy)3/xnm TPBi/1nm LiF/Al

The Last Frontier: Surface Plasmon Polariton (SPP) Modes

ηext > 80% (incl. substrate + waveguide modes)

• Waveguided light excites lossy SPPs in metal cathode • Major loss channel partially eliminated by rapid outcoupling of waveguide modes • Most difficult to eliminate cost-effectively without impacting device structure

• Simple design that does not interfere with OLED structure • Only substrate processing • Extracts all wavelengths approximately equally • 80-90% extraction within reach!

0 50 100 150

0

10

20

30

40

50

60

70

80

90

100

Power at:

460 nm

510 nm

600 nm

Air modes

Waveguide modes

Fra

cti

on

of

Po

we

r (%

)

ETL Thickness (nm)

Surface plasmon

+ Loss

SubstrateMetalmirror

ITO70nmOrganic120nm

ITO50nm

SiO2xnm

SiO2

How Much Light Can We Realistically Hope to Extract?

One possible solution: Surface corrugations

• Waveguide thickness varies due to the corrugation.

• As the thickness changes, the mode distribution changes.

• When the waveguided power travels from thin to thick areas, k|| needs to change direction to keep “being trapped”.

• Otherwise, the light is extracted.

0.8 0.85 0.9 0.95 10

0.5

1

1.5

2

2.5

3

3.5x 10

-3

k||/k

pow

er

inte

nsity

10nm

30nm

50nm

70nm

90nm

TM0

TE0

ETL thickness

Waveguide

nsub/norg=0.88

W. H. Koo, et al, Nat. Photonics 2010, 4, 222.

A possible approach: Surface buckling?

FFT

Al on resin

Substrate Au Ag Mirror on Grid Surface

SiO2 IZO Organic Anti-reflection layer

Getting Rid of SPPs Using Sub-Anode Grid + Mirror

Top Emitting OLED

3 Lift-off

Substrate Fabrication Qu

, et

al. A

CS

Ph

oto

nic

s, 2

01

7

-100 0 100 200 3000.0

0.2

0.4

0.6

0.8

1.0OrganicIZO/MoO3Dielectric

(n)

|HS

PP|2

Thickness (nm)

n=1.5,mm

n=1.8,mm

n=2.0,mm

n=2.2,mm

Metal

-100 0 100 200 3000.0

0.2

0.4

0.6

0.8

1.0

|HS

PP|2

Thickness (nm)

Ag Dielectric (n=1.5)IZO

/MoO3

a.

50 100 150 200 250 3000

20

40

60

80

100

SPPs and loss

Waveguide

modes

Po

we

r D

istr

ibu

tio

n (

%)

Dielectric thickness (nm)

air modes

Decoupling SPPs with Grid + Mirror

Thin grid regions

Thick spacer regions

Qu, et al. ACS Photonics, 2017

a. Ag

IZO/MoO3 80nm

ETL 60 nm

Substrate

EML 20 nm

HTL 40 nm

IZO/MoO3 80nm

SiO2 65 nm

Ag

IZO/MoO3 80nm

ETL 60 nm

Substrate

EML 20 nm

HTL 40 nm

IZO/MoO3 80nm

SiO2 245 nm

a. Ag

ETL 35 nm

Substrate

EML 15 nm

HTL 30 nm

Ag 15nm

Sub-electrode grid modeling

Variable Waveguide Widths Prevent Mode Propagation

0 2 4 6 8 10 12 1410

-5

10-4

10-3

10-2

10-1

100

101

102

Voltage (V)

Curr

ent D

ensity (

mA

/cm

2)

1

10

100

1000

10000

Lum

inance (

Cd/m

2)

0

20

40

60

80100

120

140

160

180

Con

Grid

0.1 1 10 1000

5

10

15

20

25

30

E

QE (

%)

Current Density (mA/cm2)

Performance with and without grid + mirror

Grid

No Grid

Qu, et al. ACS Photonics, 2017

Top emission spectra

450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

Wavelength

0o

30o

60o

450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

Wavelength (nm)

MgF2 AR Coating + Grid No AR Coating + Grid

All top emitting OLEDs show microcavity effects Effects minimized by AR coatings, diffusers, µlenses, etc.

Pt(dbq)(acac)

Isotropic Orientation

Horizontal Orientation

Manipulating Molecular Orientation

Ir(ppy)3

Factors Affecting Dipole Orientation

23

Dipole Orientation

Host-Guest interaction C.Moon et al., Chem. Mater. (2015)

Ligand Effect M.Jurow et al., Nat. Mater. (2016)

Molecular Shape J. Frischeisen et al.,

Org. Electronics (2011)

Substrate & Substrate temp. D.Yokoyama et al., Adv.Func.Mat. (2010)

T. Komino et al., Chem.Mater. (2014)

Static Dipole A.Graf et al.,

J.Mat.Chem.C (2014)

Use Templating to Force TDM

Orientation Thermodynamically

Driven Organization

(Amorphous)

Organization via

Molecular Anisotropy

Self-Templated Host

Pre-deposited Molecular Template

Organization via

Polycrystalline host

Dopant Molecule

(Random Alignment)

Host Molecule

(Random Alignment)

Dopant Molecule

(Anisotropic Alignment) Host Molecule

(Self-Template)

Advantage: Structure driven by growth process, not molecular design Disadvantage: Finding appropriate growth conditions to obtain structure

400 450 500 550 600 650 700 7500.0

0.5

1.0

1.5

2.0

2.5

3.0

P

L In

ten

sit

y (

No

rmalized

)

Wavelength [nm]

1vol.% PtD in PMMA

Neat PtD

Neat PtD

hor

= 0.91 0.01

Templated

hor

= 0.33 0.01

0 10 20 30 40 50 60 70 80 900.0

0.5

1.0

1.5

2.0

2.5

No

rma

lize

d I

nte

ns

ity

Detector Angle [Degree]

hor= 0.67

hor= 1.00

5 10 15 20 25 300

5x103

1x104

2[Degree]

Inte

ns

ity

[C

ou

nts

]

Neat PtD(200)

Top View

φ =

Ψ =100

Examples of Alignment via Templating

Collaborators: M. Thompson, M. Omari, J. Li

0.01 0.1 1 10 1000

2

4

6

8

10

12

EQ

E [

%]

JOLED [mA/cm2]

PtOO8:PtOEP

PtD:PtOEP

High Efficiency Devices via Self Templating

0 10 20 30 40 50 60 70 80 900.0

0.5

1.0

1.5

2.0

hor= 0.67

hor= 1.00No

rma

lize

d I

nte

ns

ity

Detector Angle [Degree]

PtD:PtOEP

hor

= 0.54 0.01

PtOO8:PtOEP

hor

= 0.73 0.01

Conclusions

• Outcoupling solutions should have these properties – Low cost

– Angle and wavelength independent

– Minimal impact on established OLED and materials designs

• Sub-anode grid outcouples all waveguide modes

• With top emission, SPP and waveguide modes not excited – No impact on electrical characteristics

– No significant optical effect

27

Thanks!

• OCM Group

– Yue Qu

– Jonchang Kim

– Michael Slootsky

• Mark Thompson

• Mohammed Omari

• Jian Li