Getting All the Light Out, or No Photon Left Behind

Steve Forrest

Departments of Electrical Engineering and

Computer Science, and Physics

University of Michigan

Ann Arbor, MI 48109

OLEDs: Major Remaining Challenges for Lighting

• Blue Lifetime

• Getting the Light Out

• Cost & Yield

– Patterning & Deposition

– Throughput

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 X 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

4

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

Gu, et al., Opt. Lett., 22, 396 (1997)

ηext~40%

Pyrimidal structures on substrate

Microlens arrays Polymer hemispheres Much smaller than pixel

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

6

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.

7

2mm

The Real Things

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

8

400 600 800

0.0

0.5

1.0

No

rm.

EL

(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

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

The grid is out of the OLED active region

Waveguided light is scattered into substrate and air modes

9

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)

Emission fields

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WITH GRID

WITHOUT GRID

VERTICAL DIPOLE

HORIZONTAL DIPOLE

Mostly in-plane (Waveguided)

Mostly out-of-plane (Glass + air)

Effects of Refractive Index

6

160nm Host

120nm org 70nm ITO

Grid

3µm 1µm

Simulation results • Larger Refractive index difference

gives higher enhancement • This is not a cavity effect

450 500 550 600 650 700 750 8000.0

0.5

1.0

Inte

nsity (

No

rm.)

Wavelength (nm)

Optical Characteristics

13

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

Pt(dbq)(acac)

Isotropic Orientation

Horizontal Orientation

Manipulating Molecular Orientation

Ir(ppy)3

Example results

0 10 20 30 40 50 60 70 80 900.0

0.5

1.0

1.5

2.0

2.5

Detector Angle [Degree]N

orm

alize

d I

nte

nsit

y

Isotropic (hor

= 66.7%)

Horizontal (hor

= 100%)

Fit (hor

= 45.9%)

CBP:Pt(dbq)(dpm) 8wt%

Pt(dbq)(dpm)

0 10 20 30 40 50 60 70 80 900.0

0.5

1.0

1.5

2.0

Detector Angle [Degree]

No

rmalized

In

ten

sit

y

Isotropic (hor

= 66.7%)

Horizontal (hor

= 100%)

Fit (hor

= 65.9%)

CBP:Irppy3 8wt%

Ir(ppy)3

Approach largely impractical • Added constraints on molecular design • Added constraints on process (growth) conditions: may not align as expected • Added constraints on device architecture • Alignment is never “perfect”: only modest improvements

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

One possible solution: Surface corrugations

• Waveguide thickness varies dueto the corrugation.

• As the thickness changes, themode distribution changes.

• When the waveguided powertravels from thin to thick areas,the k vector needs to changedirection to keep “beingtrapped”. Otherwise, the light isextracted.

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

• 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?

Conclusions

• Optimal criteria for outcoupling solutions

– Low cost

– Angle and wavelength independent

– Minimal impact on established OLED and materials designs

• Sub-anode grid outcouples all waveguide modes

• No impact on electrical characteristics

• No significant optical effect

• No active dead zone

• SPP Modes are the final frontier

– 80% outcoupling may be possible if appropriate techniques are employed

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