Light Extraction in OLED with Corrugated Substrates

Franky So

Department of Materials Science and EngineeringNorth Carolina State University

Raleigh, NC 27695-7907

1

Where Did the Light Go?

2

• Modes Distribution in OLED

• Optical Modes and In-plane Wave-vector

• Light Extraction Techniques

Glass

Organic

Al

ITO

TM0 (SP)

TE0

� Internal quantum efficiency (ηint) : ~ 100%

� External quantum efficiency (ηext) : ~25%• ITO/organic waveguide mode : ~50%

• Substrate mode : ~25%

• Extracted mode : ~20-25%

ITO/organic mode

Substrate mode

Air mode

Escape cone

intoutext KFK

χout : ~20-25%

Optical Modes in OLED

Optical Modes – ETL Thickness Dependence

4

Appl. Phys. Lett. 97, 253305 (2010)

In-plane Wave-vector

Wave-vector k• In the air k0 = 2π/λ• In an organic layer k = norgk0• k = kx

2 + kz2

z

x Metal

Organic

ITO

Glass Sub

kx=0

kx= k0

Air

WG

SPP

Simulated Mode Distribution

SubAir

kx(μm-1)P

hoto

n E

nerg

y (e

V)

k0 nglassk0

5

0 251.9

2.8

Optical Modes in Green OLEDSimulated Mode Distribution

• Photons couple into each optical mode • Only 30% can emit into air• 16% is trapped in waveguided mode• 26% is trapped in SPP mode

λ=520nm

Air (30%) Sub(24%)

kx (μm-1)

λ=520nm

EL Spectrum of Ir(ppy)2(acac)

MoOx (10nm)

TAPC (40nm)

CBP:Ir(ppy)2(acac)(20nm, 7%)

B3PYMPM (10nm)

Alq3: Cs2CO3 (30nm)

Cs2CO3/Al

ITO(100nm)

Simulation Structure

6

Light Extraction Techniques

• Light extraction techniques to improve OLED light outcoupling • Extracts substrate, waveguided and SPP modes

Sub-anode grids Corrugation Microlens arrays Porous substrateHalf-ball lens

7

Extracting Light with Corrugation

8

• OLED on PDMS/Al Corrugation

• OLED on Corrugated Glass Substrate

• OLED on Corrugated Sapphire Substrate

15 nm Al100 nm ITO

LiF & Al cathode

50 nm TAPC

70 nm Bphen15 nm CBP:Irppy315 nm TCTA:Irppy3

0 1 2 3 4 5 6 7 81E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10 Buckling Reference

Lum

inan

ce (c

d/m

2 )

Cur

rent

den

sity

(mA/

cm2 )

Voltage (V)

10-1

100

101

102

103

104

105

I-V-L

Corrugated OLED on PDMS/Al

9

¡ Efficiency at 1000 cd/m2

• Ref. : 50.8 cd/A, 30.3 lm/W

• Buckling : 79.2 cd/A, 56.7 lm/W

¨ 56% enhancement in cd/A and

87% enhancement in lm/W

10 100 1000 100000

20

40

60

80

100

Pow

er e

ffici

ency

(lm

/W)

Cur

rent

effi

cien

cy (c

d/A)

Luminance (cd/m2)

0

20

40

60

80

100

Efficiency

Adv. Opt. Mater. 1, 404 (2013)

Corrugated OLED on PDMS/Al

10

0

30

60

90

Reference Buckling Lambertian

Emission angle (q)0

0.5

1.0

1.5

2.0

500 550 600 650

0q 20q 40q 60q

Buckling Ref.

EL in

tens

ity

Wavelength (nm)

EL spectrum Angular dependence

¡ EL spectrum

• Enhancements over all λ & angles

¡ Angular dependence

• Lambertian distribution

Adv. Opt. Mater. 1, 404 (2013)

Angular Dependence

11

– High efficiency planar control device– Combined light extraction methods

• Corrugated epoxy glass extracts waveguided mode and SPP mode• Macrolens extracts substrate mode

Organic Layers

ITO

Al

Glass

Organic Layers

ITO

Glass

Al

Hemisphere Lens

Planar Corrugated with lens

OLED on Corrugated Glass Substrate

12

OLED on Corrugated Glass Substrate

13

Hemi Sphere Lens

Glass SubstrateCorrugated Epoxy

ITO MoOx (10nm) TAPC (40nm)

CBP:Ir(ppy)2(acac)(30nm, 7%)

3TPYMB (20nm)NBphen (20nm)LiF/Aluminum

13

• Over 60% EQE achieved on corrugated 3TYPMB device • Over 25% enhancement by corrugation • 2.7 times enhancement by combining corrugated substrate and hemi

sphere lens.

Corrugated Device with Lens

Corrugated Device

Planar Device

OLED on Corrugated Glass Substrate

14

Surface Plasmon

Waveguide

Absorption

Substrate

Air

Mode distribution

Air mode Substrate mode Absorption loss Waveguidemode SP mode

Normal glass 20~25% 20~25% 5~10% 20~25% 30~35%

High index glass 20~25% 30~35% 5~10% 0 35~40%

Normal glass substrate n = 1.5 High index glass n = 1.8

Surface Plasmon

Absorption

Substrate

Air

OLEDs on High Index Substrates

10 sccm of BCl3, 25 sccm of Cl2 and 10 sccm of Ar

Silica sphere diameter: 500 nm

Corrugated High Index Substrate

SEM images

Corrugation depth: 90 nm

Nominal periodicity: 500 nm

Corrugated Sapphire Substrate

HI glass

ITO

TAPCCBP :Irppy3

TCTA :Irppy3

3TPYMBAl/LiF

Plain Grating

Index matching

gel

Sapphire macro lens

OLED on Corrugated Sapphire Substrate

Extraction of Substrate Mode by Macrolens

19

EQE Current efficiency

¡ Peak comparison

• 63 %EQE and 225 cd/A for grating devices (20 ~25% up)

• 52 %EQE and 178 cd/A for plain devices

10 100 1000 100000

20

40

60

80

100

Plain 260nm grating

EQE

(%)

Luminance (cd/m2)

10 100 1000 100000

50

100

150

200

250

Cur

rent

effi

cien

cy (c

d/A)

Luminance (cd/m2)

Plain 260nm grating

ACS Appl. Mater Interface 7, 8974 (2015)

0

1

2

3

Norm

alize

d In

tens

ity (a

.u.)

Plain glass without macrolens with lacrolens

450 500 550 600 650 7000

1

2

3

Wavelength (nm)

Norm

alize

d In

tens

ity (a

.u.)

Plain high index glass

500nm grating glass

450 500 550 600 650 700

Wavelength (nm)

500nm grating high index glass

Plain glass Plain high index glass

500nm grating glass

500nm grating high index glass

Current efficiency (cd/A) 168 178 184 225

OLEDGlass or high index glass

Extraction of Substrate Mode by Macrolens

OLEDHigh index glass

Macrolens

Planar normalglass

Planar high index glass

Grating high index glass

Current Efficiency (cd/A) 168 173 210

EQE (%) 46 52 63

Extraction of waveguided mode

Extraction ofSPP mode

Extraction of Substrate Mode by Macrolens