No Photon Left Behind:Challenges in OLED

Outcoupling

Stephen Forrest

Departments of Electrical Engineering and Computer Science, Physics, and Materials Science and Engineering

University of Michigan

Ann Arbor, MI 48109

80% of Light is Trapped in the OLED

❑ ηEQE = ηIQE (~100%) × ηExt ≈ 20%

2

• OLED Loss Channels➢ Substrate mode

➢Waveguide mode

➢ Surface plasmon polariton (SPP)

➢Metal absorption

• Good solutions▪ Inexpensive

▪ Viewing angle & wavelength independent

▪ Independent of OLED structure

• Examples▪ Optical gratings or photonic crystals1

▪ Corrugations or grids embedded in OLED2

▪ Nano-scale scattering centers3

▪ Molecular dipole orientation management

3

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 light out, or “no photon left behind”

Three solutions explored in SSL

• Sub electrode grids and microlens arrays• Harvests most available photons

• Dielectric diffuser• Efficient and simple

• Ultrathin, roughened substrates• Good for flexible, too.

Substrate Au Ag Mirror on Grid Surface

SiO2IZOOrganicAnti-reflectionlayer

Getting Rid of SPPs Using Sub-Anode Grid + Mirror

Top Emitting OLED

3 Lift-off

Substrate FabricationQu

, et

al. A

CS

Ph

oto

nic

s, 2

01

7

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

Al OrganicITO

SEMLA

High IndexSpacer layer

Getting All the Light Out: Sub-Electrode Microlens Array (SEMLA)

• Micron-scale lens array between the bottom electrode and the glass substrate

• Flat spacer layer

• High refractive index

• Microlens array imbedded into glass

SEMLA Fabrication

8

HF: surfactant

Photoresist 0.8 μm openings with a 10 μm pitch

10 μm

As nsub/norg goes up, more waveguided light is squeezed out

Structure: 70nm ITO/ 40nm TAPC/ 20nm CBP/ 65nm Bphen/Al

Refractive Index of SEMLA

9

n=1.6 n=1.7 n=1.8nsub/norg nsub/norg

0 20 40 60 80 100 120 1400

20

40

60

80

100

Fra

ction o

f P

ow

er

(%)

ETL thickness (nm)

SPPLoss

WV

SEMLA

0 20 40 60 80 100 120 1400

20

40

60

80

100

Fra

ction

of P

ow

er

(%)

ETL thickness (nm)

SPP

WV

SubAir

Loss

SEMLAs Change the Outcoupling Landscape

0 5 1010

-7

10-3

101

Con

SEMLA

J (

mA

/cm

2)

Voltage (V)

102

103

104

0

20

40

60

80 Con_air Con_IMF

SEMLA_air SEMLA_MLA

SEMLA_IMF Sap_IMF

E

QE (

%)

Brightness (cd/m2)

-90

-60

-300

30

60

90 Lamb. Con

SEMLA MLA SEMLA HS

SEM

LA

SEM

LA+M

LA

SEM

LA+I

MF

Sap

1.0

1.5

2.0

2.5

3.0

3.5

41±3%45±4%

27±3%

20±2% 60±4%65±5%

47±4%

E F

30±3%

SEMLA Performance

Diffuse Reflectors: Low Cost & Simple• Dielectric diffusive reflector

✓ No SPP✓ Small absorption (Reflectance ~98%)✓ No angle dependence✓ Reduced w/g mode

Device Structure

Planarization

Layer

0.0 0.5 1.0 1.5-20

-10

0

10

20

Heig

ht (

m)

Distance (mm)

Surface Roughness of Diffuse Reflector

OLED grown on the substrate

PHOLED

Top contact

(ITO)

Bottom

contact (ITO)

Fabrication Sequence

Device Performance – Efficiency

• External Quantum Efficiency (EQE)➢ Mirror 15 ± 2%

➢ Diffuser 37 ± 4% (×2.5)

• Comparable J-V➢ No influence on device structure

Diffuse (Green)

Mirror (Green)

Diffuse (White)

Mirror (White)

0.01 0.1 1 100

10

20

30

40

50

E

QE

(%

)

Current Density (mA/cm2)

Diffuse

Mirror

0 4 8 12 16 2010

-4

10-3

10-2

10-1

100

101

102

Curr

ent D

ensity

(m

A/c

m2)

Voltage (V)

Device Performance – White Spectrum

• White OLED➢ No spectrum shift

✓ Weak optical micro-cavity by high-index planarization layer

➢ Lambertian light source

✓ Scattering via diffuse reflection

0.5

1.0

1.5

400 500 600 700 8000.0

0.5

1.0

1.5Diffuse

0 Degree

30 Degree

60 Degree

No

rma

lize

d I

nte

nsity

Wavelength (nm)

Mirror

90

60

300

30

60

90 Diffuse

Mirror

Lambertian

Device Area

Device Analysis(I) – Peripheral Emission

Device Analysis– Peripheral Emission

• Light emitted outside the defined active area : Peripheral Emission

• Device area ↑ → peripheral emission ↓

• Planar. layer thickness ↓ → peripheral emission ↓, EQE ↑ (68%, ×3.4 @50um)

Device Analysis– Number of Reflections

• Diffuse reflection➢ Redirect light uniformly➢ RS = 30% → air mode

➢ 5 scattering → ~80% escape

0 1 2 3 4 50

1

2

3

Inte

nsity (

a.u

.)

Number of Reflections

Air mode

Substrate Mode

x10-4

Air Mode

out TA D S = + ηTA : Light power fraction to top surface

ηD : Planarization layer efficiency

ηS : Light power fraction into planarization layer

Ultrathin, Ultra-flexible OLEDs With High Outcoupling

Corrugations by Deposition on Rough Sapphire

450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

No

rmalized

In

ten

sit

y

Wavelength (nm)

0o

30o

60o

Ultrathin, Ultra-Flexible OLED Performance

Conclusions

• Substrates can be modified to extract almost all trapped modes in OLED✓Waveguide

✓SPP

✓Substrate

• Best solutions do not interfere with OLED structure✓Wavelength and viewing angle independent

✓Low cost

✓Adaptable to both top and bottom emission

• Practical limit for outcoupling: 70 - 80%

Acknowledgements• Optoelectronic Components and Materials Group @ UM

• Jonchang Kim• Yue Qu• Xiaheng Huang

Acknowledgements• Optoelectronic Components and Materials Group @ UM

• Jonchang Kim• Yue Qu• Xiaheng Huang